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Biogeosciences An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/bg-2020-261
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2020-261
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  20 Jul 2020

20 Jul 2020

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This preprint is currently under review for the journal BG.

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

Emilio Marañón1, France Van Wambeke2, Julia Uitz3, Emmanuel S. Boss4, María Pérez-Lorenzo1, Julie Dinasquet5, Nils Haëntjens4, Céline Dimier3, and Vincent Taillandier3 Emilio Marañón et al.
  • 1Department of Ecology and Animal Biology, Universidade de Vigo, 36310 Vigo, Spain
  • 2Aix-Marseille Université, CNRS, Université de Toulon, CNRS, IRD, Mediterranean Institute of Oceanography, MIO UM 110, 13288 Marseille, France
  • 3CNRS and Sorbonne Université, Laboratoire d’Océanographie de Villefranche, 06230 Villefranche-sur-mer, France
  • 4School of Marine Sciences, University of Maine, Orono, Maine, USA
  • 5Scripps Institution of Oceanography, University of California, San Diego, USA

Abstract. The deep chlorophyll maximum (DCM) is a ubiquitous feature of phytoplankton vertical distribution in stratified waters that is relevant for 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 biomass and primary production but not in 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 carbon to chlorophyll a ratio (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 mgC m−3 at the surface to 10–15 mgC 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 fucoxanthin to total chlorophyll a ratio increased markedly with depth, as did the biomass contribution of large cells, suggesting a dominance 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 fueled 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 deep primary production peaks associated with the DCM persist across the western and central Mediterranean Sea throughout the stratification season.

Emilio Marañón et al.

Emilio Marañón et al.

Emilio Marañón et al.

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Short summary
The concentration of chlorophyll is commonly used as an indicator of the abundance of photosynthetic plankton (phytoplankton) in lakes and oceans. Our study investigates why a deep chlorophyll maximum, located near the bottom of the upper, illuminated layer, develops in the Mediterranean Sea. We find that the acclimation of cells to low light is the main mechanism involved, and that this deep maximum represents also a maximum in the biomass and carbon fixation activity of phytoplankton.
The concentration of chlorophyll is commonly used as an indicator of the abundance of...
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