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- Korth, Frederike, et al.
(författare)
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Uptake of dissolved organic nitrogen by size-fractionated plankton along a salinity gradient from the North Sea to the Baltic Sea
- 2012
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Ingår i: Biogeochemistry. - : Springer Science and Business Media LLC. - 0168-2563 .- 1573-515X. ; 111:1-3, s. 347-360
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Tidskriftsartikel (refereegranskat)abstract
- The Baltic Sea is known for its ecological problems due to eutrophication caused by high nutrient input via nitrogen fixation and rivers, which deliver up to 70% of nitrogen in the form of dissolved organic nitrogen (DON) compounds. We therefore measured organic nitrogen uptake rates using self produced N-15 labeled allochthonous (derived from Brassica napus and Phragmites sp.) and autochthonous (derived from Skeletonema costatum) DON at twelve stations along a salinity gradient (34 to 2) from the North Sea to the Baltic Sea in August/September 2009. Both labeled DON sources were exploited by the size fractions 0.2-1.6 mu m (bacteria size fraction) and > 1.6 mu m (phytoplankton size fraction). Higher DON uptake rates were measured in the Baltic Sea compared to the North Sea, with rates of up to 1213 nmol N l(-1) h(-1). The autochthonous DON was the dominant nitrogen form used by the phytoplankton size fraction, whereas the heterotrophic bacteria size fraction preferred the allochthonous DON. We detected a moderate shift from > 1.6 mu m plankton dominated DON uptake in the North Sea and central Baltic Sea towards a 0.2-1.6 mu m dominated DON uptake in the Bothnian Bay and a weak positive relationship between DON concentrations and uptake. These findings indicate that DON is an important component of plankton nutrition and can fuel primary production. It may therefore also contribute substantially to eutrophication in the Baltic Sea especially when inorganic nitrogen sources are depleted.
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| 2. |
- Zilius, Mindaugas, et al.
(författare)
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Spatiotemporal patterns of N2 fixation in coastal waters derived from rate measurements and remote sensing
- 2021
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Ingår i: Biogeosciences. - : Copernicus GmbH. - 1726-4170 .- 1726-4189. ; 18, s. 1857-1871
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Tidskriftsartikel (refereegranskat)abstract
- Coastal lagoons are important sites for nitrogen (N) removal via sediment burial and denitrification. Blooms of heterocystous cyanobacteria may diminish N retention as dinitrogen (N2) fixation offsets atmospheric losses via denitrification. We measured N2 fixation in the Curonian Lagoon, Europe's largest coastal lagoon, to better understand the factors controlling N2 fixation in the context of seasonal changes in phytoplankton community composition and external N inputs. Temporal patterns in N2 fixation were primarily determined by the abundance of heterocystous cyanobacteria, mainly Aphanizomenon flos-aquae, which became abundant after the decline in riverine nitrate inputs associated with snowmelt. Heterocystous cyanobacteria dominated the summer phytoplankton community resulting in strong correlations between chlorophyll a (Chl a) and N2 fixation. We used regression models relating N2 fixation to Chl a, along with remote-sensing-based estimates of Chl a to derive lagoon-scale estimates of N2 fixation. N2 fixation by pelagic cyanobacteria was found to be a significant component of the lagoon's N budget based on comparisons to previously derived fluxes associated with riverine inputs, sediment-water exchange, and losses via denitrification. To our knowledge, this is the first study to derive ecosystem-scale estimates of N2 fixation by combining remote sensing of Chl a with empirical models relating N2 fixation rates to Chl a.
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