Biomass–density relationships of the seagrass Zostera noltii: A tool for monitoring anthropogenic nutrient disturbance

In order to assess how anthropogenic nutrient disturbance affects the populations of the seagrass Zostera noltii, the temporal variation of the stand biomass and shoot density expressed by the population-specific biomass–density relationships were studied along nutrient gradients associated with discharge from Waste Water Treatment Works, in two coastal systems of southern Portugal. Two sites were studied in Ria Formosa lagoon (Faro NW and Tavira) and one in the Arade estuary. Four stations were sampled along the nutrient gradient in each of two sites (Faro NW along 2 years, and Arade along 1 year) and two stations were sampled in Tavira. The Z. noltii population structure reflected the anthropogenic nutrient disturbance. The nutrient-disturbed stations showed significant correlations between biomass and density, whereas at nutrient-undisturbed stations the biomass–density data were uncorrelated. The Z. noltii population parameters derived from the temporal variation of the population structure, i.e. the slope of the biomass–density relationships, the coefficient of variation of biomass and the ratio of the maximum/minimum biomass increased with nutrient availability, whereas the intercept of the biomass–density relationships decreased with nutrient availability. These patterns resulted mostly from the higher variability of biomass at nutrient-disturbed stations as shoot density varied little among stations. The higher biomass variability may reflect both the beneficial effects (availability) and the detrimental effects (toxicity) of nutrients along the year. Very high concentrations of DIN (>400 μM) caused toxic effects on plants limiting the biomass development. The described relationships and patterns of variation may be used to assess the global nutrient disturbance level of areas within the coastal systems in southern Portugal. Validation of these patterns for other geographical areas and other seagrass species may provide a general valuable tool to assess the anthropogenic nutrient disturbance in coastal areas.