Impact of open-ocean convection on nutrients, phytoplankton biomass and activity
Introduction
Open-ocean convection is a fundamental process for the formation and the renewal of deep and intermediate waters (Marshall and Schott, 1999, Testor and Gascard, 2006, Lévy et al., 2000). In the Northwestern Mediterranean Sea (NWM), cyclonic circulation induces a doming of the isopycnals in the center of the basin that weaken the stratified waters (Mertens and Schott, 1998, Ridame and Guieu, 2002, Millot, 1999) generally characterized by a three layer water column (AW: Surface water of Atlantic origin, LIW: Levantine Intermediate Water, and WMDW: Western Mediterranean Deep Water). The convection process is initiated by the combination of the geographic configuration of the area (Medoc Group, 1970, Lévy et al., 1999) and by strong winds; the Mistral in the Rhone valley and the Tramontane bypassing the Pyrenees (Marty and Chiavérini, 2010, Holmes et al., 1999). The convection area generally centered on 42 °N latitude and 5 °E longitude (Marshall and Schott, 1999, Krom et al., 2004, Testor and Gascard, 2006), and delimited to the South by the position of the Balearic front (Mertens and Schott, 1998, Strickland and Parsons, 1997).
Convection is an annual event in the Gulf of Lion, but there is a high degree of interannual variability associated with its lifetime and spatial extent (Marty and Chiavérini, 2010, Jickells, 1998). The main factor controlling the onset and the termination of the convection event is the atmospheric forcing (cooling and evaporation) of water column density. The frequency and the intensity of the convection process are susceptible to being altered by processes associated with the climate change, but the method of change is not clear. Marty and Chiavérini (2010) observed an increase of the frequency of extreme convection events as a result of a substantial decrease of precipitation and an increase of evaporation, raising the surface water salinity. This would compensate for the already observed sea surface temperature (SST) increase of 0.03–0.15 °C a−1 from 1957 to 1997 (Béthoux et al., 1998) as well as a slow warming (0.0034 °C a−1) and increases in salinity (1.05×10−3 a−1) of the WDMW during the 1993–2000 period (Béthoux et al., 2002a). Long-term warming and salinization trends in the deep layers have been confirmed by several studies (Krom et al., 2004, Vargas-Yáñez et al., 2010).
Convection processes have important repercussions for carbon sequestration. The convective vertical mixing exports particulate organic matter, estimated to 10 mg C m−2 d−1 during the winter 2008–2009 in the NWM (Stabholz et al., 2013) and dissolved organic carbon estimated to 3000 mg C m−2 d−1 during the intense NWM open-ocean convection event of 2005 (e.g. considering a convection area of 100 km of diameter, Santinelli et al., 2010). But the quality of the organic matter present in the bathypelagic zone depends on the intensity of the convective mixing (Turchetto et al., 2012, Santinelli et al., 2012). A bottom-reaching convection episode resuspends sediment, introducing refractory organic matter few meters above the sea floor (Stabholz et al., 2013). In contrast, a deep convection event that does not reach the sea floor might export only organic matter coming from the surface layer; theoretically more labile than the one from the sediment. The quantity and quality of organic matter exported have important consequences on the productivity and the diversity of bathy-pelagic microorganisms (Boutrif, 2011) and benthic fauna (Pusceddu et al., 2010).
The Mediterranean Sea is an oligotrophic region, with the eastern basin being more oligotrophic than the western basin (Mermex Group, 2011). The nutrient sources that could explain the observed primary production rates in the NWM are the Atlantic influx, riverine discharge, atmospheric deposition and deep ocean convection. However, the importance of deep ocean convections is rarely taken into account because of the lack of knowledge due to the difficulties of sampling a convection episode. Some authors have suggested that the nutrient input from deep to euphotic layers by the convection process directly determine the intensity of the spring bloom (Gacic et al., 2002, Gogou et al., 2014). In the Adriatic Sea, winter 1997 was marked by the absence of a convection event. It resulted in a reduced spring bloom (Gacic et al., 2002). In the NWM, phytoplankton blooms following convection episodes generally exceed 2 mg m−3 of chlorophyll a concentration (D׳Ortenzio and Ribera d׳Alcala, 2009). Some studies based on remote sensing or modeling estimated a primary production of this NWM bloom ranging from 106 to 213 g C m−2 y−1 (Moncheva et al., 2001, L׳Hévéder et al., 2013, Lavigne et al., 2013, Bosc et al., 2004). Hence, it contributes actively to the sequestration of carbon in the deep sea. A better estimate of the nutrient input by the convection process is necessary to improve the existing models and to propose a realistic scenario of the ecosystem functioning during and after winter convection.
In this study, we evaluated the impact of a convection event on the nutrient stoichiometry and the resulting phytoplankton bloom in the NWM during the winter 2010–2011. We analyzed (i) the relative importance of the spatial scale of the open-ocean convective event encountered during the cruise, (ii) the associated nutrient budget, (iii) the evolution of the nutrient stoichiometry during and after the convection event, and (iv) phytoplankton biomass, activity and carbon budget with respect to the biogeochemical fluxes initiated by the convection episode. We propose three mechanisms that initiate the NWM spring bloom: lateral advections, nutrient enrichment, and the ‘phyto-convection theory’.
Section snippets
Study area and sampling
The ‘CASCADE’ cruise (Cascading, Surge, Convection, Advection and Downwelling Events) took place in the Gulf of Lion from the 01 to 23 March 2011 on the R/V L׳Atalante. Two transects (CL and CM) of 13 stations each, crossed the ‘known’ dense water formation area of the NWM (Marshall and Schott, 1999, Krom et al., 2004; Thetis Group, 1994) (Fig. 1). At each station, a profile was conducted from the surface to a few meters above the seafloor using a Seabird 911Plus CTD probe with SBE 32 Carousel
Hydrography
The convective cell sampled on 4 March (CONVg) was observed by the homogeneous vertical profiles of potential temperature (12.9 °C) and potential density anomaly (σ0=29.116 kg m−3) down to 1500 m (Fig. 2A). The presence of a thermocline and a pycnocline on 9 and 16 March 2011 (Fig. 2B and C respectively), illustrated a rapid restratification of the water column after the cessation of the convection episode (Fig. 2B and C). This early March convective cell was found in the western part of the basin
Importance of the convection episode of March 2011
The late convection event sampled during the cruise corresponded to 5% of the previous convection area of winter 2010–2011. While the bottom reaching convection episode of February 2011 covered an area of ~17000 km² during about 20 days, the convective cell in March 2011 reached a depth of 1500 m, extended an additional ~1000 km² during 8 days (Fig. 4, Fig. 6). The convection episode of March 2011 was then a small episode more confined in space, shallower and shorter than the previous convective
Acknowledgments
We are grateful to the crew and officials of R/V L׳Atalante and to all the scientific and technical staff involved in the CASCADE cruise for their support during sea operations. This work was funded by the HERMIONE Project (No. 226354) under the European Commission׳s Seventh Framework Program and the French Program MERMEX under the MISTRALS framework.
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2020, Marine GeologyCitation Excerpt :The same temporal pattern was observed in the Gulf of Lions for downward fluxes of organic carbon, biogenic silica, diatoms, silicoflagellates, coccoliths and foraminifera, which showed the highest annual values during the winter-spring transition (Leblanc et al. 2005; Rigual-Hernández et al. 2010, 2012, 2013). Satellite derived chlorophyll-a abundance maps suggest that the influence of the Ebro River on chlorophyll-a abundance is only evident close to the coastline and that the chlorophyll-a biomass above the trap sites is more related to primary production taking place in the open sea (Fig. 4), probably stimulated by convective mixing, typical of the Mediterranean winter (Morel and André 1991; Estrada 1996; Severin et al. 2014). For instance, in the map of February 2011, there is a clear separation between the coastal and open sea chlorophyll-a abundance, which later in March appear to be strongly connected to a bloom developing off the Gulf of Lions (Fig. 4b).