Sedimentation flux in a large subtropical lake: Spatiotemporal variations and relation to primary productivity

Spatial and temporal heterogeneity in sedimentation of particulate organic material (POM) was studied in a large subtropical lake, and its export from the upper mixed layer was quantified. Sedimentation fluxes were measured over 4 yr with traps deployed in pelagic and littoral areas, the benthic boundary layer (BBL), and the lake interior. Analysis of fluxes and composition of collected material showed that traps deployed at the lake periphery and in the BBL notably overestimate the export flux of newly produced POM. The best estimation of the primary production (PP) export from the upper mixed layer was achieved when traps were deployed in the quiescent hypolimnion, where the effect of lake boundaries is negligible. The proportion and composition of POM exported from the upper productive stratum was dependent on lake thermal and chemical structure and on the dominant phytoplankton species. The dynamics of photopigments (chlorophyll and β-carotene) collected in hypolimnetic traps reflected the composition and abundance of phytoplankton in the upper mixed stratum. Despite large variation in algal community composition, the ratio of POM sedimentation flux to PP (export ratio) changed only slightly (average ∼ 20%) throughout the stratified period. The observed temporal and spatial variability of sedimentation flux in response to ambient conditions was influenced by secondary processes (resuspension at the lake boundaries and oversampling under turbulent conditions) as intensely as by the actual export of POM.