Phytoplankton size structure stability in a meso-eutrophic subtropical lake

The aim of the present study was to analyze seasonal and inter-annual variability, and examine the stability of several parameters describing the size structure of the phytoplankton assemblage of Lake Kinneret (Israel). Phytoplankton biomass size spectrum (BSS) patterns were analyzed using cell-volume data based on microscopic counts of samples collected biweekly over 4 years (1996-1999). A typical pattern of Lake Kinneret phytoplankton BSS emerged, as being quasi-stable in spite of unprecedented man-induced lowering of the lake's water levels during those years, atypical phytoplankton biomass dynamics, and extreme inter-annual variations in phytoplankton species composition. The present study included all phytoplankton greater than ca. 2 μm diameter, which comprise most of the lake's autotrophic biomass, and phytoplankton alone. Statistical descriptors of separate size classes elucidate two zones of pronounced variability within Kinneret BSS and a zone of stability near its center. The phytoplankton biomass variability is produced mainly by two bloom zones, at relatively large cell size classes (V = 2048-4096 and V = 65 500-131000 μm³), corresponding with proliferation of the bloom-forming species Aulacoseira granulata and Peridinium gatunense, respectively, while the stability zone at the center of the BSS, essentially a 'nanoplankton plateau', corresponds with a diverse assemblage of nanoplanktonic species, of different taxonomic composition at different times. Statistical parameters of BSS approximation provide a tool for the quantitative estimation of the stability/variability of whole phytoplankton assemblages. According to these parameters, Kinneret BSS is comparable to BSS of eutrophic lakes of Canada and Spain but differs from the more stable BSS typical of oligotrophic systems.