Mesozooplankton in the Canary Current System: The coastal–ocean transition zone
Introduction
The Canary Current System (CCS) is one of the 49 Large Marine Ecosystems of the world (Sherman, 1993). These are wide geographic regions generally larger than 200,000 km2 characterized by their bathymetry, hydrography and productivity and supporting marine populations which have adapted their feeding, reproductive and growth strategies (Sherman and Alexander, 1989). The CCS is one of the largest upwelling areas of the world and therefore supports important fisheries. However, it is important to distinguish between the North Atlantic subtropical gyre and the eastern coastal biome (Longhurst, 1998). In this work, we will consider this wide geographic area as a system with two clear sides, the oceanic (oligotrophic) waters and the coastal upwelling region. Connections between both areas are one of the most prominent features of the region in comparison with other upwelling areas. The presence of the Canarian Archipelago as a barrier of more than 600 km (Fig. 1) in the path of the Canary current and the consequent mesoscale activity generated promotes a rather different functioning of the system in comparison with similar ecosystems.
During the last two decades, knowledge of the CCS has changed due to the increase in the research effort performed by laboratories working in the region. Most of the research in the past was done by mainland European institutions. During the 1970s, the CINECA program (Cooperative Investigation of the Northern part of the East Central Atlantic) promoted an extraordinary advance in the knowledge of the area (Hempel, 1982). Recently, the European projects “Coastal Transition Zone: Islas Canarias” and “Canary Azores Gibraltar Observations” (CANIGO) added valuable information about the functioning of the exchanges between the coastal zone and the open ocean. One of the key advances in the area was the study of mesoscale phenomena, such as upwelling filaments and eddies shed by the Canary Islands. Remote sensing methods provided an important tool for the study of such structures. The enormous and complex mesoscale variability related with the cyclonic and anticyclonic eddies shed by the islands as well as the upwelling filaments which flow into the open ocean, have prompted a reappraisal of the role of ocean physics (and therefore climate) in the development of marine communities (Bakun, 1998). However, although the growth of populations is intimately related to the effect of physics (upwelling, vertical mixing, etc.) not all the biological variability can be explained by climatic phenomena. Ecosystem structure depends also on strict biological factors. Grazing and predation promote drastic changes in the composition and development of marine populations. In particular, predation has enormous implications in the structure of marine ecosystems, and the oceanic realm is no exception (Verity and Smetacek, 1996).
Zooplankton plays a central role in the food web as a link between the microbial food web and the larger organisms such as small pelagic fishes. Fish larvae feed on phyto- and zooplankton and their biomass and abundance is crucial for the development of their populations. In upwelling areas there is also a relationship between fish and zooplankton of different sizes (Cushing, 1978). Anchovies and sardines feed on copepods and jack mackerel feed on copepods as well as on euphausiids.
In the present work, the mesozooplankton and related components of the food web will be reviewed in the Coastal–Ocean Transition Zone of the Canary Current System. Emphasis will be placed on describing the planktonic components of the web, their variability and factors influencing mesozooplankton development in the oceanic area of the CCS, the upwelling and the effects of the Canary Islands in the path of the main current. These areas and their mesoscale structures will be reviewed beginning in each section with a description of the physical phenomenon, primary productivity and then zooplankton in terms of biomass and taxonomic composition. Copepods, as the main mesozooplankton organisms will be the focus of the review. Top-down effects by predators inhabiting the deep scattering layers will be also reviewed as not all the variability in zooplankton biomass and abundance is driven by physics.
Section snippets
The oceanic area of the Canary Current System
The Canary current is the easternmost branch of the North Atlantic Subtropical Gyre flowing southward. It is a relatively cold current in relation to its latitude due to cooling of the waters of the subtropical gyre during its flow through the westerly winds province at the latitude of the Azores Islands. Temperature at the surface varies from 18 to 23 °C, although off the African coast values can reach only 16 °C (Mittelstaedt, 1991). Salinity varies between 36 and 37 psu. Below the mixed layer
The upwelling system
The upwelling area off Northwest Africa extends from about 15 to 33°N covering a large area of the African coast. However, the upwelling is not continuous and homogeneous along this coast and different spots of strong upwelling are usually observed coinciding with topographic features such as capes as observed from remote sensing (Van Camp et al., 1991). The deep water in the upwelling area circulates through the continental shelf and reaches the surface at the coast where the lowest
The island effect
Plankton biomass increases near islands (Doty and Oguri, 1956). These enhancements are due to the physical perturbations generated by these physical barriers in the path of the oceanic flow. This enrichment is of importance for upper trophic levels as small and medium pelagic fish, which feed on the primary and secondary productivity, are prey for large species such as tuna (Blackburn, 1965, Sund et al., 1981). The presence of the Canary Islands distinguishes the Canary Current System from
Deep scattering layers control on epipelagic zooplankton
An important component of the pelagic fauna, unfortunately scarcely studied, are the organisms inhabiting the deep scattering layers (DSLs). In the Canary current this layer is located between 400 and 600 m depth almost permanently (Boden and Kampa, 1967). A portion of these organisms are interzonal diel vertical migrants (DVMs). Boden and Kampa, 1967, Blaxter and Currie, 1967 observed sharp DSLs to the south of Fuerteventura Island and found that DVMs follow the isolume. They showed that
Summary
The trophic scenario of mesozooplankton was depicted for the oceanic, upwelling and eddy system in the Canary Current System. The effect of mesoscale phenomena in the region is of paramount importance in order to understand the development of zooplankton. In addition to the tremendous increase in production in the coastal area off Northwest Africa, the coupling of this production with the oceanic area through filaments and eddies topographically formed in the coast or shed by the islands
Acknowledgements
The authors are indebted to three anonymous reviewers who helped to improve the manuscript. This study was funded by project ConAfrica (CTM2004-02319) from the Spanish Ministry of Education and the European Union.
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