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 increased nutrient (N1P) loading and natural grazing pressure on species composition, carbon storage, and nitrogen retention in the Baltic Sea littoral food web, composed of macroalgae, grazers (snails, isopods, amphipods), and predators (shrimps, crabs, fish). Nitrogen was depleted relative to phosphorus throughout most of the year. Increasing nitrogen (6–200% over ambient concentrations) enhanced algal productivity and cover of fast-growing annual algae, grazer, and predator densities, suggesting a three-level bottom-up effect. With increasing nitrogen loading, annual algae increasingly blocked perennial algal recruitment (65–98% decrease) and growth. Grazers counteracted the effects of nutrient enrichment on algal species composition through selective consumption of annual algae. Grazer exclusion had equivalent negative effects on perennial recruitment as a 85% increase in nitrogen loading. Nutrient 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 with increasing nitrogen loading (C: 271%, N: 274%). 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. Currently, humans are more than doubling the rate at which nitrogen and phosphorus enter the global biogeochemical cycles (Schlesinger 1991; Vitousek et al. 1997). Through river, groundwater, and atmospheric transport, a large fraction of these excess nutrients passes through estuarine and coastal ecosystems (Howarth et al. 1996; Nixon et al. 1996; Jickells 1998). Primary producers in these systems can act as efficient filters and largely control nutrient cycling and the export of nutrients to the open ocean (Jickells 1998). This function has been pointed out as one of the most important ecosystem services provided by marine ecosystems (Costanza et al. 1997). However, increasing nutrient loading can change primary producer abundance and species composition, with feedbacks on the cycling and processing of nutrients. In this study, we asked how the effects of increasing nutrient loading are mediated by food web interactions in the Baltic Sea coastal system. Detailed experimental work in freshwater systems showed that nutrients and consumer strongly influence algal abundance, biogeochemical cycling, and fisheries production (e.g., Carpenter et al. 1985, 1996; Sommer 1985, 1992; Power 1990; Brett and Goldman 1997). In contrast, very few studies in the marine environments considered nutrient and consumer effects simultaneously. Experimental evidence for 1 To whom correspondence should be addressed. Present address: Biology Department, Dalhousie University, Halifax, Nova Scotia, Canada, B3H 4J1 ([email protected]).