Sedimentation processes

Sedimentation is a major process for removal of particulate material from the water column and an important determinant accounting for the stability of aquatic ecosystems. Gross sedimentation rates (GSRs) in Lake Kinneret (Israel), regularly monitored from 1999 up to date with sedimentation traps, showed salient temporal and spatial variability. In the lake center, the annual mean GSR ranged from 1.9 to 6.0 g m^-2day^-1. The accumulation rate of sediments at the lake centrum during the study averaged from 2.6 to 4.3 mm year^-1, in agreement with values obtained by sediment core dating. Organic matter (OM) content comprised 33–42% of the sinking particulate matter in sediment traps located in the lake center and was 1.5–2 times lesser in peripheral stations. The highest seasonal values of OM content in traps were associated with collapse of algal blooms. Algae and their debris are the main components of OM and their fate in the water column can be well traced by photosynthetic pigments. Chlorophyte signature pigments display much lower degradability in the water column than those of diatoms and dinoflagellates and leave a relatively persistent residue in the buried sediments. Analysis of seasonal changes of algal signature pigments in the upper euphotic zone and those in sedimentation traps allow us to follow the fate of dominant algal phyla in the water column. We argue thatlarge individual algal cells may have better ability to survive in the deep non-stratified water column with limited light, while the ability to retain and recycle in the euphotic epilimnion under conditions of nutrient limitation may well confer an evolutionary advantage to small or buoyant algal populations. Despite large variation in algal community composition, the ratio of OM sedimentation flux to primary production (i.e., the export ratio) alters only slightly throughout the stratified period. Approximately 20% of the OM supplied to the lake by primary production found its way to the traps. A large decline of water level in recent years affected the processes of particle resuspension and offshore translocation, which caused prominent site-specific impacts on the sedimentation regime. We argue that changes in sedimentation rates observed in the lake are related to fluctuations of loads from the watershed and prominent water-level fluctuations.