Enhanced primary production at the plume/oceanic interface of the Mississippi River

Abstract Mechanistic and empirical models were used to examine relationships between primary production and environmental variables along the Mississippi River plume/oceanic gradient off Southwest Pass, Louisiana. A large proportion of variation in primary production could be explained on the basis of light and biomass ( r 2 > 0.857, N > 25). However, comparison of observed chlorophyll concentrations with those predicted using a steady-state light limitation model suggested factors in addition to light availability constrained maximum biomass levels in the plume. Factors which may have contributed to low observed biomass included growth limitation or inhibition by substances not measured, losses due to grazing and sinking, and a short residence time for plume waters, which may have prevented populations from reaching steady state. The dissipative effects associated with plume/oceanic mixing may have been enhanced by potential inhibitory effects of large and varying salinity gradients. Nutrient-salinity distributions, in conjunction with approximate calculations of primary consumption of riverine nutrient sources by phytoplankton, led to the conclusion that biomass and production were controlled by nutrient supply at salinities above 30.

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