Biogeochemistry and carbon mass balance of a coccolithophore bloom in the northern Bay of Biscay (June 2006)

Abstract

a b s t r a c tPrimary 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 CO 2 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.

Phytoplankton community dynamics during late spring coccolithophore blooms at the continental margin of the Celtic Sea (North East Atlantic, 2006–2008)

“…The higher phytoplankton biomass (chl a) in June 2006 compared to the 2007 and 2008 campaigns may have been produced thanks to partial replenishment of depleted nutrient stocks due to a mixing event prior to the campaign. This mixing event is apparent from the deepening of the MLD and a drop in SST as described by Harlay et al (2011) using modelled data by the UK Met Office National Centre for Ocean Forecasting for the North-East Atlantic. This partial supplementation of nutrients together with the prevailing high irradiance levels and SST during the 2006 campaign probably favoured phytoplankton growth (Figs.…”

Coccolithophores on the north-west European shelf: calcification rates and environmental controls

“…The 14 sampling stations ( Fig. 1; Table 1) were located at Mingulay Reef (MRf), the Atlantic coast (Atl) off Ireland, the central Celtic Sea (Cel), the Western English Channel Observatory (E1), the Bay of Biscay (BB), the sampling site for the PEACE (Role of PElagic cAlcification and export of CarbonatE production in climate change; Harlay et al, 2010Harlay et al, , 2011 project (PEA), the southern North Sea (sNS), Heligoland Roads (Hel), the central North Sea (NS), south of the Shetland Islands (Sh) and south of the Faroe Islands (sFI). The Western English Channel Observatory (E1), Bay of Biscay (BB) and central North Sea (NS) were all sampled twice during the cruise either on consecutive days (BB, NS) or within 9 days (E1).…”

“…Van der Wal et al, 1995;Head et al, 1998;Rees et al, 2002;Harlay et al, 2010Harlay et al, , 2011. However, few studies have made observations during non-bloom conditions, in the context of the biomass and production of the phytoplankton community as a whole, and with reference to potentially growth-regulating environmental factors.…”

“…Blooms associated with continental shelves often tend to be smaller in areal extent (< 100 000 km 2 ) than oceanic features, with similar levels of calcite production (Harlay et al, 2010(Harlay et al, , 2011Poulton et al, 2013) although species composition is a potentially important constraint on calcite standing stocks (Poulton et al, 2013). In contrast to oceanic regions where benthic calcite production is relatively low, benthic organisms in shallow shelf seas may exhibit significant calcite production and dominate shelf sea calcite production (Lebrato et al, 2010).…”

Enhanced rates of particulate organic matter remineralization by microzooplankton are diminished by added ballast minerals

“…This is slightly, but not unreasonably, higher than concentrations of caclium carbonate observed during coccolithophore blooms in the North Atlantic, which fall into the range of 0.4 to 0.8 mg CaCO 3 L −1 (Harlay et al, 2011). At the same time, the PIC : POC ratio at the beginning of the experiment (around 0.1 in units of g C : g C) is slightly low compared to coccolithophore blooms, which tend to have values around 0.3 to 0.7 g C g −1 C.…”