TAXON-SPECIFIC GROWTH AND LOSS RATES FOR DOMINANT PHYTOPLANKTON POPULATIONS FROM THE NORTHERN GULF-OF-MEXICO

Taxon-specific growth and sedimentation rates of dominant phytoplankto n were measured during 2 cruises (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 als o were measured during the summer cruise. During each of the cruises, a series of stations from the Mississippi River mouth to the hypoxia r egion (located ca 50 to 100 km west) were sampled to examine variabili ty 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(-1) with highest rates in the plume region during the summer cruise, where sur face rates were close to or exceeded previous mu(max), values for seve ral taxa. For all taxa, growth rates were lower in the hypoxia region (mean = 0.5 d(-1)) than in the plume region (mean = 1.1 d(-1)); solubl e nitrogen concentrations explained over 50 % of the variability in gr owth rates. Diatom growth rates were similar to non-diatoms in the plu me region, but were significantly lower in the hypoxia region, which s uggests that silica limitation may exist in this region. The fate of p hytoplankton appeared to be controlled by size and by the degree of si licification. Significant microzooplankton grazing loss rates were not ed only for small taxa (<20 mu m). For microflagellates, microzooplank ton grazing rates averaged 82 % (range 42 to 214%) of the growth rate; sedimentation rates were always <1% of the growth rate. Sedimentation was an important loss for several diatoms, with significant taxon-spe cific and seasonal differences noted. Large colonial diatoms, such as Skelelonema costatum and Thalassiosira rotula, exhibited the highest s edimentation rates in the plume region during the spring cruise(0.2 to 1.0 d(-1)), whereas the lowest rates (< 0.01 d(-1)) were noted for Rh izosolenia fragilissima and Cera tulina pelagica in the hypoxia region during the summer cruise. Our results suggest that in the northern Gu lf of Mexico, phytoplankton rate processes proceed very rapidly, with growth rates primarily controlled by the supply of nitrogen via the Mi ssissippi River and the fate controlled primarily by size and density (silicification).