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The influences of resources (carbon, nitrogen, and phosphorus) and protist bacterivory on heterotrophic bacterioplankton growth rates were investigated at 4 sampling sites in the subtrop- ical Florida Bay seagrass estuary during 3 seasons, summer 2004, winter 2005, and fall 2006. Bacter- ial growth and grazing mortality rates were determined by dilution experiments and flow cytometry, where bacterial communities were separated into high and low DNA content populations (HDNA and LDNA, respectively). In addition, methylumbelliferyl-heptanoate hydrolysis assays were used to measure ambient esterase activity and to determine the impact of resources and grazing on esterase activity. Total bacterial gross growth, grazing mortality, and net growth rates ranged from 0.41 to 1.72, 0.32 to 1.46, and -0.17 to 0.41 d-1, respectively. HDNA bacteria gross growth and grazing mor- tality rates were consistently higher than LDNA gross growth and grazing mortality rates. However, LDNA bacteria were not inactive and exhibited positive gross and net growth rates. Stepwise linear multiple regression analysis indicated that ln-transformed ammonium concentration and ambient esterase activity related significantly and positively to total bacteria net growth rates (R2 = 0.75). When total bacterial mortality rates met or exceeded gross growth rates (i.e. negative or zero net growth rates), esterase activity increased with the reduction of grazing pressure without nutrient addition. When bacterial gross growth rates exceeded bacterial grazing mortality rates (i.e. positive net growth rates), both reduction of grazing pressure and nutrient amendments were required to stimulate esterase activity. These results suggest that bottom-up processes regulated heterotrophic bacterial growth, abundance, and carbon use when ammonium concentrations were high and phos- phate concentrations were low (i.e. high inorganic N:P ratios). Under low ammonium concentrations and relatively increased phosphate concentrations (i.e. low N:P ratios), top-down processes regulated heterotrophic bacterioplankton growth, abundance, and carbon use in Florida Bay, USA.

Bottom-up and top-down control of heterotrophic bacterioplankton growth in a phosphorus-depleted subtropical estuary, Florida Bay, USA