The origin and recycling of sedimented biogenic debris in a subalphine eutrophic lake (Lake Bled, Slovenia)

The areal distribution of organic C contents, delta(13)C values, total N and P and biogenic Si contents in surficial sediments were used to study the distribution, origin and diagenetic transformations of sedimented biogenic debris in the eutrophic subalpine Lake Bled (Slovenia), which for most of the year has an anoxic hypolimnion. The influence of an allochthonous input, restricted to the western basin, was clearly traced by higher organic C and total N and P contents, higher delta(13)C values, and higher sedimentation rate in comparison to the eastern basin. The low delta(13)C values of sedimentary organic matter in the major part of the lake, lower than the delta(13)C values of different types of organic matter, suggest that this sedimentary organic matter is most probably the product of a microbial community and not a residue of primary production. The temporal variation of benthic diffusive fluxes of NH4, Si and PO4, derived from modelling the pore water profiles, was related to sedimentation of phytoplanktonic blooms, while the PO4 fluxes were also dependent on changing redox conditions at the sediment-water interface in the period of the winter-spring overturn. The removal of PO4 in pore waters is probably due to the adsorption of phosphate and precipitation of apatite and vivianite. The budget of C, N and P at the sediment-water interface revealed a high recycling efficiency (>70%), also confirmed by the rather uniform (or only slightly decreasing) vertical profiles of organic C, total N and P in sediment cores and C/N and C/P ratios. The percentage of biogenic Si recycling is low (<10%), suggesting its removal in sediments.