Phytoplankton Pigments in Baltic Sea Seston and Sediments: Seasonal Variability, Fluxes, and Transformations

We studied the temporal variability in the composition of plant pigments in water-column particulate matter and in surface sediments, with specific emphasis on bloom events. The pigment concentration of sinking and suspended particles was used to evaluate pre- and post-depositional decomposition of bloom-derived phytodetritus in the Baltic proper. Water, sediment, and sediment trap samples were collected in 1994 at a long-term coastal monitoring station with fine sediments and a water depth of 36 m, situated near the Asko¨ Laboratory in the north-western part of the Baltic Sea proper. Annual phytoplankton succession showed an early spring bloom dominated by diatoms, followed by dinoflagellates in late spring and early summer. Later in summer, this dinoflagellate community was gradually replaced by a filamentous, nitrogen-fixing cyanobacterial bloom; late summer-early autumn was characterized by diatoms. The highest chlorophyll a flux occurred in spring during peak diatom bloom conditions, which resulted in peak concentrations of chlorophyll a in sediments. Assuming that all chlorophyll a in sediments in early May was derived from the current bloom, it was estimated that approximately half (48%) the bloom-derived chlorophyll a was decomposed in two months. The highest total phaeopigment/chlorophyll a ratios (an index of the amount of chlorophyll a decay), were found in water column particulate matter (PM) during the spring bloom and occasional high-energy winter periods. The latter higher ratios were likely due to wind-induced resuspension of surface sediments into the water column and trap materials. The settling rate of pigments in the water column indicated rapid sedimentation of diatoms and slow sedimentation of dinoflagellates during the spring bloom. In summer, unicellular cyanobacteria, including picocyanobacteria, appeared to have higher sedimentation rates than filamentous nitrogen-fixing cyanobacteria, which seemed to decompose largely in the water column. Thus, this study demonstrated that plant pigments were useful biomarkers of the taxonomic composition of phytoplankton blooms and their sedimentation and subsequent decomposition in the Baltic Sea.