Pulsed export of carbon in the north-western Mediterranean Sea -- Journal of Oceanology and Limnology,2023,41(1):189-202

	Cite this paper:
	Michel DENIS, Dominique LEFEVRE, Melilotus THYSSEN, Ian R. JENKINSON, Gérald GRÉGORI. Pulsed export of carbon in the north-western Mediterranean Sea[J]. Journal of Oceanology and Limnology, 2023, 41(1): 189-202

Pulsed export of carbon in the north-western Mediterranean Sea

Michel DENIS¹, Dominique LEFEVRE¹, Melilotus THYSSEN¹, Ian R. JENKINSON^2,3, Gérald GRÉGORI¹

1 Aix Marseille Université, Université de Toulon, CNRS/INSU, IRD, Institut Méditerranéen d'Océanologie(MIO), Marseille cedex09 13288, France;
2 Key Laboratory of Marine Ecology and Environmental Sciences, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China;
3 Agency for Consultation and Research in Oceanography, Lavergne, 19320 La Roche Canillac, France

Abstract:

The short term (hourly scale) variability of heterotrophic prokaryote (HP) vertical distribution and respiratory activity, was investigated in the north-western (NW) Mediterranean Sea. HP vertical distribution was determined on board by flow cytometry analysis of seawater samples collected by series of CTD casts. Cell counts and viability were determined for all samples. HP respiratory rates were determined later in the laboratory from filtered seawater samples (23 dm³) from 300–1 150-m depth. The average cell viability was 94.8%±2.2% (n=240). There was no accumulation of dead cells, due to quick decay of damaged cells. In the epipelagic layer, three HP groups were distinguished, two (HNA1, HNA2) whose cells exhibited a high nucleic acid content and one (LNA) with low nucleic acid content cells. HNA2 was most populated at 50 m but not detected at 90 m and below, presumably aerobic anoxygenic photoheterotrophic bacteria (AAPs). The variability in HP abundance was mainly confined in the upper 80 m. A few secondary peaks of HP abundance were observed (80–150 m) in connection with abundance troughs in the surface layer. HP cells were continuously present in a wide layer around 500 m (mean 191×10³ cells/cm³). Below this layer, HP abundance randomly exhibited peaks, coupled to respiratory rate peaks. The HP abundance and variability in the water column was suppressed during a strong wind event. The observed sporadic variability was tentatively interpreted through a pulsed carbon-export mechanism induced by the microorganism production of dissolved polysaccharides, followed by flocculation and rapid sinking. This mechanism would thus contribute to (i) preventing organic matter accumulation in the epipelagic layer, (ii) seeding the water column with live HP cells, and (iii) supplying the aphotic water column with fresh and labile organic matter. This important vertical flux mechanism needs further observations and modelling.

Received: 2022-01-20 Revised:

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Articles by Michel DENIS

Articles by Dominique LEFEVRE

Articles by Melilotus THYSSEN

Articles by Ian R. JENKINSON

Articles by Gérald GRÉGORI

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