RESPONSES OF COASTAL LAGOON PLANT-COMMUNITIES TO DIFFERENT FORMS OF NUTRIENT ENRICHMENT - A MESOCOSM EXPERIMENT

We carried out an enrichment experiment to quantify the effects of dif ferent forms of nutrients on the plant communities characteristic of l agoons common along the north east coast of the USA. 'Lagoon' mesocosm s were enriched with phosphate alone, nitrate alone, nitrate plus phos phate, or ammonium plus phosphate. Nitrogen and phosphorus were added at loadings of 8200 mu mol m(-2) day(-1) and 685 mu mol m(-2) day(-1), respectively. Two control mesocosms remained unenriched. In the contr ols, as in many unenriched field systems, phytoplankton biomass remain ed low, eelgrass (Zostera marina L.) beds were sustained, and drift ma croalgae and epiphytes were abundant. In mesocosms enriched with phosp hate alone, the plant communities were similar to the controls, except that belowground eelgrass biomass was lowered. Enrichment with nitrat e alone generated moderate phytoplankton blooms (dominated by Nannochl oropsis spp.), declines of above- and below-ground eelgrass, and decli nes of drift macroalgae and epiphytes. With combined N+P enrichment th e Nannochloropsis blooms were between 4 and 10 times more intense than with nitrate alone. The eelgrass declines also started earlier, proce eded more rapidly, and were ultimately more complete. For none of the plant components were, we able to detect significant differences betwe en the N+P mesocosms enriched with nitrate and the N+P mesocosms enric hed with ammonium.