Seasonal significance of N2 fixation in coastal and offshore waters of the northwestern Baltic Sea

Abstract

Annual rates of N 2 fixation were measured over 3 yr (1998)(1999)(2000) at an open water station (BY31) and 2 coastal stations (H4 and X1) in the Baltic Sea. This is the first report on depthintegrated rates of N 2 fixation from more than one complete growth season in the Baltic Sea. Annual estimates of N 2 fixation ranged from 56 000 to 125 000 t N in the Baltic Proper, and 18 000 to 162 000 t N at the inshore stations (Himmerfjärden). Rates of N 2 fixation were measured in situ at 4 depths between 0 and 25 m using the 15 N tracer technique for size fractionated organisms larger and smaller than 20 μm. Maximum rates of N 2 fixation were found in surface waters (0 to 4 m depth), and a major part of this activity (80% in coastal and 89% in offshore waters) took place during daylight hours. Integrated rates of N 2 fixation in cells > 20 μm followed the average abundance of filamentous cyanobacteria (primarily Aphanizomenon sp.) in the water column. Molar C:N mass ratios in particles > 20 μm, i.e. filamentous cyanobacteria, suggest that this size fraction was N-sufficient during summer, whereas the molar C:P mass ratios indicated P-limitation during this period. A reduction in sewage discharge to the Himmerfjärden bay area during the study period appears not to have been compensated for by increased rates of N 2 fixation. The patchy distribution of cyanobacteria and the high seasonal variability in N 2 fixation rates emphasize the need for adequate spatial and temporal sampling strategies in studies of N 2 fixation in coastal and open waters of the Baltic Sea.KEY WORDS: Baltic Sea · Aphanizomenon · Nodularia · N 2 fixation · HimmerfjärdenResale or republication not permitted without written consent of the publisher

Carbon, nitrogen and O2 fluxes associated with the cyanobacterium Nodularia spumigena in the Baltic Sea

“…The measured carbonspecific photosynthesis of Aphanizomenon sp. collected in the Baltic Sea (Ploug et al, 2010) as well as the Chl.a.-specific photosynthesis of N. spumigena colonies of this study were in the same range as those measured in cultures of the respective cyanobacteria isolated from the Baltic Sea (Moisander et al, 2007;Degerholm et al, 2008). This study suggests that bicarbonate in addition to CO 2 must be an important carbon source in N. spumigena colonies, because the net photosynthesis rate can be as high as the potential diffusive fluxes of DIC to colonies.…”

“…The large, filamentous cyanobacteria are thought to be the main N 2 -fixing organisms in the Baltic Sea (Boströ m et al, 2007;Degerholm et al, 2008;Ploug et al, 2010). Budget calculations of combined nitrogen as well as experimental studies have shown that these cyanobacteria must fix excess N 2 relative to their own nitrogen demand and channel the surplus nitrogen through the microbial food web and to picocyanobacteria (Ohlendieck et al, 2000;Larsson et al, 2001;Stal et al, 2003).…”

Carbon and nitrogen fluxes associated with the cyanobacterium Aphanizomenon sp. in the Baltic Sea

“…The average number of cells per colony was described as Cells ¼ 1.53 Â 10 6 Â (Vol col ), where colony volume is measured in mm 3 . Alternatively, biomass of filamentous cyanobacteria is measured as meter per length (m l À1 ) (Hajdu et al, 2007;Rolff et al, 2007;Degerholm et al, 2008). The cumulative length of filaments in individual colonies varied between 7 and 246 mm, and it was described by L ¼ 11 341 Â (Vol col ); where length, L, is expressed in mm.…”

“…is a filamentous, colonyforming N 2 -fixing cyanobacterium, which frequently blooms in lakes and brackish waters. It occurs both as colonies and single trichomes during their growth, which often lasts 2-3 months during the summer in the Baltic Sea (Rolff et al, 2007;Degerholm et al, 2008). The primary productivity in the Baltic Sea by filamentous cyanobacteria, which are represented by Aphanizomenon sp., Anabaena sp., and Nodularia spumigena, represents 44% of the community primary production.…”

“…accumulates inorganic phosphate during the growth season, and blooms appear to terminate when the internal storage is used up and bulk concentrations are low in mid-August in the Baltic Sea (Walve and Larsson, 2007). N 2 fixation by filamentous cyanobacteria, including Aphanizomenon sp., is considered to be a quantitatively important source of nitrogen to the plankton community during the summer months in the Baltic Sea (Degerholm et al, 2008). Nitrogen budgets of the Baltic Sea have suggested that Aphanizomenon leaks a substantial fraction of its fixed nitrogen that is used by other members (for example picocyanobacteria and heterotrophic bacteria) in the plankton community (Larsson et al, 2001;Stal et al, 2003;Rolff et al, 2007).…”