Description: During two simultaneous cruises in the Central Baltic Sea in July 2007 we applied a 15N tracer addition approach to assess the impact of cyanobacterial N2 fixation on mesozooplankton production in the Central Baltic Sea. We determined rates of diazotrophic 15N2 fixation, as well as uptake of diazotrophic derived 15N by mesozooplankton species. Diazotrophic 15N2 fixation rates were low representing pre-bloom situations. A first order estimate using a two source mixing model of natural δ15N-PON abundance revealed that diazotrophic fixed N contributed to 27 ± 8% to mesozooplankton biomass. Additionally, the application of stable isotope tracer showed that fixed 15N was detectable in the mesozooplankton fraction within 1 h after the onset of the incubation. On a daily basis, 5% up to 100% of newly fixed 15N and 14% of cyanobacteria standing stock were incorporated by mesozooplankton species in our experimental set-ups. By applying size fractionating experiments and the usage of different control treatments, we calculated that the majority of 15N transfer (67%) was mediated by the release of nitrogenous compounds and their channelling through the microbial loop towards the mesozooplankton community. Moreover, direct grazing on filamentous cyanobacteria accounted for 33% of gross 15N incorporation. Grazing in the experiments seemed to be largely influenced by cyanobacterial species dominating the community and by the abundance of Cladoceran species like Evadne. Overall, N2 fixing cyanobacteria are ecological more important as instantaneous sources of nitrogen for higher trophic levels of the Baltic Sea food web than previously assumed.

Description: Highlights: • Elemental C:N:P variations of organic matter are simulated at monitoring site BY15. • No N2 fixation needed to explain observed PO4PO4 and pCO2pCO2 levels after spring bloom. • Model features relevance of DOP production and remineralization for N2 fixation. • Model estimates of annual N2 fixation are View the MathML source297±24mmolNm-2a-1. • Model estimates of annual total production are View the MathML source14.16±0.71molCm-2a-1. Abstract: For most marine ecosystems the growth of diazotrophic cyanobacteria and the associated amount of nitrogen fixation are regulated by the availability of phosphorus. The intensity of summer blooms of nitrogen (N2) fixing algae in the Baltic Sea is assumed to be determinable from a surplus of dissolved inorganic phosphorus (DIP) that remains after the spring bloom has ended. But this surplus DIP concentration is observed to continuously decrease at times when no appreciable nitrogen fixation is measured. This peculiarity is currently discussed and has afforded different model interpretations for the Baltic Sea. In our study we propose a dynamical model solution that explains these observations with variations of the elemental carbon-to-nitrogen-to-phosphorus (C:N:P) ratio during distinct periods of organic matter production and remineralization. The biogeochemical model resolves seasonal C, N and P fluxes with depth at the Baltic Sea monitoring site BY15, based on three assumptions: (1) DIP is utilized by algae though not needed for immediate growth, (2) the uptake of dissolved inorganic nitrogen (DIN) is hampered when the algae׳s phosphorus (P) quota is low, and (3) carbon assimilation continues at times of nutrient depletion. Model results describe observed temporal variations of DIN, DIP and chlorophyll-a concentrations along with partial pressure of carbon dioxide (pCO2)(pCO2). In contrast to other model studies, our solution does not require N2 fixation to occur shortly after the spring bloom to explain DIP drawdown and pCO2pCO2 levels. Model estimates of annual N2 fixation are View the MathML source297±24mmolNm-2a-1. Estimates of total production are View the MathML source14200±700mmolCm-2a-1, View the MathML source1400±70mmolNm-2a-1, and View the MathML source114±5mmolPm-2a-1 for the upper 50 m. The models C, N and P fluxes disclose preferential remineralization of P and of organic N that was introduced via N2 fixation. Our results are in support of the idea that P uptake by phytoplankton during the spring bloom contributes to the consecutive availability of labile dissolved organic phosphorus (LDOP). The LDOP is retained within upper layers and its remineralization affects algal growth in summer, during periods of noticeable N2 fixation.

Description: Cyanobacteria blooms in the Baltic Sea appear after upwelling events, which transport phosphate-rich intermediate water to the surface. The growth potential of diazotrophic cyanobacteria in upwelled water was studied in a mesocosm (tank) experiment in summer 2007. An Anabaena bloom was only induced in the tanks filled with upwelled surface water but not in those filled with surface water from outside the upwelling cell and with intermediate water. The low initial cyanobacteria biomass in the intermediate water could not grow to bloom concentrations within three weeks. It is concluded that mixing of upwelled water with surrounding surface water forms a precondition for a cyanobacteria bloom. An additional mesocosm experiment conducted in 2009 revealed that mixing of intermediate water with surface water had the same stimulating effect on nitrogen fixation and cyanobacteria growth as artificial phosphate input. Phosphate input stimulates the growth of Nodularia and Anabaena more than that of Aphanizomenon. We suggest that the upwelled phosphate-rich intermediate water has to be mixed with the surface water containing physiologically “young” cyanobacteria biomass of at least 20 mg/m3 as an inoculum in order to initiate a cyanobacteria bloom.