USE OF PLASTIC DRIFT CARDS AS INDICATORS OF POSSIBLE DISPERSAL OF PROPAGULES OF THE MANGROVE AVICENNIA MARINA BY OCEAN CURRENTS

Abstract

Plastic drift cards with the same buoyancy as propagules of Avicennia marina (Forssk.) Vierh. were used as an indicator of possible mangrove dispersal by ocean currents. The cards were dropped from an aircraft into the sea at the mouths of the Mhlathuze River off Richards Bay, the Mgeni River off Durban and the Nxaxo-Ngqusi rivers off Wavecrest on the east coast of South Africa. Details of the time and locality of strandings were recorded from cards returned. Of the 4 500 cards released, 133 (8.9%), 146 (9.7%) and 280 (18.7%) were returned respectively from the above locations. The high returns from Wavecrest were attributable to cards being washed ashore in the immediate vicinity of the dropping point. A high percentage of the cards dropped at Durban were transported northwards by the inshore counter-current. Approximately 68 and 32% of the cards recovered for Richards Bay and Durban respectively were transported by the Agulhas Current and were stranded farther south along the East and South-East coasts. Cards that reached the Agulhas mixing area were deposited ashore on the Cape Peninsula and the West Coast (3%), or were transported either across the Atlantic Ocean to South America or the Indian Ocean to Australasia (4%). Estimated transport rates of cards to South America and Australasia were similar to previously published values. The results indicate that the northern estuaries could provide propagative material over a considerable portion of the South African coast, which could result in a wider distribution of mangroves in the Eastern Cape.

Afr. J. mar. Sci. 25: 169–176