SPATIAL-DISTRIBUTION AND ABUNDANCE OF LARVAL AND JUVENILE FISH, CHLOROPHYLL AND MACROZOOPLANKTON AROUND THE MISSISSIPPI RIVER DISCHARGE PLUME, AND THE ROLE OF THE PLUME IN FISH RECRUITMENT

In September 1986, we collected neuston (1 x 2 m, 0.947 mm mesh) and surface chlorophyll a samples and hydrographic data at 46 stations around the discharge plume of the Mississippi River. Transects were positioned so that the 3 water masses in the plume area - plume water, Gulf of Mexico shelf water and frontal water (a mixture of the former 2) - were sampled. The plume was represented by a shallow lens of water < 34% salinity and < 29-degrees-C resting atop warmer (> 29-degrees-C) and more saline (> 34%) Gulf of Mexico shelf water. Strong turbidity fronts with a scale of 50 to 100 m form, relax and reform approximately at tidal frequencies within the frontal region that has a larger scale of 6 to 8 km. Total ichthyoplankton catch per tow, individual surface chlorophyll a values and macrozooplankton displacement volumes were all significantly greater in frontal waters than adjacent Gulf of Mexico shelf or plume waters. Hydrodynamic convergence at the continually forming and relaxing turbidity fronts most likely accounts for concentrated neustonic ichthyoplankton, and at least partially for high macrozooplankton values as well, in frontal waters. Elevated macrozooplankton displacement volumes in frontal waters may also result from higher rates of proliferation of macrozooplankton biomass. High primary production in frontal water is probably due to the mixing of nutrient-rich, but turbid, plume water (where photosynthesis is light limited) with clear, but nutrient-poor, Gulf of Mexico shelf water (where photosynthesis is nutrient limited) creating favorable phytoplankton growth conditions. Concentrations of ichthyoplankton and zooplankton offer rich trophic resources that some species utilize to gain superior growth. Faster growth will lead to increased survival and recruitment (because larvae pass through the period of greatest vulnerability to predation by gape-limited predators more quickly) if larval growth is increased disproportionately to larval mortality from predation.