SIZE STRUCTURE OF PLANKTON IN A TEMPORARY, SALINE INLAND LAKE

The study of the size structure of a planktonic community in a relativ ely extensive (1350 ha), very shallow (0.46 m maximum depth), temporar y inland saline lake during a relatively rainy year (1987-88) showed a n average biomass-size spectrum with four clear modes at nano-, micro- . meso- and macroplankton size ranges. The size-abundance distribution was dominated by medium-sized phytoplankton cells (Gymnodinium cf. ex cavatum, Tetraselmis apiculata, Dunaliella spe. and several diatom spe cies), large ciliates (Fabrea salina) and a few zooplankton species ad apted to changing temporary environments within a wide size range (Cle tocamptus retrogressus, Moina salina, Branchinella spinosa). This poly modal size distribution is compared with results obtained in other sys tems, including small physically controlled lakes, large lakes, oceans and marine benthos. Consideration of the size structure within homoge neous groups of organisms shows slopes less negative than the average for the whole size range analyzed, a feature which is common in small and physically controlled ecosystems. A seasonal study of the size str ucture revealed uneven distributions throughout the year, resulting in discontinuities in the size spectra as well as in differences of tota l size range. There were noticeable differences among the size spectra at each particular time, showing phases of primary producer predomina nce and phases with a marked predominance of consumers. The time-integ rated size spectrum did not exhibit any distinct discontinuity. The in fluence of extreme shallowness and resuspension on the shape and dynam ics of the size spectrum is examined and discussed compared to deeper aquatic systems.