Nitrogen versus phosphorus enrichment of brackish waters : responses of the submersed plant Potamogeton perfoliatus and its associated algal community

We examlned the relative effects of nitrogen (N) versus phosphorus (P) enrichment to waters overlying the submersed plant Potamogeton perfoljatus L. and its assoc~ated community. This plant species was formerly an abundant component of brackish waters of Chesapeake Bay (USA) prlor to a general decline in submersed plants associated with eutrophicat~on. An experimental study was conducted uslng plant-sedlment microcosms held in temperature-controlled baths under natural light in greenhouse facillt~es. 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 sign~ficant increases in biomass accumulation of eplphytic 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 nutnent treatment rates, comb~ned amendments (N plus P) resulted in s ign~ficantly greater increases in epiphytes and phytoplankton than did the same h ~ g h inputs of e ~ t h e r nutrient (N or P) individually Associated w ~ t h increased algal dens~t les at high nutrient l oad~ng rates, there were s~gniflcant decreases in growth and blomass of P perfoljatus. A g a ~ n , responses of plant growth to nutnent 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 b~omass as well as 11ght (PAR) attenuation A hyperbolic relation between PAR attenuation by epiphytes and plant growth suggests light availablllty as a principal mechanism for epiphytic inhiblt~on of P perfol~atus growth. In addition, plants at high nutnent treatments exhibited morphological responses characterlst~c of shade adaptation in this species, further emphasizing the importance of light at tenuatlon 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 nutrlent 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.

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