Effects of Rice Bran, Cottonseed Meal, and Alfalfa Meal on pH and Zooplankton

Abstract

Rice bran (RB), cottonseed meal (CSM), and alfalfa meal (ALF) were compared as organic fertilizers for their effects on pH and zooplankton abundance in limnocorrals that lacked fish. The weight (W) and nitrogen (N) content of CSM, which served as the control, were used to determine the other applications (RB-N, RB-W, ALF-N and ALF-W). Temperature, dissolved oxygen, chlorophyll a, ammonia nitrogen, and zooplankton densities were monitored for 8 weeks to simulate the average period for phase 1 production of striped bass Morone saxatilis (38–50 mm). Rice bran promoted better water quality than ALF or CSM when application rates were based on nitrogen content. The pH was lowest for RB-N, intermediate for ALF-N, and highest for CSM. Chlorophyll a was a function of nitrogen input, and the high-nitrogen applications (CSM, ALF-N, and RB-N) supported similar phytoplankton standing crops that were 38–62% greater than the corresponding low-nitrogen applications (ALF-W and RB-W). Water temperature and dissolved oxygen concentrations were similar among applications, and total ammonia nitrogen was below detectable levels for all applications. Overall mean densities of rotifers, nauplii, adult copepods, adult crustaceans, and total zooplankton did not differ significantly among applications. However, the mean density of cladocerans was significantly higher for RB-N and ALF-N than for ALF-W. The densities of cladocerans, adult copepods, adult crustaceans, and total zooplankton correlated strongly with chlorophyll a but not with the quantities of fertilizers applied, indicating that phytoplankton probably was the primary food for these zooplankters. Based on pH, RB was better than CSM or ALF, but all had similar effects on zooplankton when application rates were based on nitrogen.