LAKE KINNERET DISSOLVED-OXYGEN REGIME REFLECTS LONG-TERM CHANGES IN ECOSYSTEM FUNCTIONING

Lake Kinneret (Israel) has undergone several prominent chemical and bi ological changes since 1970. Between 1970 and 1991 significant, long-t erm gradual increase were recorded in epilimnetic dissolved oxygen (DO ) concentrations (about 20%), and in pH levels (0.2 units). Concomitan tly there was a significant increase in hypolimnetic H2S concentration s (about 75%) and a long-term gradual drop in zooplankton biomass (50% ). Since 1994 these trends were reversed and the levels of the three c hemical parameters have returned to those found in the 1970's and that of zooplankton to mid 1980's levels. The present study is an attempt to relate some of these long term changes by means of yearly oxygen bu dgets, based on fluxes of oxygen producing and consuming processes. Th is analysis raises the possibility that part of the long-term increase in epilimnetic DO and pH between 1970 to 1990 may be attributed to re duced inputs of organic matter from alochthonous sources and possibly to enhanced burial of organic matter in the bed sediments. However, th e major cause for the observed increase in epilimnetic DO and pH is in creased sedimentation of organic matter to the hypolimnion during stra tification. As indicated by the amount of H2S formed in the hypolimnio n during stratification added to the amount of oxygen entrapped in thi s layer at the onset of thermal stratification, between 1970 to 1991 t he sedimentation flux of organic matter increased by approximately 40% . It is estimated that during these two decades hypolimnetic respirati on increased from ca. 8% of the annual amount of oxygen evolved due to photosynthesis during the early 1970's to ca. 12.5% during the 1980's . The shift in the layer of oxidative processes is suggested to be the result of a multi-annual decline in zooplankton grazing pressure, whi ch led to increased sedimentation of organic matter. The reversed tren ds for DO, pH;md H2S since 1994 may have partially been due to the inc rease in zooplankton activity and partially due to changes in phytopla nkton community structure.