Examining the mean vertical attenuation of scalar quantum irradiance (PAR) over the Louisiana-Texas shelf (northern Gulf of Mexico)

We examined freshwater and ocean circulation effects on the distribution of vertical quantum diffuse attenuation coefficients (K-q0) of photosyntheticaly available radiation (PAR) in waters of the northern Gulf of Mexico's Louisiana-Texas shelf. Mean K-q0 coefficients were estimated from 509 vertical profiles of PAR collected during 10 cruises spanning 30 months (1992-1994). Vertical profiles of density revealed that the shelf waters are divided into two periods: a stratified period with an upper layer 10 m thick of turbid waters (0.06 <= K-q0 <= 1.18 m(-1)) and a lower layer of more transparent waters (0.01 <= K-q0 <= 0.49 m(-1)). The second or non-stratified period consists of a homogenous layer similar to 55 m thick and less turbid waters (0.03 <= K-q0 <= 1.00 m(-1)). Horizontally, the distribution of K-q0 reveals nearshore coastal or case 2 waters followed by offshore oceanic or case 1 waters that separate near the 70-m isobath regardless of time and place. The K-q0 distribution reflects the freshwater influx from the Mississippi and Atchafalaya Rivers which causes a turbid surface trapped river plume, the shelf wind-driven circulation, and ensuing mixing. To investigate K-q0 we used two regression models involving salinity, suspended particulate matter (SPM), chlorophyll-a (Chl), and water depth. The best statistical model explained 57% to 85% of the observed K-q0 variability and involved the reciprocal of water depth, salinity, and SPM. However, a more bio-optically relevant model involving salinity, SPM, and Chl, explained only 32% to 64% of the observed K-q0 variability. Estimates of K-q0 for the upper layer indicate compensation depths of 30-92 m in waters deeper than 70 m which help account for the presence of coral communities on submerged banks near the shelf edge. The observed temporal and spatial distribution of K-q0 agrees qualitatively with that of satellite-derived values of the diffuse attenuation coefficient, K-d(4 9 0) over this shelf. Published by Elsevier Ltd.