Food web architecture and basal resources interact to determine biomass and stoichiometric cascades along a benthic food web

Abstract

Understanding the effects of predators and resources on primary producers has been a major focus of interest in ecology. Within this context, the trophic cascade concept especially concerning the pelagic zone of lakes has been the focus of the majority of these studies. However, littoral food webs could be especially interesting because base trophic levels may be strongly regulated by consumers and prone to be light limited. In this study, the availability of nutrients and light and the presence of an omnivorous fish (Hyphessobrycon bifasciatus) were manipulated in enclosures placed in a humic coastal lagoon (Cabiúnas Lagoon, Macaé - RJ) to evaluate the individual and interactive effects of resource availability (nutrients and light) and food web configuration on the biomass and stoichiometry of periphyton and benthic grazers. Our findings suggest that light and nutrients interact to determine periphyton biomass and stoichiometry, which propagates to the consumer level. We observed a positive effect of the availability of nutrients on periphytic biomass and grazers' biomass, as well as a reduction of periphytic C∶N∶P ratios and an increase of grazers' N and P content. Low light availability constrained the propagation of nutrient effects on periphyton biomass and induced higher periphytic C∶N∶P ratios. The effects of fish presence strongly interacted with resource availability. In general, a positive effect of fish presence was observed for the total biomass of periphyton and grazer's biomass, especially with high resource availability, but the opposite was found for periphytic autotrophic biomass. Fish also had a significant effect on periphyton stoichiometry, but no effect was observed on grazers' stoichiometric ratios. In summary, we observed that the indirect effect of fish predation on periphyton biomass might be dependent on multiple resources and periphyton nutrient stoichiometric variation can affect consumers' stoichiometry.

Figure 1. Biomass (a), Carbon content (b), Nitrogen content (c), Phosphorus content (d) and C∶N (e), C∶P (f) and N∶P (g) molar ratios of Biomphalaria tenagophila (Gastropoda).

Treatments are represented by different nomenclatures (N0F0 (no fish or nutrients addition); N0F1 (only fish addition); N1F0 (only nutrients [N and P] addition); N1F1 (both nutrients and fish addition). Each bar represents mean values +1SE. Different letters above bars represent significant statistical differences (Two-way ANOVA with Contrast Analysis as post hoc test, P<0.05).

Figure 2. Periphyton total biomass (top – a and c) and algal biomass (bottom b and d) over the time at different light regimes (High light – a and b; Low light – c and d).

Treatment abbreviations are the same as in figure 1. Circles indicate enriched treatments and unfilled symbols indicate presence of fish. Data are means ± SE. For statistical differences see the text.. Note the differences on Y-axis scales for each graph.

Figure 3. Periphyton Carbon∶Chlorophyll-a ratio in high (○) and low (▪) light conditions.

Treatment abbreviations are the same as in figure 1. Each point represents averaged values of three weeks (n = 12) ±SE. Different letters above bars represent significant statistical differences (Two-way ANOVA with Contrast Analysis as post hoc test, P<0.05).

Figure 4. Response of C∶N∶P stoichiometric ratios in periphyton biomass to different light regimes (high light – a, b, c; low light – d, e, f) and treatments.

Treatment abbreviations are the same as in figure 1. Each bar represents mean values for each week (n = 4) +1SE.