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C, N, and P content were measured across the ontogeny of lotic aquatic insects representing a diversity of life-history characteristics. The relationship between individual mass and nutrient content was used to show ontogenetic patterns of nutrient content by species. Species analyzed for C and N content exhibited a quasihomeostatic pattern across ontogeny. Percent C and %N varied among taxa irrespective of ontogeny, with %C ranging from 47.4 to 56.2% and %N ranging from 9.6 to 11.6%. P content also varied by species but declined nonlinearly across ontogeny and was best represented by a power function. Percent P varied from >7% in 1st-instar Tabanus larvae to only 0.34% in adult male Ambrysus circumcinctus. Females had more P per unit mass than males in 6 of the 10 species that could be sexed. In the leptophlebiid mayflies, %P increased in mature female nymphs relative to the penultimate developmental class, whereas %P content of males continued to decline to eclosion. Maximum terminal mass by species was the main factor driving the magnitude of change in %P through their ontogeny. Small-bodied, rapidly growing species exhibited the sharpest decline in P content. Nonhomeostatic patterns in %P across ontogeny and between sexes has important implications for population- and community-level dynamics and ecosystem processes. First, small-bodied, high-%P taxa have faster growth rates than larger individuals, which supports one of the predictions of the growth-rate hypothesis (GRH). Second, elemental imbalance between consumers and their food changes across ontogeny, and therefore, nutrient recycling rate by a species changes with population age structure. Last, community structure may reflect nutrient availability in food such that enriched environments are more likely to be dominated by taxa with high growth rates and, thus, relatively high P demand.

Stable isotope analysis of δ13C and δ15N have been broadly used in ecological studies to access trophic interactions between organisms, and the flow of energy and contaminants among systems and through the trophic chain. These questions can be addressed based on the organic carbon assimilated by the organisms to acquire energy. However, for many kinds of samples, like suspended particulate organic matter (SPOM), inorganic carbon is also present at high proportions, and it is not assimilated by organisms during the metabolic process. In this sense, we tested the benefit of extracting inorganic carbon from SPOM samples with HCl fumigation, by comparing the δ13C and δ15N of acidified and non-acidified samples. We calculated a correction model and ran a MixSIAR model to evaluate the implications of using non-acidified samples when evaluating carbon contributions to organisms. We found a decrease of -2.78 ± 0.05‰ in the δ13C of acidified samples, which was high enough to change the results and interpretation of the simulations in the MixSIAR models. The δ15N was not altered. The correction model provided results similar to the acidified samples and could be used when acidification is not possible. We suggest acidifying SPOM samples for ecological studies assessing trophic relationships and energy flow, as the δ13C can be modified due to the content of inorganic carbon and the productivity of the environment, influencing interpretation of the ecological relationships among organisms and systems.

Eutrophication has become one of the most widespread anthropogenic forces impacting freshwater biological diversity. One potentially important mechanism driving biodiversity changes in response to eutrophication is the alteration of seasonal patterns of succession, particularly among species with short, synchronous, life cycles. We tested the hypothesis that eutrophication reduces seasonally driven variation in species assemblages by focusing on an understudied aspect of biodiversity: temporal beta diversity (βt). We estimated the effect of eutrophication on βt by sampling benthic macroinvertebrate assemblages bimonthly for two years across 35 streams spanning a steep gradient of total phosphorus (P) and benthic algal biomass (as chlorophyll a [chl a]). Two widely used metrics of β diversity both declined sharply in response to increasing P and chl a, regardless of covariates. The most parsimonious explanatory model for βt included an interaction between P and macroinvertebrate biomass, which revealed that βt was lower when macroinvertebrate biomass was relatively high. Macroinvertebrate biomass explained a greater amount of deviance in βt at lower to moderate concentrations of P, providing additional explanatory power where P concentration alone was unable to fully explain declines in βt. Chl a explained similar amounts of deviance in βt in comparison to the best P model, but only when temperature variability, which was positively related to βt, also was included in the model. Declines in βt suggest that nutrient enrichment decreases the competitive advantage that specialists gain by occupying particular temporal niches, which leads to assemblages dominated by generalists that exhibit little seasonal turnover. The collapse of seasonal variation in assemblage composition we observed in our study suggests that treating dynamic communities as static assemblages is a simplification that may fail to detect the full impact of anthropogenic stressors. Our results show that eutrophication leads to more temporally homogenous communities and therefore degrades a fundamental facet of biodiversity.

Comprehension of basic stream ecosystem function relies on an understanding of aquatic–terrestrial linkages. One major component of such linkages is the incorporation of landscape‐derived energy and nutrients into the aquatic food web via microbes. In many boreal streams, wetlands and alder are known to be primary sources of dissolved organic carbon (DOC) and dissolved inorganic nitrogen (DIN), respectively. To simulate the influence of the highly labile portion of wetland‐derived DOC subsidies on microbial production and ecosystem processes in a stream with high landscape‐derived nutrient inputs, we enriched a boreal headwater stream situated in a high‐alder, low‐wetland cover catchment (i.e., high DIN, low DOC) with low levels (~0.25 mg/L) of labile DOC (as acetate‐C) for 9 weeks. We compared nutrient uptake, bacterial biomass production, and photosynthesis of periphyton and ecosystem metabolism in physicochemically similar upstream (reference) and downstream (treatment) reaches. DIN uptake was greater in the treatment than in reference reach on six out of nine dates during the dosing period. Bacterial biomass production positively responded to C enrichment. Ecosystem respiration increased up to ~50% after dosing began. Gross primary production responded positively to DOC enrichment early in the study when riparian vegetation did not limit light availability, but negatively later on in the growing season. We conclude that even low levels of labile DOC may act as a strong subsidy to headwater stream ecosystems, particularly those with high levels of DIN inputs from alder. Headwater streams influenced by high contributions of both alder and wetlands may represent biogeochemical hotspots, and these landscape features should be viewed as vital and complementary in their roles for ecosystem function.

One important mechanism governing the temporal maintenance of biodiversity is asynchrony in co-occurring competitors due to fluctuating environments (i.e., compensatory dynamics). Temporal niche partitioning has evolved in response to predictable oscillations in environmental conditions so that species may offset competition, but we do not yet have a clear understanding of how novel anthropogenic stressors alter seasonal patterns of succession. Many primary producers are nutrient limited, and enrichment may decrease the importance of environmental fluctuations that govern which species are effective competitors under naturally low nutrient regimes. Consequently, elevated nutrient concentrations may synchronize species responses to seasonality. By studying benthic algal assemblages over 2 years from 35 streams that spanned a wide gradient of nutrient enrichment, we found that compensatory dynamics characterizing seasonal succession under natural nutrient regimes broke down at relatively low levels of total phosphorus (P) enrichment (∼ 25 μg/L). With increasing P more species were able to coexist at any given time, and seasonal variation in assemblage composition was characterized by synchronous swings in species biovolumes. We also observed much higher instability in assemblage biovolumes with declines in compensatory dynamics, which indicates that anthropogenic alteration of nutrient regimes can affect community stability by changing the dominant mode of seasonal succession. Our findings indicate that compensatory fluctuations of stream algae are driven by seasonality and provide insight about how nutrient enrichment alters evolved drivers of species coexistence.

Phosphorus (P) is a key biological element with important and unique biogeochemical cycling in natural ecosystems. Anthropogenic phosphorus inputs have been shown to greatly affect natural ecosystems, and this has been shown to be especially true of freshwater systems. While the importance of microbial communities in the P cycle is widely accepted, the role, composition, and relationship to P of these communities in freshwater systems still hold many secrets. Here, we investigated combined bacterial and archaeal communities utilizing 16S ribosomal RNA (rRNA) gene sequencing and computationally predicted functional metagenomes (PFMs) in 25 streams representing a strong P gradient. We discovered that 16S rRNA community structure and PFMs demonstrate a degree of decoupling between structure and function in the system. While we found that total phosphorus (TP) was correlated to the structure and functional capability of bacterial and archaeal communities in the system, turbidity had a stronger, but largely independent, correlation. At TP levels of approximately 55 µg/L, we see sharp differences in the abundance of numerous ecologically important taxa related to vegetation, agriculture, sediment, and other ecosystem inhabitants.

1. Nutrient over-enrichment increasingly threatens global water resources. Stressor-response studies specifically designed to identify levels of nutrients strongly associated with undesirable ecological conditions are needed to inform numeric nutrient criteria that protect inland waters. 2. Diatoms are important components of aquatic life, which support higher trophic levels and are sensitive to nutrient enrichment. We tested a framework that relies on stressor-response modelling of phosphorus (P) enrichment and stream diatom assemblages across many field locations, multiple years and seasons within years, and under controlled experimental conditions to inform nutrient criteria development. 3. Diatom species composition was nonlinearly correlated with total phosphorus (TP) throughout the 2-year field study. This occurred despite temporal shifts in species composition between two hydrologically distinct years and over eight seasons. 4. Species assemblages on rocks transplanted from a low P stream to mesocosms representing a P enrichment gradient (8,20 and 100 μg/L) shifted into two groups over time. Species composition on rocks in low (20 μg/L) and high (100 μg/L) P mesocosms was consistent with assemblages at P-enriched field sites, whereas rocks in control (8 μg/L) mesocosms had significantly different species composition, consistent with low P field sites. Species composition on rocks transplanted from high P streams did not shift as dramatically and were not significantly different after exposure to different treatments in mesocosms. 5. Threshold Indicator Taxa Analysis identified synchronous declines in several diatom species that culminated in assemblage thresholds associated with TP concentrations >20 and 25 μg/L for 2006 and 2007 respectively. 6. Synthesis and applications. Diatom assemblages show consistent responses to nutrient enrichment despite temporal shifts associated with confounding factors common in stream ecosystems. Regulators should include diatom assemblage responses when developing numeric nutrient criteria. We present a framework that includes spatial, temporal and experimental components, and has broad applicability for use in different ecological settings to evaluate ecological endpoints and set limits for a variety of contaminants threatening freshwater ecosystems throughout the world.

We studied several streams spanning a steep dissolved phosphorus (PO4–P) gradient to test the hypothesis that faster stream velocity would reduce alkaline phosphatase activity (APA) and carbon:phosphorus (C:P) of benthic periphyton because higher velocities should increase the supply rate of dissolved phosphorus at the community–water interface. We tested the hypothesis that the differences in APA and C:P between fast and slow velocity locations within a stream reach would decline as stream PO4–P concentrations increased, and, therefore, velocity effects should be the greatest at low levels of PO4–P. APA declined in response to both the increased water velocity and PO4–P, but the effect of velocity on APA was negligible at the highest levels of PO4–P. Further, we found a strong, negative relationship between periphyton C:P and PO4–P levels as hypothesized, but did not detect significant relationship between C:P and velocity after accounting for the effects of PO4–P. The lack of an effect of velocity on C:P is probably due to the higher levels of APA in low-velocity, low PO4–P reaches, as the higher APA rates reflect an alternative pathway for acquiring sufficient PO4–P to sustain periphytic growth and metabolism. These results have important implications for stream ecosystem function because of the increasing frequency of extreme weather events associated with the climate change, particularly droughts that reduce or eliminate perennial stream flow, and further illustrate the important effects of stream flow on biogeochemical processes.

Phosphorus (P) is a nutrient of primary importance in all living systems, and it is especially important in streams and rivers which are sensitive to anthropogenic P inputs and eutrophication. Microbes are accepted as the primary mineralizers and solubilizers of P improving bioavailability for organisms at all trophic levels. Here, we use a genomics approach with metagenome sequencing of 24 temperate streams and rivers representing a total P (TP) gradient to identify relationships between functional genes, functional gene groupings, P, and organisms within the P biogeochemical cycle. Combining information from network analyses, functional groupings, and system P levels, we have constructed a System Relational Overview of Gene Groupings (SROGG) which is a cohesive system level representation of P cycle gene and nutrient relationships. Using SROGG analysis in concert with other statistical approaches, we found that the compositional makeup of P cycle genes is strongly correlated to environmental P whereas absolute abundance of P genes shows no significant correlation to environmental P. We also found orthophosphate (PO₄³⁻) to be the dominant factor correlating with system P cycle gene composition with little evidence of a strong organic phosphorous correlation present even in more oligotrophic streams.

Terrestrial sources of nitrogen (N), particularly N-fixing alder, may be important for sustaining production in headwater streams that typically lack substantial subsidies of marine-derived nutrients from spawning salmon yet support upstream-dispersing juvenile salmonids. However, other physiographic characteristics, such as watershed slope and topographic wetness, also control transport of nutrients to streams and may confound apparent linkages between alder and stream N. Seasonal patterns in precipitation and temperature may interact with watershed characteristics to modulate stream N availability. We empirically modeled the effect of alder cover and other watershed physiographic variables on stream N and contrasted these relationships over the growing season among 25 first-order streams from the lower Kenai Peninsula, Alaska. For each date, percent alder cover, mean topographic wetness, and mean slope were used as watershed predictors of NOₓ-N concentration (nitrate + nitrite) and daily NOₓ-N yield using Generalized Additive Models (GAM) and compared using Akaike's Information Criterion (AIC c ). Alder cover was the only probable model and explained 75-96% of the variation in NOₓ-N concentration and 83-89% of the variation in daily NOₓ-N yield. The relationship between alder and both NOₓ-N concentration and daily NOₓ-N yield changed from constant inputs in May across the range of alder cover (linear fit) to increasing inputs in July and September (non-linear fits) implying that high-alder watersheds were Nsaturated. The strong linkage between alder and stream N coupled with the concurrent timing of maximum stream N from alder in the spring to salmon fry emergence indicates the potential importance of this subsidy to headwater stream ecosystems.