Nutrient, phytoplankton, and oyster dynamics in a highly flushed subtropical lagoon, Northeast Florida

With ever-increasing coastal development, predicting the consequences of nutrient enrichment in coastal ecosystems has become a main focus of estuarine research. The goal of this research was to characterize important processes related to the effects of nutrient enrichment in a highly flushed subtropical estuary, the Guana Tolomato Matanzas National Estuarine Research Reserve (GTMNERR) in northeast Florida. Understanding the effects of nutrients on a system first requires knowledge of the structure and function of the base of the food web. In this study, patterns in phytoplankton biomass were explored in relation to a suite of potentially regulating factors, including nutrient availability, in context with other gain and loss processes in the GTMNERR. Monthly measurements of temperature, light, nutrient concentrations, and phytoplankton standing stock over seven years (2003-2009) were examined through correlation analysis. Laboratory experiments in the spring and summer of 2009 quantified phytoplankton growth rates, nutrient limitation potential, and zooplankton grazing rates. The potential influence of oyster grazing was also examined using population metrics and filtration rate estimates. All of the gain and loss factors were correlated to some degree with phytoplankton biomass in the GTMNERR, but results indicated a temporal shift in the primary controlling factors, from temperature, light, and flushing in the winter to grazing and flushing the remainder of the year. In contrast to temperate systems, or systems dominated by riverine inputs, phytoplankton biomass in this area exhibited a regular seasonal pattern characterized by short periods of low biomass rather than by bloom events. The magnitude and interannual variability of phytoplankton biomass observed in this study were fairly small compared to estuarine and coastal ecosystems around the world.;Since traditionally monitored water quality parameters, such as nutrient and phytoplankton concentrations, often do not provide a clear indication of trophic status in estuaries with short water residence times, response to nutrient enrichment in this system was also measured at the level of benthic primary consumers. Oyster population structure was examined within two regions of the GTMNERR using measurements of oyster density, biomass, length, and condition. As expected, oysters exhibited greater population density, average biomass, and condition in the region with historically elevated nutrient loads and carbon availability than in the less urbanized region. Results suggest that oysters have the potential to be used as bioindicators of trophic state in highly flushed estuaries.;Overall, this study demonstrated that well-flushed estuaries may be more resistant to some of the negative effects of nutrient enrichment than those systems with comparatively restricted hydrodynamics. Especially in subtropical systems without major riverine inputs, primary production can exist virtually in balance with consumption, resulting in relatively low primary producer biomass and food-limited benthic consumer populations. The threshold for maintaining such a balance is undefined, however, especially since changes in nutrient loads can also affect phytoplankton species composition and be associated with other harmful impacts such as toxic contaminants.