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Animals can play important roles in cycling nutrients [hereafter consumer-driven nutrient dynamics (CND)], but researchers typically simplify animal communities inhabiting dynamic environments into single groups that are tested under relatively static conditions. We propose a conceptual framework and present empirical evidence for CND that considers the potential effects of spatially overlapping animal groups within dynamic ecosystems. Because streams can maintain high biomass of mussels and fish, we were able to evaluate this framework by testing if biogeochemical hotspots generated by stable aggregations of mussels attract fishes. We predicted that spatial overlap between these groups may increase the flux of mineralized nutrients. We quantified how different fish assemblage biomass was between mussel bed reaches and reaches without mussels. We compared fish and mussel biomass at mussel beds to test whether differences in animal biomass mediate their contributions to nutrient cycling through nitrogen and phosphorous excretion. We estimated areal excretion rates for each group by combining biomass estimates with measured excretion rates. Fish biomass was homogeneously distributed, except following a period of low flow when fish were more concentrated at mussel beds. Mussel biomass was consistently an order of magnitude greater than fish biomass and mussel areal excretion rates exceeded fish excretion rates. However, the magnitude of those differences varied spatially and temporally. Mussel excretion stoichiometry varied with changes in assemblage composition, while fish excretion stoichiometry varied little. Biogeochemical hotspots associated with mussels did not generally overlap with fish aggregations, thus, under these conditions, animal processes appear to exert additive ecosystem effects.

Animals are a critical component of biogeochemical cycles. While animal mediated fluxes of nutrients and energy have received considerable attention, the impacts of these fluxes on microbial community structure and function are comparatively understudied. Here, we investigated if freshwater mussel influences on biogeochemical cycling in stream sediment are accompanied by changes in sediment microbial community composition and ecoenzymatic activity, and if these relationships change under different nutrient regimes.We predicted that mussel effects on ecosystem function are reflected by modified sediment microbial communities. We hypothesized that if changes in either sediment ecoenzymatic function or microbial community composition are driven by mussel‐derived nutrient amendments, we should see muted changes in microbial community assemblages or function when a given nutrient is abundant. However, if microbial communities and function are influenced by other mussel functions, then we should see uniform changes regardless of nutrient availability.We transplanted freshwater mussels and natural river sediment to flow‐through mesocosms and monitored changes in microbial community composition over 1 week. Our results indicate that mussels always changed sediment microbial community composition, but the way communities changed was dependent on ambient nutrient concentrations.On the final day we measured the activity of ecoenzymes known to correlate to microbial function and nutrient availability. Mussels homogenized the stoichiometric ratios of ecoenzyme activities, indicating a consistent function of sediment microbes associated with freshwater mussels. Our results suggest that mussels may promote functional redundancy in sediment microbial communities and highlight the importance of animals in controlling biogeochemical transformations under changing nutrient conditions. Read the free Plain Language Summary for this article on the Journal blog. [ABSTRACT FROM AUTHOR]

Nitrate retention is an important ecosystem service provided by streams which reduces nitrogen loads to downstream ecosystems. However, most inferences about nitrate retention and uptake in streams are based on measurements at sub-annual scales. Our objective was to evaluate variation in nitration retention and solute concentrations at weekly to decadal time scales. We measured nitrate and chloride concentration, nitrate flux and nitrate retention in Emmons Creek, a groundwater-dominated, nitrate-rich stream in central Wisconsin during a 10-year period. We used a two-station mass balance approach to measure nitrate retention while accounting for groundwater inputs. Surface water nitrate concentration, nitrate yield and nitrate retention exhibited strong seasonal variation. Seasonal Kendall tests revealed positive trends for surface water discharge, nitrate and chloride concentrations and groundwater chloride concentration, but not nitrate retention. Generalized additive mixed models (GAMM), accounting for serial autocorrelations, also indicated positive trends in surface water nitrate and chloride concentrations but no trend for nitrate retention. Nitrate retention averaged 370 mg NO3-N m−2 d−1 (± 316 SD) and was highest during the spring (512 mg NO3-N m−2 d−1 ± 319), followed by summer (460 ± 322), fall (340 ± 281) and winter (165 ± 212). Nitrate retention rates in Emmons Creek were relatively high compared to most published estimates. However, our results suggest that Emmons Cr. may have saturated its capacity to retain nitrate during a period when nitrate yield from its watershed increased, which is an important finding given the increases in N availability in many regions of earth. [ABSTRACT FROM AUTHOR]

Stormwater drainage and wastewater disposal are primary pathways through which urbanization degrades streams. These technologies and management practices change over time, reshaping the urban template and affecting the environmental challenges a city will face in the following decades to centuries. Spatial and temporal asynchrony in the implementation and replacement of these technologies and the adoption of new management approaches means that the mechanisms of the urban stream syndrome will be heterogeneous. Thus, promoting 'one size fits all' global panaceas for urban streams may be less effective than local solutions based on the unique urban templates and socioeconomic factors governing streams. Understanding the cumulative effects of spatiotemporal changes in urban templates on streams is critical to protecting and managing them because: 1) existing water infrastructure in many countries (especially high-income countries) is aging and requires replacement, 2) urbanization in rapidly developing countries requires ever more infrastructure, and 3) demand for higher levels of environmental quality is transforming technological and management approaches globally. The management and technological decisions made during the current infrastructure replacement or new construction cycle will define the future urban template and select the trajectory and character of the urban stream syndrome during the coming decades. [ABSTRACT FROM AUTHOR]

Population growth in cities has resulted in the rapid expansion of urbanized land. Most research and management of stream ecosystems affected by urban expansion has focused on the maintenance and restoration of biotic communities rather than their basal resources. We examined the potential for urbanization to induce bottom-up ecosystem effects by looking at its influence on dissolved organic matter (DOM) composition and bioavailability and microbial enzyme activity. We selected 113 headwater streams across a gradient of urbanization in central and southern Maine and used elemental and optical analyses, including parallel factor analysis of excitation-emission matrices, to characterize DOM composition. Results show that fluorescent and stoichiometric DOM composition changed significantly across the rural to urban gradient. Specifically, the proportion of humic-like allochthonous DOM decreased while that of more bioavailable autochthonous DOM increased in the more urbanized streams. In laboratory incubations, increased autochthonous DOM was associated with a doubling in the decay rate of dissolved organic carbon as well as increased activity of C-acquiring enzymes. These results suggest that urbanization replaces upstream humic material with more local sources of DOM that turnover more rapidly and may drive bottom-up changes in microbial communities and affect the quality and quantity of downstream DOM delivery. [ABSTRACT FROM AUTHOR]