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

Marañón, Emilio; Wambeke, F. Van; Uitz, Julia; Boss, Emmanuel; Dimier, Céline; Dinasquet, Julie; Engel, Anja; Haëntjens, Nils; Pérez‐Lorenzo, María; Taillandier, Vincent; Zäncker, Birthe

doi:10.5194/bg-18-1749-2021

Cited by 46 publications

(44 citation statements)

References 102 publications

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“…Despite similar total chl. a concentrations at the three stations (Gazeau et al, 2020), PP was higher at FAST (Table 1, Gazeau et al, 2021;Marañón et al, 2021). The initial microbial communities differed substantially between the three stations as shown by pigments (Gazeau et al, 2020), 18S and 16S rDNA sequencing (this study).…”

Section: Discussionmentioning

confidence: 50%

Impact of dust addition on the microbial food web under present and future conditions of pH and temperature

Dinasquet

Bigeard

Gazeau

et al. 2021

Preprint

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Abstract. In the oligotrophic waters of the Mediterranean Sea, during the stratification period, the microbial loop relies on pulsed inputs of nutrients through atmospheric deposition of aerosols from both natural (Saharan dust) and anthropogenic origins. While the influence of dust deposition on microbial processes and community composition is still not fully constrained, the extent to which future environmental conditions will affect dust inputs and the microbial response is not known. The impact of atmospheric wet dust deposition was studied both under present and future (warming and acidification) environmental conditions through experiments in 300 L climate reactors. Three dust addition experiments were performed with surface seawater collected from the Tyrrhenian Sea, Ionian Sea and Algerian basin in the Western Mediterranean Sea during the PEACETIME cruise in May–June 2017. Top-down controls on bacteria, viral processes and community, as well as microbial community structure (16S and 18S rDNA amplicon sequencing) were followed over the 3–4 days experiments. Different microbial and viral responses to dust were observed rapidly after addition and were most of the time higher when combined to future environmental conditions. The input of nutrients and trace metals changed the microbial ecosystem from bottom-up limited to a top-down controlled bacterial community, likely from grazing and induced lysogeny. The composition of mixotrophic microeukaryotes and phototrophic prokaryotes was also altered. Overall, these results suggest that the effect of dust deposition on the microbial loop is dependent on the initial microbial assemblage and metabolic state of the tested water, and that predicted warming, and acidification will intensify these responses, affecting food web processes and biogeochemical cycles.

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Section: Discussionmentioning

confidence: 50%

Impact of dust addition on the microbial food web under present and future conditions of pH and temperature

Dinasquet

Bigeard

Gazeau

et al. 2021

Preprint

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“…Chlorophyll a concentrations were typical of oligotrophic conditions . A diatom-dominated deep chlorophyll maximum (DCM) that coincided with a maximum in biomass and primary production was well developed and observed all along the cruise track (Marañón et al, 2021). POC downward fluxes measured at 200 m depth were similar at the 3 long stations, while downward fluxes of Al and LSi, two proxies for dust, were maximum at TYR (Table 1).…”

Section: Biogeochemical Conditionsmentioning

confidence: 86%

“…For suspended particles, the median Fe/Al ratio was maximum within the surface mixed-layer, and minimum at the DCM (60-100 m; Fig. 6a), highlighting a strong contrast in Fe/Al between the diatom-dominated particle assemblage at the DCM (Marañón et al, 2021) and detrital/lithogenic particles in the rest of the water column. This contrast supports the important role played by phytoplanktons, and in particular diatoms, in accumulating Al via active uptake (Gehlen et al, 2002;Liu et al, 2019) and/or adsorption onto cell membranes (Dammshäuser and Croot, 2012;Twining et al, 2015b).…”

Section: Drivers Of the Rapid Removal Of Dalmentioning

confidence: 99%

Subsurface iron accumulation and rapid aluminium removal in the Mediterranean following African dust deposition

Bressac

Wagener

Leblond

et al. 2021

Preprint

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Abstract. Mineral dust deposition is an important supply mechanism for trace elements in the low-latitude ocean. Our understanding of the controls of such inputs has been mostly built onto laboratory and surface ocean studies. The lack of direct observations and the tendency to focus on near surface waters prevent a comprehensive evaluation of the role of dust in oceanic biogeochemical cycles. In the frame of the PEACETIME project (ProcEss studies at the Air-sEa Interface after dust deposition in the MEditerranean sea), the responses of the aluminium (Al) and iron (Fe) cycles to two dust wet deposition events over the central and western Mediterranean Sea were investigated at a timescale of hours to days using a comprehensive dataset gathering dissolved and suspended particulate concentrations, along with sinking fluxes. Dissolved Al (dAl) removal was dominant over dAl released from dust. Fe / Al ratio of suspended and sinking particles revealed that biogenic particles, and in particular diatoms, were key in accumulating and exporting Al relative to Fe. By combining these observations with published Al / Si ratios of diatoms, we show that adsorption onto biogenic particles, rather than active uptake, represents the main sink for dAl in Mediterranean waters. In contrast, systematic dissolved Fe (dFe) accumulation occurred in subsurface waters (~100–1000 m), while dFe input from dust was only transient in the surface mixed-layer. The rapid transfer of dust to depth (up to ~180 m d−1), the Fe-binding ligand pool in excess to dFe in subsurface (while nearly-saturated in surface), and low scavenging rates in this particle-poor depth horizon are all important drivers of this subsurface dFe enrichment. At the annual scale, this previously overlooked mechanism may represent an additional pathway of dFe supply for the surface ocean through diapycnal diffusion and vertical mixing. However, low subsurface dFe concentrations observed at the basin scale (< 0.5 nmol kg−1) questions the residence time for this dust-derived subsurface reservoir, and hence its role as a supply mechanism for the surface ocean, stressing the need for further studies. Finally, these contrasting responses indicate that dAl is a poor tracer of dFe input in the Mediterranean Sea.

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“…Assuming a C:P molar ratio of 130 in phytoplankton (mean value of sorted phytoplanktonic cells in P-depleted conditions, Martiny et al 2013), such new phosphate would support between 14 and 285 µmol C m -2 d -1 of new production. Based on primary production (PP) rates reported for the PEACETIME cruise (Marañón et al 2021), PP integrated over the mixed layer would range between 817 and 3611 µmol C m -2 d -1 from which 82-361 would be new production based on an average f-ratio of 0.1 previously reported for the Mediterranean Sea (Powley et al 2017 and references therein). Under these assumptions, external supply of phosphate to the surface layer would support, on average, ca.…”

Section: External Sources Of P To the Upper Waters Of The Mediterranean Seamentioning

confidence: 99%

Phosphorus cycling in the upper waters of the Mediterranean Sea (Peacetime cruise): relative contribution of external and internal sources

Pulido-Villena¹,

Desboeufs²,

Djaoudi³

et al. 2021

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Abstract. The study of phosphorus cycling in P-depleted oceanic regions, such as the Mediterranean Sea, has long suffered from methodological limitations leading to a simplistic view of a homogeneous surface phosphate pool with concentrations theoretically set to zero above the phosphacline. 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 conducted collocated measurements of phosphate pools at the nanomolar level, alkaline phosphatase activities and atmospheric deposition of phosphorus, across a longitudinal gradient from the west to central Mediterranean Sea. In the phosphate depleted layer (PDL), between the surface and the phosphacline, nanomolar phosphate was low and showed little variability across the transect spanning from 6 ± 1 nmol L−1 in the Ionian basin to 15 ± 4 nmol L−1 in the westernmost station. The low variability in phosphate concentration contrasted with that of alkaline phosphatase activity which varied over one order of magnitude across the transect. Nanomolar phosphate data revealed density gradients of phosphate concentration inside the PDL ranging between 10.6 ± 2.2 µmol kg−1 in the westernmost station to values close to zero towards the east. Using the density gradients, we estimated diapycnal fluxes of phosphate to the PDL and compared them to atmospheric deposition, another external source of phosphate to the PDL. Phosphate supply to the PDL from dry deposition and diapycnal fluxes was comparable in the western part of the transect. The contribution of atmospheric deposition to external P supply increased under the occurrence of rain and Saharan dust. This result contrasts with the longtime idea that, under stratification conditions, the upper waters of the Mediterranean Sea receive new P mainly exclusively from the atmosphere. Although this finding must be taken cautiously given the uncertainties in the estimation of diapycnal fluxes, it opens exciting questions on the biogeochemical response of the Mediterranean Sea, and more generally of marine oligotrophic regions, to expected changes in atmospheric inputs and stratification regimes. Taken together, external sources of phosphate to the PDL contributed little to total phosphate requirements which were mainly sustained by in situ hydrolysis of DOP. The results obtained in this study show a highly dynamic phosphorus pool in the upper layer of the euphotic zone, above the phosphacline, and highlight the convenience of combining highly sensitive measurements and high-resolution sampling to precisely depict the shape of phosphate profiles in the euphotic zone with still unexplored consequences on P fluxes supplying this crucial layer for biogeochemical cycles.

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

Cited by 46 publications

References 102 publications

Impact of dust addition on the microbial food web under present and future conditions of pH and temperature

Impact of dust addition on the microbial food web under present and future conditions of pH and temperature

Subsurface iron accumulation and rapid aluminium removal in the Mediterranean following African dust deposition

Phosphorus cycling in the upper waters of the Mediterranean Sea (Peacetime cruise): relative contribution of external and internal sources

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