Spatially and temporally sporadic apperance of macrophytes in the littoral zone of Lake Kinneret, Israel: taking advantage of a window of opportunity

We studied the spatial distribution and temporal appearance of submerged macrophytes in the littoral zone of Lake Kinneret in relation to various environmental factors from 1989-1992. In the summers of 1989 and 1990, following a whole decade of no significant macrophyte development, dense stands of Potamogeton pectinatus L. were recorded in limited littoral areas, mainly in the southeastern basin of the lake. Thereafter, in 1991 and 1992 no significant macrophyte growth was again recorded. We found that macrophyte stands are located at exactly the same places over the years. Specific requirements of sediment structure seem to be the key factor for this sporadic but conservative spatial distribution. The plants require clay-rich sediments covered by a 20 cm sand layer in order to develop. The variation in temporal appearance is mainly explained by the water level fluctuations and thus the related depth of the water column over a site of a given sediment structure. This depth should not be larger than 2 m. When both depth and sediment requirements are met, a window of opportunity is opened for the development of dense macrophyte stands. Otherwise, no significant development occurs. The maximal biomass gained by the macrophytes is associated with the length of the growing season, which in turn, is determined by the initial water level and the rate of lake level decline. The `windows of opportunity' model is verified by a time series analysis of historical data on macrophyte development in Lake Kinneret, and should be further tested in the coming years.