We estimated nitrogen fixation from the increase in total nitrogen (N 2 gas excluded) in the upper 20 m during the summer biomass increase of heterocystous filamentous cyanobacteria at the off-shore Landsort Deep station (BY31, 5 yr) and at 10 more stations in all major basins of the Baltic Sea proper. Estimated fixation rates were 2.3-5.9 mmol N m Ϫ2 d Ϫ1, within the range of reported direct measurements. Estimated total fixation in the Baltic Sea proper, 180-430 Gg N yr Ϫ1 taking nitrogen settling loss and atmospheric deposition into account, was sufficient to sustain 30-90% of the June-August pelagic net community production. Filamentous cyanobacteria (mostly Aphanizomenon sp.) had low C : N and C : P ratios in spring 1998, indicating internal storage of both N and P. From early June, when their biomass growth started, ratios rose gradually to the biomass peak in August and early September, when the C : N ratio (6.5 mol/mol) was close to the Redfield ratio, but the C : P ratio reached 420, almost four times Redfield. The C : N ratio of the peak biomass was 1.5 times that in spring, and the C : P ratio was 13 times higher. The high C : P ratio indicates a smaller P demand by filamentous diazotrophs than expected from Redfield ratios. Only a few percent of the P mineralized daily is needed for filamentous cyanobacterial growth in summer. Filamentous cyanobacteria incorporated 16-41 mmol N m Ϫ2 into biomass (C : N ϭ 6.2) at BY31 in summer 1998. This was less than the estimated nitrogen fixation, suggesting fixed N leaks from growing diazotrophs.

Hypoxia is a well-described phenomenon in the offshore waters of the Baltic Sea with both the spatial extent and intensity of hypoxia known to have increased due to anthropogenic eutrophication, however, an unknown amount of hypoxia is present in the coastal zone. Here we report on the widespread unprecedented occurrence of hypoxia across the coastal zone of the Baltic Sea. We have identified 115 sites that have experienced hypoxia during the period 1955–2009 increasing the global total to ca. 500 sites, with the Baltic Sea coastal zone containing over 20% of all known sites worldwide. Most sites experienced episodic hypoxia, which is a precursor to development of seasonal hypoxia. The Baltic Sea coastal zone displays an alarming trend with hypoxia steadily increasing with time since the 1950s effecting nutrient biogeochemical processes, ecosystem services, and coastal habitat.

We investigated the role of N 2 -fixation by the colony-forming cyanobacterium, Aphanizomenon spp., for the plankton community and N-budget of the N-limited Baltic Sea during summer by using stable isotope tracers combined with novel secondary ion mass spectrometry, conventional mass spectrometry and nutrient analysis. + fluxes to Aphanizomenon colonies at low bulk concentrations (o250 nM) as compared with N 2 -fixation within colonies. No N 2 -fixation was detected in autotrophic microorganisms o5 μm, which relied on NH 4 + uptake from the surrounding water. Aphanizomenon released about 50% of its newly fixed N 2 as NH 4 + . However, NH 4 + did not accumulate in the water but was transferred to heterotrophic and autotrophic microorganisms as well as to diatoms (Chaetoceros sp.) and copepods with a turnover time of 5 h. We provide direct quantitative evidence that colony-forming Aphanizomenon releases about half of its recently fixed N 2 as NH 4 + , which is transferred to the prokaryotic and eukaryotic plankton forming the basis of the food web in the plankton community. Transfer of newly fixed nitrogen to diatoms and copepods furthermore implies a fast export to shallow sediments via fast-sinking fecal pellets and aggregates. Hence, N 2 -fixing colony-forming cyanobacteria can have profound impact on ecosystem productivity and biogeochemical processes at shorter time scales (hours to days) than previously thought.

SummaryWe analysed N 2 -and carbon (C) fixation in individual cells of Baltic Sea cyanobacteria by combining stable isotope incubations with secondary ion mass spectrometry (SIMS). Specific growth rates based on N 2 -and C-fixation were higher for cells of Dolichospermum spp. than for Aphanizomenon sp. and Nodularia spumigena. The cyanobacterial biomass, however, was dominated by Aphanizomenon sp., which contributed most to total N 2 -fixation in surface waters of the Northern Baltic Proper. N 2 -fixation by Pseudanabaena sp. and colonial picocyanobacteria was not detectable. N 2 -fixation by Aphanizomenon sp., Dolichospermum spp. and N. spumigena populations summed up to total N 2 -fixation, thus these genera appeared as sole diazotrophs within the Baltic Sea's euphotic zone, while their mean contribution to total C-fixation was 21%. Intriguingly, cell-specific N 2 -fixation was eightfold higher at a coastal station compared to an offshore station, revealing coastal zones as habitats with substantial N 2 -fixation. At the coastal station, the cell-specific C-to N 2 -fixation ratio was below the cellular C:N ratio, i.e. N 2 was assimilated in excess to C-fixation, whereas the C-to N 2 -fixation ratio exceeded the C:N ratio in offshore sampled diazotrophs. Our findings highlight SIMS as a powerful tool not only for qualitative but also for quantitative N 2 -fixation assays in aquatic environments.

We studied C:N:P stoichiometry, heterocyst frequency, and biomass of the N 2 -fixing cyanobacteria Aphanizomenon sp. and Nodularia spumigena from May to September 1999 and 2000 at an offshore station in the NW Baltic Proper. In 2000, we included iron (Fe) and molybdenum (Mo) contents and biomass-specific 14 C uptake. We also complemented published C:N:P stoichiometry data from 1998 with heterocyst frequency. We found a drastic increase in C:P and N:P ratios in Aphanizomenon sp., indicating severe P deficiency at the biomass maximum. N. spumigena also had high C:P and N:P ratios at high abundances. In 2000, and 1998, the total amount of P stored in Aphanizomenon sp. biomass in early summer equalled that at the bloom peak. Only a small part of the DIP surplus remaining after the spring bloom of diatoms and dinoflagellates ended up in the peak biomass of the subsequent cyanobacterial bloom. Aphanizomenon sp. heterocyst frequency peaked in early summer when the C:P ratio was near the Redfield value, and then decreased with increasing C:P ratio, initially perhaps due to increased temperature, and later to P limitation. This decrease occurred parallel to a decrease in Aphanizomenon sp. Mo content. We found no indication of Fe limitation since there was no clear decrease in Aphanizomenon sp. Fe content as the bloom progressed. N. spumigena had considerably higher Fe content, but a high Fe:Mo ratio suggests that Fe adsorbed to cell surfaces biased the measurements. An expected reduction in growth rate due to the high C:P ratio was not mirrored in a decreased specific C uptake of Aphanizomenon sp. as measured in short-term 14 C incubations. This may indicate short-term C storage with subsequent respiration or excretion of excess carbon when growth is P limited.KEY WORDS: Cyanobacteria · Stoichiometry · Cell quota · Nitrogen · Phosphorus · Iron · Molybdenum · Heterocysts · Baltic Sea Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 49: [57][58][59][60][61][62][63][64][65][66][67][68][69] 2007 in estuaries. The Baltic Proper, the main basin of the Baltic Sea, commonly has high abundances of the heterocystous N 2 -fixing species Nodularia spumigena and Aphanizomenon sp. (hereafter referred to by their genus names) in the stratified summer period (Granéli et al. 1990, Larsson et al. 2001). New estimates indicate N 2 fixation in the Baltic Proper to be a larger source of nitrogen than previously thought, almost as large as that from river discharges or up to twice that from atmospheric inputs (Larsson et al. 2001, Wasmund et al. 2005. Thus, N 2 fixation may contribute 30 to 90% of the nitrogen needed to sustain the phytoplankton net community production during the summer (Larsson et al. 2001).The severely N-limited spring bloom of diatoms and dinoflagellates in the Baltic Proper leaves a substantial pool of dissolved inorganic P (DIP) (Larsson et al. 2001), which is generally assumed to support the succeeding large N 2 -fixing cyanobacterial blooms (e.g. Rahm et al. ...