Valett, H.M. et al. 1990. Physical and chemical characteristics of the hyporheic zone of a Sonoran Desert stream. Journal of the North American Benthological Society 9(3):201-215.

Abstract: The hyporheic zone of three reaches of Sycamore Creek, Arizona consisted of an average 63 cm depth of predominantly sand or fine gravel (0.5-5 mm). Sediments were highly porous (19-23% interstitial space) and interstitial water volume was 3-4 times that of surface water. Spatial distribution of temperature, sediment organic matter, interstitial nutrients, and subsurface oxygen indicate that physical-chemical conditions vary greatly within the hyporheic zone. Much of the observed variability may be due to repeated disturbance by flash floods. Organic matter content of sediment was low (0.08% by weight), variable, and generally declined with depth in shallow portions of the hyporheic zone. Hyporheic water temperature was higher that surface temperature in regions beneath the wetted perimeter in summer. Nutrient concentrations of interstitial water were enriched compared to surface water; ammonium-N, SRP, and nitrate-N were 269%, 174%, and 327% of surface concentration, respectively. Sub-surface velocity was low (0.62 mm/s), but vertical exchanges were pronounced. Interstitial oxygen was high in regions of infiltration (downwelling), and was generally reduced in discharge regions (upwelling), but subsurface patters were otherwise complex. Vertical linkages between surface and hyporheic zones provide a mechanism for mutual influences. Chief among these are replenishment of interstitial oxygen by downwelling (and enhancement of aerobic respiration), and nutrient enrichment of surface water at upwelling sites.

Fisher, S.G. 1990. Recovery processes in lotic ecosystems: limits of successional theory. Environmental Management 14(5):725-736.

Abstract: The concept of succession has a distinguished history in general ecology and has been applied to stream ecosystems with some success. Succession in stream is largely secondary, follows initial floristics models, and occurs through a variety of mechanisms. The process is moderately predictable but is highly influenced by "climatic" factors, particularly nutrient chemistry. In desert streams, succession does not result in climax state. While evidence is slim, succession may not be a significant process in stream of certain types or in certain regions. Successional theory is difficult to apply in spatially heterogeneous, hierarchically organized ecosystems. It also suffers in begin only one component of a better integrate concept, that of ecosystem stability, which deals more directly with disturbance and ecosystem resistance in addition to resilience (which encompasses succession). Succession has so suffered from a half century of confusion that a strong case can be made for abandoning the term, at least as it applies in streams, in favor of the broader view provided by stability theory.

Fisher, S.G. et al. 1991. Streams and disturbance: are cross-ecosystem comparisons useful? pp 196-221 in J.C. Cole, G.M. Lovett, and S.E.G. Findlay, editors. Comparative analyses of ecosystems: patterns, mechanisms and theories. Springer-Verlag, New York, New York, U.S.A.

Abstract: Disturbance is a pervasive influence in all ecosystems but its effects vary widely. This is true both when quite different ecosystems are compared and when a suite of similar ecosystems, such as streams, is considered. Comparative approaches to ecosystem studies can provide insight into a complex process such as disturbance, but the power to resolve mechanistic questions diminishes as variance among the ecosystems that are compared increases. As a result, the utility of the comparative approach in ecosystem studies will depend on the goals of the investigation. Broad principles of ecosystem science may be revealed by comparison of very different ecosystems, but an understanding of underlying mechanisms shaping ecosystem structure and functioning still requires detailed studies of one or a few similar ecosystems. This is especially true when the process of interest is itself complex. Thus a process such as disturbance may be best studied by comparing similar systems, whereas principles governing simpler attributes, such as decomposition rate or nutrient ratios, may be better elucidated by examining a wide range of ecosystem types.

Boulton, A.J. et al. 1991. Invertebrate recolonization of small patches of defaunated hyporheic sediments in a Sonoran Desert stream. Freshwater Biology 26:267277.

Abstract: Fine mesh (63 æm) traps filled with defaunated sediment and open to recolonization from upstream, downstream or below were buried 30 cm deep in the hyporheic zone of a gravel run in a desert stream and recovered after 1, 3, 7, 14 and 27 days. Temporal changes in organic matter and faunal composition and abundance were compared among treatments and with those in completely sealed "control" traps. Closed traps consistently contained little detritus (< 30 mg AFDM/5000 cm3). Traps facing upstream accumulated significantly more organic matter than those facing below or downstream, indicating downstream hyporheic transport of fine particulate detritus. Upstream facing traps also captured significantly greater numbers of taxa and individuals. Low numbers of most taxa recolonized from all three directions within one day; total densities of individuals in the traps increased before leveling off after two weeks. The prediction that taxa with planktonic epigean relatives (e.g. cyclopoid copepods) would predominate in upstream facing traps was unsupported, nor did unpigmented, blind phreatic forms preferentially colonize traps from below. Two closely-related taxa exhibited major differences in recolonization dynamics, illustrating the risk of masking such trends when pooling data for more than one taxon. Experimental manipulations such as this supplement the predominantly descriptive studies of the hyporheos, and demonstrate the complex dynamics of organic matter and biota in the hyporheic zone that must be considered in analyses of all stream ecosystems.

Boulton, A.J. et al. 1992. Oversummering strategies of macroinvertebrates in intermittent streams in Australia and Arizona. Pages 227-237 in R.D. Roberts and M.L. Bothwell, editors. Aquatic ecosystems in semiarid regions: implications for resource management. NHRI Symposium Series 7. Environment Canada, Saskatoon, Saskatchewan, Canada.

Abstract: Aquatic macroinvertebrates must adopt physiological and/or behavioural strategies to avoid desiccation during waterless periods in intermittent streams. Differential survival determines which biota are present when flow resumes and may influence the direction of subsequent succession. In two Australian intermittent streams, 23 taxa sought refuge below the stream bed in the hyporheic zone (8 taxa), under dried litter and rocks (11), in crayfish burrows (6) or in dry sediments in riffles (18). Most of these taxa were common in epigean samples collected soon after flow resumed. In a Sonoran desert stream, fewer major taxa (13) occupied these refuges and there was little overlap with the fauna collected when flow began, or with the fauna using similar refuges in Australia. These results imply that in the Australian intermittent streams, some stability of the epigean ecosystem to drying disturbances is apparently due to the presence of a variety of over-summering refuges whereas in-stream refuges in a Sonoran desert stream contribute little to the resistance of the aquatic benthos to drying. However, conclusions from inter-continental comparisons of the ecology of arid stream ecosystems must be tempered with recognition of the extreme variability inherent in these systems, even at the broad scale used here. Results from a larger number of intermittent streams subject to similar drying regimes in both continents are needed to fully test our hypothesis.

Stream Solute Workshop(N.B. Grimm with 18 others). 1990. Concepts and methodologies for studying solute dynamics in stream ecosystems. Journal of the North American Benthological Society 9:95-119.

Abstract: A stream solute workshop was held February 1-5, 1989, at The University of Mississippi with the goals of 1) suggesting a conceptual model for stream solute studies that integrates physical, chemical, and biological processes, and 2) identifying advantages and limitations of various methods for studying solute transport and exchanges. Solute dynamics refers to the spatial and temporal patterns of transport and transfers of materials that are chemically dissolved in water. Solute transport and exchange processes can be described by solute transport equations that relate solute concentration to advection, dispersion, groundwater and tributary inputs, transient storage zones, and biotic and abiotic transformations. Studies can be based on these model equations even if a full simulation of a particular system is not attempted. Although no common methodological approach can serve every investigation of solute dynamics, experimental approaches represent a range from greatest control and least realism to least control and greatest realism. The model parameters describe processes that can be investigated in laboratory, chamber, and flume experiments designed to reduce confounding experimental variables. Whole-stream studies, particularly solute injection experiments, provide estimates of solute transfer to and from the water column and can be used to calibrate the simulation models. Transport and transfer models can link experimental results obtained at different scales and increase the opportunity for inter-site comparisons and the extrapolation of results between laboratory, chamber, flume, and whole-stream studies.

Grimm, N.B. 1992. Biogeochemistry of nitrogen in arid-land stream ecosystems. Journal of the Arizona-Nevada Academy of Science 26:130-146.

Abstract: Inputs of inorganic nitrogen to streams of the arid Southwest are dominated by nitrate and ammonium in precipitation and nitrate mobilized from desert soils and transported to stream channels by overland flow. Thus, during flash floods, inorganic nitrogen concentrations are high in stream water. Once in the stream channel, inorganic nitrogen may be exported in surface runoff and subsurface flow to downstream ecosystems, stored and transformed in the hyporheic (deep sediment) zone and later released to the surface stream, or utilized by biota in the surface stream. Important nitrogen transformations within stream ecosystems include autotrophic assimilation by periphyton of surface sediments, and coupled mineralization and nitrification in deep sediments. Stream channels interact with groundwater in this region, thus nitrogen dynamics of streams may affect groundwater quality.

Peterson, C.G. et al. 1992. Temporal variation in enrichment effects during periphyton succession in a nitrogen-limited desert stream ecosystem. Journal of the North American Benthological Society 11:20-36.

Abstract: Periphyton succession was studied over 89 d in adjoining reaches (a riffle and run) in Sycamore Creek, a spatially intermittent desert stream. Effects of nitrogen limitation were assessed by comparing algal development on clay saucers containing either nitrate-enriched or unenriched agar. We evaluated potential effects of grazing on periphyton accrual by amending agar in half the substrata from each enrichment condition with an insecticide (Malathion). Early-successional (25d) communities on unenriched substrata were dominated by Epithemia sorex, a diatom capable of N2 fixation, whereas non-fixing diatoms dominated enriched communities. Nitrate-N enrichment increased algal diversity (H') and delayed the inevitable successional dominance by Calothrix, a heterocystous cyanobacterium. Replacement of diatoms by cyanobacteria was likely facilitated by temporal increases in water temperature and autogenic changes in nutrient and light conditions within the developing periphyton community.
Three measures of algal biomass exhibited nearly linear increases over the 3-month period in all treatments. Enrichment enhanced standing crops of chlorophyll a and ash-free dry mass in both reaches, an effect most pronounced within the first 3-4 wk. Total algal biovolume was unaffected by enrichment, but taxa unable to fix dinitrogen were stimulated by enrichment whereas N2 fixers were not. Biovolume of non-fixers did not change on unenriched substrata, indicating that these taxa are poor competitors in N-poor habitats and may be abundant only during early-successional periods after floods, or in areas of localized high nutrient concentration.
Spatial heterogeneity of nutrient supply in a nutrient-limited system contributes to maintenance of algal diversity, but these effects can be mitigated over long successional seres by autogenic changes that occur during succession. Our results confirm earlier observations that low nitrogen availability limits accrual of algal biomass following floods and constrains algal community structure. However, highly significant early-successional differences in algal standing crop and community structure among enrichment treatments were eliminated or much reduced in mid- to late stages of succession. We propose that this temporal change in response to nitrogen enrichment was caused by development of a thick periphyton mat that reduced access to either water column or substratum-derived nutrients, and increased reliance on internal nutrient recycling.

Boulton, A.J., et al. 1992. Spatial distribution and taxonomic composition of the hyporheos of several Sonoran Desert streams. Archiv fur Hydrobiologie 125:37-61.

Abstract: A survey of desert streams in Arizona, U.S.A. revealed a rich hyporheos (56 taxa) dominated by crustaceans, especially cyclopoid and harpacticoid copepods, amphipods, bathynellaceans, isopods, and ostracods. Interstitial water mites formed a diverse assemblage whereas insects were species-poor relative to surface benthos. There was little faunal overlap between the composition of epigean and hyporheic communities. Most hyporheic taxa appeared to be detritivores. We recognized four biotopes that were physicochemically and biotically distinctive in desert streams: a shallow hyporheic biotope down to approximately 50 cm, a phreatic biotope between 50 cm and bedrock, a parafluvial biotope comprising saturated sediments lateral to the wetted channel, and a dry channel hyporheic biotope that exists temporarily after surface water has disappeared. The shallow hyporheic zone is relatively well-oxygenated, low in nutrients, physically unstable (due to floods and drying), and harbours most taxa. Nutrients are higher whereas oxygen levels are lower in the more stable phreatic zone where larger interstitial spaces and higher hydraulic conductivity allow amphipods, isopods, and bathynellaceans free movement. In the parafluvial zone, nutrient levels vary, the water is hypoxic, and only nematodes and bathynellaceans are common. When surface water dries, a subset of the shallow hyporheic assemblage dominated by the water mites Meramecia and Neomamersa, ostracods, certopogonid larvae, and nematodes occurs. These biotopes can be viewed as functional subunits of the desert stream ecosystem. The existence of a rich hyporheic biota indicates the importance of subsurface processes in desert streams, and may provide a valuable tool for biomonitoring groundwater pollution in arid zones.

Stanley, E.H. et al. 1992. Intermittency, disturbance, and stability in stream ecosystems. Pages 271-280 in R.D. Robarts and M.L. Bothwell, editors. Aquatic ecosystems in semiarid regions: implications for resource management. NHRI Symposium Series 7. Environment Canada, Saskatoon, Saskatchewan, Canada.

Abstract: Intermittent streams are common, particularly in arid and semi-arid regions of the world, yet are seldom studied and poorly understood. We argue that more attention needs to be focused on drying as it is often a major determinant of ecosystem structure and functioning and that drying should be viewed as a natural disturbance. Intermittency is characterized by three hydrologic phases: 1) drying, marked by the development of discontinuous longitudinal flow, 2) dryness, when surface water is absent, and 3) rewetting, which can be either gradual or rapid. Each phase poses distinct challenges to the biota, and consequently studies should be designed according to these phases. Changes in state variables can then be interpreted with respect to ecosystem stability. Resistance to this disturbance is partitioned into resistance to drying (resistance I), resistance to dryness (resistance II), and resistance to rewetting (resistance III). Resilience is the rate of recovery following rewetting. By studying drying as a disturbance and measuring changes in state variables with respect to resistance and resilience, variability caused by different antecedent conditions or different drying regimes can be quantified and compared. Thus, hypotheses explaining differential effects of drying on the stream ecosystem can be generated and tested.

Boulton, A.J. et al. 1992. Stability of an aquatic macroinvertebrate community in a multi-year hydrologic disturbance regime. Ecology 73:2192-2207.

Abstract: We compared rates and directions of benthic aquatic macroinvertebrate succession following eight spates of varying magnitude that occurred in different seasons over 3 yr in Sycamore Creek, a Sonoran Desert stream. A consistent cycle of seasonal change in assemblage composition occurred each year, little altered by spates. Changes reflected variations in presence or absence rather than relative abundance of taxa. Seasonal patterns were confirmed by plotting temporal changes in densities of common taxa. Invertebrate abundance (mostly oligochaetes and mayflies) peaked in spring. "Summer" dominants included the gastropod Physella virgata and the caddisfly larva Cheumatopsyche arizonensis. Assemblage composition remained relatively consistent during spring over 3 yr when high discharge was prolonged, whereas there was a major change in autumn community structure between 1984 and 1986, probably reflecting low discharge during a drought in 1986. Drying apparently influenced assemblage composition more than spates, possibly by altering habitat availability and the intensity of biotic interactions as surface stream volume shrank.
Assemblage resistance to disturbance by spates was variable. Similarly, resistance of individual common taxa varied within and among taxa, and like assemblage resistance, was not simply a function of spate magnitude or timing (season). Resilience was generally high. Succession rate (degree of change in assemblage composition) declined during succession in all but spring sequences, which displayed no consistent trend. The two summer sequences had highest initial succession rates (in first 30 d post-spate), possibly reflecting higher water temperatures, and also exhibited late-successional increases in succession rate. Spatial variation in assemblage composition was uncorrelated with any physical variable measured.
Factors known to influence ecosystem-level processes such as primary productivity (e.g., inorganic nitrogen flux, days since spate) also affected community-level aspects such as aquatic invertebrate assemblage composition in Sycamore Creek. Discharge and water temperature had lesser but detectable effects, and probably contributed to the marked seasonality in assemblage composition. Further comparisons of collective properties of ecosystems and communities within other biomes may identify "common denominators" that characterize responses to disturbance and environmental change. This will remove the different perceptions about stability we gain by using response variables that are assessed only at a community or ecosystem level.

Valett, H.M. 1993. Surface-hyporheic interactions in a Sonoran Desert stream: hydrologic exchange and diel periodicity. Hydrobiologia 259:133-144.

Abstract: Diel variation in interstitial physical-chemical features and hydrologic exchange between the hyporheic zone and surface environment were studied in Sycamore Creek, Arizona. Wells were established beneath the wetted perimeter of the surface stream and beneath exposed alluvial sediments lateral to surface water. Dissolved oxygen (DO) was reduced in the interstitial environment (<50% saturation), but hyporheic water was enriched in nitrate nitrogen (NO3-N) and soluble reactive phosphorus (SRP). Time-of-day and location of wells interacted significantly to produce distinct patterns of diel variation in NO3-N and SRP in wells located beneath the wetted perimeter. In lateral wells, NO3-N concentration showed significant spatial variation, but exhibited no significant variation among sampling times within a day. Vertical hydraulic gradient varied from +0.29 to -0.84 over a 90m study reach and hydraulic heads varied by as much as 9cm on a diel basis. Concentration and flux of NO3-N was greatest in surface water overlying upwelling regions at night when upwelling heads were greatest. Downstream decreases in NO3-N reflected loss of groundwater discharge into surface water and strong assimilatory demand by surface algal communities. Hydrologically mediated exchange can have strong influence on physical-chemical and biologic conditions in the hyporheic zone, benthic environment and associated surface water.

Valett, H.M. et al. 1992. Hyporheic-surface water exchange: implications for ecosystem structure and function. Pages 395-405 in J.A. Stanford and J.J. Simons, editors. Proceedings of the First International Conference on Groundwater Ecology. American Water Resources Association, Bethesda, Maryland, U.S.A.

Abstract: Regions of groundwater that exchange water actively with the surface are considered part of the hyporheic zone. In Sycamore Creek, Arizona, areas of groundwater discharge (upwelling zones) and recharge (downwelling zones) occur over short (150-200m) reaches of the stream. The hyporheic zone is habitat for invertebrate assemblages that reflect differing hydrologic and physical-chemical conditions. When surface water supplies ample oxygen to the interstitial environment hyporheic water is nutrient-rich compared to surface water where fixed inorganic nitrogen is scarce. Under these conditions greater algal standing crop and elevated concentrations of nitrate nitrogen are associated with upwelling zones, reflecting nutrient input from the hyporheic zone. During periods of extensive drying (loss of surface water) reducing conditions occur in the hyporheic zone affecting nitrogen transformations. In Sycamore Creek interaction between interstitial and surface subsystems influences temporal and spatial patterns of drying and rates of system recovery following flash floods. Ecologists are now recognizing the hyporheic zone as an integral part of the stream because it influences solute dynamics, harbors a distinct fauna, and actively interacts with surface waters and the riparian zone.

Stanley, E.H. et al. 1993. Hydrology and the distribution of hyporheos: perspectives from mesic rivers and desert streams. Journal of the North American Benthological Society 12:79-83.

Abstract: Our understanding of spatial and temporal dynamics of the distribution of the hyporheos can be considerably enhanced by an appreciation of surface-hyporheic hydrologic exchange. We illustrate this relationship using two case studies from very different lotic ecosystems: a large mesic river in France and an intermittent desert stream in Arizona. In both cases, there are strong correlations between direction and magnitude of hydrologic fluxes and the composition of hyporheic invertebrate assemblages. Hydrologists are likely to find that the distribution and abundance of hyporheos will provide useful corroborative data in studies of hyporheic flow dynamics, while ecologists should recognize the effects of hydrologic exchange between the surface stream and the hyporheic zone at a variety of scales. Thus hydrologic measurements of surface water and groundwater exchange should be included routinely in future studies of hyporheic invertebrate ecology. Such exchange controls the availability of nutrients, food, and dissolved gases to the hyporheos and indicates a mechanism whereby the subsurface component influences processes occurring in the rest of the lotic ecosystem.

Wood, D.J. et al. 1992. Pools in desert streams: limnology and response to disturbance. Journal of the Arizona-Nevada Academy of Science 26:171-179.

Abstract: Lentic properties of pools and the resistance and resilience of pools and riffles to a flood-related silting event are described in a desert stream, Sycamore Creek, Arizona. Pools stratify on a diel basis. Vertical profiles reveal an average temperature decline of 2.2oC and an average oxygen decline of 2 mg/L with depth. Pools and riffles did not differ in their resistance to a silting event in terms of algal chlorophyll a reduction, and recovery of algal chlorophyll was delayed in both reach types. Macrophytes, which were located mostly in riffles, were both more resistant and more resilient than algae. Because of differential distribution of algae and macrophytes, pools and riffles may differ in stability. When collective components are considered in this study, riffles were both more resistant and resilient than pools to a silting flood disturbance. Flood disturbances which deposit silt differ from scouring floods in that recovery is delayed in the former.

Valett, H.M. et al. 1994. Vertical hydrologic exchange and ecological stability of a desert stream ecosystem. Ecology 75:548-560.

Abstract: The influence of hydrologic linkage between hyporheic and surface subsystems was investigated in sand-bottomed reaches of a desert stream. Direction of hydrologic exchange was measured as vertical hydraulic gradient (VHG) using mini-piezometers. Maps of VHG indicated upwelling (discharge from the interstitial regions into surface water) at the bases of riffles and heads of runs; downwelling (infiltration of surface water into the hyporheic zone) occurred at the bases of runs. Dissolved NO3-N in surface water was higher over or immediately downstream from upwelling zones. Loss of continued supply from the hyporheic zone and intense assimilatory demand by surface autotrophs generated longitudinal declines in NO3-N and lower nutrient concentrations in downwelling zones. Algal standing crop (as chlorophyll a) was significantly higher in upwelling zones than in areas without positive VHG. Postflood trajectories of chlorophyll a indicated that algae at upwelling zones recovered from disturbance significantly faster than those at downwelling zones. Recovery rate was related to supply of NO3-N from enriched interstitial water in the hyporheic zone. Hydrologic linkage integrates surface and hyporheic subsystems and increases ecosystem stability by enhancing resilience of primary producers following flash flood disturbance.

Valett, H.M. et al. 1993. Perspectives on the hyporheic zone: integrating hydrology and biology. Introduction. Journal of the North American Benthological Society 12:40-43.

Abstract: This introduction to six articles on the hyporheic zone notes the rise in the number of presentations on this topic at meetings of the North American Benthological Society, and traces the recent merging of "process-functional" perspectives with "population-community" approaches. Acquiring a broad understanding of the hyporheic zone and its role as a component of stream ecosystems requires the integration of ecological and hydrological techniques and perspectives.

Hakenkamp, C.C. et al. 1993. Perspectives on the hyporheic zone: integrating hydrology and biology. Concluding remarks. Journal of the North American Benthological Society 12:94-99.

Abstract: Hyporheic zone research is an area of rapidly growing interest in stream ecology. Several key points have emerged as important for consideration of future research in the hyporheic zone. Hyporheic researchers need to outline explicitly the spatial scale of their own research, from single sampling locations to entire catchments, and how research at this examined scale relates to finer or larger scaled processes. Spatial and temporal scale considerations are also important when planning sampling and experimental manipulations of hyporheic processes. Stream researchers need to examine the importance of the hyporheic zone as a boundary or ecotone that potentially controls or contributes to surface water and groundwater ecosystem dynamics. Inclusion of hydrologic considerations in the research design and analysis of hyporheic processes is a promising approach that will help to elevate hyporheic research from a descriptive science to a predictive one, and may help to make future cross-system comparisons possible.

Peterson, C.G. et al. 1993. Infection, growth, and community-level consequences of a diatom pathogen in a Sonoran Desert stream. Journal of Phycology 29:442-452.

Abstract: We describe effects of a pathogen that spread through a dense, rapidly growing, benthic diatom community during two infection periods (February and mid-April 1991) in Sycamore Creek, Arizona. Infected areas appeared as gray rings within a matrix of healthy diatom growth and spread rapidly, eventually covering all benthic substrata and causing algal sloughing (within 2 wk in February and 1wk in April). Examination of algal material with transmission electron microscopy revealed the presence of invasive bacteria within diatom cells from infected areas, suggesting a pathogenic bacterium as the most probable cause of this phenomena. Infected areas supported lower chlorophyll a concentrations and contained higher percentages of diatom cells with fragmented or reduced chloroplasts than uninfected areas. Spread of the pathogen appeared to be linked most strongly with diatom densities. The infection spread most rapidly in April, when cell densities were highest, and decimated all diatom species populations. The February infection was more species-specific in its action, affecting large motile and rosette-forming taxa more strongly than small, adnate diatoms. This latter group likely resided at the base of communities and may have been buffered from pathogen transfer by mucilage and/or detrital particles. Consequently, relative abundance of small, adnate diatom taxa increased in algal communities as a result of the February infection. Pathogen-induced alterations of diatom species composition and abundance should influence primary production in this ecosystem and affect the dynamics of organisms that exploit algae as a resource.

Grimm, N.B. 1995. Why link species and ecosystems? A perspective from ecosystem ecology. Pages 5-15 + lt. cited in Jones, C.G. and Lawton, J.H., editors. Linking species and ecosystems. Chapman and Hall, Inc., New York.

Abstract: Population, community, and ecosystem ecology historically have asked different kinds of questions about nature, and as a result have defined domains of study quite differently. In this chapter, the general question "why link species and ecosystems?" will be explored from an ecosystem ecologist's point of view. I begin by considering the subdisciplinary distinctions in ecology: how they are reflected in questions asked, what theories underlie them, and what areas are ripe for integration.
Examples drawn largely from stream ecology are presented in a discussion of redundancy and keystone species. The primary conclusion is that these phenomena occur in nature, are applicable to ecosystem processes as well as community structure, and should be predictable from ecosystem and species characteristics. Although available data do not always allow prediction of which species is likely to affect ecosystem functioning, use of variables such as biomass or production as indicators of a given species' status may lead to misinterpretation. Concepts of interaction strength are applicable to ecosystem studies.
Species occur in nature as members of interactive assemblages, and their interactions may affect ecosystem functioning in subtle or dramatic ways. Given the attention that has been paid by community ecologists to biotic interactions, does an understanding of these processes contribute to understanding ecosystem functioning? A complex stream system and conceptual model are described for the purpose of highlighting questions relevant to integrating biotic interactions, disturbance, and ecosystem functioning.
Finally, some loose ideas on the themes of disturbance and stability, patchiness, and scale as potential starting points for linking species and ecosystems are given in the final section. To facilitate interaction between population-community and ecosystem-landscape ecology, units of study should be spatially-based.

Fisher, S.G. 1996. Natural disturbance and recovery of desert streams: a challenge to wetland management. In C. Montes and G. Oliver, editors. Bases ecologicas para la restauracion de humedales en la cuenca mediterranea. Agencia de Medio Ambiente, Junta de Andalucia, Sevilla.

Abstract: While there are many wetland types in the arid southwestern United States, water is scarce and local wetlands have experienced over 150 years of intensive use, modification, degradation, and more recently, efforts at conservation. Predominant wetland types in Arizona are artificial reservoirs, playa lakes, rivers, streams (many of which are spatially and/or temporally intermittent), marshes (cienegas), and groundwater. Permanent natural lakes do not occur in Arizona. Stream-riparian ecosystems are the predominant form of wetlands in this region, are the most highly valued, and are probably the most degraded. Current efforts to manage and conserve these habitats for a variety of uses are underway. This paper focuses on the structure, functioning, and management challenges inherent in this ecosystem type.
Streams and rivers of arid lands deviate substantially from conceptual models of running waters derived in more mesic regions. In particular, aridland streams are wider and better lighted, support higher primary production, exhibit macroinvertebrate communities dominated by small, fast species, and fail to fit patterns dictated by the River Continuum Concept. Most importantly, natural disturbance is a major organizing force in desert streams, while abiotic forcers in mesic regions are much more benign. Disturbance takes many forms: flash flooding and extensive drying are probably most influential; however, siltation, cattle grazing, algal pathogens, and various human effects such as water diversion, introduction of exotic species, and recreational abuse may have strong effects.
The aridland stream-riparian concept ecosystem provides several challenges to our spatial concept of streams as well. The stream through much of the year is uncoupled from upland areas but interacts significantly with both laterally and vertically dispersed subsystems with which surface flow is intimately connected. The role and significance of subsystem connections varies as a function of both spatial and temporal scale, but at most scales, the hyporheic zone, lateral parafluvial areas (sand bars), and the zone of riparian vegetation are vital components of the ecosystem and must be incorporated in both scientific models and management schemes dealing with streams in all regions.
Spatial and temporal variability present a serious challenge to effective management of arid streams since ecosystem services or amenities are not simultaneously optimized. Furthermore, restoration or preservation effects must take into account the wide variety of states occupied by these ecosystems in space and time. While this multivariate problem provides many targets for restoration efforts, successful management must have as a primary goal, the maintenance of variability through management of normal, multiple disturbance regimes. Scientific research is providing the basis for this understanding; however, degradation has been so severe in some regions of the world that reliance on inter-continental comparisons may be necessary to guide management and restoration efforts in some areas where natural streams no longer exist.

Stanley, E.H. et al. 1994. Invertebrate resistance and resilience to intermittency in a desert stream. American Midland Naturalist 131:288-300.

Abstract: Invertebrate responses to water loss were investigated during drying, dry and rewetting phases in Sycamore Creek, an intermittent Sonoran Desert stream. Some taxa were resistant to drying because they could tolerate rapidly changing water quality and/or more upstream to avoid stranding. Community shifts occurred at one site when it became isolated from up- and downstream reaches; taxa such as beetles, hemipterans, and the snail Physella virgata became dominant. No community changes were detected at a second site which remained connected with upstream reaches by surface flow. Mortality after water loss was severe as few individuals survived longer than 10 days. Low resistance during the dry phase was associated with rapid moisture loss from sediments. Invertebrate persistence in intermittent reaches was due to recolonization after rewetting; however, density increases after floods which reestablished flow at dry sites were lower than reported values for perennial sites in Sycamore Creek. Slower rates of recovery may reflect the composition, reduced size and remoteness of macroinvertebrate colonist pools. Nonetheless, invertebrate persistence in desert streams where both flooding and drying are frequent is due largely to their ability to rapidly recolonize disturbed sites.

Peterson, C.G. et al. 1994. Mechanisms of benthic algal recovery following spates: comparison of simulated and natural events. Oecologia 98:280-290.

Abstract: We conducted a manipulative field experiment to examine individual and interactive effects of scour and short-term nutrient enrichment (4 h exposure) on post-spate recovery of benthic algae in a desert stream. We then compared recovery from these simulated-spate conditions to algal recovery patterns following a natural spate that increased water-column nutrient levels for 2 weeks. That event differentially scoured communities on artificial substrata in place for a long-term experiment, significantly reducing biomass in 49-day-old communities but causing no significant reduction of biomass in older, 133-day-old communities. Thus, we were able to examine recovery of scoured and non-scoured benthic algal communities under natural post-spate conditions. Both natural and simulated spates reduced actual and relative abundances of diatoms within communities. In the manipulative experiment, scoured communities accrued biomass more rapidly than those not subjected to scour, but short-term enrichment had no effect. Accrual of diatoms and green algae was stimulated by the scour manipulation, while cyanobacteria maintained equal rates of growth in all treatments. Following the natural spate, diatom and green-algal densities increased in scoured communities, but recovery of algal biomass was slow on both scoured and non-scoured substrata, primarily because cyanobacteria, the dominant algal group on all tiles, did not increase under exposure to highly nitrate-enriched waters. Rates of algal cell accrual were inversely correlated with the amount of algal biomass present at the start of a recovery sequence. Algal immigration rates measured immediately after the natural spate and during an interflood period in the same season did not differ, indicating that the algal drift pool was not augmented by disturbance. Benthic algal recovery following spates is strongly influenced by the degree of scour generated by the event, but recovery patterns are also affected by the length of post-spate enrichment and the taxonomic composition of the affected community.

Danielpol, D.L. et al. 1994. World subterranean ostracod biogeography: dispersal or vicariance. Hydrobiologia ?:1-12.

Abstract: Origins of the present day distribution of several freshwater and marine phyletic groups of ostracods are described using both Recent and fossil data. Six examples of subterranean ostracods distributed world-wide are discussed. The first two examples (i.e. the Candoninae Namibcypridini and the Sphaeromicolinae) seemed, in a first approach, to fit well with the "vicariance model" but a detailed study demonstrate that their present day distribution can not be seen as a consequence of any geological events. The four other examples (the Xestoleberis arcturi species group, the Tuberoloxoconcha, the Cavernocypris and Fabaeformiscandona wegelini) fit well with the "dispersionist model". We propose a biogeographical model similar to the dispersal one which focus on the ecological processes occurring at local and/or regional scales. Some present day species or their epigean ancestors may originally have been more widely dispersed. These species were predisposed to colonize subsurface habitats; a process that could occur polytopically and at various times. It is the degree of ecological flexibility, the width of ecological tolerance, the type of preadaptations, and the capacity to perceive and successfully invade new environments that allow subsurface ostracods to migrate actively or be dispersed passively through both subterranean and epigean aquatic systems and to settle in new places. But no centers of origin and direction of dispersal can be identified in our data. There is little known about the autecology of subterranean ostracod taxa with broad geographical ranges. Samples should be collected at fine (habitat) and broad scales (regional surveys) so that we can better understand the models of ostracod dispersal across a range of spatial scales.

Holmes, R.M. et al. 1994. Nitrogen dynamics along parafluvial flowpaths: importance to the stream ecosystem. Pages 47-56 in J.A. Stanford and H.M. Valett, editors. Proceedings of the Second International Conference on Groundwater Ecology. American Water Resources Association, Bethesda, Maryland, USA.

Abstract: Nitrogen availability in stream ecosystems depends upon processes occurring in the surface stream as well as upon nutrient exchanges with adjacent ecosystems. In nitrogen-limited desert streams, the hyporheic zone is a source of nitrate to the surface stream, alleviating nutrient limitation in regions where hyporheic water upwells into surface water. We investigated nitrogen dynamics along parafluvial flowpaths in Sycamore Creek, Arizona, and found that parafluvial gravel bars also are sources of nitrate. The magnitude of nitrate production varies temporally and at times the parafluvial zone is a major source of nitrate to the surface stream. Similar studies in other stream ecosystems have found that parafluvial gravel bars may be either sources or sinks of nitrate. In either case, the parafluvial zone is an active site for nitrogen transformation and should be considered in studies of stream nutrient dynamics.

Holmes, R.M. et al. 1994. Parafluvial nitrogen dynamics in a desert stream ecosystem. Journal of the North American Benthological Society 13:468-478.

Abstract: We investigated nitrogen dynamics over a 15-mo period in the parafluvial zone (the part of the active channel without surface water) of Sycamore Creek, Arizona, a nitrogen-limited Sonoran Desert stream. The parafluvial zone and surface stream are linked hydrologically; thus, nitrogen dynamics in the parafluvial zone potentially influence whole-system functioning. We identified discrete parafluvial flowpaths by following the movement of fluorescent dye through gravel bars over time, sampled subsurface water along these flowpaths, and collected parafluvial sediments for measurement of nitrification rate. Water samples were analyzed for nitrate-N, ammonium-N, dissolved oxygen, temperature, and conductivity. Nitrate-N concentration increased along parafluvial flowpaths, with the largest increases occurring in summer. Although ammonium-N concentration was low and did not vary with season or location on flowpath, dissolved oxygen declined as water moved through parafluvial gravel bars. Net nitrification rate was highest in the summer and at the heads of flowpaths where surface water entered the parafluvial zone, suggesting that nitrification may be dependent upon ammonium, dissolved organic nitrogen, or particulate organic nitrogen imported from the surface stream. Overall, the parafluvial zone of Sycamore Creek was a source of nitrate to the nitrogen-limited surface stream, and may play an important role in the productivity of the stream ecosystem.

Jones, J.B. et al. 1995. Vertical hydrologic exchange and ecosystem metabolism in a Sonoran Desert stream. Ecology 76:942-952.

Abstract: Hyporheic metabolism in a Sonoran Desert stream was examine, focusing on the sources of detritus supporting hyporheic respiration. Two alternative hypotheses were specifically addressed: (1) organic matter derived from the surface stream supports hyporheic respiration, and (2) detritus buried during flash floods supports hyporheic respiration. As predicted for the surface-derived organic matter hypothesis, respiration was lowest immediately following flash floods and increased significantly with time after flood (P < 0.001). Hyporheic respiration ranged from 0.05 mgO2-L sediments-1 h-1 immediately following a flash flood to as high as 4.41 mgO2-L sediments-1 h-1 late in algal succession. Respiration was significantly correlated with surface algal biomass during two spring/summer successional sequences (P < 0.05; partial correlation coefficients 0.58 and 0.88). Respiration was also consistently higher in downwelling that upwelling zones with overall mean rates of 1.12 and 0.46 mgO2-L sediments-1 h-1, respectively. Respiration exhibited a distinct diel pattern with highest rate coinciding with time of maximum photosynthesis and was also significantly correlated with dissolved organic carbon concentration (P < 0.05), further supporting the hypothesis of hyporheic dependence on algal production. Flash floods bury organic matter that is also respired in the hyporheic zone; however, based upon storage or organic carbon immediately following floods, an average of only 15% of the observed respiration could be supported. We conclude that hyporheic respiration in Sycamore Creek is tightly linked to surface production. It is spatially distributed in biotic "hot spots" where surface waters enter hyporheic sediments and is most likely supported by organic matter that is supplied as dissolved organic carbon, perhaps from algal production.

Jones, J.B. et al. 1994. Chemoautotrophic production and respiration in the hyporheic zone of a Sonoran Desert stream. Pages 329-338 in J.A. Stanford and H.M. Valett, editors. Proceedings of the Second International Conference on Groundwater Ecology. American Water Resources Association, Bethesda, Maryland, USA.

Abstract: Chemoautotrophic production and respiration (aerobic and anaerobic) were examined along flowpaths in three subsystems in Sycamore Creek, Arizona. Chemoautotrophic production was highest where surface waters enter parafluvial sediments (64 to 76 mgC.m-2.d-1) and lowest in anoxic bank sediments (14 to 16 mgC.m-2.d-1). Aerobic respiration was considerably greater than chemoautotrophy in oxygenated hyporheic and parafluvial zones (2400 to 4900 mgC.m-2.d-1). In anaerobic (i.e., methane production; 3500 mgC.m-2.d-1). Weighting subsystems by areal extent, the largest proportion of aerobic respiration and chemoautotrophic production occurred in parafluvial sediments (64 to 76%), whereas anoxic bank sediments were most important for anaerobic respiration (94% of total anaerobic respiration). Overall, chemoautotrophic production was only 1.0 to 1.3% of respiration and methane production was only 5% of total sediment respiration.

Boulton, A.J. and E.H. Stanley. 1995. Hyporheic processes during flooding and drying in a Sonoran Desert stream. II. Faunal dynamics. Archiv fur Hydrobiologie 134:27-52.

Abstract: Spatial and temporal changes in assemblage structure of the hyporheos of a Sonoran Desert stream were monitored after re-wetting occurred in winter at three sites with different flow regimes. Diverse assemblages of hyporheic invertebrates were found at all sites within two days of re-wetting. At two sites, drying represented a gradual press disturbance to the hyporheos, altering the assemblage composition from a predominance of cyclopoid copepods and chironomids to one dominated by microturbellarians, ceratopogonid larvae, ostracods, nematodes, and just before the hyporheic zone dried, several species of water mites. There were fewest hyporheic taxa at the most intermittent site where cyclopoid copepods were most numerous. The site that remained inundated during the entire study harbored a speciose hyporheos whose composition varied vertically and laterally. Amphipods, isopods, and bathynellaceans were found in the "phreatic" (deeper that 50 cm) zone whereas cyclopoids, cladocerans, chironomids, and water mites were common in the "shallow" (< 50 cm) hyporheic habitat. In wells sunk in the stream bank, nematodes, ostracods, and bathynellaceans characterized the "parafluvial" zone. Spates temporarily altered the assemblage composition of the parafluvial and shallow hyporheic habitats but recovery was rapid, implying a resilient fauna. Phreatic assemblages were not immediately influenced by spates. At the scale of an individual reach (122 m), a distinct longitudinal pattern in assemblage composition was correlated with trends in water chemistry and the gradual transition from zones of hyporheic upwelling (discharge) to downwelling (recharge). Upwelling areas were nitrate-rich, hypoxic, and harbored a typically "phreatic" fauna whereas downwelling zones were well-oxygenated, low in nitrate, and were characterized by taxa found in the shallow hyporheos. This longitudinal pattern was consistent over time but became truncated as surface water receded upstream, altering the exchange of surface and subsurface water. Recognition of temporal changes in the vertical, lateral, and longitudinal variation in subsurface biota and how they respond to surficial hydrologic extremes is essential to understanding the way these subsystems interact with the surface stream ecosystem; the next goal is to determine the causes of these patterns.

Stanley, E.H. and A.J. Boulton. 1995. Hyporheic processes during flooding and drying in a Sonoran Desert stream. I. Hydrologic and chemical dynamics. Archiv fur Hydrobiologie 134:1-26.

Abstract: We examined the effects of flooding and prolonged drying on interstitial water chemistry in the hyporheic zone of a Sonoran Desert stream at three sites with different flow regimes. Sampling began immediately after a winter flood rewetted the previously dry stream, and ended when a summer flash flood destroyed many sampling wells 170 days later. Nitrate-N, soluble reactive phosphorus (SRP), and dissolved oxygen concentrations were high whereas ammonium-N was low at all sites at the beginning of the study period. As discharge declined and drying began, nutrient and dissolved oxygen concentrations declined in phreatic (> 50 cm deep) and parafluvial (lateral to the active channel) subsystems while shallow hyporheic (< 50 cm) samples remained well-oxygenated. Hyporheic water chemistry in areas lacking surface flow (the dry channel hyporheic) varied between sites; nitrate-N and dissolved oxygen decreased at one site but increased at another. With continued drying, phreatic and parafluvial habitats became extremely hypoxic (< 0.50 mg/L dissolved oxygen) and low in nitrate-N (< 0.02 mg/L) due to reduced hydrologic exchange between surface and hyporheic waters. The increased area of hypoxia apparently enhanced nitrate-reducing transformations in phreatic and parafluvial habitats because interstitial nitrate-N concentrations often were lower than surface values; thus, drying caused a functional shift in this N-limited stream as the hyporheic zone changed from a nitrate source to a nitrate sink. Boundaries of hyporheic subsystems fluctuated considerably in response to flooding and drying, underscoring the importance of hydrologic extremes to the strength of chemical linkages between the surface stream and the hyporheic zone.

Jones Jr., J.B. et al. 1995. Nitrification in the hyporheic zone of a desert stream ecosystem. Journal of the North American Benthological Society 14:249-258.

Abstract: Nitrification in the hyporheic zone of Sycamore Creek, a Sonoran Desert stream, was examined, focusing on the association between respiration and nitrate production. Subsurface respiration in Sycamore Creek is highest in regions of hydrologic downwelling where organic matter derived from the stream surface is transported into the hyporheic zone. Similarly, nitrification was closely related to hydrologic exchange between the surface and hyporheic zone. Nitrification in downwelling regions averaged 13.1 gNO3-N.L sediments-1 h-1 immediately following floods to 38.5 gNO3-N.L sediments-1 h-1 late in succession. Nitrification was significantly correlated with hyporheic respiration, supporting the hypothesis that nitrification is fueled by mineralization of organic nitrogen to ammonium. The coupling between subsurface respiration and nitrification is one step in a cyclic interaction between surface and hyporheic zones and serves to transform nitrogen from an organic to inorganic form.

Jones Jr., J.B. et al. 1995. Methanogenesis in Arizona, USA dryland streams. Biogeochemistry 31:155-173.

Abstract: Methanogenesis was studied in five streams of central and southern Arizona by examining the distribution of methane in interstitial water and evasion of methane in three subsystems (hyporheic, parafluvial and bank sediments). In Sycamore Creek, the primary study site (studied during summer and early autumn), methane content of interstitial water exhibited a distinct spatial pattern. In hyporheic (sediments beneath the wetted channel) and parafluvial zones (active channel sediments lateral to the wetted channel), which were well oxygenated due to high hydrologic exchange with the surface stream and had little particulate organic matter (POM), interstitial methane concentration averaged only 0.03 mgCH4-C/L. Bank sediments (interface between the active channel and riparian zone), in contrast, which were typically vegetated, had high POM, low hydrologic exchange and concomitantly low dissolved oxygen levels, had interstitial concentration averaging 1.5 mgCH4-C/L. Methane emission from Sycamore Creek, similar to methane concentration, averaged only 3.7 mgCH4-C m-2 d-1 from hyporheic and parafluvial zones as opposed to 170 mgCH4-C m-2 d-1 from anoxic bank sediments. Methane in four additional streams sampled (one sampling date during late winter) was low and exhibited little spatial variation most likely due to cooler stream temperatures. Interstitial methane in parafluvial and bank sediments of all four streams ranged from only 0.005 to 0.1 mgCH4-C/L. Similarly methane evasion was also low from these streams varying from 0 to 5.7 mgCH4-C m-2 d-1. The effects of organic matter and temperature on methanogenesis were further examined by experimentally manipulating POM and temperature in stoppered flasks filled with hyporheic sediments and stream water. Methane production significantly increased with all independent variables. Methane production is greatest in bank sediments that are relatively isolated hydrologically and lowest in hyporheic and parafluvial sediments that are interactive with the surface stream.

Jones Jr., J.B. et al. 1996. A long-term perspective of dissolved organic carbon transport in Sycamore Creek, Arizona, USA. Hydrobiologia 317:183-188.

Abstract: Dissolved organic carbon (DOC) dynamics were examined over five years (1989-1993) in Sycamore Creek, a Sonoran Desert stream, specifically focusing on DOC concentration in surface and hyporheic waters, and rates of export. In 1989 and 1990, the years of lowest stream discharge (0.08 and 0.04 m3 s-1 annual mean of daily discharge, respectively), DOC was high, averaging 7.37 and 6.22 mgCl-1 (weighted annual means). In contrast, from 1991 through 1993, a period of increased flow (1.1, 1.2 and 4.3 m3 s-1), concentration was significantly lower (P<0.001) with annual mean concentrations of 3.54, 3.49 and 3.39 mgCl-1. Concentration exhibited little spatial variation between two sampling stations located 6 km apart along the mainstream or between surface and hyporheic waters. Annual export of DOC from Sycamore Creek varied 100-fold over the five year period from a mean rate of only 24 kgC d-1 in 1990 to 2100 kgC d-1 in 1993. Ninety percent of DOC was exported by flows greater than 2.8 m3 s-1, and 50% during flow greater than 24 m3 s-1; flows of 2.8 and 24 m3 s-1 occurred only 9 and 1% of the time. The export of organic matter in Sycamore Creek appears to be coupled to El Nino-Southern Oscillation phenomena. The years of highest export, 1991-1993, had El Nino conditions while 1989 and 1990 had medial conditions.

Jones Jr., J.B. 1995. Factors controlling hyporheic respiration in a desert stream. Freshwater Biology 34:101-109.

Summary: 1) Experimental manipulations were performed to determine the biological, chemical and physical attributes that govern sediment respiration in the hyporheic zone of Sycamore Creek, a Sonoran Desert stream. 2) Hyporheic respiration per unit volume of sediment was inversely related to diameter of sediment particles, indicating that respiration is affected by availability of substrate for microbial colonization (i.e. sediment surfaces). Respiration rate per unit surface area on sediments was positively correlated with particle diameter, indicating greater metabolic activity of microbes on larger sediments. 3) Hyporheic respiration was more than twice as high in water collected from the surface flow than from subsurface flow. Further, hyporheic respiration was highest immediately following exposure of sediments to surface water and declined over time, presumably due to exhaustion of labile organic matter. 4) Microbial activity was stimulated by addition of algal leachate; however, amendments of leaf leachate had little effect. Respiration was also elevated with dextrose and leucine amendments, but not with inorganic nitrogen additions, indicating hyporheic respiration is carbon limited. 5) Water from the stream surface is probably enriched in labile organic matter derived from algae and stimulates respiration at points of hydrologic downwelling where surface water enters hyporheic sediments. The physical structure of sediments further affects metabolism by affecting the area available for microbial attachment.

Holmes, R.M. et al. 1996. Denitrification in a nitrogen-limited stream. Biogeochemistry 33:125-146.

Abstract: Denitrification was measured in hyporheic, parafluvial, and bank sediments of Sycamore Creek, Arizona, a nitrogen-limited Sonoran Desert stream. We used three variations of the acetylene block technique to estimate denitrification rates, and compared these estimates to rates of nitrate production through nitrification. Subsurface sediments of Sycamore Creek are typically well-oxygenated, relatively low in nitrate, and low in organic carbon, and therefore are seemingly unlikely sites of denitrification. However, we found that denitrification potential (C & N amended, anaerobic incubations) was substantial, and even by our conservative estimates (unamended, oxic incubations and field chamber nitrous oxide accumulation), denitrification consumed 5-40% of nitrate produced by nitrification. We expected that denitrification would increase along hyporheic and parafluvial flowpaths as dissolved oxygen declined and nitrate increased. To the contrary, we found that denitrification was generally highest at the upstream ends of subsurface flowpaths where surface water had just entered the subsurface zone. This suggests that denitrifers may be dependent on the import of surface-derived organic matter, resulting in highest denitrification rate at locations of surface-subsurface hydrologic exchange. Laboratory experiments showed that denitrification in Sycamore Creek sediments was primarily nitrogen limited and secondarily carbon limited, and was temperature dependent. Overall, the quantity of nitrate removed from the Sycamore Creek ecosystem via denitrification is significant given the nitrogen-limited status of this stream.

Grimm, N.B. et al. 1996. Sensitivity of aquatic ecosystems to climatic and anthropogenic changes: the Basin and Range, American Southwest, and Mexico. Hydrological Processes: in press.

Abstract: Variability and unpredictability are characteristics of the aquatic ecosystems, hydrologic patterns, and climate of the largely dryland region that encompasses the Basin and Range, American Southwest, and western Mexico. Neither hydrologic nor climatologic models for the region are sufficiently developed to describe the magnitude or direction of change in response to increased carbon dioxide, thus an attempt to predict specific responses of aquatic ecosystems is premature. Instead, we focus on sensitivity of rivers, streams, springs, wetlands, reservoirs, and lakes of the region to potential changes in climate, especially those including a change in hydrologic patterns such as amount, timing and predictability of streamflow.
The major sensitivities of aquatic ecosystems are their permanence and even existence in the face of potential reduced net basin supply of water, stability of geomorphic structure and riparian ecotones with alterations in disturbance regimes, and water quality changes due to modified water balance. In all of these respects, aquatic ecosystems of the region also are sensitive to the extensive modifications imposed by human use of water resources, which underscores the difficulty of separating this type of anthropogenic change from climate change. We advocate a focus in future research on reconstruction and analysis of past climates and associated ecosystem characteristics, long-term studies to discriminate directional change vs. year-to-year variability (including evidence of aquatic ecosystem responses or sensitivity to extremes), and studies of ecosystems affected by human activity.

Grimm, N.B. and K.C. Petrone. Nitrogen fixation in a desert stream ecosystem. Biogeochemistry: in press.

Abstract: Few measurements of nitrogen fixation exist for streams. Desert streams are warm, well lighted, and often support abundant cyanobacterial populations; thus N2 fixation may be significant in these N-poor ecosystems. N2 fixation was measured in situ by acetylene reduction for two patch types (Anabaena and an epilithic assemblage). Patch-specific rates were high compared with published values (maximum 775 g N2 [83 mol C2H4] mg chl a-1 h-1 or 51 mg N2 [5.4 mmol C2H4] m-2 h-1). Daytime fixation was higher than nighttime fixation, and temperature, light and inorganic N concentration explained 52% of variance in hourly rates over all dates. Diel input-output budgets were constructed on five dates when cyanobacteria were present in the stream. Diel N2 fixation rates were measured for comparison with reach-scale diel nitrogen retention, to assess the importance of this vector to N economy of the stream. Fixation accounted for up to 85% of net N flux to the benthos, but its importance varied seasonally. Finally, we applied biomass-specific fixation rates to 1992 and 1993 biomass data to obtain seasonal and annual N2 fixation estimates. Cyanobacteria were absent or rare during winter and spring, thus most of the annual N2 fixation occurred during summer and autumn. Annual rates of nitrogen fixation for 1992 and 1993 (8.0 g/m2 and 12/5 g/m2) were very high compared to other streams, and moderately high compared to other ecosystems. Like other phenomena in this disturbance-prone ecosystem, nitrogen fixation is strongly influenced by the number and temporal distribution of flood events.

Stanley, E.H. et al. Ecosystem expansion and contraction: a desert stream perspective. BioScience: in press.

Abstract: We examined spatial patterns of surface water distribution, particularly during low precipitation periods, in a Sonoran Desert drainage (Sycamore Creek, AZ). Changes in surface water distribution were viewed in the context of a hierarchial model of stream structure with four levels (reaches, sections, geomorphic phases, and basins). Within each spatial scale (hierarchial level) considered, we observed differential susceptibility to drying among habitat types. Riffles were more likely to dry than runs or pools at the reach scale, while channel areas with broad (unconstrained) valley floors dried sooner and stayed dry longer than channels with narrow (constrained) valley floors at the section level. Surface water extent varied dramatically over time at all spatial scales, with dry periods characterized by extreme habitat shrinkage and fragmentation. For a 12 km length of stream, wetted channel length varied from its maximum of 12 km to a minimum of 4 km over 4 months. Similarly, wetted channel length varied from 23 to 175 km across the 505 km2 basin during a 5-month period. These hydrologic patterns demonstrate that drying is extreme and therefore is likely a key determinant of ecosystem structure and function (e.g., algal and invertebrate biomass and productivity) in the Sycamore Creek basin. While system expansion and contraction are extreme in desert settings, such changes in surface water extent are not unique to arid-zone streams. Lotic ecosystems should properly be viewed as expanding, contracting, and fragmenting habitats; this spatial dynamic is as fundamental to streams as is discharge. Because size change is a defining feature of streams, it has the potential to be a key ecological determinant of structure and function, and should be incorporated into our understanding of stream ecology.