Satellite Remote Sensing of Chlorophyll-a Concentrations in the Galveston Bay, Texas

Friday, 19 December 2014
Shuai Zhang1, Guanheng Zheng2, Huilin Gao1 and Daniel Roelke3, (1)Texas A & M University, Zachry Department of Civil Engineering, College Station, TX, United States, (2)Tsinghua University, Department of Hydraulic Engineering, Beijing, China, (3)Texas A & M University, Department of Wildlife and Fisheries Sciences, College Station, TX, United States
Galveston Bay (Texas) is characterized by the unique and complex mixing of waters allowing the bay to produce more seafood than any bay in the United States with the exception of Chesapeake Bay (Maryland). The concentration of Chlorophyll-a plays a key role in ecosystem productivity. Since field measurements are complicated and expensive, satellite remote sensing provides an opportunity to observe real-time variations of Chlorophyll-a at a large spatial scale. However, because of the notable influences caused by the absorption and scattering of other substances (such as total suspended solids, dissolved organic matters and tripton), the use of ocean color remote sensing images to estimate Chlorophyll-a in coastal waters is usually limited. In order to monitor the variation of Chlorophyll-a with high accuracy (and at a high temporal resolution), an algorithm was developed using Terra Moderate Resolution Imaging Spectroradiometer (MODIS) reflectance data, Landsat Thematic Mapper (TM) images, and in-situ measurements. This approach mainly contains two steps. First, based on the relatively high spatial resolution of Landsat images (30m), an exponential regression relationship was found between the Landsat reflectance ratio (i.e., band 3/band 4) and the in-situ measured Chlorophyll-a concentrations (R2 = 0.81) during the two-year sampling period (2005- 2006). By using an automatic outlier detection technique for quality control, a Landsat Chlorophyll-a concentration product from 2000- 2011 was generated. Second, in order to improve the temporal resolution over the bay area, an 8-day MODIS product at 500 m spatial resolution from 2000 to 2013 was created. Landsat results from the first step were employed to calibrate the parameters used in NASA’s existing operational product algorithm. This way, the MODIS product from this study can best represent the phytoplankton productivity in Galveston Bay. Using the remotely sensed Chlorophyll-a product, further comparisons were made among phytoplankton productivity, climate indices, and inflows.