The seasonal ecophysiology of Undaria pinnatifida (Harvey) Suringar in Otago Harbour, New Zealand

Abstract

Since the discovery of the Asian kelp, Undaria pinnatifida, in Wellington, New Zealand in 1987, interest has been expressed in the wild harvest or cultivation of this edible species. As there is little data on the ecology of this species outside its native habitat, the objectives of this study were to investigate the seasonal ecophysiology of Undaria pinnatifida in Otago Harbour, New Zealand, to enable a more informed appraisal of the potential impacts of U. pinnatifida on New Zealand's coastal ecosystems. Field investigations showed that sea surface temperature correlated with blade growth rate of Undaria pinnatifida, however, evidence for the environmental control of sporophyte senescence was equivocal. Seasonal variation in blade and stipe growth rate, and sporophyll development, produced a temporal cline of morphological traits characteristic of both f. typica and f. distans. Phenotypic response to environmental factors produced an extreme morphology of U. pinnatifida f. typica. (= f. narutensis), which was morphologically similar to U. undarioides. The seasonal growth pattern of U. pinnatifida sporophytes in Carey's Bay was typical of a seasonal annual (sensu Sears and Wilce 1975). Also, the seasonal changes in sporophyte population structure did not indicate the persistence of successive cohorts through to the next season. Recruitment of U. pinnatifida sporophytes showed a variable response to environmental factors, which may enable U. pinnatifida to colonise regions over a wide latitudinal range and environments which differ from those experienced in its endemic habitat. The development of gametophyte cultures indicated that the physiological requirements of vegetative and reproductive development occured for U. pinnatifida over the temperature range similar to that of Otago Harbour (7 - 16°C). This may indicate an adaption to sub-optimal temperatures, enabling U. pinnatifida to complete its lifehistory at temperatures which are lower than those experienced in its native habitat.