MESOSCALE SPATIAL HETEROGENEITY IN CHAETOGNATH POPULATIONS DURING UPWELLING ABATEMENT IN THE NORTHERN BENGUELA REGION

In April 1986 abatement of upwelling in the northern Benguela coincide d with strong penetration by Angolan waters. The combination of these 2 phenomena generated considerable spatial heterogeneity in the physic al structure of the region. In addition, the vertical location and wid th of the pycnocline varied over the study area. This affected the spa tial distribution of the chaetognaths, shifting the axis of spatial va riation from one running inshore-offshore to one running latitudinally , north-south. A total of 19 species were present, with Sagitta setosa accounting for more than 70% of all individuals collected. Chaetognat h species exhibited 4 distinct distribution patterns: some species wer e widely distributed throughout the entire region, though they tended to be concentrated preferentially in a certain area, while the remaini ng species were distributed exclusively in the southern, central, or n orthern parts of the region. On the whole, however, specific distribut ion patterns were closely related to the spatial extensions of one of 3 water masses identifiable in the region. The species in the southern part of the region had the deepest and most oceanic distribution, whi le the species in the northern part of the region had a more inshore d istribution in the upper layers of the water column. The adaptability of chaetognaths to conditions of high turbulence like those in the pyc nocline layer makes the location and strength of the pycnocline in eac h region one of the main factors responsible for the spatial heterogen eity observed.