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1 INTRODUCTION

from east to west. The strength of these gradients rises with stronger upwelling (KNOLL et al. 2002). The area of influence can be clearly seen in Figure 1a-f with an enhanced chlorophyll production. Simultaneously with the trade winds, the upwellings show a seasonal pattern, (HERNÁNDEZ-GUERRA et al. 2002; NAVARRO-PÉREZ and BARTON 2001;VAN CAMP et al. 1991).The main determinant of the seasonal variation of the trade winds and subsequently for the intensity of the upwelling is the meridional shift of the Azores High (HERNÁNDEZ-GUERRA et al. 2002; MITTELSTADT 1983). The Azores High reaches its most northern position in summer and in autumn, resulting in the strongest winds and for the upwelling most favourable wind direction in summer. Consequently, the upwelling is most intense in summer north of 25°N (ARÍSTEGUI et al. 1997; KNOLL et al. 2002; NAVARRO-PÉREZ and BARTON 2001; vAN CAMP et al. 1991) at the Canary Islands. During the winter the upwelling is low or non existent due to weak winds and their variability in direction (BARTON et al. 1998), see Figure 1a, d and e. Even a warm onshore flow due to onshore winds may occur (VAN CAMP et al. 1991). However, despite the low intensity of the winds in winter, upwelling generally occurs throughout the year (NAVARRO-PÉREZ and BARTON 2001). Furthermore, upwelling can be observed west of the islands, depicted in Figure 1 b, c and f, with an enhanced chlorophyll production west of Gran Canaria and Fuerteventura. Upwelling varies with wind and coastal currents due to irregularities of coastline, bathymetry and influence of ocean currents (MITTELSTAEDT 1983). In general, variations in the offshore flow of up to 20 cm per second and the onshore flow from 5 – 15 cm per second can occur (MITTELSTAEDT 1983).Vertical velocities of up- and downwellings amount to approximately 50 m per day (BASTERRESTXEA et al. 2002; MITTELSTAEDT 1983). By meandering the Canary Current develops so-called upwelling filaments, which are mostly formed at the cups of the African coast and can reach more than 100 km offshore and so influence the Canary Islands directly (BARTON et al. 1998; JOHNSON and STEVENS 2000; KNOLL et al. 2002; NAVARRO-PÉREZ and BARTON 2001). The filament generated between Cap Yubi and Cape Bojador can extend to the east and southeast or towards the south of the island Gran Canaria (ARÍSTEGUI et al. 1997; BARTON et al. 1998; BASTERRESTXEA et al. 2002; vAN CAMP et al. 1991), see e.g. Figure 1d. A strong filament of semi-permanent nature is located at Cape Ghir near 31°N. It has its highest intensity in summer and autumn and reaches beyond 13°W (JOHNSON and STEVENS 2000; VAN CAMP et al. 1991). From the interaction of the meandering upwelling filaments and perturbations of the islands in the flow of the Canary Current, so-called eddies develop. These are vortices of waters and can be orientated in a clockwise (anticyclonic) or in a counterclockwise (cyclonic) direction. In the area of the Canary Islands the island induced eddies are common, especially at the island Gran Canaria (ARÍSTEGUI et al. 1997; BASTERRESTXEA et al. 2002). The island perturbs the flow of the Canary Current resulting in anticyclonic eddies to the south and cyclonic eddies to the southwest of the island (ARÍSTEGUI et al. 1997). One feature of a cyclonic eddy is its enhanced phytoplankton productivity at the periphery while at the core the amount is lower (ARÍSTEGUI et al. 1997). Cyclonic eddies bring deep nutrient rich waters to the surface in their core and move it away from the centre to the periphery, providing nourishment for phytoplankton. In older eddies the pump get weaker and the amount of chlorophyll increases at the core (ARÍSTEGUI et al. 1997). Cyclonic eddies are associated with low values of zooplankton biomass, causing further phytoplankton accumulation (ARÍSTEGUI et al. 1997). Contrarily, anticyclonic eddies submerge water at the core accumulating chlorophyll at a depth of about 150 m (ARÍSTEGUI et al. 1997). Zooplankton biomass accrues around the phytoplankton with high feeding and growth rates (HERNÁNDEZLÉON 1991). Anticyclonic eddies are therefore associated with sinking phytoplankton and picoplankton, concentrating biomass at their core (YEBRA et al. 2004)

16

Inf. Téc. Inst. Canario Cienc. Mar. n°13

Spatial and seasonal patterns in species composition of fish larvae in the Canary Islands  

Technical report consisting on a comprehensive annotated larvae taxa list with the most important taxonomic characters of this region

Spatial and seasonal patterns in species composition of fish larvae in the Canary Islands  

Technical report consisting on a comprehensive annotated larvae taxa list with the most important taxonomic characters of this region

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