ALBERT

Description: This study investigates the turnover of polysaccharides by heterotrophic bacterioplankton in the northern Bay of Biscay, a productive marine system on the continental margin of the temperate Atlantic Ocean. Bacterial biomass production (BBP) near the surface ranged from 0.5 to 25.7 nmol C L−1 h−1 during small phytoplankton blooms in May and June that occurred after the main spring bloom. A direct relationship between BBP and total polysaccharides strongly suggests the dependence of bacterial growth on the availability of semi-labile organic matter. Concentrations of combined glucose as well as rate constants of extracellular glucosidase activity and glucose uptake were determined to estimate the actual carbon fluxes from bacterial polysaccharide turnover. Results reveal that the degradation of polysaccharides in the upper 100 m of the water column sustained a glucose flux of 15.2–32.3 mg C m−2 d−1 that was available for bacterial consumption. The mean turnover time for polysaccharides was 170 and 165 days for α- and β-glycosidic linked polymers, respectively. Incorporation of free glucose supported 0.4–19.6% of BBP. The availability of nitrate plus nitrite (NOx) was identified as one factor increasing bacterial incorporation of glucose in most samples. Our results demonstrate that the bacterial recycling of polysaccharides generated a significant flux of organic carbon in microbial food-webs and biogeochemical processes.

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.

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.

Description: Coccolithophores (Prymnesiophyceae) such as Emiliania huxleyi belong to the most productive calcifying organisms in the oceans. During two consecutive years we assessed bacterial diversity and dynamics during the course of spring phytoplankton blooms dominated by coccolithophores in the northern part of the Bay of Biscay. Bacterioplankton community composition was assessed by means of denaturing gradient gel electrophoresis (DGGE) in combination with 16S rRNA gene clone libraries. We used ordination analysis to relate bacterioplankton community dynamics to phytoplankton pigment data and environmental parameters (nutrient concentrations, total alkalinity, concentration of transparent exopolymeric particles (TEP), pCO2). We found a clear difference in composition between the free-living and the particle-associated bacterial assemblage, with the identified Flavobacteria and Sphingobacteria phylotypes being characteristic for the particle-associated bacterial assemblage and Alfaproteobacteria and members of the SAR86 cluster dominating the free-living bacterial assemblage. Stations along the continental margin, at different stages in the coccolithophore bloom, were characterized by distinct bacterial assemblages which correlated well with changes in phytoplankton community composition and TEP abundance. We hypothesize that coccolithophore bloom dynamics shape both the free-living and the particle associated bacterial assemblages through phytoplankton group-specific associations and TEP production

Description: Carbohydrates comprise a large fraction (~30 %) of organic matter in the ocean (Pakulski and Benner, 1994). The production of polysaccarides (CCHO) in seawater is mainly linked to autotrophic processes. CCHO serve as structural and storage compounds, or are released by exudation, cell lysis or leakage to the dissolved organic matter pool. Concentrations of total hydrolyzable neutral sugars range from 200-800nM and 20-170nM in surface and deep waters, respectively (Benner, 2002). It has been shown that CCHO, in particular acidic polysaccharides, are involved in aggregation processes, e.g. the formation of transparent exopolymer particles (TEP). Recently, it has been suggested that ocean acidification increases TEP production, and therefore potentially enhances particle aggregation (Engel 2002, Mari 2008). Here, we present data on the abundance and composition of neutral and acidic CCHO and on TEP, obtained from field investigations and from CO2-controlled chemostat experiments that were conducted during a coccolithophore bloom in the Bay of Biscay 2006. Based on our findings, we discuss the partitioning of CCHO into gel particles, potential effects of ocean acidification and related consequences for organic matter export.

Description: Carbon cycling processes (primary production (PPp), calcification (CAL), bacterial production and pelagic community respiration (PCR)) and variables (partial pressure of CO2 (pCO2) and total alkalinity (TA)) were measured in early June 2006 at several stations in the northern Bay of Biscay. These measurements were characterized with respect to the coccolithophorid blooming (growth or decline) based on satellite remote sensing (high reflectance (HR)) and other biogeochemical measurements i.e. inorganic nutrients, chlorophyll-a (Chl-a), phaeopigments (Phaeo), particulate inorganic carbon (PIC), particulate organic carbon (POC) and particulate nitrogen (PN)). The major HR patch was located over the shelf, along the continental margin and corresponded to declining bloom conditions characterized by moderate Chl-a 〈1.0 µg L-1, dissolved phosphate (PO4) depletion, low (〈2.0 µmol L-1) dissolved silicate (DSi), low potential primary production (〈0.25 µmol C L-1 h-1) and calcification rates (0.02-0.10 µmol C L-1 h-1). Yet, surface waters were undersaturated in CO2 with respect to atmospheric equilibrium. We present a coherent scheme of the C dynamics of a coccolithophorid bloom along the continental margin of the Bay of Biscay, an active hydrodynamic area, based on standing stocks and processes including 14C-based particulate primary production, CAL and PCR. A carbon budget obtained by integrating PPp, CAL and PCR over the water column highlights the importance of C extracellular production to sustain the bacterial demand in the twilight zone, which has also several repercussions on the fate of organic and inorganic C production in the photic zone during the different stages of the bloom.