Description: Highlights: • Activity of Arctic bacterioplankton in summer is regulated by concentration and composition of organic matter. • Bacterial production in Fram Strait is significantly related to concentrations of total amino acids. • Bacterioplankton in Polar Water show enhanced enzymatic hydrolysis of combined carbohydrates compared to Atlantic Water. Abstract The bacterial turnover of organic matter was investigated in Fram Strait at 79°N. Both Atlantic Water (AW) inflow and exported Polar Water (PW) were sampled along a transect from Spitsbergen to the eastern Greenland shelf during a late successional stage of the main annual phytoplankton bloom in summer. AW showed higher concentrations of amino acids than PW, while organic matter in PW was enriched in combined carbohydrates. Bacterial growth and degradation activity in AW and PW were related to compositional differences of organic matter. Bacterial production and leucine-aminopeptidase along the transect were significantly correlated with concentrations of amino acids. Activity ratios between the extracellular enzymes β-glucosidase and leucine-aminopeptidase indicate the hydrolysis potential for polysaccharides relative to proteins. Along the transect, these ratios showed a higher hydrolysis potential for polysaccharides relative to proteins in PW than in AW, thus reflecting the differences in organic matter composition between the water masses. Q10 values for bacterial production ranged from 2.4 (± 0.8) to 6.0 (± 6.8), while those for extracellular enzymes showed a broader range of 1.5 (± 0.5) to 23.3 (± 11.8). Our results show that in addition to low seawater temperature also organic matter availability contributes to the regulation of bacterial growth and enzymatic activity in the Arctic Ocean.

Description: Two cruises were conducted after the diatom spring bloom in the northern Bay of Biscay (2006, 2007), to assess the contribution of combined carbohydrates to organic carbon partitioning. Partitioning of total organic carbon (TOC) into particulate organic carbon (POC) and dissolved organic carbon (DOC) differed between the two years, particularly for depths above 60 m, and was related to the vernal development of the system: a post spring-bloom system in 2007, and a more stratified summer system with higher coccolithophore abundance in 2006. In general, contribution of POC to TOC ranged between 4% and 28% and decreased with depth. Concentration of high-molecular-weight (HMW;〉1 kDa) dissolved combined carbohydrates (dCCHO) ranged from 0.6 to 1.4 μmol L−1 and contributed between 4% and 11% to DOC. Concentration of particulate combined carbohydrates (pCCHO) varied between 0.03 and 1.3 μmol L−1. A high contribution of pCCHO to POC was observed in 2007, i.e. 22–60% C compared to 3–10% C in 2006, and coincided with a higher abundance of transparent exopolymer particles (TEP). TEP accounted for 0.4–2.0 μmol C L−1 in 2007 and 0.5–1.5 μmol C L−1 in 2006. Above 60 m, differences in contribution of TEP-C to POC were most pronounced yielding 15.4±3.0% in 2007 compared to relatively low 4.8±1.4%, in 2006. TEP-C could explain about 60% in 2007 and about 40% of pCCHO-C in 2006. Hence, TEP were identified as a substantial component of pCCHO and POC, particularly in the wake of the spring bloom. Molecular composition of CCHO, i.e. HMW−dCCHO+pCCHO, revealed little difference between the years but strong variation over depth. Uronic acids (URA) were identified as a major component of CCHO (20–40%). Our study indicates that the distribution and composition of CCHO in surface seawater are determined by biogeochemical processes on a seasonal scale. A better knowledge of CCHO cycling and molecular signature has therefore a high potential for a better tracing of carbon dynamics in shelf sea ecosystems. Highlights: ► Role of combined carbohydrates for DOC–POC partitioning assessed (Bay of Biscay). ► TEP comprised substantial carbon fraction of particulate carbohydrates. ► Molecular composition of polysaccharides revealed organic matter diagenesis. ► Uronic acids comprised 20–40% of total and dissolved combined carbohydrates.

Description: The distribution of transparent exopolymer particles (TEP) was investigated during a coccolithophorid bloom in the northern Bay of Biscay (North Atlantic Ocean) in early June 2006. MODIS chlorophyll-a (Chl-a) and reflectance images before and during the cruise were used to localize areas of important biological activity and high reflectance (HR). TEP profiles along the continental margin, determined using microscopic (TEPmicro) and colorimetric (TEPcolor) methods, showed abundant (6.1×106–4.4×107 L−1) and relatively small (0.5–20 μm) particles, leading to a low total volume fraction (0.05–2.2 ppm) of TEPmicro and similar vertical profiles of TEPcolor. Estimates of carbon content in TEP (TEP-C) derived from the microscopic approach yielded surface concentration of 1.50 μmol C L−1. The contribution of TEP-C to particulate organic carbon (POC) was estimated to be 12% (molar C ratio) during this survey. Our results suggest that TEP formation is a probable first step to rapid and efficient export of C during declining coccolithophorid blooms.

Description: Primary production (PP), calcification (CAL), bacterial production (BP) and dark community respiration (DCR) were measured along with a set of various biogeochemical variables, in early June 2006, at several stations at the shelf break of the northern Bay of Biscay. The cruise was carried out after the main spring diatom bloom that, based on the analysis of a time-series of remotely sensed chlorophyll-a (Chl-a), peaked in mid-April. Remotely sensed sea surface temperature (SST) indicated the occurrence of enhanced vertical mixing (due to internal tides) at the continental slope, while adjacent waters on the continental shelf were stratified, as confirmed by vertical profiles of temperature acquired during the cruise. The surface layer of the stratified water masses (on the continental shelf) was depleted of inorganic nutrients. Dissolved silicate (DSi) levels probably did not allow significant diatom development. We hypothesize that mixing at the continental slope allowed the injection of inorganic nutrients that triggered the blooming of mixed phytoplanktonic communities dominated by coccolithophores (Emiliania huxleyi) that were favoured with regards to diatoms due to the low DSi levels. Based on this conceptual frame, we used an indicator of vertical stratification to classify the different sampled stations, and to reconstruct the possible evolution of the bloom from the onset at the continental slope (triggered by vertical mixing) through its development as the water mass was advected on-shelf and stratified. We also established a carbon mass balance at each station by integrating in the photic layer PP, CAL and DCR. This allowed computation at each station of the contribution of PP, CAL and DCR to CO2 fluxes in the photic layer, and how they changed from one station to another along the sequence of bloom development (as traced by the stratification indicator). This also showed a shift from net autotrophy to net heterotrophy as the water mass aged (stratified), and suggested the importance of extracellular production of carbon to sustain the bacterial demand in the photic and aphotic layers.