Horizontal sedimentation differences in a eutrophic Swiss lake

Abstract

Settling fluxes measured by means of sediment traps were 887 g dry wt m −2 yr −1 , 113 g POC m −2 yr −1 , and 2.6 g PP m −2 yr −1 in 1982–1983 at a central station in eutrophic Lake Hallwil, Switzerland. Nearshore settling rates were higher by a factor of 1.4–3.4. Since particulate organic carbon (POC), particulate phosphorus (PP), and biomass concentration in the lake water showed vertical rather than horizontal differences, this result was attributed to permanent bottom sediment resuspension and redeposition in the littoral zone. Horizontal sediment transport (sediment focusing) and sediment resuspension in the profundal zone in winter, when wind and turbulence are increased, accounted for only about 20% of the sedimentation rates. Because 70–80% of the total sedimentation for the year occurred during summer, such processes are of minor importance in Lake Hallwil, although a “nepheloid” layer with increased POC and PP concentrations 5–8 m above the bottom was present. It is stressed that the mechanisms of sediment transport must be known to permit interpretation of trap results and to understand lake metabolism.