Alterations of Vertical Distribution and Migration of Zooplankton in Relation to Temperature

Abstract

-Several crustacean zooplankters (Tropocyclops prasinus, Mesocyclops edax, Pseudosida bidentata ahd Bosmina longirostris) exhibited diurnal vertical migrations in early June at Belews Lake, North Carolina. These species spent daylight hours within the thermocline and nights in the epilimnion. Aeration of the lake hypolimnion lowered it from a depth of 11 m to 17 m by mid-July. After this restratification, B. longirostris, P. bidentata and M. edax migrated to greater depths, indicating that temperature may exert an important influence on vertical migratory behavior for these three species. Although vertical migration of Skistodiaptomus pallidus, Diacyclops thomasi and copepod nauplii was not demonstrated, these zooplankters consistently were found within or below the thermocline during day and night in June, and in July after restratification, suggesting distinct habitat preferences for colder waters. INTRODUCTION The vertical distribution and migration of zooplankton are often influenced by the temperature-depth profile in stratified systems and, indeed, thermal gradients may pose a barrier to migration (Wells, 1960; Hutchinson, 1967). Frequently, lakes are host to a number of different species exhibiting different thermal preferences (see Wells, 1960; Carter, 1974; Lane, 1978; Balcer et al., 1984, for examples). Of the species found in Belews Lake, North Carolina, the cyclopoid copepod, Diacyclops thomasi, is usually associated with cold waters (Yeatman, 1959; Carter, 1974; Moore, 1979), whereas Mesocyclops edax and Tropocyclops prasinus generally prefer warm waters (Yeatman, 1959; Carter, 1974; Balcer et al., 1984). The calanoid copepod, Skistodiaptomus pallidus, also is found and reproduces in warm conditions (Yeatman, 1959; Chapman et al., 1985). The cladoceran, Pseudosida bidentata, is restricted in geographic distribution to the southern United States (Brooks, 1959), which suggests that it also tends to be found in warm waters. In addition, some of the zooplankton species in Belews Lake have been reported elsewhere to undergo vertical migration. These groups include the cladoceran, Basmina longirostris (Wells, 1960; Wilson and Roff, 1973) and the copepods, Mesocyclops edax (Woodmansee and Grantham, 1961; Williamson and Magnien, 1982), Tropocyclops prasinus (Wilson and Roff, 1973) and D. thomasi (Wells, 1960; Wilson and Roff, 1973; Peacock and Smyly, 1983), as well as copepod nauplii (Wilson and Roff, 1973). Belews Lake serves as a cooling reservoir for the Belews Lake Steam Station operated by Duke Power Company. The thermocline is normally located between 7 and 11 m during the summer. Water intake valves at the steam station are located at approximately 9 m. To increase cooling efficiency, the power company installed a pair of air injectors at 17 m near the intake valves (Mitch Griggs, Duke Power Co., pers. comm.). Operation of the air injectors, which commenced on 9 June 1986, caused mixing of the cooler hypolimnetic waters with the warm epilimnion near the intake valves, thus reducing water temperature and increasing plant efficiency. As a result, the temperature profile in the rest of the main I Present address: Dept. of Fisheries and Oceans, Maurice Lamontagne Institute, C.P. 1000, MontJoli, Quebec, Canada G5H 3Z4.