Quantifying seagrass light requirements using an algorithm to spatially resolve depth of colonization.

Depth of colonization ( ) is a useful seagrass growth metric that describes seagrass response to light availability. Similarly, percent surface irradiance at (% SI) is an indicator of seagrass light requirements with applications in seagrass ecology and management. Methods for estimating and % SI are highly variable making meaningful comparisons difficult. A new algorithm is presented to compute maps of median and maximum , and , respectively, for four Florida coastal areas (Big Bend, Tampa Bay, Choctawhatchee Bay, Indian River Lagoon). Maps of light attenuation ( ) based on MODIS satellite imagery, PAR profiles, and Secchi depth measurements were combined with seagrass growth estimates to produce maps of % SI at and . Among estuary segments, mean varied from (±s.e.) 0.80±0.13 m for Old Tampa Bay to 2.33±0.26 m for Western Choctawhatchee Bay. Standard errors for were 1-10% of the segment means. Percent SI at averaged 18% for Indian River Lagoon (range = 9-24%), 42% for Tampa Bay (37-48%) and 58% for Choctawhatchee Bay (51-75%). Estimates of % SI were significantly lower in Indian River Lagoon than in the other estuaries, while estimates for Tampa Bay and Choctawhatchee Bay were higher than the often cited estimate of 20%. Spatial gradients in depth of colonization and % SI were apparent in all estuaries. The analytical approach could be applied easily to new data from these estuaries or to other estuaries and could be incorporated routinely in assessments of seagrass status and condition.