DESCRIPTION OF THE BIOGEOCHEMICAL AND BIOLOGICAL DIVERSITY IN THE MEDITERRANEAN SEA

The main objective of the BOUM cruise is to give a description of the biogeochemical parameters and biological diversity along two gradients (1) North - South from the mouth of the Rhone to the centre of the Western basin and (2) West-east from Gibraltar to the coast of Syria, during a marked period of stratification

Objective 1.1. : Biogeochemical description

The classic variables temperature, salinity, dissolved oxygen concentration, along with alkalinity, pigments, organic and mineral carbon and other biogenic elements (C, N, P, Si) pools, will be measured as well as other specific variables at selected depths (nitrogen fixation, diazotroph diversity, recycling times for dissolved phosphate...). The aim is to describe the variations in the biogeochemical characteristics (nutrient availability in the photic layer, depth of the nutricline, ratios of dissolved/particulate and organic/mineral in the biogenic elements pools, drawdown of CO 2 of anthropic origin, phytoplankton biomasses and diversity, nitrogen fixation rates...) starting at the two principal sources of nutritive elements: the entry of Atlantic surface water from Gibraltar and the contributions of the Rhone river. This biogeochemical description will extend to the Levantin basin of the Mediterranean where oligotrophic conditions are the most extreme (not sampled during the PROSOPE cruise of 1999). Our recently acquired techniques for biogeochemical measurements in ultra-oligotrophic waters should enable us to significantly improve our understanding of the biogeochemistry of the Mediterranean Sea . We do not have a good insight into the spatial distribution of certain biogeochemical variables, such as nitrogen fixation, or absolute phosphate concentrations in surface waters, which have a predominant role in controlling planktonic production. Recent studies infer the importance of the Mediterranean Sea in the drawdown of anthropogenic carbon (Alvarez et al. 2005, Aït-Ait-Ameur & Goyet, 2005) from data in/off the Gulf of Cadiz . It is necessary to quantify the drawdown of the anthropogenic carbon in the whole Mediterranean Sea ( A 1.1 ) and to evaluate its transport to the Atlantic Ocean through the Strait of Gibraltar

Specific question: What is the actual longitudinal distribution of carbon and nutrients in the Mediterranean Sea?

Collaboration within the SESAME program

Within the framework of the European program SESAME, 10 Oceanographical Ships (Countries of origin: Bulgaria, Spain, France, Greece, Israel, Italy, Romania, Russia, Tunisia, Turkey) will simultaneously, on a survey study in the Mediterranean Sea and the Black Sea (Fig. 3). A quasi synoptic biogeochemical description of the biogeochemical parameters for all the Mediterranean is anticipated. This description will be compared with previous data and used as the basis for future work observing biogeochemical changes, in particular the nutrient availability in the Mediterranean Sea. It is envisaged that within the framework of WP2, in the SESAME program, that two oceanographical cruises will be conducted, one during March-April 2008 and the other during August-September of the same year. This has the objective of obtaining a physical and biogeochemical "snap-shot" of the oceanographical variables of the Mediterranean and Black Sea over 2 contrasting seasons, the spring bloom period and the period of marked stratification. French participation involves the acquisition of biogeochemical parameters and is envisaged to take place in March-April 2008 during the ARCHIMED cruise whose principal objective is to study contaminants and during the BOUM cruise in August-September 2008, whose objective is to study production and subsequent fate in contrasting oligotrophic environments. These two cruises, the European stage of SESAME, are under the responsibility of Thierry Moutin. Discussions concerning this subject were carried out with Daniel Cossa of the IFREMER in order to establish a common strategy for sampling the Western Mediterranean.

Fig. 3.

Map of the transect locations and the countries responsible as set out in the European program SESAME and the approximate routes of the BOUM cruises (in black dotted line).

The priority of each variable was discussed within the SESAME community. It is the result of a compromise between feasibility by the various partners and the specifics of each cruise. Priority one variables are: temperature, salinity, fluorescence, concentration of dissolved oxygen and nutrients (Nitrate, Nitrite, Phosphate and Silicate) and chlorophyll a concentrations. Priority two variables are: particulate organic carbon, particulate organic nitrogen and phosphorus, dissolved organic carbon, dissolved organic nitrogen and phosphorus, pC0 2 , pH and alkalinity, dissolved mineral carbon, bacterial biomass, density and phytoplankton diversity, size fractionated pigments and zooplankton biomass and diversity. 3rd priority variables are bacterial production, primary production, zooplankton and the tracer radioelements of water masses whose analysis is envisaged in some transects in the Eastern Mediterranean Sea.

Objective 1.2. Biological Diversity

A major objective will be to produce an overall description of the planktonic community structure in Mediterranean pelagic ecosystems, providing a “functional” structure for specific analysis. A detailed description of the biological diversity is essential as it is hypothesised that biodiversity increases the functional redundancy of marine ecosystems. Such redundancy may play an important role in an ecosystems ability to withstand natural and anthropogenic disturbances (Fonseca and Ganade, 2001). The need to describe a more “operational” functional structure should not replace having a “complete” description of the species composition. Variation in species composition probably remains the most effective tool in identifying natural and / or anthropogenic perturbations.

Special attention will be given to studying the diversity of diazotrophs ( (A 1.2 ). Recent studies have shown that a large number of pico and nanoplanktonic prokaryotes are able to fix -nitrogen. This function was previously attributed to the Trichodesmium sp ., a large bloom forming microplanktonic cyanobacteria (Zehr et al. 2001) and to Richelia , a heterocystous endosymbiont found in certain species of diatoms (Gomez et al., 2005). Surprisingly, Trichodesmium sp . have never been observed in the Mediterranean Sea, where di-nitrogen fixation rates are suspected to be high (Gruber and Sarmiento 1997, Karl et al. 2002). It is likely that, the recently discovered small sized diazotrophs (Zehr et al. 2001, Montoya et al. 2004) are responsible for this important biochemical activity . Recent data aquired thanks to the use of molecular tools revealed the existence of new very small diazotrophic cyanobacteria, belonging to the picoplankton size fraction (<3 µm), (Biegala et al, submitted, in prep). Only heterotrophic bacteria were previously thought to exert nitrogen fixation in this size fraction. Although these every small cyanobacteria were discovered in the south west Pacific, il should be interesting to investigate their presence in Mediterranean ( ( A 1.2 ) ). Nevertheless, the relatively high accumulation of biogenic silica associated with the deep chlorophyll maximum, observed during the PROSOPE cruise in the Ionian Sea (Leblanc, pers. com.), indicates that we should not dismiss the role of diatoms, associated with Richelia ( (A 1.3 ) ), as possible key players in nitrogen fixation. If predominant, a new link between the nitrogen and silicone biogeochemical cycles must be considered.

P icoplankton (i.e. cells passing through 3 µm filters) are at the base of all marine food webs. They are composed of prokaryotes ( Bacteria and Archaea ) and eukaryotes. Among both groups, organisms can be either autotrophic (e.g. essentially relying on photosynthesis) or heterotrophic (relying on organic material) or possibly both (mixotrophs). The marine cyanobacteria Prochlorococcus and Synechococcus dominate the prokaryotic photosynthetic biomass particularly in oligotrophic areas of the world’s oceans (Partensky et al. 1999a, Partensky et al. 1999b) . The vast distribution of these two genera, suggest that they possess efficient strategies for responding to environmental stresses. However, the fine-tuning of these responses is still largely unknown ( (A 1.4 ) ) . Recent reports suggest that aerobic anoxygenic phototrophic bacteria (AAnPB) are abundant in pelagic marine systems (Kolber et al ., 2000; 2001), and may be more abundant in oligotrophic areas (preliminary results from the BIOSOPE cruise, in the south pacific gyre, suggest that they can represent up to 20% of the total heterotrophic bacteria: work in progress P. Lebaron). Given the large potential impact that AAnPB may have on marine carbon cycling and the recent conflicting reports concerning their abundance in the world’s ocean (Schwalbach & Furhman, 2005), further work is necessary in order to determine the ecological significance of these photoheterotrophs ( (A 1.5 ) ). Among the autotrophs, eukaryotes often dominate over prokaryotes in terms of biomass and production (Worden et al. 2004). In recent years, molecular methods have revealed a wide diversity among picoeukaryotes, in particular, the heterotrophs. However, we still have very little information concerning the dominant groups and their ecology in most oceanic waters (A 1.6 ) with the exception of coastal waters (Not et al. 2004).

Coccolithophores, calcifying haptophytes (Thierstein and Young 2004) are the key organisms responsible for the transfer of materials in oligotrophic areas (de Vargas et al. in press). Their diversity is maximal in these conditions. Their adapted strategies enable them to capture light from all depths, even those below the deep chlorophyll maximum. Their micro-skeleton enables the aggregation of organic particulate matter, which could be the major conveyor of the biological pump (Klaas and Archer 2002).

The residence time of water in the Mediterranean Sea is about a hundred years (Lacombe, 1990) which is less than that for other vast oceans. Thus, the effects of acidification related to the global increase in anthropogenic CO 2 should be observed more quickly in the Mediterranean Sea (C. Lee conference, Marseille 2006). It is therefore particularly important to study coccolithophore diversity (A 1.7 ) as their calcification is highly dependant on pH.

A pilot study in the Bay of Villefranche showed that the abundance of cyanophages increased along with abundance of cyanobacteria (the spring phytoplankton bloom of 2005, unpublished data). This suggests a co-variation in the diversity of the hosts and phages, and could explain certain mechanisms of cyanobacteria control. Despite the importance of this control, the data on viral diversity in oligotrophic areas, in particular in the Mediterranean Sea, are sparse (A 1.8 ).

Diversity in low trophic levels, i.e. primary producers, is often assumed to drive diversity in the higher trophic levels. This means that resource diversity may underly consumer diversity. However, this relationship remains hypothetical as it has rarely been shown. In fact, some have claimed that there is no such relationship in plankton (Irigoien et al. 2004), a claim that has been challenged (Dolan 2005). Among the consumers of phytoplankton, microzooplankton generally have the dominant role. They are composed predominantly of protists capable of growth rates equal to that of their prey. Thus, microzooplankton communities ( A 1.9 ) are potentially as dynamic as their phytoplankton prey.

Specific questions: Which community structures currently characterize the Mediterranean’s pelagic ecosystems during oligotrophic conditions at the end of the stratified period? Which dominating species or group is responsible for specific functions (nitrogen fixation, dissolved organic phosphate utilization, bacterial production, silification rates, grazing)?