Dissolved Inorganic Carbon dynamics in the northern Bay of Biscay during a Coccolithophore bloom

(1)

Coccolithophores

are considered to be the most

pro-ductive calcifying

organisms in the oceans and

contri-bute significantly to global biogeo-chemical cycles. From the oceanic carbon cycle point of view, the effect of a calcifying bloom deviates from other non-calcifying primary produ-cers.

HARLAY, J.

1

; van der ZEE, C.

1

; SCHIETTECATTE, L.-S.

2

; ROEVROS, N.

1

; REBREANU, L.

1

; LAPERNAT, P.-E.

3

;

BORGES, A.V.

2

; CHOU, L.

1

_{Université Libre de Bruxelles, Laboratoire d’Océanographie Chimique et Géochimie des Eaux}

2

_{Université de Liège, Unité d’Océanographie Chimique}

3

_{Vrij Universiteit Brussel, Laboratorium voor Ecologie en Systematiek}

Corr. Auth.: jharlay@ulb.ac.be

Dissolved Inorganic Carbon dynamics in the

northern Bay of Biscay during a

Coccolithophore bloom

Fig. 1 : composite image (NASA) of a Coccolithophore bloom off Brittany (France, 15 June 2004)

SCOPE OF THE STUDY

In the framework of the Belgian Global Change Programme, we have developed a project devoted to the study of the inorganic carbon cycle in the Bay of Biscay where coccolithophore blooms occur frequently (Fig. 1). Real time remote sensing (Fig. 2-left) allowed us to pinpoint a coccolithophore bloom that was sampled in June 2004 during a multidisciplinary investigation, on board the

R.V. Belgica

.

The fixation via photosynthesis of dissolved inorganic carbon (DIC) in the photic zone causes a drawn down of

CO₂ near the surface and the organic

matter is subsequently exported to the deeper ocean, where it undergoes remineralization. This process, termed

Biological pump

, contributes to drawn

down of CO₂ from the atmosphere to

the ocean.

The calcification has an opposing ef-fect, the

Carbonate counter

_{Carbonate counter}

-

_-

pump

_pump

, by the removal of Total Alkalinity (TA) from the sunlit layer and releases CO₂ to the surrounding waters.

The relative strength of both pumps, the so-called

Rain Ratio

_{Rain Ratio}

(particulate inorganic carbon to particulate organic carbon ratio PIC:POC), determines the ,

overall flux of CO₂ across the

atmosphere-ocean interface as well as the yield and intensity of the related carbon sequestration.

Until now, studies have focused on the sunlit production in correlation with environmental parameters. The origi-nality of these preliminary results is to integrate

Transparent

_Transparent

Exopolymeric

_Exopolymeric

Particles

(TEP) as a possible pathway for both microbial activity and physical removal of surface produc-tion.

OVERVIEW

RESULTS

The western continental shelf and edge of the Bay of Biscay undergo strong hydrodynamic conditions that sustain high rates of productivity (Fig. 2-right). Among the 11 stations sampled for biogeochemical processes, 4 are presented here for various parameters (Fig. 3); their geographic locations are shown on figures 5 and 6 along the 200m isobath.

Station 2 (red) displays typical features of a well-developed coccolithophore bloom : decrease in TA accompanied by high Chl-a concentration.

Conversely, Station 12 (purple) exhibits an early bloom-forming situation. Features observed at Stations 7 (deep-water station) and 10 (shallow-water station) are due to the transition between diatom and coccolithophore-dominated communities, as shown by the decrease of TA in shallow waters coupled to an elevated concentration of PIC in the upper photic zone.

Fig. 2 : Backscattering (left) and SeaWIFs (right) pictures received at the NERC Dundee University Receiving Station, and processed by the Remote Sensing Group at Plymouth Marine Laboratory (12 June 2004)

Fig. 6 : Air-sea CO₂ fluxes (mmolCO₂.m-2_.d-1_{) from the composite}

pCO₂, temperature and salinity in June 2004 and wind speed data from NOAA, calculated according to Wanninkhof (J. Geophys. Res. 97(C5), 1992). -12°W -10°W -8°W -6°W -4°W -2°W 0°W 47 N 48 N 49 N 50 N 51 N 52 N 2 7 10 12 -24 -22 -20 -18 -16 -14 -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12 14 AKNOWLEDGMENTS

This study is funded by the Belgian Federal Science Policy Office under contract n° EV/11/5A. We would like to thank the captain, officers and crewmembers of the R/V Belgica for their logistic support. J. Backers, J.-P. De Blauwe, G. De Schepper and R. Van den Branden from the Management Unit of the North Sea and Scheldt Estuary Mathematical Model (M.U.M.M.) in Ostend are greatly acknowledged for their assistance in the operation of the CTD rosette and in data acquisition.

http://www.ulb.ac.be/sciences/dste/ocean/carbonate/frame.html

Fig. 3 : vertical profiles of Temperature, Chlorophyll-a, Oxygen saturation, TA (upper panel) and PIC, POC, TEP and Bacterial abundance (lower panel).

TEP (µgXeq/l) 0 20 40 60 80 100 120 140 0 50 100 150 POC (µg/l) 0 20 40 60 80 100 120 140 0 50 100 150 200 PIC (µg/l) 0 20 40 60 80 100 120 140 0 20 40 60 Bacterial abundance (106/ml) 0 20 40 60 80 100 120 140 0 1 2 3 Seawater Temperature (°C) 0 20 40 60 80 100 120 140 10 11 12 13 14 15 2 7 10 12

Seawater Total Alkalinity (µmol/kg) normalized to salinity 35 0 20 40 60 80 100 120 140 2280 2290 2300 2310 2320 Chl-a (µg/l) 0 20 40 60 80 100 120 140 0 0.5 1 1.5 2 O2 saturation (%) 0 20 40 60 80 100 120 140 80 90 100 110 120

Fig. 5 : distribution of seawater pCO₂ (µatm) in June 2004 (direct measurement) -12°W -10°W -8°W -6°W -4°W -2°W 0°W 47 N 48 N 49 N 50 N 51 N 52 N 2 7 10 12 180 200 220 240 260 280 300 320 340 360 380 400 420 440

At a mature stage, the coccolithophore community (St2) in the upper photic zone is characterized by large amounts of TEP that originate from phytoplanktonic and/or bacterial exudation of C-rich compounds. Whether TEP correlate with high phytoplankton or bacterial biomasses remains an open question, whereas their presence is a common feature in late-bloom situations. The important role of TEP in the composition and fate of the sinking particles becomes increasingly evident.

0 5 10 15 20 25 30 St 10 St 2 St 7 St 12 µg C l -1 d -1 0 0.5 1 1.5 2 2.5 x10 6 ml -1 Bact. Prod. Bact. Abund 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 St 10 St 2 St 7 St 12 g 14 C m -2 d -1 0 10 20 30 40 50 60 70 80 mg C h la .m -2 Org. Uptake Inorg. Uptake Integr. Chl-a

Fig. 4 : (left) daily uptake of 14C for organic and inorganic C synthesis

and Chlorophyll stock. (right) Bacterial production (based on 3_H-Leu

incorporation) and abundance in surface waters.

Radioactive Leu-based bacterial production in surface waters is shown in figure 4 (right). A high production level is observed at Station 12 in spite of a low bacterial biomass that is characteristic of the early bloom-forming situation. The utilization of DIC for organic and inorganic phytoplanktonic carbon production (Fig. 4-left) can be compared to the partial pressure of CO₂ (pCO₂) in surface waters (Fig. 5). pCO₂ values ranging from 280 to 370 µatm could be attributed to a « carbonate counter-pump » effect resulting from the development of

coccolitho-phorid blooms. In contrast, pCO₂ values decrease

considerably around 4°W where a non-calcifying coastal phytoplankton bloom occurred (data not shown).

CONCLUSIONS

The results on air-sea CO₂ flux (Fig. 6) indicate that the phytoplanktonic communities investigated during this

study behave as a sink for atmospheric CO₂. However,

the CO₂ sequestration is lowered by calcification,

resulting in a smaller air-sea CO₂ flux.

Further investigations should consider the “ballast effect” of CaCO₃ lithes and TEP, as well as the substrate effect that could provide TEP for microbial activity and biogeochemical transformation during settling.

Representing a continuum between dissolved and particulate matter due to their gel and stickiness properties, (1) to what extent could TEP act in the transformation and export of biogenic material and (2) how would they affect the vertical distribution of carbonate alkalinity?

Addressing these two questions could be of major importance in the understanding of future interactions between CaCO₃ cycle and climatic changes.