Characterization of the underwater light environment and its relevance to seagrass recovery and sustainability in Tampa Bay, Florida.

摘要

The availability of light is a primary limiting factor for seagrass recovery and sustainability. Understanding not only the quantity but the quality of light reaching the bottom is an important component to successful seagrass management and the key focus of this study. This study explores the spectral properties of the sub-surface light field in four shallow Seagrass Management Areas (SMA) in Tampa Bay. Wavelength-specific photosynthetically active radiation (PAR(lambda)) and the spectral light attenuation coefficient (K d(lambda)) are used to estimate the percent blue, green, and red light remaining at the bottom relative to the surface. LIDAR Bathymetry is combined with Kd(lambda) to produce high-resolution maps of percent subsurface light along the seagrass deep edge. The absorptance spectra from two seagrass species together with PAR(lambda) is used to calculate the photosynthetically usable radiation (PUR(lambda)), a term describing the actual wavelengths of light being used by the seagrass. Based on the average annual , 32%--39% percent of PAR reached the bottom at the seagrass deep edge, while only 14%--18% of blue light reached bottom, suggesting that seagrass may be blue-light limited. Analysis of PUR(lambda) data further confirmed that seagrass are blue-light limited.Each SMA was characterized in terms of the inherent optical properties (IOP) of absorption and scatter. Tampa Bay is considered a chlorophyll-dominated estuary. However, in this study, colored dissolved organic matter (CDOM) was the major absorber of blue light, accounting for 60% of the total absorption. To infer past light conditions, the IOPs were related to parameters more commonly used in routine monitoring programs. To estimate Kd(lambda) an empirically-derived model using only the total absorption and scatter coefficients was used and resulted in a good fit between measured Kd(480) and modeled Kd(480).A deck-mounted flow-through system was used to survey each SMA for CDOM and chlorophyll a fluorescence, among other properties. A series of SMA-specific predictor equations were empirically derived to relate raw fluorescence to the IOPs. The Kitchen SMA was used as a case study. Survey results show a strong connection between CDOM-rich waters and the mangrove-dominated shoreline.