Increases in nutrient concentrations in the Mississippi River over the past 35 yr have led to speculation that primary production of organic carbon has been elevated as a result of increased nutrient fluxes that have occurred in the northern Gulf of Mexico coastal ecosystem. However, studies thus far have not provided direct demonstration of temporal relationships between measured primary production in continental shelf waters and river-borne nutrient fluxes. This investigation compared temporal variations in primary production with associated annual and interannual changes in river-borne nutrient inputs. Primary production in shelf waters near the river delta were found to be significantly correlated with nitrate (NO3-) + nitrite (NO2-) concentrations and fluxes over a 6 yr period from 1988 to 1994. Although light limitation was probably an important factor during winter months, a positive correlation was demonstrated between river inputs of NO3-+NO2- and primary production for data collected from other times of the year. Peak nutrient inputs generally occurred in the spring. The magnitude of the riverborne NO3-+NO2- inputs averaged 106% of estimated nitrogen requirements for phytoplankton in the river-impacted region, considerably greater than in Amazon shelf waters, which have been less subject to anthropogenic nutrient increases. The possibility exists that further increases in anthropogenic nutrients in the Mississippi River could lead to higher and more widespread primary production, and this may intensify and extend the depletion of oxygen that has already been observed in the Louisiana shelf ecosystem. However, such a prediction is difficult because relationships between increasing nutrient inputs and primary production are unlikely to be Linear, and a complete understanding of processes intermediate between primary production of organic matter and oxygen depletion in bottom waters on the Louisiana shelf is still lacking.

Taxon-specific growth and sedimentation rates of dommant phytoplankton were meas- ured during 2 cruses (summer 1990 and spring 1991) in the northern Gulf of Mexico as part of the NOAA Nutrient-Enhanced Coastal Ocean Productivity (NECOP) program. Microzooplankton grazing rates also were measured during the summer cruise. During each of the cruises, a series of stations from the Mississippi River mouth to the hypoxia region (located ca 50 to 100 km west) were sampled to examme variability of growth and loss processes along a strong environmental gradient. Significant taxa- and group-specific differences were noted for both growth and loss rates Growth rates ranged from <0.1 to 3.0 d-I with highest rates in the plume region during the summer cruise, where surface rates were close to or exceeded previous urnax values for several taxa. For all taxa, growth rates were lower in the hypoxia region (mean = 0.5 dl) than in the plume region (mean = 1.1 dl); soluble nitrogen concentrations explained over 50 % of the variability in growth rates. Diatom growth rates were similar to non-diatoms in the plume region, but were significantly lower in the hypoxia region, which suggests that silica limitation may exist in this region. The fate of phytoplankton appeared to be controlled by size and by the degree of silicification. Significant mcrozooplankton grazing loss rates were noted only for small taxa (<20 pm). For microflagellates, microzooplankton grazing rates averaged 82 % (range 42 to 214 %) of the growth rate; sedimentation rates were always <I% of the growth rate. Sedimenta- tion was an important loss for several diatoms, with significant taxon-specific and seasonal differences noted. Large colonial diatoms, such as Skeletonema costatum and Thalassiosira rotula, exhibited the highest sedimentation rates in the plume region dunng the spring cruise(0.2 to 1.0 dl), whereas the lowest rates (<0.01 dl) were noted for Rhizosolenia fragilissirna and Ceratulinapelagica in the hypoxia region dunng the summer cruise. Our results suggest that in the northern Gulf of Mexico, phyto- plankton rate processes proceed very rapidly, with growth rates primarily controlled by the supply of nitrogen via the Mississippi River and the fate controlled primarily by size and density (silicification).