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

Abstract

We examined freshwater and ocean circulation effects on the distribution of vertical quantum diffuse attenuation coefficients (Kq0) of photosyntheticaly available radiation (PAR) in waters of the northern Gulf of Mexico's Louisiana–Texas shelf. Mean Kq0 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 10m thick of turbid waters (0.06≤Kq0≤1.18m−1) and a lower layer of more transparent waters (0.01≤Kq0≤0.49m−1). The second or non-stratified period consists of a homogenous layer ∼55m thick and less turbid waters (0.03≤Kq0≤1.00m−1). Horizontally, the distribution of Kq0 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 Kq0 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 Kq0 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 Kq0 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 Kq0 variability. Estimates of Kq0 for the upper layer indicate compensation depths of 30–92m in waters deeper than 70m which help account for the presence of coral communities on submerged banks near the shelf edge. The observed temporal and spatial distribution of Kq0 agrees qualitatively with that of satellite-derived values of the diffuse attenuation coefficient, Kd(490) over this shelf.