Shallow-water, nearshore current dynamics in Algoa Bay, South Africa, with notes on the implications for larval fish dispersal

Nearshore currents play a vital role in the transport of eggs and larval stages of fish. However, little is known about their complexity and the implications for dispersal of fish larvae. The study describes the complexity of the shallow nearshore environment in eastern Algoa Bay, on the south-east coast of South Africa, and its effect on larval fish ecology. An ADCP was used to assess short-term spatial current variability across the nearshore (4–20 m depth) during intensive one-day-transect profiling surveys. Data showing long-term temporal variability was collected half-hourly from a bottom-moored (15–20 m depth) ADCP over the course of one year (May 2006–May 2007). The short-term profiling revealed complexity in both current speed and direction with largely wind-driven flow patterns identified. The long-term data showed that the currents were aligned approximately west/east with prevailing winds and local shoreline. Modal speeds of 8 cm s–1 near the surface (4 m) and 6 cm s–1 near the bottom (14 m) of the water column are much lower than the average swimming speeds of postflexion larvae known to occur in the area. Mean current speed decreased with depth from ~30 cm s–1 (4 m) to ~10 cm s–1 (14 m), suggesting opportunities for depth refuge from current displacement in older larvae. Potential nett displacements were greater during the spring and summer, coinciding with peak fish breeding with passive eggs and early stage larvae travelling distances of up to 475 km near the surface of the water column. These maximum dispersal distances recorded during the present study differ considerably by half that from other calculations made for the area, highlighting a data error in a previous study. These predictions provide preliminary indications of potential transport from a point source and do not factor in anomalous local oceanography, benthic topographic impediments to transport or the complexities of larval fish behaviour.Keywords: connectivity, dispersal, ichthyoplankton, larval fish behaviour, marine protected area, nearshore currentsAfrican Journal of Marine Science 2013, 35(2): 269–28

Upwelling and ocean structures off Algoa Bay and the south-east coast of South Africa

As the Agulhas Current flows along the south-east coast of South Africa, a number of processes operate that bring cold, deep water up onto the narrow shelf. As a consequence, upwelling along the coastline is enhanced farther southward and downstream. This situation is investigated off Algoa Bay and along the south-east coast to Port Alfred, where measurements demonstrate that marked temperature variability occurs at the coastline, particularly in summer when temperature structures are more intense and easterly-component winds more common. There is no indication that upwelling is more prevalent at Port Alfred; increasing variability farther south is evident at Woody Cape/Cape Padrone, where the coastline veers westwards, forming the eastern boundary of Algoa Bay. Here it is found that, after a wind change to north-easterly, cold water is upwelled along the shoreline between 19 hours and 2.5 days later. Such upwelling progresses north-eastwards with the movement of the wind and weather systems, although colder water also moves south-westwards into Algoa Bay. Winds, currents, sea level and sea temperatures are highly correlated, with fluctuations in sea level measuring >50 cm being associated with coastal trapped waves (CTWs). Such barotropic wind-driven CTWs are frequently active during upwelling, although it is unclear whether there is any interaction between the two phenomena.Keywords: coastal trapped waves, currents, Port Alfred, temperatureAfrican Journal of Marine Science 2012, 34(4): 525–53