Responses of very shallow marine ecosystems to nutrient enrichment

The success of simple predictive relationships such as the Vollenweider plot in limnology has encouraged marine ecologists to attempt to develop similar models relating pollutant inputs to ecological conditions in estuaries. Most of these efforts have focused on relatively deep (>5 m) river mouth estuaries and embayments where primary production is dominated by phytoplankton. Experimental nutrient enrichment studies of phytoplankton-based mesocosms at the Marine Ecosystems Research Laboratory (MERL) have confirmed that simple Vollenweider type relationships can be found between the rate of input of inorganic nutrients and annual mean chlorophyll concentrations and primary production. However, much of the coastline of the U.S. is characterized by estuarine ecosystems that are very shallow, and where most of the primary production is carried out by angiosperms, such as eelgrass, Zostera marina, epiphytic algae, drift and attached macroalgae, and epibenthic microalgae, rather than by phytoplankton. We have not been able to find useful relationships between nutrient input and the type of plant providing most of the primary production or between nutrient input and the amount of primary production in such shallow lagoon systems. Attempting to adjust nutrient loading for varying hydraulic residence time did not improve the models. Experimental studies using shallow lagoon mesocosms have shown that there is a large variation in the abundance of the various plant forms in these very shallow systems, and that simple Vollenweider models are not likely to emerge for this type of environment. However, it does seem that total system production increases with nutrient enrichment at very low rates of input, and that eelgrass does not persist when exposed to even moderate levels of fertilization. Zostera responds to inorganic nitrogen enrichment and to shading by increasing the rate of leaf elongation and decreasing the allocation of resources to below ground roots and rhizomes. This reduces or eliminates lateral branching of the rhizomes and causes a decline in the density of shoots. Based on mesocosm studies, we propose several indicators of eelgrass health, including the rate of leaf elongation, plant density, and the shoot: root biomass ratio that all deserve further study and field testing.