Seasonal succession of diatoms in the coastal Baltic Sea: insights into diatom-based proxy for past environmental change

Diatoms are excellent environmental proxies due to their often species-specific, narrow environmental tolerances, but the annual succession of diatom communities in coastal seas remains deficiently known. This knowledge gap constrains our ability to reconstruct past seasonal changes in marine systems, as variation in seasonal conditions strongly impacts the composition of diatom species assemblages, and consequently assemblages preserved in the sediments. In particular species dynamics related to sea ice are still underexplored, as ice-cover restricts undisturbed access. Automated sequencing sediment traps offer an effective solution to overcome these challenges, even during the ice-cover period.

 

In this PhD project, the seasonal succession of coastal diatom communities, as well as their contribution to vertical particulate organic matter fluxes and sedimentation are studied over multiple years in Tvärminne Storfjärden, Gulf of Finland. The aim is to enhance our understanding of the seasonal patterns of diatom species succession and sedimentation at a high temporal resolution, with a focus on understanding seasonal environmental drivers of species assemblage composition and the development of paleoenvironmental reconstruction methods. The data is collected with automated sequencing sediment traps, moored to the sea floor to continuously collect vertical material fluxes settling from the sea surface at a two-week temporal resolution. Two sediment traps are deployed at depths of 15 m and 27m (approximately 3 meters above the seabed) to assess how processes like decomposition and predation impact the vertical sediment flux. Diatom assemblages are analysed by microscopic identification and compared to simultaneous environmental measurements of e.g., sea-surface temperature, salinity and sea-ice cover to assess species-specific seasonal ecologies and deposition patterns. In addition, bulk organic geochemical analysis renders information about carbon flux and sources to the seafloor. Enhanced seasonal ecological information will improve diatom-based methods, enabling more accurate reconstructions of past and predictions of future coastal environments. Also, the advancement provides valuable insights into the impacts of ongoing environmental change and anthropogenic pressure on aquatic systems and, ultimately human well-being.