Deep maxima of phytoplankton biomass, primary production and bacterial production in the Mediterranean Sea

Type Article
Date 2021
Language English
Author(s) Maranon Emilio1, Van Vambeke France2, Uitz Julia3, Boss Emmanuel S4, Dimier Céline3, Dinasquet Julie5, Engel Anja6, Haentjens Nils4, Perez-Lorenzo Maria1, Taillandier Vincent3, Zancker Birthe6, 7
Affiliation(s) 1 : Department of Ecology and Animal Biology, Universidade de Vigo, 36310 Vigo, Spain
2 : Mediterranean Institute of Oceanography, Aix-Marseille Université, CNRS, Université de Toulon, CNRS, IRD, MIO UM 110, 13288 Marseille, France
3 : Laboratoire d'Océanographie de Villefranche, Sorbonne Université, CNRS, 06230 Villefranche-sur-Mer, France
4 : School of Marine Sciences, University of Maine, Orono, Maine, USA
5 : Scripps Institution of Oceanography, University of California, San Diego, San Diego, California, USA
6 : GEOMAR, Helmholtz Centre for Ocean Research Kiel, 24105 Kiel, Germany
7 : The Marine Biological Association of the United Kingdom, Plymouth, PL1 2PB, United Kingdom
Source Biogeosciences (1726-4189) (Copernicus GmbH), 2021 , Vol. 18 , N. 5 , P. 1749-1767
DOI 10.5194/bg-18-1749-2021
Note Special issue | Atmospheric deposition in the low-nutrient-low-chlorophyll (LNLC) ocean: effects on marine life today and in the future (BG/ACP inter-journal SI)(BG/ACP inter-journal SI) Editor(s): Christine Klaas, Cecile Guieu, Karine Desboeufs, Jan-Berend Stuut, Mark Moore, Paraskevi Pitta, Silvia Becagli, and Chiara Santinelli

The deep chlorophyll maximum (DCM) is a ubiquitous feature of phytoplankton vertical distribution in stratified waters that is relevant to our understanding of the mechanisms that underpin the variability in photoautotroph ecophysiology across environmental gradients and has implications for remote sensing of aquatic productivity. During the PEACETIME (Process studies at the air-sea interface after dust deposition in the Mediterranean Sea) cruise, carried out from 10 May to 11 June 2017, we obtained 23 concurrent vertical profiles of phytoplankton chlorophyll a, carbon biomass and primary production, as well as heterotrophic prokaryotic production, in the western and central Mediterranean basins. Our main aims were to quantify the relative role of photoacclimation and enhanced growth as underlying mechanisms of the DCM and to assess the trophic coupling between phytoplankton and heterotrophic prokaryotic production. We found that the DCM coincided with a maximum in both the biomass and primary production but not in the growth rate of phytoplankton, which averaged 0.3 d−1 and was relatively constant across the euphotic layer. Photoacclimation explained most of the increased chlorophyll a at the DCM, as the ratio of carbon to chlorophyll a (C:Chl a) decreased from ca. 90–100 (g:g) at the surface to 20–30 at the base of the euphotic layer, while phytoplankton carbon biomass increased from ca. 6 mg C m−3 at the surface to 10–15 mg C m−3 at the DCM. As a result of photoacclimation, there was an uncoupling between chlorophyll a-specific and carbon-specific productivity across the euphotic layer. The ratio of fucoxanthin to total chlorophyll a increased markedly with depth, suggesting an increased contribution of diatoms at the DCM. The increased biomass and carbon fixation at the base of the euphotic zone was associated with enhanced rates of heterotrophic prokaryotic activity, which also showed a surface peak linked with warmer temperatures. Considering the phytoplankton biomass and turnover rates measured at the DCM, nutrient diffusive fluxes across the nutricline were able to supply only a minor fraction of the photoautotroph nitrogen and phosphorus requirements. Thus the deep maxima in biomass and primary production were not fuelled by new nutrients but likely resulted from cell sinking from the upper layers in combination with the high photosynthetic efficiency of a diatom-rich, low-light acclimated community largely sustained by regenerated nutrients. Further studies with increased temporal and spatial resolution will be required to ascertain if the peaks of deep primary production associated with the DCM persist across the western and central Mediterranean Sea throughout the stratification season.

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Maranon Emilio, Van Vambeke France, Uitz Julia, Boss Emmanuel S, Dimier Céline, Dinasquet Julie, Engel Anja, Haentjens Nils, Perez-Lorenzo Maria, Taillandier Vincent, Zancker Birthe (2021). Deep maxima of phytoplankton biomass, primary production and bacterial production in the Mediterranean Sea. Biogeosciences, 18(5), 1749-1767. Publisher's official version : , Open Access version :