A decline in water clarity in Florida Bay has been observed following the seagrass dieoffs starting in the late 1980's. Algal blooms and discolored water have been reported in Florida Bay over the last several years and factors such as resuspension of material and nutrients from the bottom have been suggested as a cause. Monthly monitoring programs by Florida International University (FIU) and Florida Department of Environmental Protection (FDEP) have provided documentation of blooms through chlorophyll measurements. This study is using remote sensing to examine resuspension events, the distribution of turbid water and changes in the patterns of water clarity in the Bay.

The Advanced Very High Resolution Radiometer (AVHRR) on NOAA meteorological satellites has been used in this study. Currently, over 600 usable scenes are available from some 1500 covering a period from December 1989 to the present. AVHRR has a pixel size of about 1.1 km. The data sets are processed for water reflectance, which is related to water turbidity variables such as attenuation, Secchi depths, total particulate matter, and nephelometric turbidity. Tentative relationships with these variables have been made. (Sea surface temperature is also determined.) High reflectance corresponds to high attenuation or particulate loads or shallow Secchi depth. The individual scenes are also processed to obtain monthly and seasonal means, with winter corresponding to the period of December to March and summer to June through September. Initial analyses include the points corresponding to fixed stations occupied at a monthly interval by FIU or FDEP.

In examining the average reflectance of the entire Bay, the satellite imagery does not show a trend between December 1989 and September 1996. The seasonal pattern of high turbidity in winter and low in summer is evident. Trends over the time period appear in subsections of the Bay. A substantial increase in water reflectance is evident in the north-central Bay (which has been documented in field studies), this region includes Rankin Lake and Johnson Key. However these appear to have different phasing, with Johnson Key showing an increase in turbidity about two years earlier than Rankin Lake. Both sites show the decline in both winter and summer. Twin Key, which has the clearest water in the Bay, has shown a slight increase. The southwest portion of the Bay, west of Sprigger Bank, has shown a decrease in reflectance, indicating clearer water.

The project is conducting comparisons between chlorophyll values collected from the shipboard monitoring programs and pre-cruise reflectances to assess whether there is a link between resuspension events and algal blooms. The next stage in the project is to expand the AVHRR data set backward to before the seagrass dieoffs and to incorporate Landsat data for limited high resolution analysis.

(This abstract was taken from the Florida Bay Science Conference Proceedings, 1996)

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