Relationship between light availability and benthic community structure in the Florida Keys

Background/Question/Methods

Coastal ecosystems are particularly susceptible to anthropogenic impacts because of their proximity to large populations.  In 1996, the Seagrass Ecosystems Research Lab began a monitoring program in the Florida Keys National Marine Sanctuary (FKNMS).  As part of this program, community structure (richness, abundance) was assessed using benthic surveys, and water quality parameters including nutrient content and light attenuation coefficient (k_d) were measured to monitor the status of the system. I calculated percent light reaching the benthos (%I₀) across FKNMS using Lambert-Beer Law [%I₀=exp(-1*k_d*z)*100] and evaluated the relationship between light availability at depth and benthic community structure.

Results/Conclusions

While light availability varied significantly across FKNMS, temporal differences were not significant.  The amount of light required by each of the 4 dominant species of South Florida seagrass differed significantly.  Thalassia testudinum, Syringodium filiforme, and Halodule wrightii inhabited sites where %I₀ was a minimum of 13%-22% but averaged 54%-69%.  Halophila sp. (combined H. decipiens and H. engelmannii) was identified in sites with %I₀ as low as 10% but averaged 40%.  Increased light attenuation caused by elevated nutrient levels from increased freshwater flow might negatively impact coastal ecosystems.  The observed results identify regions of FKNMS that could be particularly susceptible to changes in water quality.