Flux of lipophilic photosynthetic pigments to the surface sediments of Lake Baikal

Abstract

The pigment flux to the sediment surface was studied in Lake Baikal to evaluate the validity of approaches reconstructing long-term variations in standing crops of phytoplankton by fossil pigment analysis. Chlorophylls and carotenoids were analysed by HPLC in suspended and settling matter and in the surface sediment of the central south basin (c. 1400-m water depth). Sedimentation rates of dry matter, organic carbon and nitrogen were also determined. The flux of particulate matter in 40 m, directly below the euphotic zone, amounted to 14.9 g m −2 month −1 with a carbon content of 21.9%, and an atomic C/N ratio of 14.8. The pigment flux was 12.1 μmol m −2 month −1 chlorophyll a, 40.8 μmol m −2 month −1 pheophorbide a, 6.5 μmol m −2 month −1 pheophytin a, 2.1 μmol m −2 month −1 chlorophyllide a, and 0.3 μmol m −2 month −1 pyropheophytin a. The decay during sedimentation can be described by two-exponential or decay regression models for organic carbon, total nitrogen, chlorophyll a, pheophorbide a, chlorophyllide a, chlorophyll b, and most carotenoids, but not for pheophytin a, pheophytin b, and pyropheophytin a. The two-phase character of the models outlined that, for the former components, the flux diminished strongly in a first phase down to 250-m water depth and remained rather stable below 250 m. The chlorophyll a/carbon ratio also decreased with depth, whereas the pheophytin a/carbon ratio and the pyropheophytin a/carbon ratio increased with depth. From chlorophyll a, plus its degradation products, 28% reached the lake bottom when compared to the sedimentation below the euphotic zone. Based on the marker pigments fucoxanthin, chlorophyll b, and zeaxanthin, the contribution of the main phytoplankton groups to the settled chlorophyll a was estimated as 87% Bacillariophyceae+Chrysophyceae, 11% Chlorophyta, and 2% cyanobacterial picoplankton. These relationships changed only little during the sedimentation through the whole water column, but diverged from compositions calculated for the summer standing crop.