Human-mediated introductions of non-indigenous species now threaten to homogenize the biota of the Globe, causing huge economic and ecological damage. This thesis studies the ecological role of 3 invasive planktonic crustaceans, the omnivorous copepod Acartia tonsa (western Atlantic and Indo-Pacific) and the predatory cladocerans, Cercopagis pengoi (Ponto-Caspian) and Bythotrephes longimanus (Eurasian). B. longimanus invaded the North American Great Lakes in 1982, C. pengoi the Baltic in 1992 and the Great Lakes in 1999, while A. tonsa has an extensive invasion history that includes the Baltic.

We review current knowledge on feeding biology of the predatory cladocerans. A study of stable C and N isotope ratios indicated mesozooplankton as the main food source of C. pengoi in the northern Baltic Sea proper, with young C. pengoi also eating microzooplankton, such as rotifers. Young-of-the-year herring did eat C. pengoi and herring trophic position shifted from 2.6 before the invasion to 3.4 after, indicating that C. pengoi had been “sandwiched” into the modified food web between mesozooplankton and fish.

Salinity tolerance experiments on Acartia tonsa and co-occurring Acartia clausi showed the formers euryhaline character and high grazing potential. Energy partitioning between ingestion, production and respiration was rather constant over the tested salinity range of 2 to 33, with small differences in gross growth efficiency and cost of growth, but maximum ingestion at 10-20. Egg hatching in A. tonsa was only reduced at the lowest salinity. Extreme changes in salinity were needed to cause significant mortality of A. tonsa in the field, but its feeding activity could be severely reduced by salinity changes likely to occur in estuaries. A study of a hypertrophic estuary showed that A. tonsa can sustain a population despite very high mortality rates, caused by predation, high pH and low oxygen, helping explain the success of A. tonsa as an invader of estuaries.

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 δ¹⁵N-value of near 8‰ close to the discharge point, compared to only 2‰ in the open Baltic. In contrast, filamentous nitrogen-fixing cyanobacteria had δ¹⁵N-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 δ¹⁵N 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).