SEAGRASS DEPTH LIMITS IN THE INDIAN-RIVER-LAGOON (FLORIDA, USA) - APPLICATION OF AN OPTICAL WATER-QUALITY MODEL

A model of spectral diffuse attenuation coefficient for downwelling ir radiance in terms of the inherent optical properties of optically impo rtant water quality parameters was calibrated near two seagrass beds i n the Indian River Lagoon, Florida, U.S.A. One of the seagrass sites w as near the outflow of a canal discharging highly coloured water, and is regularly inundated by a plume of coloured water. Attenuation coeff icients for photosynthetically active radiation predicted by the model agreed with observations within about 15%. Observed ecological compen sation depth for the seagrass bed more distant from the colour source agreed well with the central tendency of the 20% penetration depth pre dicted by the optical model using the observed distributions of water quality parameters. Depth distributions of the seagrass bed near the c oloured water source were about 0.5 m shallower, a result predicted by the optical model assuming the seagrass bed is inundated by the colou red water plume 50-70% of the time. At depths less than their ecologic al compensation depths, the two seagrass beds had similar species comp osition, shoot densities and biomass characteristics. Application of t he model to hypothetical scenarios to reduce the impact of coloured wa ter on the depth distribution of seagrasses near the discharge site in dicated that any improvement in water quality would improve conditions for seagrass growth. The results indicate the utility of optical mode lling in conjunction with limited field surveys of seagrasses for sett ing water quality objectives. (C) 1996 Academic Press Limited