Seagrass Depth Limits in the Indian River Lagoon (Florida, U.S.A.): Application of an Optical Water Quality Model

A model of spectral diffuse attenuation coefficient for downwelling irradiance in terms of the inherent optical properties of optically important water quality parameters was calibrated near two seagrass beds in the Indian River Lagoon, Florida, U.S.A. One of the seagrass sites was near the outflow of a canal discharging highly coloured water, and is regularly inundated by a plume of coloured water. Attenuation coefficients for photosynthetically active radiation predicted by the model agreed with observations within about 15 %. Observed ecological compensation depth for the seagrass bed more distant from the colour source agreed well with the central tendency of the 20 % penetration depth predicted by the optical model using the observed distributions of water quality parameters. Depth distributions of the seagrass bed near the coloured water source were about 0 •5 m shallower, a result predicted by the optical model assuming the seagrass bed is inundated by the coloured water plume 50 –70 % of the time. At depths less than their ecological compensation depths, the two seagrass beds had similar species composition, shoot densities and biomass characteristics. Application of the model to hypothetical scenarios to reduce the impact of coloured water on the depth distribution of seagrasses near the discharge site indicated that any improvement in water quality would improve conditions for seagrass growth. The results indicate the utility of optical modelling in conjunction with limited field surveys of seagrasses for setting water quality objectives.