Dynamics and biological implications of the Port St Johns cyclonic lee eddy
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This project describes the current dynamics of the cyclonic lee eddy off Rame Head, Port St Johns and Waterfall Bluff, and how its physical forcing may influence the biological communities. Three research surveys were conducted during July 2013, December 2013 and July 2014 during which environmental data (temperature, oxygen, salinity and chlorophyll) and biological samples (zooplankton) were collected from 18 stations spanning latitudes 31.4 S - 32 S and longitudes 29.3 E - 30 E. A 300 kHz Acoustic Doppler Conductivity Profiler (ADCP) mooring deployed off Port St Johns revealed maximum current velocities 1.5 m s-1, associated with the Agulhas Current. Forty-one current reversals were observed over a 12 month period, with most of these reversals noted on the bottom boundary layer. Warming and cooling events were associated with these current reversals. Ship-ADCP measurements also revealed inshore counter-currents and cyclonic flow along the surface layers. There may have been a spin-up of a cyclonic eddy between Rame Head and Port St Johns during July (2013 and 2014), which could have influenced the productivity on the shelf during the winter months. Satellite altimetry also revealed the presence of both cyclonic and anticyclonic eddies over and offshore of the study area during the cruises.