The distribution of Macrocystis (Phaeophyta: Laminariales) as a biological indicator of cool sea surface temperature, with special reference to New Zealand waters

At its geographical limits, the distribution of Macrocystis C. Agardh is related to highest summer sea surface temperatures. This paper discusses New Zealand populations of M. pyrifera (Linnaeus) C. Agardh, the distribution of which may be correlated closely with the Southland Current. In New Zealand, M. pyrifera does not persist in areas where maximum temperatures exceed 18-19°C for several days, and where the warmest monthly isothere does not exceed 16-17°C. An apparent easterly retraction of Macrocystis distribution within Cook Strait since 1942 is described, and the possibility that this may reflect slight warming in ambient sea surface temperature over the last forty years is discussed. The intolerance of the kelp to extreme temperatures above 18-19°C was confirmed by experimental transplantation of kelp plants within Queen Charlotte Sound. Collectively these observations suggest M. pyrifera may indicate patches of relatively cool water, and on a very small geographic scale. In an area such as the Marlborough Sounds, clearly close to the plant's limits of distribution, the presence of Macrocystis may be useful in considering potential sites for locating net cages for the commercial "ranching" of chinook salmon. The upper temperature limit for both kelp and salmon appear to be similar.