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Coccolithophorid calcium carbonate dissolution in surface waters

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J. Harlay

J. Harlay

11

, C. Koch

, C. Koch

22

, J.R. Young

, J.R. Young

22

, N. Roevros

, N. Roevros

11

, A.V. Borges

, A.V. Borges

33

, L.

, L.

-

-

S. Schiettecatte

S. Schiettecatte

33

, C. van der Zee

, C. van der Zee

11

, S. Groom

, S. Groom

44

, L. Rebreanu

, L. Rebreanu

11

, R. Godoi

, R. Godoi

55

& L. Chou

& L. Chou

11 1

1UniversitUniversitéé Libre de Bruxelles (ULB) Libre de Bruxelles (ULB) -- OcOcééanographie Chimique et Ganographie Chimique et Gééochimie des Eaux, Campus de la Plaine, Boulevard du Triomphe ochimie des Eaux, Campus de la Plaine, Boulevard du Triomphe BB--1050 Brussels, B1050 Brussels, B

2

2The Natural The Natural HistoryHistory MuseumMuseum PalaeontologyPalaeontology DepartmentDepartment, Cromwell Road, London SW7 5BD, UK, Cromwell Road, London SW7 5BD, UK

3

3UniversitUniversitéé de Lide Lièège (ge (ULgULg) ) OcOcééanographie Chimique, Institut de Physique, Allanographie Chimique, Institut de Physique, Alléée du 6 Aoe du 6 Aoûût, 17 B5a) t, 17 B5a) -- BB--4000 Li4000 Lièège 1, Bge 1, B

4

4Plymouth Marine Plymouth Marine LaboratoryLaboratory (PML)(PML)-- RemoteRemote sensingsensing Group, Prospect Place, The Group, Prospect Place, The HoeHoe, Plymouth, UK, Plymouth, UK

5

5University of University of AntwerpAntwerp (UIA) (UIA) Micro and Trace Micro and Trace AnalysisAnalysis Centre (Centre (MiTACMiTAC), ), DeptDept. of . of ChemistryChemistry, , UniversiteitspleinUniversiteitsplein, 1,B, 1,B--2610 2610 VilrijkVilrijk--AntwerpenAntwerpen, B, B

Contact : jharlay@ulb.ac.be

Contact : jharlay@ulb.ac.be

Coccolithophorid calcium carbonate dissolution

Coccolithophorid calcium carbonate dissolution

in surface waters

in surface waters

ACKNOWLEDGEMENTS

This work was supported by the CCCC project (Contract no. EV/11/5A) and the PEACE project (Contract no. SD/CS/03A), funded by the Belgian Science Policy Office. This is also a Belgian contribution to SOLAS. Some figures were made with the Ocean Data View software, for which we are grateful to Dr R. Schlitzer (http://odv.awi-bremerhaven.de/). The travel grant to JH provided by SOLAS.be is acknowledged.

CONCLUSIONS

CONCLUSIONS

In the light of these results, CaCO3 dissolution may occur within coccolithophore blooms

when nutrient limitation leads to the production of TEP. The continuum between the release of TEP precursors by coccolithophores and the export of particles to depth, through the formation and reworking of aggregates by bacteria (particularly from the Clade Roseobacter), is thought to constitute a mechanism through which coccolith

preservation is altered. CaCO3 dissolution contributes to the restoration of carbonate

alkalinity and is a sink for dissolved CO2 in seawater. Such a process in surface or

intermediate waters counteracts the CO2 fluxes of respiration and could act as a pump

for atmospheric CO2. It affects the exchange of CO2 through the air-sea interface

and the rain ratio of particulate matter to depth, and could thus play a potentially important role in the C cycle on the Global scale.

Abundance of

Abundance of

coccospheres

coccospheres

and

and

liths

liths

SEM analysis of samples showed that the investigated area was dominated by the Prymnesiophyte

Emiliania

huxleyi

(E. huxleyi), which could reach densities close to 107 cells .l-1 (e.g. station 5, at 10m, Figure 3). The

abundance of E. huxleyi, compared to other coccolithophores, was close to 65% at stations 10 and 8 and increased to more than 90% at stations 5, 2 and 7.

Figure 2: Longitudinal transect along the shelf-break showing

Ωcal (saturation state with respect to calcite), total alkalinity (in µmol .kgsw-1) and particulate inorganic carbon concentrations (in

µg .l-1).

Primary production, calcification and bacterial density

Primary production, calcification and bacterial density

Instantaneous daily rates integrated over the photic depth (based on 14C

incubations) ranged from 0.21 gC.m-2.d-1 and 0.68 gC .m-2 .d-1 for net

primary production and from 0.01 gC .m-2 .d-1 to 0.14 gC .m-2 .d-1 for

calcification (Figure 4).

Bacterial density, measured by epifluorescence after DAPI staining, varied between 0.5x 106 cells .ml-1 and 1.5x 106 cells .ml-1 from surface to

depth, during this survey. Relatively constant value of 0.5x 106 cells .ml-1

was found below the Chl-a maximum. The higher density (> 3x 106 cells

.ml-1) was found in surface waters at station 5 (data not shown).

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 1 10 8 5 2 12 7 4 Stations gC .m -2 .d -1 Primary Production Calcification

Figure 4: Daily rates of primary production (in

green) and of calcification (in blue) integrated over the photic depth (in gC.m-2.d-1) for the continental

plateau (station 1), the shelf-break (stations 10 to 12) and the slope (stations 7 and 4).

Transparent

Transparent

Exopolymer

Exopolymer

Particles (TEP)

Particles (TEP)

TEP are formed from cellular releases during and after cellular growth. The aggregation of particles during the decay phase of a coccolithophore bloom (see poster of C. De Bodt

et al.) is a mechanism that

contributes to the export of particulate matter to depth and could lead to the formation of marine snow under certain conditions (Passow 2002b). The composition of the aggregates may be different if they come from cellular growth (labile) or bacterial growth (refractory) because bacterial polysaccharides are designed for sticking bacteria to their substrates (Azam

et al., 1999; Passow, 2002a).

TEP concentrations were measured colorimetrically (Passow & Alldredge 1995). The highest TEP concentrations were found in association with the highest Chl-a, at the top of the water column and could be attributed to phytoplankton. Variable concentrations (from 40 to >100 µg Xeq .l-1) in surface waters

were observed (Figure 5). TEP concentrations were lower at depth (±25 µg Xeq .l-1) and appeared to be

constrained by bacterial density.

In contrast to stations 8 and 2, station 5 displayed unexpectedly low TEP concentration in surface waters, which was associated with higher bacterial density and production (up to 25 µgC. l-1 .d-1 in surface). This

could be the result of the transformation of its labile fraction into a more refractory one and its export to depth. During this phase, the aggregation of particles, related to bacterial activity, may act as the microenvironments needed for biologically mediated dissolution of calcium carbonate above the chemical lysocline, according to Milliman

et al.

(1999).

TEP (µgXGeq.l-1) 0 20 40 60 80 100 120 140 0 20 40 60 80 100 120 de pth (m) St 5 St 8 St 2

Figure 5: Vertical distribution

of transparent exopolymer particles (TEP) within the high reflectance patch (stations 8, 5 and 2). Emiliania huxleyi 0 2 4 6 8 10 1 10 8 5 2 12 7 4 Millions Stations ce ll s . l -1 0 100 200 300 400 500 Millions li th s .l -1 Coccospheres Coccoliths

Figure 3: E. huxleyi cells (left axis) and shed liths (right Y-axis) concentrations at 10 m depth for the continental plateau (station1), the shelf-break (Stations 10 to 12) and the slope (stations 7 and 4).

Figure 7: Diagram summarizing the degree of coccoliths preservation in seawater (see figure 6 for

explanation of the preservation index). ‘Aliénor Samples’ refer to samples collected in surface waters during the Aliénor cruise (7th June to 17th June 2004). ‘Belgica Samples’ refer to samples collected at various depth (from 3m to 40m, sample number corresponds to station number) during the Belgica

cruise 2004/13 (1st June to 17th June 2004). Small numbers below each sample indicate numbers of coccoliths graded.

Figure 6: Index of coccolith preservation in

samples based on morphological observations by SEM. Five stars correspond to intact coccoliths. Decreasing star numbers indicates a decrease in the preservation state of coccoliths.

References

Azam F., Fonda Umani S. and Funari E., 1999. Ann. Ist. Super. Sanità 35(3): 411-419.

Milliman J. D., Troy P. J., Balch W. M., Adams A. K., Li Y.-H. and Mackenzie F. T., 1999. Deep-Sea Research I

46(10): 1653-1669.

Passow U., 2002a. Marine Ecology-Progress Series 236:

1-12.

Passow U., 2002b. Progress In Oceanography 55(3-4):

287-333.

Passow U. and Alldredge A. L., 1995. Limnology and Oceanography 40(7): 1326-1335.

Wollast R. and Chou L., 1998. Aquatic Geochemistry

4(3-4): 369-393. 10 8 5 2 12 -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 1 2 3 4 5 6 7 8 10 12 -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 1 2 3 4 5 6 7 8 10 12 5 2

Figure 1: Reflectance satellite image on the 2nd of June. The

white line indicates the isobath 200 m that separates oceanic waters from the continental shelf.

In June 2004, a biogeochemical survey of a coccolithophore bloom in the Bay of Biscay was conducted onboard the R.V. Belgica in the framework of the Belgian Global Change Programme. This region is characterized by a steep continental slope that separates the open ocean from the wide continental shelf. Such a bathymetric feature induces vertical mixing that transports nutrient-rich deeper water to the photic zone, enhancing biological activity. Real-time remote sensing allowed us to pinpoint coccolithophore blooms located on the continental shelf and along the continental margin as shown by the high reflectance patches in Figure 1.

Various degrees of CaCO3 preservation were observed by SEM in the cruise samples (Figures 6 and 7), but they globally appeared to be well preserved (good-4 stars- to excellent-5 stars). A contrario, bad preservation of coccoliths was encountered within the high reflectance patch, where 4 or 5 stars preservation index was rarely found in the top 40 meters of the water column. Such a low preservation may represent dissolution of CaCO3 above the lysocline, as shown by Wollast & Chou (1998) in the same area.

SEM observation of CaCO

SEM observation of CaCO

33

preservation

preservation

Biogeochemical context

Biogeochemical context

The high reflectance patches spread along the 200 m isobath to the continental shelf. In June 2004, the coverage of the blooms extended from station 10 to station 12, representing respectively its north-western and south-eastern boundaries. Within the study area, surface waters were characterized by low nutrient levels ([PO43-] close to 0 µM at surface and [Si] < 2 µM).

Sea-surface temperatures ranged between 13 and 15°C at the beginning of June, and reached 17°C at the end of the cruise (mid-June).

Chlorophyll-a (Chl-a) concentrations varied from 0.5 µg .l-1 to 1.5 µg .l-1. The production of CaCO

3 by

coccolithophores led to a depletion in total alkalinity in surface waters (Figure 2). Nevertheless, the photic zone never became under-saturated with respect to calcite (Ωcal>1).

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