Thesis (MTech (Oceanography))Cape Peninsula University of Technology, 2010 / The current dynamics along the Tsitsikamma coast is described from a combination of
acoustic current measurements. satellite-tracked surface drifters and underwater
temperature recordings made between November 2006 and March 2008. The Tsitsikamma
coast is largely a Marine Protected Area (MPA) that protects a rich marine biodiversity. The
nearshore currents are important in the dispersal of eggs and larvae of many marine species.
including the paralarvae of the commercially caught chokka squid. LoNgo reynaudii. Changes
in the environment, including the currents. can affect the successful recruitment of chokka
squid, and can bring about large annual fluctuations in biomass that creates economic
uncertainty in the squid fishery. Results confirm the existence of a predominantly alongshore
current off the Tsitsikamma coast. At Middelbank eastward flow was slightly dominant, with a
percentage occurrence of 58% vs. 41% westward flow near the surface. The percentage
eastward flow decreased with depth, with 41% vs. 58% westward flow near the seabed. At
Thyspunt westward and eastward flow occurred at near equal percentages. but westward
flow was slightly dominant throughout the water column. The alongshore current was
strongest near the surface during eastward flow (maximum = 141 crn.s1: average = 27 crn.s
'). while westward surface currents were weaker (maximum velocity = 78 cm.s1: average =
19 crn.s1). Current speed generally decreased with depth and opposing surface and bottom
currents, associated with a thermal stratified water column, were occasionally recorded. The
nearshore flow regime was characterised by frequent barotropic alongshore reversals that
occurred year round. An increase in strong eastward episodes, and opposing surface and
bottom currents during spring and summer months have implications for the dispersal of
squid paralarvae during the summer and winter spawning seasons. In summer, the
combination of strong eastward pulses in the current and upwelling at the capes favoured
dispersal onto the midshelf of the Agulhas Bank. In winter, alongshore oscillations without
the offshore displacement associated with upwelling. restricted offshore dispersal which
caused surface particles to be retained inshore. Drifter trajectories show that both the
eastward and westward nearshore current can link the inshore spawning grounds with the
nursery grounds, offshore on the central Agulhas Bank; and that passive, neutrally buoyant
material in the surface layer can reach the vicinity of the cold ridge in as little as eight days.
The wind-driven processes of upwelling and coastal trapped waves (CTWs). and the
influence of the greater shelf circulation are discussed as possible driving forces of variability
in the currents off the Tsitsikamma coast. The occurrence of coastal trapped waves during
thermal stratification appears to drive the jet-like, eastward pulses in the current. and results
suggest that the propagation of CTWs may regulate and even enhance upwelling and
downwelling along the Tsitsikamma coast.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:cput/oai:localhost:20.500.11838/2035 |
| Date | January 2010 |
| Creators | Hancke, Lisa |
| Contributors | Sammaal, Y., Vi |
| Publisher | Cape Peninsula University of Technology |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Rights | http://creativecommons.org/licenses/by-nc-sa/3.0/za/ |
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