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Studies of Baltic Sea plankton - spatial and temporal patterns
Stockholm University, Faculty of Science, Department of Systems Ecology.
2004 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

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).

Place, publisher, year, edition, pages
Stockholm: Institutionen för systemekologi , 2004. , 43 p.
Keyword [en]
Phytoplankton, Vertical distribution, Sedimentation, Baltic Sea, Cyanobacteria, Stable istopes, Cercopagis pengoi, Zooplankton
National Category
Ecology
Identifiers
URN: urn:nbn:se:su:diva-323ISBN: 91-7265-989-0 (print)OAI: oai:DiVA.org:su-323DiVA: diva2:192484
Public defence
2005-01-28, Nordenskiöldsalen, Geovetenskapens hus, Svante Arrhenius väg 8 C, Stockholm, 10:00
Opponent
Supervisors
Available from: 2004-12-21 Created: 2004-12-21 Last updated: 2010-08-10Bibliographically approved
List of papers
1. Vertical distribution and settling of spring phytoplankton in the offshore NW Baltic Sea proper
Open this publication in new window or tab >>Vertical distribution and settling of spring phytoplankton in the offshore NW Baltic Sea proper
2004 In: Marine Ecology Progress Series, ISSN 0171-8630, Vol. 283, 15-27 p.Article in journal (Refereed) Published
Identifiers
urn:nbn:se:su:diva-23523 (URN)
Note
Part of urn:nbn:se:su:diva-323Available from: 2004-12-21 Created: 2004-12-21Bibliographically approved
2. Vertical distribution and composition of phytoplankton in Baltic Sea cyanobacterial blooms
Open this publication in new window or tab >>Vertical distribution and composition of phytoplankton in Baltic Sea cyanobacterial blooms
In: Marine Ecology Progress SeriesArticle in journal (Refereed) Submitted
Identifiers
urn:nbn:se:su:diva-23524 (URN)
Note
Part of urn:nbn:se:su:diva-323Available from: 2004-12-21 Created: 2004-12-21Bibliographically approved
3. Nitrogen-fixing cyanobacteria in a eutrophicated Baltic Sea bay
Open this publication in new window or tab >>Nitrogen-fixing cyanobacteria in a eutrophicated Baltic Sea bay
Manuscript (Other academic)
Identifiers
urn:nbn:se:su:diva-23525 (URN)
Note
Part of urn:nbn:se:su:diva-323Available from: 2004-12-21 Created: 2004-12-21 Last updated: 2010-01-13Bibliographically approved
4. Stable isotopes show food web changes after invasion by the predatory cladoceran Cercopagis pengoi in a Baltic Sea bay
Open this publication in new window or tab >>Stable isotopes show food web changes after invasion by the predatory cladoceran Cercopagis pengoi in a Baltic Sea bay
2005 (English)In: Oecologia, ISSN 0029-8549, E-ISSN 1432-1939, Vol. 143, no 2, 251-259 p.Article in journal (Refereed) Published
Abstract [en]

Cercopagis pengoi, a recent invader to the Baltic Sea and the Laurentian Great Lakes, is a potential competitor with fish for zooplankton prey. We used stable C and N isotope ratios to elucidate trophic relationships between C. pengoi, zooplankton (microzooplankton, 90–200 m, mostly copepod nauplii and rotifers; mesozooplankton, >200 m, mostly copepods), and zooplanktivorous fish (herring, size range 5–15 cm and sprat, 9–11 cm) in a coastal area of the northern Baltic Sea. The isotope ratios in C. pengoi and fish were much higher than those of zooplankton, showing general trends of enrichment with trophic level. Young-of-the-year (YOY) herring had a significantly higher 15N/14N ratio than C. pengoi, suggesting of a trophic linkage between the two species. To evaluate the possible relative importance of different food sources for C. pengoi and YOY herring, two-source isotope-mixing models for N were used, with micro- and mesozooplankton as prey for C. pengoi and mesozooplankton and C. pengoi as prey for YOY herring. These models indicate that mesozooplankton was the major food source of both species. However, microzooplankton may be important prey for young stages of C. pengoi. Comparative analyses of the herring trophic position before and after the invasion by C. pengoi showed a trophic level shift from 2.6 to 3.4, indicating substantial alterations in the food web structure. Our findings contribute to a growing body of evidence, showing that C. pengoi can modify food webs and trophic interactions in invaded ecosystems.

Keyword
Diet, Effects of preservation and storage, Invasive species, Zooplankton, Zooplanktivorous fish
National Category
Ecology
Identifiers
urn:nbn:se:su:diva-25730 (URN)10.1007/s00442-004-1791-0 (DOI)
Available from: 2009-02-19 Created: 2009-02-11 Last updated: 2011-02-16Bibliographically approved

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