Abstract
Storm events can have strong effects on marine biological dispersal. Here, we used geological evidence to assess the speed of biological rafting driven by a major cyclonic storm (Cyclone Gita) in southern New Zealand during February 2018. We undertook a comparative analysis of coastal rock types to assign beach-cast macroalgal specimens to their geological source regions. Approximately 10% (23 of 250) of Lepas-encrusted beach-cast kelp (Durvillaea) specimens sampled from Dunedin beaches in the immediate aftermath of Cyclone Gita (22 and 23 February 2018) bore holdfast-clasts consistent with origins in the distant Fiordland region of southwestern South Island. Based on Lepas barnacle growth data, we estimated that this distinctive 'pulse' of rafted material achieved the 350 to 500 km journey from Fiordland to Dunedin in as little as 10 d, with many of these rafts thus inferred to have travelled 30 to 50 km d(-1). These rapid rafting events were facilitated by westerly gales in Foveaux Strait (12 to 17 February), followed by cyclonic southeasterly winds off eastern South Island (19 to 21 February). The rafting-speed estimates from this study are substantially faster than mean current-flow estimates for the same coastal region (12 to 20 km d(-1)), and also exceed the speeds of previously documented kelp-rafting events in this region (estimated 10 to 15 km d(-1)) and elsewhere. Together, these data highlight the key role of storms in driving 'wind slippage' of buoyant material, facilitating pulses of dispersal at speeds far exceeding the predictions of oceanographic models.