Changes to the phytoplankton assemblage of Lake Kinneret after decades of a predictable, repetitive pattern

Abstract

Summary1. Phytoplankton abundance and species composition in Lake Kinneret, Israel, have been monitored at weekly or fortnightly intervals since 1969. This paper summarises the resulting 34‐year phytoplankton record with a focus on the last 13 years of new data, and reassesses an earlier conclusion that the lake phytoplankton shows remarkable stability despite a wide range of external pressures.2. The Kinneret phytoplankton record can be split into two major periods. The first, from 1969 till 1993, was a period of distinct stability expressed by a typical annual pattern revolving around a spring bloom of the dinoflagellate Peridinium gatunense that repeated each year. The second period, starting around 1994 and ongoing, is characterised by the loss of the previously predictable annual pattern, with both ‘bloom years’ and ‘no‐bloom years’.3. In the second period, deviations from the previous annual pattern include: the absence of the prevailing spring P. gatunense blooms in some years and increased variability in the magnitude of the bloom in others; intensification of winter Aulacoseira granulata blooms; higher summer phytoplankton biomass with replacement of mostly nanoplanktonic, palatable forms by less palatable forms; new appearance and establishment of toxin‐producing, nitrogen fixing cyanobacteria in summer; increase in the absolute biomass and percentage contribution of cyanobacteria to total biomass; and fungal epidemics attacking P. gatunense.4. The 34‐year record serves to validate Schindler's (1987) assessment that phytoplankton species composition will respond to increased anthropogenic stress before bulk ecosystem parameters.