Nutrient limitation and the eutrophication of coastal lagoons

An ecosystem-level experiment was conducted to identify the nutrient most limiting to productivity and biomass in the marine lagoons of the northeast United States. Mesocosms containing a complex of species characteristic of shallow coastal marine environments were enriched with P alone, N alone, or combined N plus P, at loadings typical of highly enriched natural lagoons. The mesocosms showed significant responses to ennchment with N alone but not P alone, indicating limitation by N. Enrichment with N alone caused increased water column concentrations of chlorophyll a and particulate nitrogen [PN), increased water column daytime net production (NP), and increased rates of growth of juvenile winter flounder. It also caused eelgrass beds and mats of drift macroalgae to decline, apparently in response to phytoplankton shading. Comparison of the N-alone and combined N + P treatments indicated that when enriched with N alone, the limitation of the systems shifted to P limitation of total system metabolism and of phytoplankton production and standing crop, and to light limitation of eelgrass and macroalgal growth. In the combined N + P mesocosms, water column concentrations of chlorophyll a, PN, and particulate P, rates of total system and water column NP and night-time respiration, and growth rates of juvenile winter flounder and killlfish were all increased relative to the N-alone mesocosms. Declines of eelgrass and macroalgae were also more severe.