Research Product

Light attenuation (kd), chlorophyll a (chl a), and primary production (PP) were measured across the Louisiana shelf, encompassing the area of the shelf where summer hypoxia forms, on 7 spring/summer cruises from 2005 to 2007. Shelf-wide average kd (1/m) co-varied with Mississippi and Atchafalaya River Basin (MARB) discharge (R2 = 0.67), but kd was low enough that the euphotic depth (1% light level) almost always exceeded the surface mixed-layer depth across most of the shelf. Commonly, the euphotic depth intersected the bottom. Similar to kd, shelf-wide average chl a concentrations (range = 1.4 to 5.9 mg/m3) varied coincident with MARB discharge (R2 = 0.90) and MARB nitrate (NOx) load (R2 = 0.64). Chl a concentrations were significantly higher in plume versus nonplume regions of the shelf. On the contrary, shelf-wide average vertically integrated chl a (chl aint, range = 26.3 to 47.6 mg/m2) did not correlate with either MARB discharge or nutrient loads and plume versus nonplume averages were not statistically different. The elevated chl aint in the higher salinity waters resulted from frequent sub-surface chl a maxima (SCM). Over the 7 cruises 54% of the stations had a SCM. PP rates were highest in the vicinity of the Mississippi River plume (average for this plume region = 2.1 g C m-2 d-1) but were not statistically different from one another across the rest of the shelf. Owing to the prevalence of SCM and deep euphotic depths a substantial fraction of PP occurred below the pycnocline (range = 25-50% per cruise). These results suggest that MARB freshwater and nutrient inputs regulate shelf-wide kd and, subsequently, the vertical distribution of PP, thus affecting sub-pycnocline O2 concentrations.