Physiological responses of seagrasses used to identify anthropogenic nutrient inputs

Fertilization experiments have established that seagrass growth in Moreton 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 concentrations and delta(15)N ratios) of Zostera capricorni Aschers. in Moreton Bay, close to and distant from nutrient sources, were compared. 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 the five sites distant from nutrient sources had physiological characteristics representative of low nutrient availability. Differences in sediment nutrient concentrations (NH4+ and PO43-), seagrass morphology and growth were not related to proximity to nutrient sources. However, the nutrient content of the seagrasses and their amino acid concentrations were consistently higher at sites close to a nutrient source. The amino acids glutamine and asparagine were the most responsive to elevated nutrient availability, and delta(15)N values of seagrasses reflected the source of N rather than the nutrient load. These results demonstrate that physiological characteristics of seagrasses can be used to identify the nutrient load and source affecting marine ecosystems.