Winter Pond Fertilization Can Increase Phytoplankton Density in Aquaculture Ponds

Effective winter fertilization would be beneficial under certain production systems in the southeastern USA. Maintaining a phytoplankton bloom during the winter has the potential to shade macrophytes, discourage filamentous algae, and provide natural foraging opportunities for fish during warmer days; this would be advantageous. An additional benefit of a winter bloom would be to produce turbidity, thereby possibly reducing bird predation. The objective of this study was to determine if winter fertilization could increase phytoplankton to greater biomass than in nonfertilized control ponds. Six ponds served as controls and six ponds received an initial dose of nitrogen at 20kg/ha and phosphorus at 2kg/ha in mid-November. Over the following five weeks, treatment ponds received one-half the initial dose once per week, and then continued to receive the half dose once per month in January, February, and March. Fertilization led to increases in total ammonia nitrogen, nitrite, and nitrate. The water temperature ranged from a low of 5 degrees C to a high of 16.8 degrees C (mean of 10.2 degrees C) over the entire study. Even though water temperatures were low and light intensity is less in winter, fertilization caused a significant increase in chlorophyll a over the study. The increase in total algal biomass was not due to an increase in any specific algal group, but rather a general biomass increase in most algal groups. Cryptophyta tended to be the most common phytoplankton division throughout the study. Although this fertilization protocol increased algal biomass, there were no differences in zooplankton abundance between fertilized and nonfertilized ponds. In contrast to the currently held belief that winter fertilization is ineffective, this study shows that winter fertilization can increase algal blooms. Therefore, future studies should determine if blooms created from winter fertilization can increase growth of planktivorous fish, shade macrophytes, discourage filamentous algae, or produce turbidity which could reduce bird predation. Received August 13, 2013; accepted October 7, 2013