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Allochthonous matter: an important factor shaping the phytoplankton community in the Baltic Sea
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. UMEÅ MARINE SCIENCE CENTRE, SE-905 71 HÖRNEFORS, SWEDEN. (Marine ecology group ; EcoChange)
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. UMEÅ MARINE SCIENCE CENTRE, SE-905 71 HÖRNEFORS, SWEDEN; DEPARTMENT OF FOOD AND ENVIRONMENTAL SCIENCES, DIVISION OF MICROBIOLOGY AND BIOTECHNOLOGY, VIIKKI BIOCENTER I, UNIVERSITY OF HELSINKI, HELSINKI, FINLAND. (Marine ecology group ; Arcum ; EcoChange)
DHI, Environment and Toxicology, Agern Allé 5, 2970 Hørsholm, Denmark.
Center of Ecology and Evolution in Microbial Model Systems, EEMiS, Department of Biology and Environmental Sciences, Linnaeus University, SE-391 82 Kalmar, Sweden. (EcoChange)
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2017 (English)In: Journal of Plankton Research, ISSN 0142-7873, E-ISSN 1464-3774, Vol. 39, no 1, 23-34 p.Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
Oxford University Press, 2017. Vol. 39, no 1, 23-34 p.
Keyword [en]
phytoplankton, allochthonous dissolved organic matter, humic substances, nitrogen, phosphorus, structuring factors
National Category
Ecology
Identifiers
URN: urn:nbn:se:umu:diva-126382DOI: 10.1093/plankt/fbw081ISI: 000397101400004OAI: oai:DiVA.org:umu-126382DiVA: diva2:1014945
Note

Originally published in manuscript form.

Available from: 2016-10-03 Created: 2016-10-03 Last updated: 2017-08-30Bibliographically approved
In thesis
1. Phytoplankton drivers in a marine system influenced by allochthonous organic matter – the Baltic Sea
Open this publication in new window or tab >>Phytoplankton drivers in a marine system influenced by allochthonous organic matter – the Baltic Sea
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Climate change scenarios predict that seawater temperature and precipitation will increase in the Baltic Sea region during the next century. In the northern part of the Baltic Sea, increasing inflows of the terrestrial allochthonous dissolved organic matter (ADOM) are expected to be a major consequence of elevated rainfall, which can alter light and nutrient availability for phytoplankton. The aim of my thesis was to elucidate effects of ADOM on phytoplankton production, community, size-structure and nutritional strategy along offshore south-north gradients in the Baltic Sea, as well as in estuarine systems exposed to seasonal variation in river discharge. Field studies, a mesocosm experiment and a modeling study were used to explore these issues.

Results from the field studies and mesocosm experiment illustrated that the nutritional strategy, size-structure and cellular pigment content of the phytoplankton were governed by changes in ADOM, and thus in light and nutrient availability. A summer study along an offshore south-north gradient showed that the proportion of mixotrophic phytoplankton increased towards the north. In this area the concentrations humic substances (proxy for ADOM) were high, while the light availability and phosphorus concentrations were relatively low. The phytoplankton cells responded to reduced light availability by increasing their chlorophyll a: carbon ratio. Additionally, the levels of photoprotective pigments decreased from south to north, as a result of acclimation to a low-light environment and reduced exposure to ultraviolet radiation. According to ecological assumptions picophytoplankton should be favored in light- and nutrient-limited environments. However, the results did not follow that pattern, the proportion of picophytoplankton being highest in the relatively nutrient rich Baltic Proper. The study was performed during the decline of an extensive bloom of filamentous cyanobacteria, a successional phase in which picophytoplankton often dominate the phytoplankton community.

The estuarine studies performed in the Bothnian Bay (Råne estuary) and in the Bothnian Sea (Öre estuary) showed different successions. In the Råne estuary no spring phytoplankton bloom occurred and highest primary production was observed during the summer. This absence of a spring bloom was explained by low phosphorus and high ADOM concentrations, while the summer maximum could be explained by higher temperature and nutrient concentrations. In the Öre estuary a marked phytoplankton spring bloom was observed as well as an ADOM sustained bacterial production phase. The later secondary peak of bacterial production observed in summer, concomitant with an extended secondary primary production peak, suggests that autochthonous dissolved organic matter supported the bacterial growth Furthermore, the photosynthetic efficiency (i.e. phytoplankton growth rates) was lower during spring, indicating that high ADOM, and thus lower light and phosphorus availability, disfavored phytoplankton growth.

Our modeling study showed that climate change can impact the food web; however effects will be different between basins. In the southern Baltic Sea elevated temperature and nutrient discharge may promote nutrient recycling and oxygen consumption, potentially extending anoxic areas, sediment nutrient release and cyanobacteria blooms. In the north, increased inflow of ADOM may promote heterotrophic bacterial production and decrease primary production due to light attenuation and lower phosphorus availability. This will favor the heterotrophic microbial food web and consequently lead to lower food web efficiency of the ecosystem.

Place, publisher, year, edition, pages
Umeå: Umeå University, 2016. 31 p.
Keyword
phytoplankton, nutrients, allochthonous organic matter, humic substances, structuring factors
National Category
Ecology
Identifiers
urn:nbn:se:umu:diva-126389 (URN)978-91-7601-571-1 (ISBN)
Public defence
2016-10-28, Lilla Hörsalen (KB3A9), KBC-huset, Umeå, 09:30 (English)
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Available from: 2016-10-07 Created: 2016-10-03 Last updated: 2017-09-01Bibliographically approved

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