Harmful cyanobacterial blooms are an increasing threat to water quality. The interactions between... more Harmful cyanobacterial blooms are an increasing threat to water quality. The interactions between two ecophysiological functional traits of cyanobacteria, diazotrophy (nitrogen [N]‐fixation) and N‐rich cyanotoxin synthesis, have never been examined in a stoichiometric explicit manner. We explored how a gradient of resource N : phosphorus (P) affects the biomass, N, P stoichiometry, light‐harvesting pigments, and cylindrospermopsin production in a N‐fixing cyanobacterium, Aphanizomenon. Low N : P Aphanizomenon cultures produced the same biomass as populations grown in high N : P cultures. The biomass accumulation determined by carbon, indicated low N : P Aphanizomenon cultures did not have a N‐fixation growth trade‐off, in contrast to some other diazotrophs that maintain stoichiometric N homeostasis at the expense of growth. However, N‐fixing Aphanizomenon populations produced less particulate cylindrospermopsin and had undetectable dissolved cylindrospermopsin compared to non‐N‐fixing populations. The pattern of low to high cyanotoxin cell quotas across an N : P gradient in the diazotrophic cylindrospermopsin producer is similar to the cyanotoxin cell quota response in nondiazotrophic cyanobacteria. We suggest that diazotrophic cyanobacteria may be characterized into two broad functional groups, the N‐storage‐strategists and the growth‐strategists, which use N‐fixation differently and may determine patterns of bloom magnitude and toxin production in nature.
ABSTRACT Fish-mediated nutrient recycling influences nutrient dynamics in stream ecosystems, but ... more ABSTRACT Fish-mediated nutrient recycling influences nutrient dynamics in stream ecosystems, but its consequences for smaller-scale microbial processes in benthic habitats are not well understood. We quantified the effect of nutrient recycling by the grazing fish, Campostoma anomalum, on downstream periphyton in 12 flow-through stream mesocosms. We compared periphyton nutrient ratios and algal biomass (as chlorophyll a [chl a]) between tiles upstream and downstream of enclosures with and without fish to measure nutrient-cycling effects in streams with low (11) and high (177) surface-water dissolved N∶P molar ratios. No upstream–downstream changes in periphyton nutrient ratios were observed in low N∶P streams with or without fish. In high N∶P streams, periphyton C∶N decreased and C∶P and N∶P increased on tiles downstream of enclosures. In high N∶P streams, downstream changes in periphyton nutrient ratios were greater in streams with than without fish, and chl a significantly increased downstream of enclosures with fish. We linked nutrient-recycling effects to downstream microbial processes by comparing bacterial biomass production (BBP), photosynthesis (PS) rates, and the degree of coupling between the 2 processes on tiles downstream of enclosures. We estimated the degree of coupled production between algae and bacteria downstream of enclosures as the covariance between PS and BBP among replicates within each stream (COVPS–BBP). In high N∶P streams, areal BBP and PS rates and COVPS–BBP were higher downstream of enclosures with fish. Chl a and COVPS–BBP declined with increasing periphyton C∶N content, resulting in a positive relationship between COVPS–BBP and algal biomass across all treatments. Our results indicate that grazing fish alter stream ecosystem N and P dynamics through consumer-mediated recycling pathways, but downstream responses depend on background nutrient regimes. Fish-mediated changes in nutrient dynamics and algal biomass influence reliance of heterotrophs and autotrophs on nutrients recycled within periphyton communities to support benthic production.
ABSTRACT Excessive nutrient inputs and grazers can influence biomass and elemental composition of... more ABSTRACT Excessive nutrient inputs and grazers can influence biomass and elemental composition of primary producers in freshwater ecosystems. How interactions between nutrient enrichment and grazing fish alter benthic habitats through effects on periphyton autotrophy, biomass, and elemental composition has been studied rarely. We compared the effects of grazing by central stonerollers (Campostoma anomalum) on autotrophic and total periphyton biomass, sediment mass, and C, N, and P stoichiometry of periphyton in 12 flow-through stream mesocosms randomly assigned to 1 of 3 different PO4-P concentrations (control: 8 µg/L, low: 20 µg/L, high: 100 µg/L). Fish grazing suppressed periphyton ash-free dry mass (AFDM) and sediment accumulation, regardless of P treatment. However, grazing also increased the proportion of algal biomass in the periphyton, evidenced by a reduction in benthic C:chlorophyll a on grazed substrates. The response of periphyton stoichiometry to experimental P enrichment was stronger on grazed substrates because central stonerollers maintained a higher proportion of algae in the periphyton matrix. Grazing enhanced the response of P standing stocks to enrichment, reduced C∶P and C∶N in high-P streams, and increased N∶P in control and low-P streams. Shifts from detritus- and sediment-bound nutrients to algal resources probably increase the palatability of benthic food resources and nutrient availability for other grazing organisms. Grazing fish may play a stronger role in benthic processes, such as nutrient cycling, than is currently recognized. Our results suggest that fish drive periphyton toward autotrophy, enhance sequestration of excess nutrients in periphyton and, thus, may relax stoichiometric constraints on fast growing organisms in stream communities.
One important mechanism governing the temporal maintenance of biodiversity is asynchrony in co‐oc... more 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.
The fate and transport of triclosan (5-chloro-2-[2,4-dichlorophenoxy]phenol), a widely used antim... more The fate and transport of triclosan (5-chloro-2-[2,4-dichlorophenoxy]phenol), a widely used antimicrobial agent in personal care products, in aquatic ecosystems is a growing environmental concern. At ecosystem scales, triclosan potentially interacts with co-occurring nutrient stressors to affect overall biogeochemical cycling through consumer-mediated nutrient recycling pathways. We examined N- and P-excretion rates of snails (Physella spp.) in 12 outdoor experimental stream mesocosms dosed with 3 P treatments crossed with 3 triclosan treatments and a methanol carrier control. Snail N- and P-excretion rates increased with decreasing periphyton C∶N and C∶P ratios across the P-enrichment gradient. N- and P-excretion rates were significantly higher in the high-triclosan than in the methanol control treatments on day 14, but only in high-P-enrichment streams. However, methanol had positive effects on N- and P-excretion rates in low- and high-P-enrichment treatments compared to nonsolvent controls, but no effect at background P concentrations. Multiple inferences can be drawn from our study. First, whereas many investigators have confirmed that using methanol as a carrier below established regulatory levels does not influence laboratory and mesocosm ecotoxicology responses, our results suggest that low-level methanol concentrations can influence measures of ecosystem function. Second, higher use of triclosan and other antimicrobial agents in commercial products coupled with predicted urbanization and surface-water shortages in the future have the potential to increase triclosan concentrations and shift patterns in consumer-mediated nutrient cycling in aquatic systems because of interactive effects of antimicrobial agents and nutrient enrichment.
Harmful cyanobacterial blooms are an increasing threat to water quality. The interactions between... more Harmful cyanobacterial blooms are an increasing threat to water quality. The interactions between two ecophysiological functional traits of cyanobacteria, diazotrophy (nitrogen [N]‐fixation) and N‐rich cyanotoxin synthesis, have never been examined in a stoichiometric explicit manner. We explored how a gradient of resource N : phosphorus (P) affects the biomass, N, P stoichiometry, light‐harvesting pigments, and cylindrospermopsin production in a N‐fixing cyanobacterium, Aphanizomenon. Low N : P Aphanizomenon cultures produced the same biomass as populations grown in high N : P cultures. The biomass accumulation determined by carbon, indicated low N : P Aphanizomenon cultures did not have a N‐fixation growth trade‐off, in contrast to some other diazotrophs that maintain stoichiometric N homeostasis at the expense of growth. However, N‐fixing Aphanizomenon populations produced less particulate cylindrospermopsin and had undetectable dissolved cylindrospermopsin compared to non‐N‐fixing populations. The pattern of low to high cyanotoxin cell quotas across an N : P gradient in the diazotrophic cylindrospermopsin producer is similar to the cyanotoxin cell quota response in nondiazotrophic cyanobacteria. We suggest that diazotrophic cyanobacteria may be characterized into two broad functional groups, the N‐storage‐strategists and the growth‐strategists, which use N‐fixation differently and may determine patterns of bloom magnitude and toxin production in nature.
ABSTRACT Fish-mediated nutrient recycling influences nutrient dynamics in stream ecosystems, but ... more ABSTRACT Fish-mediated nutrient recycling influences nutrient dynamics in stream ecosystems, but its consequences for smaller-scale microbial processes in benthic habitats are not well understood. We quantified the effect of nutrient recycling by the grazing fish, Campostoma anomalum, on downstream periphyton in 12 flow-through stream mesocosms. We compared periphyton nutrient ratios and algal biomass (as chlorophyll a [chl a]) between tiles upstream and downstream of enclosures with and without fish to measure nutrient-cycling effects in streams with low (11) and high (177) surface-water dissolved N∶P molar ratios. No upstream–downstream changes in periphyton nutrient ratios were observed in low N∶P streams with or without fish. In high N∶P streams, periphyton C∶N decreased and C∶P and N∶P increased on tiles downstream of enclosures. In high N∶P streams, downstream changes in periphyton nutrient ratios were greater in streams with than without fish, and chl a significantly increased downstream of enclosures with fish. We linked nutrient-recycling effects to downstream microbial processes by comparing bacterial biomass production (BBP), photosynthesis (PS) rates, and the degree of coupling between the 2 processes on tiles downstream of enclosures. We estimated the degree of coupled production between algae and bacteria downstream of enclosures as the covariance between PS and BBP among replicates within each stream (COVPS–BBP). In high N∶P streams, areal BBP and PS rates and COVPS–BBP were higher downstream of enclosures with fish. Chl a and COVPS–BBP declined with increasing periphyton C∶N content, resulting in a positive relationship between COVPS–BBP and algal biomass across all treatments. Our results indicate that grazing fish alter stream ecosystem N and P dynamics through consumer-mediated recycling pathways, but downstream responses depend on background nutrient regimes. Fish-mediated changes in nutrient dynamics and algal biomass influence reliance of heterotrophs and autotrophs on nutrients recycled within periphyton communities to support benthic production.
ABSTRACT Excessive nutrient inputs and grazers can influence biomass and elemental composition of... more ABSTRACT Excessive nutrient inputs and grazers can influence biomass and elemental composition of primary producers in freshwater ecosystems. How interactions between nutrient enrichment and grazing fish alter benthic habitats through effects on periphyton autotrophy, biomass, and elemental composition has been studied rarely. We compared the effects of grazing by central stonerollers (Campostoma anomalum) on autotrophic and total periphyton biomass, sediment mass, and C, N, and P stoichiometry of periphyton in 12 flow-through stream mesocosms randomly assigned to 1 of 3 different PO4-P concentrations (control: 8 µg/L, low: 20 µg/L, high: 100 µg/L). Fish grazing suppressed periphyton ash-free dry mass (AFDM) and sediment accumulation, regardless of P treatment. However, grazing also increased the proportion of algal biomass in the periphyton, evidenced by a reduction in benthic C:chlorophyll a on grazed substrates. The response of periphyton stoichiometry to experimental P enrichment was stronger on grazed substrates because central stonerollers maintained a higher proportion of algae in the periphyton matrix. Grazing enhanced the response of P standing stocks to enrichment, reduced C∶P and C∶N in high-P streams, and increased N∶P in control and low-P streams. Shifts from detritus- and sediment-bound nutrients to algal resources probably increase the palatability of benthic food resources and nutrient availability for other grazing organisms. Grazing fish may play a stronger role in benthic processes, such as nutrient cycling, than is currently recognized. Our results suggest that fish drive periphyton toward autotrophy, enhance sequestration of excess nutrients in periphyton and, thus, may relax stoichiometric constraints on fast growing organisms in stream communities.
One important mechanism governing the temporal maintenance of biodiversity is asynchrony in co‐oc... more 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.
The fate and transport of triclosan (5-chloro-2-[2,4-dichlorophenoxy]phenol), a widely used antim... more The fate and transport of triclosan (5-chloro-2-[2,4-dichlorophenoxy]phenol), a widely used antimicrobial agent in personal care products, in aquatic ecosystems is a growing environmental concern. At ecosystem scales, triclosan potentially interacts with co-occurring nutrient stressors to affect overall biogeochemical cycling through consumer-mediated nutrient recycling pathways. We examined N- and P-excretion rates of snails (Physella spp.) in 12 outdoor experimental stream mesocosms dosed with 3 P treatments crossed with 3 triclosan treatments and a methanol carrier control. Snail N- and P-excretion rates increased with decreasing periphyton C∶N and C∶P ratios across the P-enrichment gradient. N- and P-excretion rates were significantly higher in the high-triclosan than in the methanol control treatments on day 14, but only in high-P-enrichment streams. However, methanol had positive effects on N- and P-excretion rates in low- and high-P-enrichment treatments compared to nonsolvent controls, but no effect at background P concentrations. Multiple inferences can be drawn from our study. First, whereas many investigators have confirmed that using methanol as a carrier below established regulatory levels does not influence laboratory and mesocosm ecotoxicology responses, our results suggest that low-level methanol concentrations can influence measures of ecosystem function. Second, higher use of triclosan and other antimicrobial agents in commercial products coupled with predicted urbanization and surface-water shortages in the future have the potential to increase triclosan concentrations and shift patterns in consumer-mediated nutrient cycling in aquatic systems because of interactive effects of antimicrobial agents and nutrient enrichment.
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