PHYSIOLOGICAL-RESPONSES OF SEAGRASSES USED TO IDENTIFY ANTHROPOGENIC NUTRIENT INPUTS

Fertilization experiments have established that seagrass growth in Mor eton Bay can be limited by the supply of both N and P. In the present study, morphological and physiological characteristics (canopy height, shoot density, biomass, growth, tissue nutrient content, amino acid c oncentrations and delta(15)N ratios) of Zostera capricorni Aschers. in Moreton Bay, close to and distant from nutrient sources, were compare d. Z. capricorni at the four sites close to nutrient sources (sewage, septic or prawn-farm effluent, or river discharge), had physiological characteristics representative of high nutrient availability and at th e five sites distant from nutrient sources had physiological character istics representative of low nutrient availability. Differences in sed iment nutrient concentrations (NH4+ and PO43-), seagrass morphology an d growth were not related to proximity to nutrient sources. However, t he nutrient content of the seagrasses and their amino acid concentrati ons were consistently higher at sites close to a nutrient source. The amino acids glutamine and asparagine were the most responsive to eleva ted nutrient availability, and delta(15)N values of seagrasses reflect ed the source of N rather than the nutrient load. These results demons trate that physiological characteristics of seagrasses can be used to identify the nutrient load and source affecting marine ecosystems.