LIGHT REQUIREMENTS OF SEAGRASSES HALODULE WRIGHTII AND SYRINGODIUM FILIFORME DERIVED FROM THE RELATIONSHIP BETWEEN DIFFUSE LIGHT ATTENUATION AND MAXIMUM DEPTH DISTRIBUTION

The correspondence between maximum depth of growth (Z(max)) for two se agrases, Halodule wrightii and Syringodium filiforme, and the attenuat ion of diffuse photosynthetically active radiation (K(d)PAR) were eval uated over a 3.5-yr period in the southern Indian River Lagoon, Florid a. The lower limit of seagrass depth distribution was controlled by li ght availability. Both species grew to the same maximum depth, indicat ing they have similar minimum light requirements. Based on average ann ual values of K(d)PAR, estimates of seagrass maximum light requirement s ranged from 24% to 37% of the light just beneath the water surface ( I-0), much higher than a photic zone for many phytoplankton and macroa lgae (1-5% incident light). In less transparent waters of Hobe Sound, where turbidity (NTU) and color (Pt-Co) had their highest concentratio ns, minimum light requirements for growth were greatest. These results suggest that more sophisticated optical models are needed to identify specific water quality constituents affecting the light environment o f seagrasses. Water quality criteria and standard needed to protect se agrasses from decreasing water transparency must be based on parameter s that can be routinely and reasonably managed.