CAUSES OF LIGHT ATTENUATION IN TAMPA BAY AND CHARLOTTE HARBOR, SOUTHWESTERN FLORIDA

Vertical attenuation of photosynthetically active radiation (PAR) in clear waters of central Florida theoretically can vary almost 50 percent during a sunny summer day as a result of changing solar elevation. We used a simple formula to partially adjust the attenuation coefficient in Tampa Bay and Charlotte Harbor for changing solar elevation of the direct beam and then used multiple regression analysis to estimate the relative contribution of different water properties or constituents to the adjusted attenuation coefficient, k(adj). Color, on an average, was responsible for 18 percent of k(adj), chlorophyll a for 21 percent, nonchlorophyll suspended matter for 55 percent, and seawater for the remaining 6 percent. In both estuaries, k(adj) increased with decreasing salinity as a result of freshwater runoff adding color, suspended matter, and nutrients. Nutrients affected attenuation by stimulating phytoplankton growth and increasing concentrations of chlorophyll a. Reduced nutrient loading to upper Tampa Bay (Hillsborough Bay) in the early to mid-1980's appears to have decreased concentrations of chlorophyll a, increased water clarity, and increased seagrass recolonization. Assuming other attenuating substances remained unchanged, the decrease in the average concentration of chlorophyll from 30 to 15 mug L-1 would correspond to an increase in the depth of light penetration necessary for seagrass survival (> 10 percent incident light) from 1.0 to 1.5 m, which, on a relatively flat sea bed (slope of 2 m/km), would increase the area potentially available for seagrass recolonization by 0.25 km2/km of shoreline.