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Effects of Combined Nitrogen on Baltic Sea Diazotrophic Cyanobacteria
Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences. (Marin ekologi)
Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences. (Marin ekologi)
Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences. (marin ekologi)
(English)Manuscript (preprint) (Other academic)
Abstract [en]

We used planned variations in the discharge of combined nitrogen and the depth of outfall discharge from a sewage treatment plant (STP) to study the regulation of nitrogen fixation by filamentous cyanobacteria in Himmerfjärden Bay, northern Baltic Sea Proper. Using stable nitrogen isotopes, we found nitrogen fixation to be almost insensitive to combined nitrogen, with similar low δ15N-values found in cyanobacteria (mostly Aphanizomenon sp.) at all stations. This was in spite of significant gradients along the bay in both δ15N in seston (<10 µm), used as a proxy for δ15N in combined nitrogen, and total nitrogen. Only late in the productive season, when cyanobacterial biomass was declining or already low, did δ15N increase slightly in filaments, indicating some uptake of combined nitrogen. This coincided with an increase in the proportion of Anabaena spp. in the diazotrophic biomass and may indicate that Anabaena spp. has higher uptake of combined nitrogen than Aphanizomenon sp.. Overwintering, heterocyst-free filaments of Aphanizomenon sp. had as low δ15N as during maximum growth in summer, indicating no uptake of combined nitrogen from the water even in winter, when supplies were ample. These results show that almost all nitrogen used for growth by nitrogen-fixing cyanobacteria in the study area comes from nitrogen fixation, and very little from uptake of dissolved combined nitrogen. δ15N in seston had a similar seasonal signal at all stations, declining from spring to a minimum around week 33 followed by an increase, which lagged an increase in δ15N in diazotrophs by two weeks, presumably mirroring a reduced nitrogen fixation. During this increase, the abundance of Anabaena spp. increased at all stations.

Keyword [en]
Baltic Sea, cyanobacteria, nitrogen fixation, Aphanizomenon, eutrophication, sewage, waste water treatment, Nodularia, stable isotopes
National Category
Ecology
Research subject
Marine Ecology; Marine and Brackish Water Ecology
Identifiers
URN: urn:nbn:se:su:diva-90101OAI: oai:DiVA.org:su-90101DiVA: diva2:622791
Projects
Himmerfjärden eutrophication study
Available from: 2013-05-23 Created: 2013-05-22 Last updated: 2013-05-23
In thesis
1. External Growth Control of Baltic Sea Cyanobacteria
Open this publication in new window or tab >>External Growth Control of Baltic Sea Cyanobacteria
2013 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

In the Himmerfjärden Bay a large excess of nitrogen over phosphorus in the discharge from a large sewage treatment plant (STP) has suppressed growth of diazotrophic cyanobacteria in its inner parts. Implementation of nitrogen removal in the STP in 1997 drastically reduced nitrogen load and triggered growth of diazotrophs, mainly Aphanizomenon sp. This study is part of a long-term series of experiments with the overall aim to test how algal biomass and production in a receiving area can be reduced, without stimulating nitrogen fixation and biomass growth by diazotrophic cyanobacteria. Nitrogen removal was discontinued in the STP during two years (2007-8) and resumed in 2009, and the discharge shifted from 25 to 10 m depth, above the seasonal pycnocline. Cellular 15N showed that N2 was the most important N source for diazotrophic cyanobacteria, and that uptake of combined nitrogen was insignificant. As biomass was declining and at the end of the productive season, we could detect a small, but significant, increase in cellular δ15N at the inner bay stations (H3 and H4). However, this coincided with an increased proportion of Anabaena spp. of the total diazotrophic biomass. This may indicate that Anabaena spp. has a higher uptake of combined nitrogen compared with Aphanizomenon sp. or that declining populations of Aphanizomenon sp. take up combined nitrogen. We also found no evidence of uptake of combined nitrogen during the winter months when nitrogen supply is ample and Aphanizomenon sp. is devoid of heterocysts. During the first half of summer (week 21-27) heterocyst frequencies were higher at the outer stations B1 and H2, compared to the inner bay stations (H4 and H5). The lower frequencies at the inner bay stations are likely due to the reduced growth rate suffered by the Aphanizomenon sp. due to stronger competition for phosphorus by non-diazotrophs at these stations and hence lower need for heterocysts. Towards the end of summer conditions even out along the bay, as the surplus phosphorus from the spring bloom is used up at the outer stations and no heterocyst gradient is present. Heterocyst frequency varied significantly over the summer, with minimum values in the beginning of July, coinciding with the highest water temperatures, and higher frequencies in early and late summer. We suggest this is primarily due to a more efficiently functioning nitrogenase enzyme at high temperatures with a reduced need for “expensive” heterocysts. Spring heterocyst differentiation occurred around 4-6 weeks after depletion of dissolved inorganic nitrogen (DIN) and only when water temperature was 5-9 oC and a pycnocline established. It seems that temperature and light in concert will initiate growth, leading to an internal nitrogen deficiency which starts heterocyst differentiation.

Place, publisher, year, edition, pages
Stockholm: Department of Ecology, Environment and Plant Sciences, 2013. 21 p.
Series
Licentiate in Philosophy Thesis / Department of Ecology, Environment and Plant Sciences, ISSN 1401-4106 ; 2013:1
Keyword
Baltic Sea, cyanobacteria, nitrogen fixation, heterocysts, Aphanizomenon, Nodularia, water treatment, Himmerfjärden, sewage, climate
National Category
Ecology
Research subject
Marine Ecology
Identifiers
urn:nbn:se:su:diva-90086 (URN)
Presentation
2013-05-30, Room 107, Dept. Ecology, Environment and Plant Sciences, Frescati Backe, Svante Arrheniusv. 21A, Stockholm, 11:53 (English)
Opponent
Supervisors
Projects
Himmerfjärden eutrophication study
Available from: 2013-05-22 Created: 2013-05-22 Last updated: 2017-11-22Bibliographically approved

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