Benthic-pelagic coupling and nutrient cycling across an experimental eutrophication gradient

Abstract

Sediment-water exchange rates of dissolved inorganic nitrogen (NH:, NO;, NO;) and phosphorus (PO:-), water column concentrations of both dissolved and particulate forms of N and P, and net primary production (14C) were measured during a summer period in large (13 m3) experimental mesocosms that had been subjected to continuous daily nutrient additions (N, P, Si) for over 1 yr. The concentrat~on of combined dissolved inorganic plus particulate nutrient forms (N, P) was linearly related to the nutrient input rate across a loading range from 596 to 34,100 mmols N m-2 yr-l. Benthic nutrient regeneration generally increased, although not uniformly, with loading. Average summer fluxes for treatments ranged from about 150 to 1200 pmols NH: m-2 h' , 188 to 989 pmols DIN m-2 h' , and 8 to 76 pmols PO:m-2 h' , with highest rates being I-ecorded at the 2 highest loading levels. Benthic nutrient regeneration d ~ d not increase in direct proportion to loading, but appeared strongly related to net primary production rates across the enrichment gradient. Comparison, across the experimental gradient, of the external nutrient supply rate with the benthic feedback of dissolved nutrients to the water column indicated that beyond a loading of about 2000 to 5000 mmols N m-? yr-' the water column nutrient dynamics became dominated by the external supply. Results suggest that the importance of benthic-pelagic interactions to biogeochemical and ecological cycles of coastal areas may decrease markedly with nutrient enrichments which exceed the needs of the autotrophic components of the ecosystem.