Phytoplankton

Abstract

The phytoplankton assemblage of Lake Kinneret is dominated by dinoflagellates. Secondary phytoplankton biomass contributors are cyanobacteria, chlorophytes, and diatoms, whereas cryptophytes are always present at a low biomass. The main bloom-forming dinoflagellate, Peridinium gatunense, bloomed every spring till the mid-1990s, but since then, it bloomed only during high-rainfall years. This change and a suite of additional changes in phytoplankton dynamics since 1994, changes that occurred after more than two decades of recorded constancy, were interpreted as early responses to increasing stress at the ecosystem level. In particular, the loss of the previously predictable annual pattern (spring bloom of P. gatunense, summer low-biomass, high-diversity assemblage), the appearance and establishment of toxin-producing cyanobacteria, a major loss in species richness, and a shift to dominance of less grazed species in summer are all manifestations of this change that can be viewed as a regime shift.

Surface and remote sensing of chlorophyll a (Chl a)—a proxy of algal biomass—indicates that phytoplankton concentration extremes at the lake surface mostly do not surpass a ratio of 1:2. Exceptional is a situation when Lake Kinneret is dominated by P. gatunense, and then the spatial heterogeneity of phytoplankton increases to a ratio of 1:50 between extreme concentrations. Chl a concentrations were often higher in the northern part of the lake, near the Jordan River inflow, where nutrient enrichment apparently boosts the growth of phytoplankton. The unique Chl a optical properties enable the identification and quantification of that substance using reflectance spectra emerging from the water surface and are recorded by sensors carried onboard satellites. Since 2006, images derived from optical information acquired by MERIS are utilized for mapping of the spatial distribution of phytoplankton in Lake Kinneret.