Taxonomic Shift Over a Phosphorus Gradient Affects the Stoichiometry and Fatty Acid Composition of Stream Periphyton

Abstract

Phosphorus enrichment of stream ecosystems generally increases primary production in the benthos, but the consequences of eutrophication for the nutritional quality of periphyton for grazers are less clear. On short time scales, high phosphorus inputs may lead to reduced C:P ratios and high essential fatty acid contents of periphyton, which are both considered important determinants of food quality for grazers. However, nutrient enrichment may alter the taxonomic composition of periphyton and favor the growth of less palatable algal taxa. In this study, periphyton was grown under a gradient of dissolved phosphorus availability from 5 to 100 µg P • L-1 , to investigate eutrophication effects on periphyton taxonomy, C:N:P stoichiometry and fatty acid composition. After one month, periphyton grown under oligotrophic conditions was mainly composed of diatoms (~86%). With increasing phosphorus availability, diatoms were gradually outcompeted by chlorophytes and cyanobacteria, which were the predominant taxon under eutrophic conditions. Unexpectedly, periphyton C:P ratios increased with greater phosphorus supply, from ~280 under oligotrophic conditions up to ~790 at 100 µg • L-1 , reflecting a tendency of chlorophytes and cyanobacteria to produce more biomass per unit of assimilated phosphorus compared to diatoms. Periphyton content of essential polyunsaturated fatty acids relative to biomass followed a unimodal relationship with phosphorus availability and peaked at intermediate phosphorus levels, likely as a result of both taxonomic and nutrient effects. Our results demonstrate that phosphorus-driven eutrophication of freshwater ecosystems may worsen periphyton nutritional quality due to taxonomic sorting, which may further lead to lower growth and reproduction of herbivores.

Cascading effects of predators on algal size structure

“…While we predicted that a less edible energy pathway (i.e., larger cells) would mute the strong top-down control of algal abundance, the magnitude of the size effect was not large enough to prevent the transfer of energy to consumers. This finding is interesting given the overall shift toward larger cells in the presence of nutrient enrichment irrespective of trophic-level manipulation (Iannino et al 2020).…”

“…This finding is interesting given the overall shift toward larger cells in the presence of nutrient enrichment irrespective of trophic‐level manipulation (Iannino et al. 2020).…”

“…2016, Sanchez and Trexler 2018, Iannino et al. 2020). Selective feeding by herbivores for example may cause changes in algal assemblage structure by reducing the abundance of more edible algal taxa in favor of more resistant forms (Feminella and Hawkins 1995, Persson et al.…”

“…A growing number of studies have demonstrated that enhanced nutrient availability during intermittent wet periods will alleviate bottom-up constraints on aquatic primary production by microalgae (Wyatt et al 2012, Lougheed et al 2015. The magnitude of energy transfer from this basal resource to higher trophic levels is not yet clear because algal taxonomic groups vary widely in size and morphology, making some taxa more easily consumed than others (M€ uller-Navarra et al 2000, Bell 2002, Guo et al 2016, Sanchez and Trexler 2018, Iannino et al 2020. Selective feeding by herbivores for example may cause changes in algal assemblage structure by reducing the abundance of more edible algal taxa in favor of more resistant forms (Feminella and Hawkins 1995, Persson et al 2001, Grac ßa et al 2018.…”

Resource-dependent foraging behaviour of grazers enhances effects of nutrient enrichment on algal biomass

“…The C:P values measured in the P− treatment were within range of C:P ratios measured in this stretch of the Holtemme (266-460, Weitere et al 2021 ). Moreover, in the P− treatment, periphyton was composed of > 90% diatoms, which tend to maintain low C:P ratios compared to green algae and cyanobacteria (Quigg et al 2003 ; Iannino et al 2020 ). In the phosphorus-enriched treatment, nutrient addition led to a significant increase in the proportion of green algae, an effect frequently observed with phosphorus enrichment (Leland and Porter 2000 ; Whorley and Wehr 2016 ; Iannino et al 2020 ).…”