The vertical distribution and diurnal migration of some zooplankton in the Bay of Eilat (Red Sea)
- H. -E. Schmidt1
© Biologischen Anstalt Helgoland 1973
1. During summer 1970, plankton samples were taken in the Bay of Eilat (Red Sea) to study vertical distributions and diurnal migrations of zooplankton. In June, a collecting programme was conducted over 24 hours. Samples were taken with a closing net every 2 hours, at dawn and dusk every hour. The following depth ranges were sampled: 300 to 200 m, 200 to 150 m, 150 to 100 m, 100 to 75 m, 75 to 50 m, 50 to 25 m, and 25 m to water surface.
2. The plankton concentration (accumulated displacement volume of all samples from a certain depth) decreases slowly from the surface to 100 m, then rapidly to 300 m depth. Maximum concentrations are found between 25 and 50 m.
3. In Copepoda, Gastropoda and Chaetognatha, the concentration of individuals decreases with increasing depth. Gastropoda and Chaetognatha may exhibit reductions of 50% over a single 25-m step. The Appendicularia concentration is high between 25 and 50 m; it decreases towards the surface and with increasing depth.
4. As far as it is possible to ascertain the diurnal vertical migrations of these animal groups without separation into species, the four groups can be said to avoid the upper layers during daytime; during this time they occur below 50 m. Around sunset and sunrise, gastropods, chaetognaths and appendicularians tend to accumulate above 25 m.