Calibration of biological lake sediment records: Tracing Diatom assemblages through the water column into the sediment

Background/Question/Methods

Paleolimnological studies assume that a sediment core, taken from the deepest point of a lake represents the lakes biological past. Diatoms, siliceous unicellular algae, react directly on changes in their surrounding water body. Seasonal changes in abiotic (e.g. light climate, water temperature or nutrients) as well as biotic (e.g. grazer abundance) factors determines their assemblage. The sensitivity of Diatoms towards those direct environmental parameters and their durable cell walls predestinates them as widly applied biological parameters in paleolimnology. In this study we aim at calibrating a recent varved sediment core series with a bi‑weekly plankton survey and sediment trap record, spanning a period of 10 years, in order to reveal underrepresented seasonal physical or ecological processes in the interpretation of sediment records in peleolimnology. The annually laminated sediment of the boreal forest lake Nylandssjön in northern Sweden provides a very high temporal resolution, which allows us, even on a seasonal scale, a gapless comparison between in situ production and the sediment deposition. We follow diatom assemblages through the water column into the sediment.

Results/Conclusions

The analysis of the ten year record of net tow samples and corresponding trap samples showed a diatom assemblage dominated by Asterionella formosa, Tabellaria flocculosa and Fragilaria taxa. The plankton data reveal large shifts from season to season, but also from year to year. The sediment trap diatom record indicates comparable overall abundance patterns for main taxa. Pronounced peaks observed in the plankton survey data are often recorded with a time lag in the sediment trap. In addition to the unrepeated seasonal patterns of the main abundant species within seasons in the pelagial, spring peaks are underrepresented in the corresponding trap samples compared to autumn peaks of species blooming in the lakes epilimnion, potentially due to differing grazing pressure throughout the year. Tabellaria flocculosa appears to be under a higher grazing pressure then the other main abundant species, also peaking in spring month. These findings point out that ecological dynamics are possibly underestimated in the interpretation of paleolimnological data.