Predicting rates of consumer-mediated nutrient cycling by a diverse herbivore assemblage

Abstract

Herbivores mediate the abundances of primary producers both from the top-down, by consuming them, and from the bottom-up, by recycling nutrients. Whereas the top-down effects of herbivores on algae in marine ecosystems are well-documented, less is known about their roles as mediators of local-scale nutrient availability. We conducted a series of surveys and measurements of tide pools and the grazers in those pools between October of 2016 and June of 2017 at an intertidal site on the coast of Southern California, USA (33°35′16.3″N, 117°52′1.5″W). We surveyed grazer abundances in the field, measured biomass of representatives from four different grazer groups (littorine snails, limpets, chitons, and turban snails), measured ammonium excretion rates, and quantified ammonium accumulation rates in tide pools at our study site. We found that different grazer groups were characterized by different per-biomass ammonium excretion rates. Some grazer groups—turban snails and chitons—contributed more ammonium than predicted by their biomass, whereas other grazer groups—littorine snails and limpets—contributed less ammonium than predicted by biomass. Because of these differences between grazer groups, ammonium accumulation rates in tide pools at our study site were effectively predicted based on the ammonium excretion rates of the different grazer groups. However, ammonium accumulation rates were not related to total herbivore biomass. Our results highlight the importance of grazer identity—and particularly the role of species such as turban snails that contribute disproportionately to nutrient recycling—in understanding the contributions of grazers as mediators of bottom-up processes in marine systems.