| Abstract: | Autotrophic picoplankton (APP) are critically important in the production of organic matter in plankton food webs. A number of obvious environmental factors, including light, temperature, nutrient concentrations, and predation, have been identified as influencing APP abundance and growth rates. However, few experiments have investigated more than one factor at a time for their relationships to APP abundance or growth rate. We conducted a series of single and multi‐factor experiments to examine the relative importance of several biotic and abiotic variables, and their interactions, for their effects on APP abundance and growth rates. Experiments were conducted using APP samples collected during summer from the riverine and lacustrine zones of a southeastern U.S. reservoir. Growth rates of APP responded rapidly to nutrient addition, but the magnitude of the response was dependent on location of collection, and on the presence of grazers and other phytoplankton. Growth rates of APP from the riverine zone were limited by nitrogen, whereas lacustrine zone APP responded positively to both phosphorus and nitrogen addition. Regulation of APP population size by heterotrophic nanoflagellates appeared to be most effective when APP growth rates were relatively slow. APP cell chlorophyll concentrations varied inversely with the percentage of surface light in the mixed layer of the reservoir. We suggest that due to spatial differences in the rate and depth of vertical mixing, lacustrine zone APP receive less light on average, or light at less frequent intervals, than APP in the more turbid but shallower riverine zone. Longitudinal declines in both nutrient and light availability during summer may reduce APP growth rates and population size in the lacustrine zone compared to the more productive riverine zone of this reservoir. |
