Seasonal and mesoscale variability of primary production in the deep winter-mixing region of the NW Mediterranean
Introduction
Winter mixing is one of the main mechanisms bringing nutrients to the euphotic zone throughout the Mediterranean. However, its intensity and subsequent biological effects present a marked variability. D’Ortenzio and Ribera d’Alcalà (2009) used SeaWIFS imagery to conclude that, in the open sea, a marked late winter–early spring bloom, typical of a temperate regime, was only observed regularly in the Liguro-Provençal basin of the NW Mediterranean. The cyclonic circulation in this region, together with wind and temperature forcing, favor intense winter convection, which in some years spans all the way to depths exceeding 2000 m and originates the Western Mediterranean Deep Water (MEDOC-Group, 1970; Siokou-Frangou et al., 2010). The development of the phytoplankton bloom in the Ligurian Sea has been described by a number of remote sensing studies (Morel and André, 1991, Antoine et al., 1995, Bosc et al., 2004, Morales, 2006), which typically show a period of blue water followed by the appearance of chlorophyll patches in late winter early spring. Years with more intense convection tend to present more phytoplankton biomass due to factors like increased surface nutrient concentrations derived from deeper mixing and the greater spatial extension, duration and recurrence of the mixing events (Volpe et al., 2012, Marty and Chiavérini, 2010). The surface concentration of phytoplankton is reduced by deep mixing, but phytoplankton proliferation may take place as soon as conditions allow growth to exceed losses. The interplay between vertical mixing and phytoplankton bloom development has been modeled by Lévy et al. (1998), who showed the importance of mesoscale features in shaping phytoplankton production.
Although the phytoplankton bloom in the Liguro-Provençal deep convection region (the so-called MEDOC area, between 3° 30′ to 6° E and 41° to 43°N, Gascard, 1978) represents one of the main fertilization mechanisms in the Mediterranean (Bosc et al., 2004), most in situ measurements of phytoplankton biomass and primary production, based on some oceanographic cruises and the visits to the DYFAMED time series station, have been concentrated on the Ligurian side of the basin (Jacques et al., 1973, Jacques et al., 1976, Vidussi et al., 2000, Marty and Chiavérini, 2002, Marty et al., 2008). Primary production data are scarce in the southwestern part of the MEDOC area and there is little information on the characteristics of the photosynthesis-irradiance relationships of the phytoplankton in the region, a knowledge that would enhance our understanding of basic phytoplankton ecophysiology and could help to improve primary production modeling from remote sensing studies. In addition, there are few in situ data of the effects of this phytoplankton bloom on the other trophic levels and on the fluxes of carbon through the water column and the atmosphere. Answering this question was the main objective of the FAMOSO project, which included repeated cruises to the southwestern part of the deep convection zone during three periods of 2009 covering winter-spring bloom, post-bloom and late-summer stratification situations. In this paper, we examine primary production data from these surveys and from a previous one carried out in March–April 2005 in the same region. Our aims were to ascertain the importance of seasonal and short-term variability (including both temporal changes in the biological populations and the effects of mesoscale or sub-mesoscale spatial processes) on primary production estimates in the area and on the photosynthetic response of the phytoplankton assemblages. Given the importance of the winter-spring bloom of the NW Mediterranean, knowledge of the C fluxes during this period is a prerequisite to learn whether this region acts as a source or a sink of atmospheric C. In addition, because the NW Mediterranean has been identified as a sensitive region to global change (Somot et al., 2006), information on its biogeochemical and ecological processes is needed for establishing baseline conditions and allowing a reliable assessment of the potential effects of climate change in this marine ecosystem.
Section snippets
Material and methods
Several oceanographic cruises were conducted in the southwestern part of the Liguro-Provençal Basin, within the region delimited approximately by coordinates 41°30′ to 42°N and 4° to 5°E, an area with depths exceeding 2000 m and typically subjected to deep convection in winter (Fig. 1, Table 1). The cruises were carried out on board the R.V. Cornide de Saavedra, in March to early April 2005 [EFLUBIO 2, (E2)], and on board the R. V. Sarmiento de Gamboa in mid-March, late April–May and September
Hydrography and phytoplankton
The two winter–early spring cruises E2 (2005) and F1 (2009) presented fairly homogeneous or weakly stratified profiles of temperature, salinity and potential density anomaly (σz), as a result of winter mixing (Fig. 2, Fig. 3). There was considerable mesoscale and submesoscale hydrographical variability in the region, as can be observed in both the SST and Chl a distributions shown in Fig. 4A–C. During F1, the interweaving of relatively cool and high salinity waters with more stratified regions
Hydrography and phytoplankton
The marked stratification increase between winter and late summer and the hydrographical characteristics of the cruises studied here are typical of the seasonal variation in the NW Mediterranean. In addition, the physico-chemical variables showed substantial short-term variability, as can be seen in Fig. 2, Fig. 3, Fig. 4, Fig. 5, Fig. 6. In F1 and E2, the variability shown in the mesoscale heterogeneity of the surface fields (Fig. 4) and the interleaving of high and low salinity waters in the
Acknowledgments
This work was supported by the Spanish Projects EFLUBIO REN2002-04151-C02, FAMOSO (CTM2008-06261-C03) and TURBIMOC (CTM2009-06712-E/MAR), and partially by the Grup de Recerca of the (2009 SGR 588). B.F.-C. holds a FPU fellowship of the Spanish Government. We thank the crew and colleagues on board the R.V. “Cornide de Saavedra”, the R.V. “García del Cid” and the R.V. “Sarmiento de Gamboa” for their help during the cruises. Màxim Galindo contributed to the nutrient measurements and Paloma
References (57)
- et al.
Open sea hydrographic forcing of nutrient and phytoplankton dynamics in a Mediterranean coastal ecosystem
Estuar., Coast. Shelf Sci.
(2013) - et al.
Phytoplankton growth formulation in marine ecosystems: should we take into account photo-acclimation and variable stoichiometry in oligotrophic areas?
J. Mar Syst.
(2013) - et al.
Carbon fluxes through major phytoplankton groups during the spring bloom and post-bloom in the Northwestern Mediterranean Sea
Deep-Sea Res. I
(2010) - et al.
The onset of a bloom after deep winter convection in the northwestern Mediterranean sea: mesoscale process study with a primitive equation model
J. Mar. Syst.
(1998) - et al.
The onset of the spring bloom in the MEDOC area: mesoscale spatial variability
Deep-Sea Res. I
(1999) - et al.
Combined effects of mesoscale processes and atmospheric high-frequency variability on the spring bloom in the MEDOC area
Deep-Sea Res. I
(2000) - et al.
Seasonal and interannual variations in phytoplankton production at DYFAMED time-series station, northwestern Mediterranean Sea
Deep-Sea Res. II
(2002) - et al.
Seasonal and interannual dynamics of nutrients and phytoplankton pigments in the western Mediterranean Sea at the DYFAMED time-series station (1991–1999)
Deep-Sea Res. II
(2002) - et al.
Phytoplankton dynamics and primary production under late summer conditions in the NW Mediterranean Sea. Deep-sea Research I
Nature
(2008) - et al.
Winter pelagic photosynthesis in the NW Mediterranean
Deep-Sea Res. I
(2005)