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Developing Nitrogen Load-Eelgrass Response Relationshups for New England Estuaries
Citation:
LATIMER, J. S. AND S. A. REGO. Developing Nitrogen Load-Eelgrass Response Relationshups for New England Estuaries. Presented at Status, Trends, and Conservation of Eelgrass in Atlantic Canada and Northeastern United States, Portland, ME, February 24 - 25, 2009.
Impact/Purpose:
The purpose of the research is to provide the scientific tools and models to aid in the development of nutrient criteria for estuaries. This science provides a useful approach to develop nitrogen loading limits for classes of estuaries. It provides evidence that relates a valued ecosystem component (seagrass) to loading levels of nitrogen for one class of estuaries in New England.
Description:
We have accumulated and analyzed eelgrass areal extent data for 67 estuaries from three New England states. To our knowledge this is the largest data set of its kind. Previous comparative studies have utilized data from a far smaller number of estuaries (ten or less) to develop empirical relationships between nitrogen inputs and eelgrass areal extent. Cause-effect mechanisms of excess nitrogen loading on seagrass ecosystems have been published in the literature. It is thought that the dominant mechanism is the following: starting from nitrogen driven increases in phytoplankton and epiphytic growth, decreases in light result in diminished seagrass health. This progression leads to decreased density and ultimately areal extent of seagrass habitat. Based on this mechanism, we predicted, for a set of similar semi-enclosed estuaries along the New England coast, that surface area-weighted nitrogen loading rates would be inversely proportional to the extent of eelgrass (Zostera marina). GIS seagrass polygon coverages were derived for the study estuaries in New England from aircraft-acquired, orthorectified digital imagery and analyzed to obtain areal extent of Zostera for each estuary. Nitrogen loading rates were estimated for each estuary from data on fertilizer use, atmospheric deposition rate, and human wastewater input. The data were consistent with the scientific literature showing that extent is inversely related to nitrogen inputs. However, the considerable observed variability suggests that other factors, besides nitrogen, are contributing to the magnitude of eelgrass in these small-medium sized estuaries.