Effects of Nutrients and Physical Forcing on Satellite-Derived Optical Properties Near the Mississippi River Delta
2007
Green, Rebecca | Gould, Richard | Martinolich, Paul
We investigated the effects of various chemical and physical forcing mechanisms on optical properties near the Mississippi River delta using a multi-year time series of satellite imagery. Recent cruise data has provided in situ measurements to ground-truth satellite estimates of partitioned absorption (phytoplankton, detritus, sediment, and colored dissolved organic matter components). In our time series analyses, we addressed long-term time scales of variability (2002-2004) using monthly composite SeaWiFS imagery and fifteen different physical forcing variables. Correlation and stepwise regression analyses, performed on each image pixel, revealed which forcing mechanisms were most responsible for optical variability. For example, Mississippi River nitrate concentration explained only a portion of the seasonal variability observed in phytoplankton absorption on the Louisiana shelf, and physical factors, such as river discharge and wind speed were as important in determining variability. Such observations have management implications for hypoxia in terms of mandates to decrease nutrient loading to the Mississippi River watershed. Our goal is to develop a robust statistical model for optical prediction. To this end, we applied our stepwise regression model to physical properties for 2005, a year not included in model development. Our model fairly well predicted a (ph)(443) on the Louisiana-Texas shelf, with an average error of ~30%. In the future, we hope to improve our predictions using seasonally specific models and to analyze shorter time scales (days to weeks) of variability.
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