Nutrients, irradiance, and mixing as factors regulating primary productionin coastal waters impacted by the Mississippi River plume

Relationships among primary production, chlorophyll, nutrients, irradiance and mixing processes were examined along the salinity gradient in the Missi ssippi River outflow region. A series of six cruises were conducted during 1988-1992 at various times of year and stages of river discharge. Maximum v alues of biomass and primary production were typically observed at intermed iate salinities and coincided with non-conservative decreases in nutrients along the salinity gradient. Highest values of productivity (>10 gC m(-2) d (-1)) and biomass (>30 mg chlorophyll a m(-3)) were observed in April 1988, July-August 1990 and April-May 1992; values were lower in March and Septem ber 1991. Rates of primary production were apparently constrained by low ir radiance and mixing in the more turbid, low salinity regions of the plume, and by nutrient limitation outside the plume. Highest values of primary pro duction occurred at stations where surface nutrient concentrations exhibite d large deviations from conservative mixing relationships, indicating that depletion of nutrients was related to phytoplankton uptake. Mixing and adve ction were important in determining the location and magnitude of primary p roduction maxima and nutrient depletion. In addition to growth within plume surface waters, enhanced growth and/or retention of biomass may have occur red in longer residence time waters at the plume edge and/or beneath the su rface plume. Vertical structure of some plume stations revealed the presenc e of subsurface biomass maxima in intermediate salinity water that was depl eted in nutrients presumably by uptake processes. Exchange between subsurfa ce water and the surface plume apparently contributed to the reduction in n utrients at intermediate salinities in the surface layer. DIN (= nitrate nitrite + ammonium) : PO4 ( = phosphate) ratios in river water varied seaso nally, with high values in winter and spring and low values in late summer and fall. Periods of high DIN : PO4 ratios in river nutrients coincided wit h cruises when surface nutrient concentrations and their ratios indicated a high probability for P limitation. N limitation was more likely to occur a t high salinities and during late summer and fall. Evidence for Si limitati on was also found, particularly in spring. (C) 1999 Elsevier Science Ltd. A ll rights reserved.