Stoichiometry of nutrient excretion by fish: interspecific variation in a hypereutrophic lake

Abstract

This study investigates how nutrient cycling rates and ratios vary among fish species, with a particular focus on comparing an ecologically dominant detritivore (gizzard shad) to other fishes in a productive lake. We also examined how nutrient cycling rates are mediated by body size (as predicted by allometry theory), and how variation in nutrient cycling is related to body and food nutrient contents (according to predictions of ecological stoichiometry). As predicted by allometry, per capita nitrogen and phosphorus excretion rates increased and mass-specific excretion rates decreased, with increasing mass. Body phosphorus content was correlated with body mass only in one species, bluegill. Contrary to stoichiometric predictions, there was no relationship between body P and mass-normalized P excretion rate, or between body N:P and excreted N:P, when all individuals of all species were considered. However, at the species level, we observed some support for a body nutrient content effect on excretion as predicted by stoichiometry theory. For example, gizzard shad had lower body P (high body N:P) and also excreted P at higher rates (lower N:P) than bluegill, which had high body P (lower body N:P). We applied the Sterner (1990) homeostatic stoichiometry model to the two most common species in the study Á gizzard shad and bluegill-and found that food N:P had a greater effect than consumer body N:P on excreted N:P. This indicates that, in terms of variation among these species, nutrient excretion may be more of a function of food nutrient content than the nutrient content of the consumer. These results suggest that stoichiometry can provide a framework for variation among species in nutrient cycling and for evaluating the ecosystem consequences of biodiversity loss. Human-induced decreases in biodiversity may cause functional shifts in ecosystem processes, leading to a debate among ecologists as to the relative importance of how biodiversity mediates ecosystem functioning (Hoo-per et al. 2005). Some authors have concluded that differential impacts of species (i.e. species identity) on their environment and the types of interactions that take place may be the most important mechanisms by which biodiversity enhances ecosystem functioning (Hooper et al. 2005). To evaluate the importance of species identity in regulating ecosystem processes, more information is needed on how different species mediate such processes. Ecological stoichiometry theory (sensu Sterner and Elser 2002) has emerged as a conceptual framework for how species vary in mediating nutrient cycling, an important ecosystem process. This theory predicts that nutrient release by a consumer is a function of the imbalance between the consumer's body nutrient content and its food's nutrient content as well as the efficiency at which the consumer assimilates nutrients into biomass (Sterner and George 2000, Vanni et al. 2002). Of particular interest is the ratio at which nutrients are released. A model proposed by Sterner (1990) suggests that the N:P ratio of nutrient release (hereafter excretion N:P) by a consumer will increase curvilinearly with food N:P and will decrease with consumer N:P, assuming that the consumer is homeo-static with respect to body N and P contents and has an equal maximum gross growth efficiency (GGE max) for each nutrient. Animals such as fish and invertebrates are often important in nutrient cycling within freshwater ecosystems , but few studies have explicitly examined inter-259