Disturbance of Opportunistic Small-Celled Phytoplankton in Lake Kinneret

In spite of the chaotic dynamics of specific populations, similarity of annual species-abundance distributions was proven for phytoplankton assemblage during a “stable” period (1985–1994) of Lake Kinneret (Israel). This similarity declined during the “extreme” years (1995–1999) that followed, characterized by explicit changes in the phytoplankton annual-succession pattern. The rank-abundance distributions of species exhibit a pronounced difference between the taxonomically rich central region, producing the reliable assemblage backbone and highly variable tails of a few species. Therefore, the distribution pattern comparison enhances the importance of ubiquitous small disturbances valuable for diagnostics. Some phyla (in this case, Cyanophyta) were especially vulnerable to structural changes. A simple disturbance index was constructed, based on opportunistic small-celled species. The fine-structure disturbances, which can provide early-warning information, are discussed.