Transport of drifting fucoid algae: Nearshore transport and potential for long distance dispersal

Abstract

Processes responsible for transporting detached macroalgae through the nearshore environment and offshore to where long distance dispersal (LDD) can occur have rarely been examined. Here, we test the influence of nearshore winds, tidal currents and position of release (low, mid or high tidal zone) on the dispersal of drifting fucoid algae were tested. “Drift sets” (tagged Hormosira banksii, Durvillaea antarctica, Cystophora torulosa and GPS drifters) were tracked over single tidal cycles. Wind direction had the greatest effect on movement of drift sets, but interacted with tidal direction. Overall, offshore winds and outgoing tides were most favourable for LDD, but their effect differed between species. Approximately 90% of H. banksii, D. antarctica and GPS-tracked drifters were beach-cast after one tidal cycle during onshore winds, while 19% were beach-cast during offshore winds. In contrast, 50–75% of C. torulosa were beach-cast after one tide, regardless of wind direction. Displacement of drifters was affected by tidal zone of release, but interacted with wind and tidal direction. Drifting velocities varied between drifter types, with surface drifters travelling further and faster than the benthic-drifting species. Analysis of 20years of wind data found seasonal differences in the percentage of hourly winds, with greater periods of south-westerly and north-westerly winds, and fewer onshore north-easterly winds, during autumn and winter periods. Conditions for successful offshore dispersal from the major algal dominated peninsulas of southern New Zealand are, therefore, more likely to occur if detachment of algae occurs during outgoing tides in autumn and winter.