4.3 Article

Allochthonous matter: an important factor shaping the phytoplankton community in the Baltic Sea

Journal

JOURNAL OF PLANKTON RESEARCH
Volume 39, Issue 1, Pages 23-34

Publisher

OXFORD UNIV PRESS
DOI: 10.1093/plankt/fbw081

Keywords

phytoplankton; allochthonous dissolved organic matter; humic substances; nitrogen; phosphorus; structuring factors

Funding

  1. Marine Strategic Research Environment ECOCHANGE (the Swedish Research Council Formas)
  2. research program WATERS
  3. Swedish Agency for Marine and Water Management
  4. Swedish Environmental Protection Agency

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It is well-known that nutrients shape phytoplankton communities in marine systems, but in coastal waters allochthonous dissolved organic matter (ADOM) may also be of central importance. We studied how humic substances (proxy of ADOM) and other variables influenced the nutritional strategies, size structure and pigment content of the phytoplankton community along a south-north gradient in the Baltic Sea. During the summer, the proportion of mixotrophs increased gradually from the phosphorus-rich south to the ADOM-rich north, probably due to ADOM-fueled microbes. The opposite trend was observed for autotrophs. The chlorophyll a (Chl a): carbon (C) ratio increased while the levels of photoprotective pigments decreased from south to north, indicating adaptation to the darker humic-rich water in the north. Picocyanobacteria dominated in phosphorusrich areas while nanoplankton increased in ADOM- rich areas. During the winter-spring the phytoplankton biomass and concentrations of photoprotective pigments were low, and no trends with respect to autotrophs and mixotrophs were observed. Microplankton was the dominant size group in the entire study area. We conclude that changes in the size structure of the phytoplankton community, the Chl a: C ratio and the concentrations of photoprotective pigments are indicative of changes in ADOM, a factor of particular importance in a changing climate.

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