Studies of Baltic Sea plankton - spatial and temporal patterns

Abstract: This thesis examines the spatial and temporal patterns of Baltic Sea phyto- and zooplankton. Studies of such patterns are important to gain a better understanding of the offshore nutrient dynamics of the Baltic Sea (Papers I and II) and how to manage eutrophication and invasive species in coastal areas (Papers III and IV).During the spring phytoplankton bloom, diatoms rapidly depleted inorganic nitrogen in the shallow mixed layer, but were replaced by dinoflagellates when a seasonal pycnocline developed early in spring due to riverine freshwater inputs (Paper I). The settling of diatoms from the mixed water column was species-specific (Paper I). Vertically migrating dinoflagellates were able to exploit nutrients to considerable depth. The settling of primary produced carbon was low, only 12% as particulate organic carbon (POC) at 100 m and 2% as identifiable cells (Paper I). Profound alterations in species composition during settling will hinder interpretation of paleoecological records.During summer cyanobacterial blooms, vertically separated diazotrophic and mixotrophic species dominated the phytoplankton community. Unicellular and colony-forming picocyanobacteria, a hitherto underestimated group, made up a substantial part of phytoplankton biomass late in the bloom (Paper II). Dominant species during the blooms exhibited species-specific depth distributions, suggesting niche-separation to decrease competition.Management of coastal eutrophication is of major concern, particularly in estuaries and enclosed seas with limited water exchange. In Himmerfjärden, a eutrophicated Baltic Sea bay, filamentous nitrogen-fixing cyanobacteria increased substantially following a drastic reduction in nitrogen load from a modern sewage treatment plant (STP). Seston <10 µm responded to the 15N-enriched nitrogen from the STP with a ?15N-value of near 8‰ close to the discharge point, compared to only 2‰ in the open Baltic. In contrast, filamentous nitrogen-fixing cyanobacteria had ?15N-values of -3.0‰ to -0.2‰ (average -1.5‰) independent of distance to the STP (Paper III), indicating that they used only dinitrogen for growth. When cyanobacteria were abundant, total nitrogen concentrations increased and ?15N decreased in seston <10 µm and zooplankton >200 µm (Paper III), indicating that fixed nitrogen was transferred to other trophic levels. During a later deliberate experimental discharge of nitrogen designed to suppress cyanobacterial growth, cyanobacteria used some dissolved inorganic nitrogen but their biomass was not markedly reduced (Paper III).The cladoceran zooplankton Cercopagis pengoi, a recent invader to the Baltic Sea, is a potential competitor with fish for zooplankton prey. Stable nitrogen isotope studies imply that invasion by C. pengoi has changed the pelagic food web structure in the bay Himmerfjärden by competing with small pelagic fishes for zooplankton, and by providing a new prey for these fishes (Paper IV). These results indicate the risk of cascading effects on other trophic levels and the necessity of whole ecosystem analysis when evaluating effects of invasive species (Paper IV).