Sedimentary plant organic matter in a southern Baltic coastal lagoon: The importance of habitat constraints and temporal variability

The Baltic Sea coastal lagoons are shallow reservoirs affected by eutrophication which is manifested by, inter alia, persistent phytoplankton blooms the remains of which (phytodetritus) sink to the bottom. The research in the Szczecin Lagoon (a component of the River Odra/Oder estuary in the south-western Baltic Sea) was aimed at finding out whether (a) the autochthonous primary production, specifically the resultant phytodetritus sedimentation, was the main pathway of the sediment organic enrichment, (b) the deposition of the phytoplankton material elicited temporal and spatial responses in the Lagoon's sediment visible as changes in its organic enrichment metrics. The data were collected monthly (April–November) in 2010 (a ‘wet’ year, with a higher river water supply) and 2011 (a ‘dry’ year), from 5 stations differing in the hydrodynamic regime (two stations situated in erosional and three in depositional areas). The study revealed the presence of climatic controls over the Lagoon system manifested as differences in the phytoplankton biomass between the wet and the dry year, with a higher biomass in the latter. There were also hydrodynamic controls (depositional vs. erosional bottoms) over the ability of the sediment to accumulate and retain organic material supplied primarily by the autochthonous primary production of the phytoplankton and microphytobenthos. Differences between the depositional and erosional areas were visible also as differences in the dominant marker pigments, with fucoxanthin (diatoms) being characteristic of the erosional stations and zeaxanthin (cyanobacteria) and lutein (chlorophytes) being typical of depositional areas. In addition, the study provided evidence for the persisting eutrophication of the Lagoon, manifested as high values of the phytoplankton biomass marker (chlorophyll a). The high organic enrichment of the sediments was shown to be decoupled from, but sustained by, the autochthonous plant biomass supply.