Cape Columbine Research Articles

Physical and biological features across an upwelling front in the southern Benguela

The upwelling front of the Cape Columbine upwelling centre was intensively studied, physically and biologically, along a repeated transect during December 1984 following a quiescent phase in the upwelling cycle. Three distinct zones were evident, an inshore zone influenced by upwelling, an offshore warm oligotrophic zone and a transitional frontal zone separating the two. Salinity proved to be a useful indicator of recent water movements. There was evidence of intrusions and mixing of water types within the frontal zone, possibly accounting for the elevated phytoplankton biomass recorded there. Floral and faunal changes occurred between the frontal and offshore zones, corresponding to the thermal front. The predominant flow was alongshore, with strong equatorward jet currents, making the interpretation of cross-shelf gradients difficult in this dynamic area. Aspects of the distributions of organisms and their productivity across the upwelling front are described with respect to the hydrographic parameters...

Mesoscale variations in summer winds over the Cape Columbine — St Helena Bay region, South Africa

Spatial and temporal variations in the near-surface wind field and their relation to upwelling in the lee of the Cape Columbine headland were investigated during the summers of 1979–1980. Time series of meteorological data were collected at an automatic weather station situated on the northern tip of the headland. Intensive data collection by means of pilot balloons and hand-held sensors permitted description of thermally-driven wind oscillations and, on a larger scale, gradient wind cycles during January and February 1979. Results of aerial surveys were used to document wind shear and upwelling over St Helena Bay in November and December 1980. Synoptic weather systems further south controlled the depth and trajectory of wind flow. During cool westerly flow, homogeneous patterns prevailed with limited Seabreeze enhancement along the coast north of St Helena Bay. The prevailing equatorward wind was concentrated in a "jet" over the Cape Columbine headland. This wind jet reached maximum intensity in the 100–300 m layer just before sunset at 19h00, fanning out and decelerating with cyclonic curvature downstream over St Helena Bay. The influence of these mesoscale variations in wind stress is spatially correlated to coastal upwelling patterns in case studies.

Distribution and abundance of eggs, larvae and early juveniles of sauryScomberesox saurus scombroides(Richardson) off the south-western Cape, South Africa, 1977/78

The distribution patterns of saury eggs, larvae and early juveniles have been investigated over the continental shelf off the South-Western Cape from August 1977 to August 1978. Young saury tend to occupy water of 18°C or warmer in all seasons and are found offshore of the Benguela oceanic front when inshore upwelling is evident. Their distribution in the region appears to be linked to seasonal hydrography. Spawning in the study area occurs mainly between Cape Columbine and Cape Point, and to a lesser extent east of Cape Point. The ratio of large larvae and early juveniles to small larvae is greater off the West Coast than off the South Coast. Most spawning appears to take place in summer, with a peak abundance of larvae and early juveniles towards the end of this season. Instantaneous natural mortality was calculated: 5 – 30 mm, Z = 0,14·day−1; 30 – 100 mm, Z = 0,02·day−1. There is therefore a marked but unexplained decline in mortality shortly after the transition from the larval to the juvenile phase.

A comparison of the performance of four zooplankton samplers

The sampling performance of four zooplankton samplers was compared in the southern Benguela Current 65 km north-west of Cape Columbine. The samplers compared were a vertically towed WP.2 net (200 μm mesh), a double obliquely towed Bongo net (300 and 500 μm mesh), a horizontally towed Miller net (300 μm mesh) and a 7,6-cm diameter centrifugal pump (200 μm mesh). The variability in the biomass and the size and numbers of seven copepod taxa and one euphausiid species were examined in respect of avoidance, mesh size and mode of fishing. There was no evidence of avoidance of any sampler by either the cope pods or euphausiid larvae. Avoidance appeared to be partly responsible for the lower catches of juvenile and adult euphausiids by the WP.2 net and the pump when compared with the catches of the Miller and Bongo nets. Despite their different modes of sampling, the Miller and Bongo nets caught similar numbers and size ranges of juvenile and adult euphausiids. Total retention only occurred if the width of the copepods was considerably larger than the mesh width. Factors to correct for mesh selectivity are given and their limitations discussed.

Diel variation in the vertical distribution of some common zooplankton species off the west coast of South Africa

The vertical distribution and the diel variation of seven species of copepods and the euphausiid Euphausia lucens were examined over a 28-hour period covering 20—21 May 1978 in an area approximately 65 km north-west of Cape Columbine. The majority of the copepod species remained within the sampling depth (0—85 m) and showed no marked diel movement. Those that did migrate (Nannocalanus minor and Metridia lucens) showed evidence of reverse migratory behaviour. There was ontogenetic layering in the vertical range of Euphausia lucens. While the calyptopis larvae remained near the surface and did not migrate, the older stages did, with the juvenile and adults moving through a greater depth range than the younger furcilia stages. The data are discussed with reference to previous depth records from work carried out mainly in the Atlantic Ocean, and specifically to records from local waters.