Allochthonous contribution to seasonal and spatial variability of organic matter sedimentation in a deep oligotrophic lake (Lake Constance)

Previous studies analysing the chemical composition of profundal sediments of Lake Constance suggest a overall large contribution of allochthonous material to total sedimentation but also a large spatial variability in the relative importance of allochthonous versus autochthonous sedimentation. Here we analyse sedimenting particulate organic matter (POMsed) during an annual cycle at two sites differing in their position relative to the major inflow and thus in their proposed contribution of allochthonous matter to the sedimentary flux, i.e., site [AL] characterised by a more allochthonous contribution und the site [AU] characterised by a more autochthonous contribution.Chemical and mineralogical composition (chlorite contents) of sedimenting matter were used for discrimination of autochthonous and allochthonous sources in addition to stable isotope signatures (δ13C, δ15N) of POMsed. Generally, δ13C values were “heavier” at site AL (−26.5 to −30.9‰) than at site AU (−33.10 to −29.30‰), especially during the main sedimentation period from April to September. In contrast differences between sites AL und AU in respect to δ15N-values were small (averages: +5.10‰ and +5.80‰, respectively). δ13C values were shown to be inversely correlated to contents of chlorophyll a of POMsed (i.e. lighter values with higher concentration), whereas they were positively correlated to chlorite contents and the C:N ratio of sedimenting material. The contribution of autochthonous and allochthonous POMsed changed also seasonally at both sampling sites. These differences resulted on the one hand from the seasonal succession of plankton, and on the other hand from the discharge characteristics of the inflowing rivers with regular (snow melting) and irregular maxima (flood events). A mixing model suggests that on average the allochthonous POMsed from river loads contributes at site AL 73.40% of total organic matter sedimentation and at the site AU 33.80%.