Dinoflagellate cysts as indicators of primary productivity along a Baltic Sea–North Sea–Atlantic transect

Abstract. Dinoflagellate cysts (dinocysts) have a long-standing use as proxies due to their global distribution and ecological significance. Their application to reconstruct past primary productivity has been frequently debated in the literature, and different dinocyst-based proxies have been proposed. Moreover, region-specific calibrations of modern dinocyst assemblages in relation to water parameters and sediment properties are needed to strengthen the reconstructions, specifically in regions where multiple environmental parameters co-vary. This study evaluates the potential of dinocysts as proxies for net primary productivity (NPP) using 37 core-top samples from the Baltic Sea, North Sea, and Atlantic Ocean. Our calibration dataset spans a wide gradient of NPP and other parameters, including sea surface temperature, salinity and nutrients, seasonality, bottom water oxygen, depth of the water column, distance to shoreline, sediment total organic carbon (TOC) content and origin, and grain size. After confirming recent sediment deposition via 234Thorium measurements and excluding inner Baltic Sea assemblages due to preservation bias in low-oxygen bottom waters, the canonical correspondence analysis (CCA) identified NPP as the main variable associated with dinocyst distribution in this region. Our results regarding established dinocyst-based productivity proxies show that (i) total dinocyst concentration and flux correlate better with TOC than with NPP, (ii) the heterotrophic / phototrophic (H/A) ratio correlates weakly with NPP and thus seems not to be suitable to reconstruct NPP, and (iii) heterotrophic dinocyst concentration and flux correlate strongly with NPP and hence emerge as the best proxies to reconstruct marine primary productivity in this region. We applied the modern analogue technique (MAT) to predict NPP, which resulted in a good agreement between observed and estimated NPP values, with an uncertainty of approximately 11 %.