Carbon isotope fractionation and changes in the flux and composition of particulate matter resulting from biological activity during a sediment trap experiment in Lake Greifen, Switzerland1

Abstract

Sediment traps were deployed in Lake Greifen, a hard‐water eutrophic, Swiss lake, for 2 weeks in May and June 1983. Oxygen, pH, and suspended particle concentrations indicated a sharp increase in productivity during the experiment. Carbon isotope composition of the dissolved inorganic carbon recorded the balance between photosynthesis and respiration and showed possible evidence of diffusion of atmospheric CO2 across the water‐atmosphere interface, most likely to replace CO2 used during photosynthesis. Allochthonous mineral debris from resuspended sediments or river input was a major component of the sedimenting material. Due to removal of CO2, large, newly formed calcite crystals precipitated when photosynthesis increased. The carbon isotope ratio of trap calcite and the composition of trap amino acids showed the influence of the detrital input on the more dominant isotopic signal of authigenic calcite. Sediment calcite is enriched in 13C relative to authigenic calcite in the sediment traps. This enrichment is likely due to fractionation during precipitation in the surface waters or during dissolution in the water and sediments due to CaCO3 undersaturation. We found extensive loss of material from unpoisoned sediment traps in the cuphotic zone, most likely due to zooplankton feeding.