Vertical distribution and feeding activity of Epischura baicalensis Sars (Copepoda) nauplii in response to two predators in Lake Baikal in winter

Abstract

In March 1999 and 2000, we carried out synchronous observations for species representing the entire pelagic food web in lake Baikal: picoplankton, phytoplankton, copepods (Epischura baicalensis), amphipods (Macrohectopus branickii) and sculpin larvae (Comephoridae). During daytime, the layer with the maximal density of Epischura nauplii coincided with the maximal distribution of pico- and phytoplankton in the 0-15 m layer. In this layer, 60-70 % of the nauplii were feeding. The diurnal distribution of Epischura nauplii coincided with that of fish larvae, that feed intensively on Epischura nauplii during daytime, but did not overlap with the second predator, the amphipod Macrohectopus. At night, the decrease of feeding activity and the descent of the maximal distribution of nauplii coincided with the ascent of Macrohectopus to upper layers. We propose, that the Epischura nauplii respond to the threat from the invertebrate predator with anti-predation behaviour (reduced feeding and dispersal/descent), while the threat from fish larvae during day is ignored, probably because of the low of fish larval density.