Phytoplankton development and turbulent mixing in Lake Kinneret

Abstract. We have utilized data from a recently developed three-dimensional velocity fluctuation meter to compute the dissipation of turbulent kinetic energies (TKE) and the intensity of turbulent mixing in horizontal and vertical planes in the pelagic, epilimnic water of Lake Kinneret, Israel. These characteristics of wind-induced turbulent movement have been monitored from January 1992 through December 1996. The turbulence parameters were strongly correlated to wind energy inputs, calcu-lated daily as 5 day cumulative inputs. There have been dramatic changes in the annual and seasonal development of phytoplankton, together with unusually high levels of primary production in this lake since 1994. We observed different patterns of vertical and horizontal turbulent movement and of TKE dissipation rates during the years when 'unusual ' phytoplankton development occurred (1994-1996) compared to 'normal ' years (1992, 1993). The first appearance of the filamentous cyanobacterium Aphanizomenon in this lake in August-September 1994 coincided with a period of markedly lower rates of TKE dispersion and a shift from vertical to horizontal dominance of the turbulent eddy spins. The absence of a regular winter-spring bloom of the dinoflagellate, Peridinium, in 1996 occurred when dissipation rates of TKE were extremely high, while record high amounts of dinoflagellates (1994, 1995) appeared when dissipation rates were very low. Correlations were shown between phyto-plankton parameters (chlorophyll, primary production and the ratio of primary production to chloro-phyll) and both the dissipation rate of TKE and the intensity of water turbulent mixing in the vertical plane. We suggest that the changes in the 'turbulence climate ' of Lake Kinneret were an important factor in determining shifts in phytoplankton succession and the population composition of the algal assemblage