Evaluation of limnocorrals for studying the effects of phytotoxic compounds on plankton and water chemistry in aquaculture ponds

Two studies were conducted to evaluate the potential use of limnocorrals (i n situ enclosures) for evaluating the effects of phytotoxic compounds on ph ytoplankton community structure and water quality. Limnocorrals consisted o f open-bottomed, fiberglass cylinders that were 2.44 m in diameter and 1.53 m high. The enclosures were placed in an aquaculture pond and allowed to s ettle 10-20 cm into the bottom mud, forming a watertight seal that isolated approximately 5.5 m(3) of pond water. The First study evaluated the effect of water mixing on environmental conditions within limnocorrals. Mixing wa s accomplished by injecting air through airstones suspended inside the encl osures. Conditions in unmixed limnocorrals rapidly deviated from conditions in the pond and in aerated enclosures, with overall phytoplankton biomass decreasing while abundance of cyanobacteria and concentrations of soluble r eactive phosphorus increased. Conditions in limnocorrals with continuous mi xing also deviated from conditions in the pond. Environmental conditions am ong replicate mixed enclosures were, however, relatively consistent and sta ble for at least 2 wk. The second study evaluated the use of limnocorrals f or testing the effects of phytotoxic compounds on phytoplankton community s tructure and water quality. A commercial chelated copper algicide was added to randomly selected, mixed limnocorrals at the label-recommended rate. Th e algicide killed nearly all phytoplankton in the treated enclosures within 1 wk; however, treated limnocorrals were rapidly recolonized by green alga e and diatoms. Conditions in untreated limnocorrals remained relatively sta ble and consistent among replicates for 16 d, after which total phytoplankt on biomass began to decrease, possibly due to nutrient depletion within the enclosures. Although conditions inside the enclosures deviated with time f rom those in the surrounding pond water, mixed limnocorrals appear to provi de a convenient and reliable method for short-term studies of algicides and other water quality manipulations.