IMPACTS OF DIFFERENT FORMS OF NUTRIENTS ON THE ROLES OF COASTAL LAGOONS AS NUTRIENT SOURCES OR SINKS - A MESOCOSM STUDY

An experiment was conducted to quantify the effects of enrichment with dissolved inorganic nutrients on the N and P mass balances of coastal lagoons. The experiment was conducted during summer using mesocosms d esigned as living models of the shallow coastal lagoons of Rhode Islan d, USA. The experiment involved five treatments, each conducted in dup licate. Two mesocosms served as controls, and remained unenriched. Two mesocosms were enriched with phosphate alone, two with nitrate alone, two with nitrate plus phosphate, and two with ammonium plus phosphate . The unenriched controls functioned as small sources of N and P. The mesocosms enriched with phosphate alone showed net releases of N (as i n the controls), but a small net retention of P. The inverse pattern a pplied in the mesocosms enriched with nitrate alone. The mesocosms enr iched with N plus P showed net retention of both elements. For both nu trients, the percent retention of the nutrients added to the system wa s small (ca. 20%), and in the same order as reported for deeper, phyto plankton-based marine bays. The mass balances for the lagoon mesocosms were most sensitive to additions of N, and were less affected by whet her the N was added alone or with P. The mass balances were unaffected by the form in which the N was added, as ammonium or nitrate.