Research Product

The water column diffuse attenuation coefficient (Kd) of the Louisiana Continental Shelf (LCS) was examined during ten years to characterize the spatial and temporal variations on monthly scales from 1998 to 2007. This region is well-known for summer hypoxia (dissolved oxygen < 2 mg L-1) that occurs in bottom waters over large sections of the shelf. This report validates Sea-viewing Wide Field-of-View Sensor (SeaWiFS) daily integrated surface PAR and Kd490 diffuse attenuation for the LCS. The SeaWiFS Kd490 diffuse attenuation coefficient ranged from 0.06 to 3.32 m-1. Stepwise multiple linear regression analysis suggested spatial and temporal patterns in SeaWiFS Kd490 were influenced by wind speed, nutrient loading and river discharge from the Mississippi and Atchafalaya River Basin. We used SeaWiFS derived Kd490 to estimate the amount of photosynthetically active radiation (PAR, 400-700 nm) that reaches the sediment water interface, finding that large portions of the shelf bottom are euphotic. This finding implies that significant primary production likely occurs beneath the pycnocline. An observed increase in SeaWiFS Kd490 during the summer months of hypoxia development suggests that changes in the diffuse attenuation coefficient, and hence changes in the euphotic depth may contribute to the onset of hypoxia. Therefore the spatial and temporal distribution of hypoxic waters is likely influenced by the availability of light. This is also the first known report of euphotic depth corresponding to the depth at which the water column turns hypoxic on the LCS