Coastal food web structure, carbon storage, and nitrogen retention regulated by consumer pressure and nutrient loading

By factorial field experiments we analyzed the relative effects of increase d nutrient (N+P) loading and natural grazing pressure on species compositio n, carbon storage, and nitrogen retention in the Baltic Sea littoral food w eb, composed of macroalgae, grazers (snails, isopods, amphipods), and preda tors (shrimps, crabs, fish). Nitrogen was depleted relative to phosphorus t hroughout most of the year. Increasing nitrogen (6-200% over ambient concen trations) enhanced algal productivity and cover of fast-growing annual alga e, grazer, and predator densities, suggesting a three-level bottom-up effec t. With increasing nitrogen loading, annual algae increasingly blocked pere nnial algal recruitment (65-98% decrease) and growth. Grazers counteracted the effects of nutrient enrichment on algal species composition through sel ective consumption of annual algae. Grazer exclusion had equivalent negativ e effects on perennial recruitment as a 85% increase in nitrogen loading. N utrient enrichment increased algal nitrogen content and decreased tissue C: N ratios in spring and summer but not in fall. Carbon storage and nitrogen retention, measured as C and N retained in plant biomass at the end of the growth season, were increased by grazers (C: 39%, N: 24%) but decreased wit h increasing nitrogen loading (C: -71%, N: -74%). Our results emphasize the important role of grazers in buffering moderate eutrophication effects and illustrate how food web interactions and shifts in species composition are tightly linked to coastal ecosystem function.