SOME ENVIRONMENTAL CONSEQUENCES OF VERTICAL MIGRATION IN MARINE ZOOPLANKTON1

Abstract

The spatial distribution of plankton is highly aggregated, so one might expect that a vertically migrating animal enters a biotic environment that is markedly different each evening and that the changes in environment between daylight periods spent at depth are equally great. To measure the extent of these changes, as typified by the species composition of the zooplankton, a model zooplankter was followed in the area southeast of Guadalupe Island, Baja California, for 52 hr. Net tows for zooplankton were taken nearby every 2 hr. A 48‐hr series of plankton samples at a single geographic position was taken afterward for comparison of the rates of change of species composition as observed by drogue‐guided and ordinary station sampling. There were 250 species and other categories in the 23 nighttime samples from 10 m and 280 in the 20 daytime samples from 100 m. Contrary to the radical change expected, there was a slow, steady change in composition from the beginning to the end of the combined drogue and single position series. Some evidence was found of a limit to this change, implying a limit to the uncertainty a migrating animal experiences in the species composition of the two surrounding communities in which it lives. Several peculiar features of the change of species composition with time are discussed.