A short history of stability and change in phytoplankton populations in Lake Kinneret

From the beginning of detailed routine monitoring in 1968 (and probably before) until the end of 1993, annual and seasonal development of phytoplankton in Lake Kinneret showed a relatively constant pattern. In the late winter and early spring there were extensive blooms of the dinoflagellate Peridinium gatunense, followed in summer and autumn by much lower standing stocks of predominantly nannoplanktonic chlorophytes. Although Microcystis spp. were prominent (usually concomitant with the dinoflagellates) in the early 1970s, subsequently this genus and other cyanobacteria were generally unimportant contributors to phytoplankton biomass. Since early 1994, however, a marked change in seasonal phytoplankton development has been noted. There were record high blooms of Peridinium in 1994 and 1995; in the following years, 1996 and 1997, no dinoflagellate bloom appeared. In the late summer and fall of 1994 an exceptional bloom of Aphanizomenon ovalisporum occurred; this was the first recorded instance of an outgrowth of a potentially toxic and N-2-fixing filamentous cyanobacterium in this lake. This organism has been observed in low numbers in each subsequent year. In 1995, 1996, and 1997 Microcystis also became prominent. Levels of primary production since 1994 have generally been higher than previous multi-annual averages. The quality of water in Lake Kinneret is to a large extent determined by the nature and activity of its phytoplankton community. Because of the lake's central role as a national supply source of good-quality water, the apparent breakdown of previously regular patterns of the annual and seasonal algal development is a cause for concern. Integrated research and monitoring are needed to fully explain the causes of the observed and ongoing changes and to provide guidelines for future management policies aimed at maintaining water quality in Lake Kinneret.