Relative influence of various water quality parameters on light attenuation in Indian River Lagoon

Six synoptic sampling events were conducted in the Indian River Lagoon (IRL) between April and June, 1997 to collect TSS (total suspended solids), color (dissolved organic matter), chl a (chlorophyll a), and light (photosynthetically active radiation (PAR)) data. These data were used to develop our understanding of light attenuation dynamics in the IRL and for verification of a numerical light model. Data from our study show that tripton (non-algal particulate matter calculated from TSS and chl a corrected for pheophytin) has a dominant effect on light attenuation in the IRL. For each synoptic event, there exists a positive correlation between the eventaveraged downwelling light attenuation coefficient, Kd(PAR), and event-averaged tripton concentration. A negative correlation is found between the event-averaged Kd(PAR) and the event-averaged color, while a positive correlation is found between eventaveraged Kd(PAR) and event-averaged chl a concentration. The correlation between event-averaged tripton and event-averaged Kd(PAR) is the only one of the three to show significance at the 0.05 level. Relative contributions of color, chl a, and tripton to light attenuation were found to be 5–25%, 10–26%, and 59–78% of Kd(PAR), respectively. These values depend on the method for partitioning Kd(PAR) and the method for obtaining average value of relative partitioned Kd(PAR) from all the data points. These values show that tripton has a more dominant influence on light attenuation in the IRL than in Tampa Bay and Charlotte Harbor, but comparable to that in Florida Bay. The effect of suspended chlorophyll on light attenuation in the IRL is less than that in Tampa Bay, comparable to Charlotte Harbor, but more than that in Florida Bay. A numerical process-based light attenuation model has been developed to calculate Kd(PAR) based on measured or simulated values of TSS, color, and chl a. The model was found to give reasonable estimates of Kd(PAR) throughout the IRL.