Coastal currents and temperatures along the eastern region of Algoa Bay, South Africa, with implications for transport and shelf–bay water exchange

Abstract

Boat ADCP surveys and an eight-month deployment of a 1 200 kHz ADCP were used to study the nearshore (5–25 m depth) current and temperature patterns along the eastern coastal region of Algoa Bay, on the south-east coast of South Africa. The boat surveys showed three distinct alongshore current patterns: all eastward, all westward, and a complex mix of flows. High variance in the upper layers of the water column, possibly caused by rip currents, eliminated the use of ADCP mooring data between 0–9 m, but analysis of the remainder of the data (10–19 m) indicated a dominant alongshore current alternating between eastward and westward flows, with velocities of up to 75 cm s^–1 at a depth of 10 m and 65 cm s^–1 at 19 m. Average velocities for these depths were 17.6 cm s^–1 and 10.8 cm s^–1 respectively. The frequency of eastward flow almost equalled westward flow (56% vs 47% respectively) over this period. A good correlation etween wind and current direction indicated wind to be the primary driving force of the nearshore currents in eastern Algoa Bay. Net monthly displacement in the midwater layer (10 m) ranged between 60 km and 260 km, with eastward advection in seven of the eight months under study. Net monthly displacement in the bottom layer (19 m) was eastwards for all study months, and ranged from 8 km to 296 km. These results imply a strong likelihood that neutrally buoyant, passive material such as eggs and larvae of fish and invertebrates will be exported beyond the boundaries of a proposed marine reserve (the Greater Addo Marine Protected Area) in eastern Algoa Bay and onto the Transkei shelf. Bottom temperature data from the mooring and SST satellite imagery indicated that a warm-water plume from the Agulhas Current entered Algoa Bay and influenced the coastal current and unusually increased water temperature over a period of three days. Similarly, cold water from the adjacent Port Alfred upwelling area also entered the eastern sector of Algoa Bay, more commonly than warm-water intrusions.

Keywords: Algoa Bay; Greater Addo Marine Protected Area; ichthyoplankton; nearshore currents; temperature

African Journal of Marine Science 2010, 32(1): 145–161