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: Highlights: • Coagulation efficiency of the coccolithophorid Emiliania huxleyi was determined with Couette flow devices. • Higher coagulation efficiencies of cells were observed at lower growth rates. • Coagulation efficiency increases with the extracellular polysaccharides fraction. Abstract: Coagulation of small particles results in the formation of larger aggregates that play an important role in the biological pump, moving carbon and other elements from the surface to the deep ocean and seafloor. In this study, we estimated the efficiency of particle coagulation of the coccolithophore Emiliania huxleyi at different growth rates using Couette flow devices at a natural shear rate. To determine the impacts of chemical and biological factors involved in aggregate formation, we investigated how variance in organic matter composition, and in particular the presence of extracellular polysaccharides (EP), including transparent exopolymer particles (TEP) and acidic polysaccharides attached to the coccolith surface, affect the coagulation efficiency (α). When E. huxleyi was grown in a chemostat at different growth rates, coagulation efficiency increased from ~ 0.40 to 1 as cell growth rates declined and nutrients became more limited. With declining growth rate the concentration of EP and the number of detached coccoliths increased. Overall a close correlation between coagulation efficiency of E. huxleyi and the ratio of EP to total particle volume was observed. The minimum value of α of ~ 0.4 determined during this study is higher than estimates published for other phytoplankton cells, and may be related to the presence of EP attached to coccoliths. Based on our findings, we suggest that E. huxleyi is more prone to form aggregates, particularly during the decline of blooms, when increased production of EP and enhanced shedding of coccoliths coincide. This may be one explanation for why blooms of E. huxleyi play an important role in the biological carbon pump, efficiently enhancing the vertical flux of particles, as has been suggested by sediment trap studies.

Description: Highlights: • TEPs and CSPs showed different production patterns and particle-association behaviors. • TEPs and CSPs had different vertical distributions in the Sargasso Sea. • CSP as well as TEP gels are linked by cation bridging. • FlowCAM can be used for in-situ visualization and imaging of TEPs and CSPs in parallel-stained samples. • In-situ visualization of TEPs and CSPs led to new insights about particle interaction and their role in aggregation. Abstract: The discovery of ubiquitous, abundant and transparent gel-like particles, such as the polysaccharide-containing transparent exopolymer particles (TEP) and protein-containing Coomassie stainable particles (CSP) has changed our conception of particle–organism interaction and created new questions about the origin, composition, and role of these particles in aquatic systems. Using both standard and novel staining methods, we studied these gel-like particles to determine whether CSP and TEP are sub-units of the same particle, are distinct particles with different characteristics and behaviors, or are both. Our seawater mesocosm results show that phytoplankton produce both TEP and CSP; however, their highest abundances occur at differences phases in the phytoplankton bloom. We developed a new technique for visualizing stained transparent material in unfiltered aqueous samples with the FlowCAM; this technique allows in-situ visualization and imaging of TEP and CSP in parallel stained samples. Visual examination of stained and unstained TEP and CSP from seawater microcosms, marine algal cultures, and freshwater showed that TEP and CSP have different shape, size and particle-association behavior. In a diatom-dominated microcosm, TEP concentrations were higher than CSP concentrations and unlike CSP, TEP were usually associated with diatom cells or aggregates. The cyanobacteria culture, however, showed higher CSP than TEP concentrations and aggregates of those cells appeared to be CSP-rich. Vertical and seasonal distributions of TEP and CSP in the Sargasso Sea were different. Even though both types of particles were most abundant in the upper 100 m of the water column, CSP closely followed fluorescence and total particle concentration, while the highest TEP concentration was always in the shallowest sample collected. Thus, we conclude that TEP and CSP are different particles, produced by different species at different growth phases and rates. They have different roles and are affected by different processes according to the community composition and environmental conditions.