Competitive interaction between monoecious Hydrilla and American eelgrass on sediments of varying fertility

Monoecious hydrilla [Hydrilla verticillata (L.f.) Royle] from the Potomac River and American eelgrass (Vallisneria americana Michx.) were grown in monoculture and in mixed culture on aquatic sediments of decreasing fertility to assess their ability to compete for nutrients. Biomass yield in hydrilla decreased with decreasing fertility and increased an average of 45 % when grown intermixed with eelgrass. This was interpreted as evidence of within species competition. Biomass yield only decreased in eelgrass when grown on sand, evidence of lower nutrient requirements than hydrilla. Sediment fertility had little effect on nutrient concentrations in tissues, except that concentrations were depressed when plants were grown on sand alone. Reduced growth of hydrilla was not related to tissue concentrations of measured nutrients. Nutrient accumulation in plant tissues was most closely related to dry weight and was greatest in hydrilla. Potassium accumulation in plant tissues exceeded supplies in sediments indicating potassium uptake occurred from ambient water. Sediment potassium concentrations were increased by plant growth in some sediment treatments.