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.

The several faces of fear: ecological consequences of predation risk in a lagoon model system

“…This conclusion is obtained from observations of the imbalance in the content of C relative to N and P between herbivores and primary producers. In coastal lagoons, P is generally one of the most limiting nutrients to primary productivity (Sundareshwar et al, 2003) and even secondary productivity (Guariento et al, 2011). Our results showed that the ortho-phosphate excretion by tadpoles decreased with increased predation risk, however there was no effect observed for the excretion of ammonium (i.e., nitrogen).…”

“…We argue that this is a very plausible risk gradient, even though previous studies have indicated changes in the stoichiometry of the body of animals subject to predation risk (Hawlena and Schmitz, 2010). Although the notion that consumers are strictly homeostatic, i.e., they do not vary regardless of their nutritional content, has been contested (Guariento et al, 2011), there is a very large phylogenetic variation in the level of homeostasis observed in each taxon. Most studies that have looked at the effects of predation risk on the stoichiometry of prey have mostly used invertebrates as intermediate consumers.…”

Interactive effects of predation risk and conspecific density on the nutrient stoichiometry of prey

“…Despite the notion that consumers are strictly homeostatic (i.e., that they do not vary their elemental composition regardless of their nutritional content), some studies have shown that this is not entirely true (Guariento et al. 2011a,b) and that stoichiometric plasticity is indeed expected due to food restriction.…”

“…2010; Guariento et al. 2011a, 2011b). Consequently, it is possible to trace directly an individual prey's stoichiometry plasticity in response to predation risk, which alters the prey's nutrient body and excretion stoichiometry and can reverberate throughout ecosystem‐level processes (Sterner and Elser 2002; Schmitz 2008; Hawlena and Schmitz 2010b; Leroux et al.…”

The effect of predation pressure and predator adaptive foraging on the relative importance of consumptive and non‐consumptive predator net effects in a freshwater model system

“…We found optimal prey habitat choices using a dynamic state variable model (Clark and Mangel 2000). The model was designed (see Supplementary material Appendix 1A for details) and the parameters were chosen based on empirical studies of small fish species or aquatic insects interacting over a short time scale in shallow aquatic ecosystems (Hirai and Hidaka 2002, Guariento et al 2011). We assumed that there is a 60‐day long season and prey (e.g.…”