NITROGEN VERSUS PHOSPHORUS ENRICHMENT OF BRACKISH WATERS - RESPONSES OF THE SUBMERSED PLANT POTAMOGETON-PERFOLIATUS AND ITS ASSOCIATED ALGAL COMMUNITY

We examined the relative effects of nitrogen (N) versus phosphorus (P) enrichment to waters overlying the submersed plant Potamogeton perfoliatus L. and its associated community. This plant species was formerly an abundant component of brackish waters of Chesapeake Bay (USA) prior to a general decline in submersed plants associated with eutrophication. An experimental study was conducted using plant-sediment microcosms held in temperature-controlled baths under natural light in greenhouse facilities. The experimental design consisted of N additions at 2 levels, simulating the range of loading rates to the Bay, and N: P ratios at 3 levels, representing a variety of input conditions from runoff (N: P = 50) to sewage effluents (N: P = 2). Additions of both N and P caused significant increases in biomass accumulation of epiphytic and phytoplanktonic communities. The effects of N and P on algal densities were synergistic in that responses to N addition were greatest at high P loading and visa versa. At the highest nutrient treatment rates, combined amendments (N plus P) resulted in significantly greater increases in epiphytes and phytoplankton than did the same high inputs of either nutrient (N or P) individually. Associated with increased algal densities at high nutrient loading rates, there were significant decreases in growth and biomass of P perfoliatus. Again, responses of plant growth to nutrient treatments were most pronounced when both N and P were added together. Pooling data from all treatments, significant inverse correlations were found between epiphyte density and plant growth and biomass as well as light (PAR) attenuation. A hyperbolic relation between PAR attenuation by epiphytes and plant growth suggests light availability as a principal mechanism for epiphytic inhibition of P. perfoliatus growth. In addition, plants at high nutrient treatments exhibited morphological responses characteristic of shade adaptation in this species, further emphasizing the importance of light attenuation as a control mechanism. These results suggest that the experimental plant communities were poised in a condition where both N and P were potentially limiting for algal growth. Previous studies have attributed the loss of submersed plants from Chesapeake Bay's brackish waters largely to nutrient enrichment. The present results suggest that management efforts to restore submersed plants such as P. perfoliatus by reducing eutrophication, and associated light attenuation by algae, should consider reducing inputs of both N and P.