IntroductionThe Mozambique Channel: From physics to upper trophic levels
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
References (67)
- et al.
Global observations of nonlinear mesoscale eddies
Prog. Oceanogr.
(2011) - et al.
Ocean control of the breeding regime of the Sooty Tern in the southwest Indian Ocean
Deep-Sea Res. I
(2007) - et al.
Assessment of the seabird community of the Mozambique Channel and its potential use as an indicator of tuna abundance
Estuar. Coast. Shelf Sci.
(2005) - et al.
The interpretation of satellite chlorophyll observations: the case of the Mozambique Channel
Deep-Sea Res. I
(2009) - et al.
Forage fauna in the diet of three large pelagic fish (lancetfish, swordfish, and yellowfin tuna) in the western equatorial Indian Ocean
Fish. Res.
(2007) - et al.
Eddies in the southern Mozambique Channel
Deep-Sea Res. II
(2004) - et al.
Moored current observations in the Mozambique Channel
Deep-Sea Res. II
(2003) - et al.
Eddies and variability in the Mozambique Channel
Deep-Sea Res. II
(2003) - et al.
The satellite and in situ observations of the bio-optical signatures of two mesoscale eddies in the Sargasso Sea
Deep-Sea Res. II
(2008) - et al.
Patterns of variability of sea surface chlorophyll in the Mozambique Channel: a quantitative approach
J. Mar. Syst.
(2009)