Structure and dynamics of zooplankton communities maintained in closed systems, with special reference to the algal food supply

Abstract

Seasonal changes in the structure of the zooplankton communities of two large (18 500 m ³ ) experimental enclosures (‘Lund Tubes’) are reviewed. The diets of the two most prominent crustaceans, Daphnia hyalina and Diaptomus gracilis , were investigated by direct analyses of gut contents and in a series of short term experimental manipulations of communities isolated in 6 m ³ bags, placed in one of the enclosures. The relative abundance of Daphnia hyalina generally coincided with warmer water temperatures between late spring and late summer, but its numbers fluctuated within that period, apparently in response to the availability of smaller algae (4 μm ³ unit volume: chiefly cryptomonads, chlorellids and Ankyra ). Daphnia numbers declined steadily through the summer in the enclosure overwhelmingly dominated by Microcystis , although it did feed on smaller colonies (< 10 ⁵ μm ³ in volume). Diaptomus gracilis and most rotifers were also prominent in the first half of the year, but peak abundances of Keratella quadrata followed the presence of large populations of small (3 μm ³ ) algae. Under conditions of limiting food availability, mortality was greatest amongst the smaller species and the juvenile instars of the large Crustacea.