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

Taxon-specific growth and sedimentation rates of dominant phytoplankton 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 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 examine 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(-1) with highest rates in the plume region during the summer cruise, where surface rates were close to or exceeded previous mu(max), values for several 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)); 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 microzooplankton grazing loss rates were noted only for small taxa (<20 mu m). For microflagellates, microzooplankton 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-specific and seasonal differences noted. Large colonial diatoms, such as Skelelonema costatum and Thalassiosira rotula, exhibited the highest sedimentation 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 Rhizosolenia fragilissima and Cera tulina pelagica in the hypoxia region during the summer cruise. Our results suggest that in the northern Gulf of Mexico, phytoplankton 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).