The temporal pattern of river metabolism was estimated for high-order rivers (5–7^th) in the Daly watershed, tropical Australia, during the dry season (May–October) when discharge was supplied predominantly by groundwater. Rates of photosynthesis (P) and respiration (R) were calculated at 4 sites using the open-channel method based on a model of the river's O₂ budget and measured diurnal cycles of dissolved O₂ concentrations and temperatures. The rivers were shallow (average depth  =  0.8 m), clear (1–2 NTU), and had low concentrations of nutrients (≤15 µg/L soluble N and P at most sites) and generally open canopy. At the reach scale, P was limited by light with no evidence of light saturation. An increase in primary producer biomass over the dry season probably underpinned an approximate doubling of P at the 4 sites over the dry season, but increased water temperatures would have contributed, too. P (0.1–4.6 g O₂ m⁻² d⁻¹) in the Daly watershed was similar to rates in a shaded tropical Puerto Rican stream and some temperate rivers but was lower than in nutrient-enriched temperate rivers. We surmise that most P resulted in production of dissolved organic C (DOC), rather than growth of primary producer biomass, which was nutrient limited. R exceeded P (P/R ≈ 0.5), and increased approximately linearly with P (r²  =  0.79–0.99) over the dry season with no statistically significant difference among sites. The similar environmental setting of the 4 sites underpinned their similar temporal pattern of metabolism. Bacterial metabolism of photosynthetically produced DOC (PDOC) could partially explain the tight coupling of R and P but could not account for the river's overall net heterotrophy. The priming effect of bacterial degradation of labile PDOC to increase the mineralization of recalcitrant DOC (e.g., humic acids) provides an explanation for the river's heterotrophy and tight coupling between P and R.

We examined latitudinal patterns of generic richness in stream insects across the Mackenzie River system in northern Canada. We focused on low- to mid-order tributaries flowing into the river and spanning ∼11° of latitude. Physical-habitat characteristics and water chemistry were examined as factors affecting diversity both at the local and regional scale. No overall latitudinal trend was found in richness, but trends were apparent at the order level. Local generic richness of Ephemeroptera and Plecoptera declined toward the north, Trichoptera generic richness was lowest in the middle of the latitudinal range, and Diptera generic richness increased with latitude. Most orders of aquatic insects followed the expected decline in richness with increasing latitude, but the family Chironomidae was an exception, probably because of their ability to tolerate subarctic environmental conditions and because of the relative absence of potential predation and competition from other invertebrates. Streams were characterized along principal component axes corresponding to stream size and terrain, substrate, water source, and turbidity. Most groups were slightly more diverse in larger streams. Ephemeroptera and Plecoptera had higher richness on coarse substrates, and Trichoptera had lower richness in streams in the Norman Range that possessed unique hydrology and water-chemistry characteristics because of the presence of taliks. Chironomid richness was positively correlated with latitude and stream size. Latitudinal gradients in regional diversity largely followed the trends in local richness, but all taxa had small regional numbers of genera near the middle of the latitudinal range, and a decline in overall aquatic insect richness between the south and north was more apparent at the regional scale. We conclude that regional environmental factors control the maximum richness that can be expected in a locality at a given latitude, whereas local environmental factors control the number of taxa present at a site. Spatial gradients in benthic insect richness are likely to affect the applicability of biomonitoring metrics that use groups whose richness varies with latitude.

The hyporheic zone is of great interest for stream ecologists because of its role in stream biogeochemical processing. Our study addresses the effects of leaf-litter inputs and varying discharge on surface–hyporheic water exchange and their possible consequences for the hyporheic zone biogeochemistry. Our study was conducted during autumn in Riera de Santa Fe (northeastern Iberian Peninsula), a stream with a well developed deciduous riparian canopy. We placed 15 wells spaced at 5-m intervals longitudinally down the study reach and measured surface and hyporheic nutrient and dissolved O₂ (DO) concentrations on 23 sampling dates (15 during the leaffall period and 8 after a flood that washed out 65% of the accumulated leaf biomass). We assessed changes in surface-water exchange and in hyporheic NH₄-N and soluble reactive P (SRP) uptake via coinjection of a conservative tracer and nutrients. Compared to surface water, hyporheic water had lower DO, higher SRP and NO₃-N concentrations, and similar NH₄-N concentration. Hyporheic water had higher DO saturation (p  =  0.00) and higher NH₄-N concentration (p  =  0.00) in downwelling than in upwelling wells, whereas SRP and NO₃-N concentrations did not differ significantly between well types (p > 0.05). Hydrologic connectivity was higher in downwelling than in upwelling wells and decreased with leaf-litter accumulation in the stream channel and increased with stream discharge. Increased connectivity after a flood reduced the difference in DO between surface and hyporheic compartments in upwelling and downwelling wells and in NO₃-N in upwelling wells. NH₄-N and SRP uptake responded differently to these changes. Hyporheic SRP uptake rate was controlled by hyporheic SRP concentration, which did not vary with changes in connectivity, whereas NH₄-N uptake rate was indirectly affected by changes in connectivity through the influence of connectivity on DO availability. Last, although no NO₃-N was added during the solute injections, we observed an increase in hyporheic NO₃-N that probably was caused by nitrification. Together these results illustrate how the combination of stream hydrology and organic matter accumulation can dictate seasonal changes in hyporheic biogeochemistry.

The invasive clam Corbicula fluminea has become a widespread benthic invertebrate in many freshwater ecosystems throughout Europe and North and South America. Invasive bivalves can dramatically alter the structure of native benthic communities, so understanding the factors responsible for successful invasion is important. We investigated C. fluminea nutritional requirements for essential lipids in a standardized growth experiment. Juvenile clams were fed different cyanobacteria (Aphanizomenon flos-aquae, Anabaena variabilis, Synechococcus elongatus) or eukaryotic algae (Scenedesmus obliquus, Cryptomonas sp.). Somatic growth rates were then correlated with elemental (C∶N and C∶P) and biochemical (sterol and fatty acid content) components of the food sources and clam tissue. Somatic growth rates were significantly higher when juveniles were fed eukaryotic algae than when fed cyanobacteria. Linear regression analyses revealed significant positive relationships between somatic growth rates and dietary sterol and polyunsaturated fatty acid content. Somatic growth rates also were highly correlated with the total sterol and α-linolenic acid content of clam tissues. This result suggests that the growth of C. fluminea is partially dependent on the availability of these essential lipids in the diet. Algal nutritional value may influence the successful geographic spread of this highly invasive species because food quality and quantity are changing as a result of global warming.

Quantitative biological assessment protocols are needed for monitoring river status and evaluating river rehabilitation efforts. We conducted a standardized macroinvertebrate survey at 100 sites on 38 nonwadeable rivers in Wisconsin to construct, test, and apply an index of biotic integrity (IBI) intended to be such a bioassessment tool. We assigned independent samples to IBI development (n  =  75) and IBI validation (n  =  25) data sets. We placed Hester–Dendy artificial substrates at the sites for 6 wk and processed the samples of colonizing macroinvertebrates in the laboratory with a 500-target subsampling procedure plus a large–rare taxon search. Independent of the biota, we assigned an environmental disturbance score to each site based upon water chemistry, land cover, flow modification, and point-source pollution. Ten metrics that represent macroinvertebrate assemblage structure, composition, and function constitute the IBI: the number of taxa in: 1) Insecta or 2) Ephemeroptera, Plecoptera, Trichoptera (EPT); % individuals that were: 3) Insecta, 4) intolerant EPT, 5) tolerant chironomids, 6) gatherers, 7) scrapers, or from 8) the dominant 3 taxa; 9) the mean pollution tolerance value; and 10) the number of unique ecological functional trait niches. Analyses on both the validation subset of sites and all sites inclusive confirmed that least-disturbed sites had the highest IBI scores, severely disturbed sites had the lowest scores, and moderately disturbed sites had intermediate scores. Chironominae and Hydropsychidae taxa known to tolerate nutrient enrichment and overall degraded conditions dominated samples with low IBI scores. In contrast, a diverse assemblage that thrives in relatively undisturbed conditions was present in samples with high IBI scores. Comparison of the new macroinvertebrate IBI with an existing fish IBI suggested that the indices respond to different environmental stressors and illustrated the limitations of using only one taxonomic group for bioassessment. We discuss new macroinvertebrate methods, an IBI development process, and the refinement of metrics that may be useful in tailoring assessment tools for large rivers or wadeable streams in other regions. We also present applications of the IBI, including its potential use in comprehensive large river monitoring programs and for evaluating management efforts.

Differences in the raw data used in bioassessments and choices regarding how those data are analyzed and summarized can affect inferences regarding the status of ecological resources and, thus, the degree to which we can trust individual ecological assessments, compare assessments across different programs and regions, or share data when developing or refining new endpoint indices. Progress in addressing these issues has been hindered by lack of consensus regarding what a general definition of comparability should be in the context of bioassessments and what measures of comparability are appropriate for ecological data. In this paper, we review the state of knowledge regarding the comparability of assessments as affected by differences in raw data (composition and relative abundance of taxa), derived measures (biotic metrics and endpoint indices), and assessment levels (condition classes). We specifically address the extent to which the comparability of assessments can be compromised by systematic differences in data, discuss the factors known to affect data comparability, and consider the techniques available to evaluate and improve comparability. Rigorous assessment of data comparability should be a standard aspect of quality assurance when developing and applying biological indices.

Juvenile unionid mussels disperse in the water column after detachment from their host fish. The settling velocity (w_s) of juvenile mussels is an important component of their dispersion in the water column but has not been measured for unionid mussels. The goal of our study was to measure the w_s of juvenile mussels in the laboratory and to examine how w_s varied within and among related species. The w_s of Actinonaias ligamentina and Ptychobranchus fasciolaris were significantly lower (2.4 ± 0.1 mm/s vs 2.5 ± 0.1 mm/s, respectively) than those of the larger-sized Lampsilis fasciola and Epioblasma triquetra (4.2 ± 0.2 mm/s vs 4.6 ± 0.2 mm/s, respectively). Overall, w_s increased with juvenile size, but considerable variation (∼10×) was found within species. Observations indicated that foot movement of juvenile mussels was responsible for reductions in w_s, and this behavior may provide a potential mechanism for habitat selection at small-spatial scales. Observed w_s differed considerably from w_s predicted from Stokes' law using empirically determined shell size and density (ρ = 1.22 ± 0.003 g/cm³ for A. ligamentina), which indicates some of the limitations in predicting w_s from size measurements.

A major effect of urbanization on streams is the input of stormwater (SW) runoff from impervious surfaces. This water may contain excess nutrients, heavy metals, and other organic contaminants. These inputs are potential stressors or stimulants for algae, invertebrates, and fish within the aquatic community. We conducted a controlled experiment in mesocosms (∼1300-L tanks) to evaluate the effect of SW quality on different trophic levels. SW runoff was collected before it entered Little Black Creek, which is a tributary draining an urbanized subcatchment in western Michigan. The captured runoff was used to create the following treatments: 0% SW (control, n  =  4), 50% SW (n  =  4), or 100% SW (n  =  4). Nested within each mesocosm were exclosure treatments containing different combinations of algae, snails (Physa sp.), and fish (pumpkinseed: Lepomis gibbosus). SW did not have an overall effect on algal biomass or metabolic activity in the mesocosms. Algal community composition in the 0% SW treatment was significantly different from the 100% treatment at the end of the experiment. Mortality and growth rates of the fish and snails were not significantly affected by SW treatment. The absence of a strong effect of SW on algal biomass and metabolism or fish and snail growth in the mesocosms might have been the result of relatively low concentrations of contaminants in the SW. Changes in algal community composition in the mesocosms suggest that community composition is a more sensitive measure of water quality than either algal biomass or mortality and growth rates of fish and snails. SW quality is extremely variable, even among different storm events at the same location, so extrapolating results from one storm or one system to another should be done with caution. Our findings show that SW quality does not necessarily have negative impacts on stream biota.

Acid mine drainage (AMD) is a legacy of historical coal mining that affects several thousand stream kilometers around the world by contributing high loads of acidity, SO₄²⁻, metals, and other cations. Most research has focused on how AMD affects diversity and structure of various biological assemblages, whereas considerably less has focused on functional processes in streams. We investigated how AMD from abandoned coal mines affects epilithic biofilm development and function. Algal biomass and accrual rates were significantly lower in AMD-affected stream reaches than in control streams. Biomass and accrual rates were lowest at intermediate AMD-affected sites (pH  =  5.1–5.9), probably because copious amounts of AlOH₃ precipitates smothered the benthic habitat. Ratios of β-glucosidase (GLU):β-xylosidase (XYLO) were significantly correlated with algal biomass (r  =  0.60, p < 0.01), probably because algae are important sources of C that is readily broken down by bacterial GLU activity, whereas XYLO is mostly associated with C of allochthonous origin. Thus, reduced algal biomass could indirectly exacerbate AMD effects on bacterial function by providing less C needed for cellular functions. Ratios of phosphatase∶leucine aminopeptidase significantly increased as pH decreased (R²  =  0.88, p < 0.01). This increase indicated potential P limitation or stressful conditions for microbial communities because PO₄-P readily adsorbs to metal hydroxides, rendering it biologically inaccessible. Ratios in 3 of 4 control streams indicated that N limitation may be more typical in unaffected streams of the region. Low algal biomass and potential P limitation of biofilms may indicate that AMD-affected streams have reduced capabilities to retain nutrients and energy needed to support healthy ecosystems. Considering the links between biofilm structure and function can provide a framework for developing management strategies to restore and conserve ecosystem processes, such as nutrient retention and spiraling, energy flow, food webs, and biodiversity.

Urbanization is degrading stream ecosystems worldwide. Tropical island streams may respond to urbanization differently than temperate streams because of their overall climate differences, and they may respond differently than continental tropical urban streams because of their reduced biological diversity and short drainages. We characterized the physicochemistry, physical habitat, and macroinvertebrate assemblages of 16 stream tributaries in the Rio Piedras Watershed (San Juan, Puerto Rico). We also described landuse patterns upstream from each sampling site for the entire subwatershed and for riparian buffers of 5- and 100-m width. Urbanization had a negative effect on the physicochemical and biological condition of the Rio Piedras. Streams were distributed in ordination space along a strong physicochemical gradient that was related to concentrations of K , Mg² , dissolved O₂ (DO), and PO₄³⁻. Along this gradient, DO and Mg² decreased and PO₄³⁻ and K increased with higher % urban cover in the subwatershed. Macroinvertebrate assemblages also were related to urbanization, and more macroinvertebrate families and pollution-sensitive taxa were found at sites where physicochemistry reflected less urban cover. Family richness and pollution-sensitive taxa were positively associated with greater % forest cover in the 5-m riparian buffer zone, a result that supports the use of riparian buffers to ameliorate the effects of urbanization on stream biointegrity in the Rio Piedras. Our results are similar to findings in urban streams in temperate zones and in tropical continental streams. Therefore, despite island characteristics, tropical island stream physicochemistry and macroinvertebrate assemblages responded to urbanization in ways that are in general agreement with the predictions of the Urban Stream Syndrome.

Aquatic research historically has focused on separate aquatic ecosystems (i.e., freshwater, estuarine, and marine). We argue that this separation into subdisciplines is artificial and may even be counterproductive. Instead, scientists should consider the physical linkages between different aquatic ecosystems and the many similarities in properties and processes among those ecosystems (conceptual linkages). The 4 papers in this J-NABS BRIDGES cluster demonstrate the value of integrating information from different aquatic ecosystems. For example, the papers illustrate that transformation and transportation of nutrients and energy physically and conceptually link all aquatic ecosystems and are facilitated by the characteristics of the medium that defines them all—water. To promote the exchange of information within aquatic science, more interecosystem studies should be published in journals and books so that scientists will see parallels and linkages among freshwater, estuarine, and marine systems. Over the longer term, such studies would benefit from the funding and teaching of aquatic science as an integrated whole.

Exudates are ubiquitous in marine and fresh waters. They include Extracellular Polymeric Substances (EPS; including mucus, slimes, and biofilm matrices), silk, and chitin. EPS have many uses for the organisms that produce them—attachment, locomotion, feeding, and protection. They also act as glues to bind other materials, including the egesta within fecal pellets. Silk, an exudate produced only in fresh waters, is used in tube construction, to make nets used in feeding, and as a means of attachment. Chitin is the basis of the peritrophic matrix that surrounds the gut contents of some invertebrates and may act as a binding surrounding their fecal pellets. EPS are found free in the water column and in and on the substratum, as well as in close contact with the organisms that exude them. Free EPS have an essential role in particle formation and aggregation, processes that have been studied primarily in marine systems. Like EPS, silk is highly adsorptive and is found free within substrata, but rarely in the water column. Understanding the role of exudates requires collaboration between marine and freshwater biologists, physical geographers, and chemists, especially those working on surface processes and colloids. The results of these collaborations should expand our understanding of the many vital roles played by exudates in aquatic ecosystems.

Benthic invertebrates have important ecosystem engineering functions (bioturbation and biodeposition) in freshwater and marine benthic systems. Bioturbation and biodeposition affect the metabolism of the water–sediment interface through modification of water–sediment fluxes or organic-matter enrichment of sediments by biodeposits. The functional significance of these processes depends strongly on the type of invertebrate activities (the functional traits of the invertebrates) and on the modulation of this activity by environmental conditions. The aim of my article is to propose a common framework for the role of bioturbation/biodeposition in benthic habitats of both marine and freshwater environments. In these ecosystems, hydrological exchanges between the water and sediments (interstitial flow rates) control the microbial activity inside sediments. The ability of ecosystem engineers to influence benthic microbial processes differs strongly between diffusion-dominated (low interstitial flow rates) and advection-dominated (high interstitial flow rates) habitats. Bioturbation/biodeposition may play a role in diffusion-dominated habitats where invertebrates can significantly modify water and particle fluxes at the water–sediment interface, whereas a slight influence of ecosystem engineers is expected in advection-dominated habitats where fluxes are predominantly controlled by hydrological processes. A future challenge will be to test this general framework in marine and freshwater habitats by quantifying the interactions between the functional traits of species and the water–sediment exchanges.

The literature regarding freshwater and marine exchanges of organic matter (OM) focuses predominantly on the unidirectional delivery of allochthonous and autochthonous material from freshwater to the marine environment. Another ecologically significant exchange occurs when anadromous organisms move into coastal and interior watersheds and bring marine-accrued OM and its incorporated marine-derived nutrients (MDN). We use the example of Pacific salmon streams to discuss bidirectional transfers of OM in the upstream and downstream directions with specific reference to the role of flocculation. A fish-floc feedback loop has recently been presented as a mechanism that allows transfer of salmon OM to the gravel-bed matrix in the form of flocs. In the proposed fish-floc feedback loop, the OM source is salmon, but the OM source will change with fish species. Once fish-based flocs enter the gravel bed they can be stored or used by benthic organisms. A salmon disturbance regime that includes redistribution of gravel, fine sediment, and biofilm during redd construction and release of salmon OM to the stream is integral to the fish-floc feedback loop because it provides the inorganic fine sediment and OM required for floc formation. The MDN subsidy provided by returning salmon is important for natal watershed functioning as illustrated by a conceptual OM feedback loop that links these freshwater and marine exchanges. Retention of floc-bound OM in the gravel bed afforded by settling flocs allows slower downstream transfer of nutrients en route to the ocean and a correspondingly increased period for uptake in food webs downstream.

We evaluated the potential for grazers to regulate benthic algal biomass and taxonomic composition in an Alaskan marsh after enrichment with nutrients that are expected to increase in the region with ongoing climate change. We nested caged and uncaged substrates together inside mesocosm enclosures with natural abundances of snails or no snails and with or without nutrient enrichment (NO₃ PO₄ Si). Algal biomass was greater in all nutrient-enriched enclosures than in controls. Algal biomass was greater in enclosures where grazers were present but excluded by a cage than in enclosures where grazers were allowed to graze or where grazers were absent. In the presence of nutrients, grazed communities were dominated by small coccoid green algae and cyanobacteria, which were overgrown by filamentous green algae when grazers were excluded. In the absence of nutrients, grazers had little effect on algal biomass or taxonomic composition. However, grazers recycled a small but potentially important amount of nutrients in their waste, suggesting that consumer-driven nutrient recycling may have played a role in maintaining algal biomass when grazers were present. Our data show that grazers regulate algal responses to nutrients by suppressing algal accumulation but increasing productivity through nutrient recycling in a northern boreal wetland.

Multilevel hierarchical regression was used to examine regional patterns in the responses of benthic macroinvertebrates and algae to urbanization across 9 metropolitan areas of the conterminous USA. Linear regressions established that responses (intercepts and slopes) to urbanization of invertebrates and algae varied among metropolitan areas. Multilevel hierarchical regression models were able to explain these differences on the basis of region-scale predictors. Regional differences in the type of land cover (agriculture or forest) being converted to urban and climatic factors (precipitation and air temperature) accounted for the differences in the response of macroinvertebrates to urbanization based on ordination scores, total richness, Ephemeroptera, Plecoptera, Trichoptera richness, and average tolerance. Regional differences in climate and antecedent agriculture also accounted for differences in the responses of salt-tolerant diatoms, but differences in the responses of other diatom metrics (% eutraphenic, % sensitive, and % silt tolerant) were best explained by regional differences in soils (mean % clay soils). The effects of urbanization were most readily detected in regions where forest lands were being converted to urban land because agricultural development significantly degraded assemblages before urbanization and made detection of urban effects difficult. The effects of climatic factors (temperature, precipitation) on background conditions (biogeographic differences) and rates of response to urbanization were most apparent after accounting for the effects of agricultural development. The effects of climate and land cover on responses to urbanization provide strong evidence that monitoring, mitigation, and restoration efforts must be tailored for specific regions and that attainment goals (background conditions) may not be possible in regions with high levels of prior disturbance (e.g., agricultural development).

Former agricultural fields that are converted to wetlands can serve as either a source or a sink of P to connecting downstream water bodies. Understanding the P dynamics associated with these flooded fields can lead to appropriate mitigation strategies. We investigated P dynamics in flooded celery fields that are adjacent to and connect with a major tributary that eventually flows into Lake Michigan. We measured the P concentrations and loads in the tributary upstream and downstream of the flooded celery fields. Mean total P (TP) load was 2.6× greater downstream than upstream of the flooded celery fields. Turbidity, chlorophyll a, and soluble reactive P (SRP) concentration all were significantly greater at the downstream sampling site than at the upstream site. Mean water-column TP concentrations in the flooded celery fields ranged from 368 to 810 µg/L, compared to mean TP concentrations of 29 and 46 µg/L at the upstream and downstream sampling sites, respectively, in the tributary. Equilibrium P concentrations of flooded celery-field sediments were significantly greater than the water-column SRP concentrations, a result suggesting that the sediments were a source of P to the overlying water. Last, sediment pore-water concentrations of P were very high based on in-situ pore-water samplers, with maximum values of ∼1.6 mg/L. Our results show that wetlands converted from former agricultural fields can be a significant source of nutrients to downstream receiving water bodies. We recommend that the concentrations and release rates of sediment nutrients be measured before areas are converted to wetlands to ensure they serve as nutrient sinks instead of nutrient sources.

The ecological effects of spawning runs of native Pacific salmon (Oncorhynchus spp.) on stream ecosystems of the Pacific Northwest and Northern Pacific Rim have been studied extensively. However, little is known about how nonnative Pacific salmon affect stream ecosystems in the Great Lakes Basin, especially given the difference in environmental context between the regions. Mechanisms by which salmon spawners alter stream ecosystems include nutrient enrichment from excretion by live adults, carcass decomposition, and physical disturbance of the substrate during redd construction. The objective of our study was to quantify changes in water chemistry and benthic periphyton in 3 streams in northern Michigan that have spawning populations of Pacific salmon. In each stream, dissolved nutrients (soluble reactive P [SRP], NH₄ , dissolved organic C [DOC], NO₃⁻), and periphyton on gravel were sampled before, during, and after the spawning run in reaches upstream and downstream of a salmon barrier. Nutrients increased in reaches downstream of the barrier when salmon were present, but the magnitude of increase was low relative to increases observed in Pacific Rim streams. During and after the spawning run, periphyton biomass declined significantly in reaches where high densities of salmon spawners were present. Our results suggest that disturbance by spawning salmon may override their enrichment effects in northern Michigan streams, but this pattern may in part be driven by environmental context, especially the presence of finer substrates.

Anthropogenic N loadings and perturbations of macroconsumer communities impair ecological and economic services provided by streams. Organisms are adapted to natural disturbances, such as flooding and desiccation, but how anthropogenic and natural disturbances interact is poorly understood. We used large outdoor mesocosms to study the effect of Campostoma anomalum, a common prairie headwater-stream minnow, and NH₄ additions (to simulate fish excretion) on the recovery of ecosystem structure and function following a flood, highlighting the potential for Campostoma (and other macroconsumers) to affect denitrification. Campostoma and NH₄ treatments differentially affected particulate organic matter size and filamentous algal structure. Ecosystem structure responded differently to mesocosm treatment over time, a result suggesting that grazers or NH₄ -N availability may be especially important during early recovery periods. The presence of Campostoma did not influence denitrification, but NH₄ additions altered the response of denitrifiers to nutrient and energy amendments, and denitrification rates decreased following the recovery of mesocosms. Temporal changes in denitrification probably were caused by increasing hyporheic dissolved O₂ concentrations, which led to potentially fewer anoxic microsites for production of denitrification enzymes. Our study shows that grazers affect the recovery of ecosystem structure, but denitrification in the context of these prairie-stream mesocosms appears to be unaffected by Campostoma.

Parasites can regulate host abundance and influence the composition and structure of communities. However, host–parasite interactions might be context-specific if environmental conditions can alter the outcome of parasitism and disease. An understanding of how host–parasite interactions might change in different contexts will be useful for predicting and managing disease against a background of anthropogenic environmental change. We examined the ecology of Myxobolus cerebralis, the parasite that causes whirling disease in salmonids, and its obligate host, Tubifex tubifex, in geothermally variable stream reaches in Yellowstone National Park. We identified reaches in 4 categories of geothermal influence, which were characterized by variable substrates, temperatures, specific conductivities, and pH. In each reach, we measured aspects of host ecology (abundance, relative abundance, size, and genotype of T. tubifex), parasite ecology (infection prevalence in T. tubifex and abundance of M. cerebralis-infected T. tubifex), and risk to fish of contracting whirling disease. Tubifex tubifex abundance was high all in reaches characterized by geothermal influence, whereas abundance of M. cerebralis-infected T. tubifex was high only in reaches characterized by intermediate geothermal influence. We suggest that habitat had a contextual effect on parasitism in the oligochaete host. Abundance of infected hosts appeared to depend on host abundance in all reach types except those with high geothermal influence, where abundance of infected hosts depended on environmental factors.