Zooplankton Community Response to Salinity Addition

Abstract

The primary objective of this study was to determine the effect of salinity stress on a mixed Cladocera and Copepoda community, including shifts in zooplankton densities, percent composition of populations, and changes in percent composition of females carrying eggs or young. The researcher collected zooplankton samples at depths of 0.5m to 3.0m from a lake in Western New York as the water approached the temperature selected for each phase of the project. Samples were taken with a hand pump or a #20 mesh plankton tow net. Organisms were then concentrated into 4L of lake water and transported to the laboratory, where they were immediately placed in a Percival incubator set at the temperature at which they were collected (+- 2C) and aerated for 24 hours. The culture was then randomly sub-sampled to provide 15 sub-cultures (250-300ml each). The researcher replaced the water of each subculture with one of five salt solutions (0, 500, 1000, 1500, and 2000 ppm NaCl in native lake water), resulting in three replicates for each salinity value. Subcultures were maintained in the incubator under a 12-hour photoperiod at the selected temperature. The subcultures were then immediately partitioned and examined with a dissecting microscope for changes in the composition of the zooplankton community and reexamined at 1-2 day intervals thereafter. Only obviously living organisms were counted and classified as to generic makeup and reproductive condition. The salinity-temperature combinations appeared to be within the zooplanktons’ zone of tolerance. However, the researcher observed that long-term exposure to elevated salinity had negative effects on large segments of the zooplankton community. Cladocera were particularly affected and were eliminated at salinity values of 1000 ppm NaCl or greater. The researcher observed that the decline in numbers did not appear to be the result of salinity-induced death, but rather of a lower rate of reproduction/replacement among affected populations. The researcher concludes that higher chloride concentration selectively and significantly reduces biotic potential in specific genera or groups, resulting in lowered diversity.