Spatially and temporally sporadic appearance 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 ma crophytes in the littoral zone of Lake Kinneret in relation to various envi ronmental factors from 1989-1992. In the summers of 1989 and 1990, followin g a whole decade of no significant macrophyte development, dense stands of Potamogeton pectinatus L. were recorded in limited littoral areas, mainly i n the southeastern basin of the lake. Thereafter, in 1991 and 1992 no signi ficant macrophyte growth was again recorded. We found that macrophyte stand s are located at exactly the same places over the years. Specific requireme nts of sediment structure seem to be the key factor for this sporadic but c onservative spatial distribution. The plants require clay-rich sediments co vered 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 structur e. This depth should not be larger than 2 m. When both depth and sediment r equirements are met, a window of opportunity is opened far the development of dense macrophyte stands. Otherwise, no significant development occurs. T he maximal biomass gained by the macrophytes is associated with the length of the growing season, which in turn, is determined by the initial water le vel and the rate of lake level decline. The 'windows of opportunity' model is verified by a time series analysis of historical data on macrophyte deve lopment in Lake Kinneret, and should be further tested in the coming years. (C) 1999 Elsevier Science B.V. All rights reserved.