Long-Term Impacts of Draining a Watershed Wetland on a Downstream Lake, Lake Kinneret, Israel

Abstract

During the 1950s, the shallow Lake Hula and its adjacent marshes, located in the watershed of Lake Kinneret (LK), were drained in order to increase arable land. The drainage resulted in increasing loads of nutrients in the Jordan River and in oxidative degradation of the underlying peat, followed by Aeolian (mostly) and riverine transport to LK. Peat had been accumulating in LK bed sediments since the 1950s, with peak deposits recorded in the mid-1970s. The routine monitoring of Norg and Corg, associated with the peat, in the watershed streams and in LK water (initiated in 1970) also showed the highest levels in the mid-1970s. During the 1980s, a succession of dense natural vegetation inhibited soil surface erosion and a decline in Aeolian transport of peat to LK was recorded. Inversed temporal patterns appear for oxygen and pH, and this is suggested to result from the dependence of these parameters on the magnitude of respiratory process, induced by peat availability. During the 1970s allochthonous peat resources supported about half of the zooplankton activity in LK and therefore the decline in peat availability during the 1980s was accompanied by a two-fold decline in zooplankton biomass. The subsequent lessening of grazing pressure together with the elevated inflows of bio-available P resulted in a significant rise in LK phytoplankton biomass. The 1980s decline in Norg associated with the peat led to lower N/P ratios in the external sources of nutrients to LK. These conditions may have favored the appearance of N2 fixing cyanobacteria noted in the lake since 1994.