Introduction
The Mozambique Channel: From physics to upper trophic levels

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Abstract

A multidisciplinary programme, MESOBIO (Influence of mesoscale dynamics on biological productivity at multiple trophic levels in the Mozambique Channel) was undertaken in the Mozambique Channel within the framework of a scientific partnership between France and South Africa. MESOBIO focused on the signature of the highly energetic eddy dynamics in the Mozambique Channel. The Channel, which is known to be one of the most turbulent areas in the world ocean, has a great diversity of marine organisms and is the site of active pelagic fisheries. MESOBIO was mostly based on observations at sea during 12 multidisciplinary cruises between 2002 and 2010. Hydrographic measurements, sampling of biological organisms ranging from phytoplankton to top predators, and experiments on primary production and energy transfer through the food web, were conducted onboard various research vessels. The data were analysed in relation to eddy field characteristics for the periods of the cruises, including seasonal or inter-annual variability in mesoscale activity. A modelling approach was also developed within MESOBIO for both the circulation in the Channel and the biogeochemical response to eddy forcing. This paper introduces the suite of articles on the MESOBIO investigations by summarizing background knowledge for the different disciplines and the key issues that were addressed within the programme.

Introduction

The Mozambique Channel in the southwestern Indian Ocean is bordered by the coast of Mozambique in the west and Madagascar to the east. It is limited by the Comoros archipelago to the north and is connected to the greater Agulhas Current system to the south. Its particular geographic shape and location make the Mozambique Channel very specific in terms of ocean circulation, as it is considered to be one of the most turbulent areas in the world ocean. The Channel also contributes to inter-ocean exchanges due to its connection to the climatologically important Agulhas Current system that interacts between the Indian and Atlantic Oceans. Ecological concerns related to the Mozambique Channel focus particularly on how biodiversity loss might affect many areas (e.g. the Coral Triangle in the north, south and east of Madagascar, the almost undisturbed Iles Eparses) and how mesoscale dynamics impact population connectivity. Furthermore, improved knowledge of the complex ecosystems in the Mozambique Channel are likely to contribute to better management of marine resources exploited by regional and international (European and Asian) fisheries.

The Mozambique Channel is characterised by complex and variable surface and sub-surface circulation (Fig. 1) that is dominated by mesoscale activity (e.g. Biastoch and Krauss, 1999, De Ruijter et al., 2002, Ridderinkhof and De Ruijter, 2003, Schouten et al., 2003, Lutjeharms, 2006) and related to the large scale circulation in the Indian Ocean (Penven et al., 2006, Palastanga et al., 2006, Ridderinkhof et al., 2010, Backeberg and Reason, 2010). Eddies are generally formed at the channel narrows (~16°S) between the northern and central basins and migrate southward mostly along the Mozambique coast (e.g. Schouten et al., 2003). Eddies are also formed at the southern tip of Madagascar, some of them entering the channel northwards along the west coast of Madagascar (Quartly and Srokosz, 2004). Due to the shape of the channel, eddy–eddy or eddy–shelf interactions are common and they contribute to different enhancement processes that are highlighted by sea surface chlorophyll distribution (Quartly and Srokosz, 2004, Tew Kai and Marsac, 2009, Omta et al., 2009).

Indeed, mesoscale eddies are known to have a strong structuring effect on biological production, firstly at the lowest trophic level (McGillicuddy et al., 1998, Oschlies and Garçon, 1998, Rodriguez et al., 2001, Lévy and Klein, 2004) by the injection of nutrients into the euphotic zone and the resulting phytoplankton blooms. Such processes strongly depend, however, on eddy maturity and advection within property gradients (Bakun, 2006, Siegel et al., 2008), eddy–eddy interaction (Lima et al., 2002, Tew Kai and Marsac, 2009) or eddy–wind interaction (McGillicuddy et al., 2007). Statistics based on satellite observation of thousands of cyclonic and anticyclonic eddies showed that several mechanisms have generally to be considered for understanding the bio-optical signatures of mesoscale eddies (Siegel et al., 2011, Chelton et al., 2011).

Mesoscale structures are supposed to affect the distribution and behaviour of intermediate and upper trophic levels. Acoustic surveys of micronekton (i.e. small swimming pelagic organisms preyed upon by marine top predators) indicated potential relationships with mesoscale eddies (Sabarros et al., 2009, Drazen et al., 2011, Godø et al., 2012). In turn, the influence of mesoscale structures on forage fauna impacts the presence and catchability of marine top predators (Domokos et al., 2007, Tew-Kai and Marsac, 2010), and the behaviour of seabirds (Nel et al., 2001, Weimerskirch et al., 2004, Tew-Kai and Marsac, 2010) and sea turtles (e.g. Polovina et al., 2004, Luschi et al., 2006). Few programmes have considered the “entire food web”, however, from the physical driving forces to the lower and intermediate trophic levels, to the upper predators at the top of the food chain. A joint research initiative was therefore developed between France and South Africa, with the name of MESOBIO (Influence of mesoscale dynamics on biological productivity at multiple trophic levels in the Mozambique Channel.). This programme focussed on mesoscale eddies as ecosystem structuring features in the Channel (Fig. 2). New observations at sea (two cruises in 2009 and 2010) complemented previous work within the French ECOTEM programme (Ecologie Trophique en Environnement Marin, 2002–2004), the South African ACEP programme (African Coelacanth Ecosystem Project) with “eddy cruises” in 2005 and 2007, and the more recent ASCLME programme (Agulhas and Somali Currents Large Marine Ecosystem) where cruises were staged in 2008 and 2009.

This suite of 17 articles focussed on the different scientific components of the MESOBIO programme. The objective in this introductory article is to (1) to summarise the current knowledge for each discipline and the impact of mesoscale activity on the trophic compartments, (2) present a synopsis of research cruise activities, and (3) summarise the objectives of the research undertaken within MESOBIO. The final article of this suite summarises the major results of the programme and also proposes new avenues of research to expand our knowledge of the functioning of the ecosystem in the Mozambique Channel. One paper in this suite does not result directly from MESOBIO activities, namely the article by Pripp et al., but it naturally fitted with the objectives of the programme.

Section snippets

Physics

Early studies relying on nonsynoptic hydrographic observations concluded that circulation was dominated by large and highly variable circulation cells (e.g. Saerte and Jorge da Silva, 1984, Donguy and Piton, 1991). Remote sensing observations (e.g. Schouten et al., 2003, Quartly and Srokosz, 2004) and ocean models (Biastoch and Krauss, 1999) indicated the dominance of mesoscale dynamics on the circulation and Schouten et al. (2003) estimated a mean rate of 4–7 eddies a year moving southward

Research cruises

Four ECOTEM cruises (Table 1) were undertaken in the Mozambique Channel during different seasons (Fig. 3A–D). These cruises focused on the spatial and temporal variability of biomass and specific diversity of the prey related to marine top predators (forage fauna of tunas, swordfish and sharks) and included hydrographic profiling, acoustic surveys and seabird observations. Biological sampling was conducted using mesopelagic trawling (MC02) and longline fishing (MC03, MC04A, and MC04B). The MC02

Physical and biogeochemical dynamics

Questions addressed during MESOBIO using in situ data and satellite imagery, concerned the occurrence of eddy (cyclonic) induced upwelling, the offshore entrainment of shelf biomass due to eddy–shelf or eddy–eddy interactions, the occurrence and origin of coastal upwelling, and the potential dispersal corridors for larval and juvenile fish species offered by eddy field dynamics within the Mozambique Channel. Modelling experiments on eddy dynamics were also undertaken using a high resolution

Acknowledgements

MESOBIO (2009–2011) was funded by the MASMA programme of the WIOMSA (West Indian Ocean Marine Association) and supported by both the ASCLME (Agulhas Somali Current Large Marine Ecosystem) and SWIOFP (South West Indian Ocean Fisheries Project) programmes. Other contributing programmes included ECOTEM (2002–2004) funded by IRD (Institut de Recherche pour le Développement) and the South African ACEP programme (African Coelacanth Ecosystem Project). We are grateful to the officers and crews of the

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