A SHORT HISTORY OF STABILITY AND CHANGE IN PHYTOPLANKTON POPULATIONS IN LAKE KINNERET

From the beginning of detailed routine monitoring in 1968 (and probabl y before) until the end of 1993, annual and seasonal development of ph ytoplankton in Lake Kinneret showed a relatively constant pattern. In the late winter and early spring there were extensive blooms of the di noflagellate Peridinium gatunense, followed in summer and autumn by mu ch lower standing stocks of predominantly nannoplanktonic chlorophytes . Although Microcystis spp. were prominent (usually concomitant with t he dinoflagellates) in the early 1970s, subsequently this genus and ot her cyanobacteria were generally unimportant contributors to phytoplan kton biomass. Since early 1994, however, a marked change in seasonal p hytoplankton 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 199 4 an exceptional bloom of Aphanizomenon ovalisporum occurred; this was the first recorded instance of an outgrowth of a potentially toxic an d N-2-fixing filamentous cyanobacterium in this lake. This organism ha s been observed in low numbers in each subsequent year. In 1995, 1996, and 1997 Microcystis also became prominent. Levels of primary product ion since 1994 have generally been higher than previous multi-annual a verages. The quality of water in Lake Kinneret is to a large extent de termined by the nature and activity of its phytoplankton community. Be cause of the lake's central role as a national supply source of good-q uality water, the apparent breakdown of previously regular patterns of the annual and seasonal algal development is a cause for concern. Int egrated research and monitoring are needed to fully explain the causes of the observed and ongoing changes and to provide guidelines for fut ure management policies aimed at maintaining water quality in Lake Kin neret.