Optical Water Quality and Seagrasses in Florida’s Big Bend and Apalachee Bay, Northeast Gulf of Mexico

The abundance and health of seagrasses in Florida's Big Bend and Apalachee Bay are highly dependent on the amount and spectral quality of light reaching the seafloor. Since 2009, staff at the Florida Fish and Wildlife Research Institute (FWRI) and the University of South Florida Optical Oceanography Laboratory (OOL) have collaborated to monitor seagrass abundance, measure in situ optical water quality, and estimate phytoplankton chlorophyll, turbidity, CDOM, and water column transparency using MODIS Aqua imagery in this optically complex coastal region. Discharge from the Suwannee River (and smaller rivers such as the Ochlockonee, Aucilla, Econfina, and Steinhatchee) controls water clarity in Apalachee Bay by delivering nutrients, CDOM, and suspended sediment. Considerable spatial and temporal variation in water column transparency and optical depth occur as the result of variation in rainfall and runoff patterns.

With funding from the NASA ROSES program, we are continuing our longterm in situ measurements of optical water quality and seagrass abundance. In quarterly sampling at up to 60 sites throughout Apalachee Bay in 2018 and 2019, we have encountered phytoplankton chlorophyll concentrations ranging from 1 ug/L to more than 12 ug/L. Turbidity values have ranged from 0.3 to 3.9 NTU, corresponding to total suspended solid concentrations from 1.1 to 8.9 mg/L. The large range in optically active constituents over time and space also provides an opportunity to test existing algorithms and develop new algorithms for the retrieval of phytoplankton chlorophyll, turbidity, and CDOM. To date, fair (eg. log-log) results have been obtained using existing algorithms on MODIS Aqua and VIIRS imagery. Effective water quality management for seagrass protection and restoration will require more accurate remote sensing measurements especially during the growing season from April through October. Very high CDOM concentrations limit the retrieval of turbidity and chlorophyll, but, from a seagrass management standpoint, high ag440 values, by themselves, have serious impacts on seagrass survival and growth, so we are developing seagrass growth models that treat CDOM, chlorophyll, and turbidity stepwise, attempting retrieval of chlorophyll and turbidity only when CDOM concentrations permit more accurate estimates.