NUTRIENT DISTRIBUTIONS IN THE GULF OF MAINE: AN ANALYSIS OF SPATIAL AND TEMPORAL PATTERNS OF DISSOLVED INORGANIC NITRATE AND SILICATE

The concentrations of dissolved nutrients in the waters of the Gulf of Maine are ultimately what determine the amount of plankton biomass production in the ocean. Until now, the descriptions of the nutrient distributions and dynamics have provided only brief glimpses of restricted areas in space and time.  The primary objectives of this study are to compile and evaluate the historical nutrient data within the region to determine the spatial and temporal variability of nutrient distributions, identify the environmental influences responsible for these patterns, and quantify the variability that exists within nutrient fluxes throughout the Gulf of Maine.

 The average seasonal cycles of one nutrient, dissolved inorganic nitrogen, from five different regions in the Gulf of Maine are similar, although there are important small-scale differences in the cyclic behavior.  Seasonal variability exists at all depths, however the fluctuations are greater interannually than seasonally at depths greater than 50 meters.  Seasonal surface anomalies can be explained by the timing of the spring phytoplankton bloom which removes dissolved nutrients, correlated with salinity anomalies that identify waters of a different source, and even related to zooplankton populations that regulate the phytoplankton. All regions and depths within the Gulf of Maine are positively correlated, suggesting the Gulf typically changes as a cohesive unit.

 A statistical interpolation was used to generate a high-resolution average state and to identify physical features that determine nitrate concentrations in the water.  The analysis also provides an objective measure of the historical sampling effort and indentifies locations and seasons lacking descriptive observations.  A coupled nitrate-silicate box model for the Gulf of Maine shows that while nitrate is influenced heavily by offshore inputs, silicate has a dominant continental source.  The historical data suggest biologically available nutrient fluxes vary significantly interannually and are highly dependent on outflows of Maine Intermediate Water.  While the variability in nutrient concentrations is not the only factor determining biological productivity, these results show the possible influence of nutrient variability from a bottom-up trophic perspective within the Gulf of Maine.