Variations in the elemental ratio of organic matter in the central Baltic Sea: Part I-Linking primary production to remineralization

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 (N-2) 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 (pCO(2)). In contrast to other model studies, our solution does not require N-2 fixation to occur shortly after the spring bloom to explain DIP drawdown and pCO(2) levels. Model estimates of annual N-2 fixation are 297 +/- 24 mmol N m(-2) a(-1). Estimates of total production are 14200 +/- 700 mmol C m(-2) a(-1), 1400 +/- 70 mmol N m(-2) a(-1), and 114 +/- 5 mmol P m(-2) a(-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 N-2 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 N-2 fixation. (C) 2014 Elsevier Ltd. All rights reserved.