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Nitrogen fixation rates were determined at approximately monthly intervals during spring/summer 2001 in the eastern Gotland Sea by means of the 15 N 2 method. Additionally, a comprehensive set of variables were measured in order to apply 3 different budget approaches for the determination of integrated nitrogen fixation: (1) the nutrient concept based on excess PO 4 owing to the low DIN/PO 4 ratios (6.8) in the winter surface water, (2) the CO 2 concept using a combined carbon/nitrogen mass balance and (3) the biomass concept based on the increase in total nitrogen during the cyanobacteria bloom. For the spring season (March to May) the budget approaches gave rise to postulating an additional nitrogen input of about 100 to 200 mmol m -2 . However, this was not attributed to nitrogen fixation because 15 N 2 rate measurements from several years did not show any significant nitrogen fixation during spring. Rate measurements integrated from May to August yielded a nitrogen fixation of 138 mmol m -2 , which was consistent with the estimate based on the biomass concept. The CO 2 concept resulted in a nitrogen input of 268 mmol m -2 from May to August. The nutrient concept was questionable because the excess PO 4 was consumed during spring and not transferred to the summer bloom. High PON/POP ratios in summer indicated that PO 4 release from particulate organic matter provided some of the PO 4 necessary for the primary production based on nitrogen fixation. The estimates obtained from the rate measurements and from the CO 2 concept are considered to represent the lower and upper limit for the nitrogen fixation in the Baltic Proper and correspond to a total nitrogen input of 434 000 and 792 000 t yr -1 respectively. KEY WORDS: Nitrogen fixation · Cyanobacteria bloom · Nutrient · Redfield ratio · Baltic Sea Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 297: [23][24][25][26][27][28][29][30][31] 2005 gen-fixing cyanobacteria are already present in the trophogenic layer in June (Hübel & Hübel 1995, Ohlendieck et al. 2000. Hence, neglecting this period may lead to an underestimation of the annual nitrogen fixation. The purpose of this study was to determine nitrogen fixation rates by the 15 N method with an enhanced temporal resolution of about 1 mo for the period March to October. Since cyanobacteria colonies show a patchy distribution, integration of the rates over time and space involves uncertainties which can hardly be quantified. Therefore, we also consider different integrating approaches to determine the annual nitrogen fixation. These are based on (1) the excess PO 4 available for nitrogen fixation after the spring bloom ('nutrient concept ', Rahm et al. 2000), (2) a combined surface water carbon/nitrogen budget ('CO 2 concept', Schneider et al. 2003) and (3) the temporal increase of total nitrogen compounds in the euphotic zone ('biomass concept . The results of these approaches are compared and their applicability is discussed. MATERIALS AND METHODSInv...

The sea-surface microlayer (SML) is considered to be an 'extreme' environment. However, it is still unclear how bacteria that inhabit the SML (bacterioneuston) react to conditions within this interface. This deficiency is partly caused by the difficulty in obtaining representative samples. Our aim was to examine different sampling devices and characterize bacterioneuston activity in the Baltic Sea. Initial in situ studies revealed a decreased incorporation of 3 H-thymidine ( 3 H-TdR) by up to 90% in both glass-plate and metal-screen samples compared to the underlying bulk water. However, a series of tank experiments showed selective inhibition of bacterial productivity with either of these sampling devices, although bacterial cell counts and community composition were unaltered. The inhibition introduced by the glass plate could not be nullified by different cleaning treatments, but by the wiping technique used to scrape off the sample. Even with this modified, unbiased glassplate technique, 3 H-TdR incorporation of the bacterioneuston was still reduced by 50 to 80% compared to that in the underlying water, whereas the abundance of 5-cyano-2, 3-ditolyl tetrazolium chloride (CTC)-positive cells was not affected. Our in situ study thus revealed that in the Baltic Sea the presence of a pronounced bacterioneuston community different from that in the underlying water is unlikely. Reduced bacterial activities within the SML support the concept of a demanding habitat. Additionally, this study emphasizes the need to carefully evaluate the sampling devices used when measuring bacterial parameters. Furthermore, it supports the view that caution is required in comparisons of results from different studies.

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