Morphology and growth of the giant kelp Macrocystis pyrifera in New Zealand and California

At three sites (Papanui Inlet, Aquarium Point and Seacliff) on the Otago coast of New Zealand and at one site (Point Loma) off southern California, morphometric and shortterm growth measurements were made of single fronds of plants of Macrocystis pyrifera (L.) C. Ag. near the time of the autumn equinox, in 1977. Near the apex, internodes and laminae were shorter in wave-exposed Seacliff and California populations than in those of Papanui Inlet and Aquarium Point in strong currents, although lamina thickness was similar. In the immature region near the apex, linear relationships were apparent from double logarithmic plots of stipe distance from apex vs node number, internode fresh weight vs length (different in the two areas), lamina width vs length, fresh weight vs lamina area and immature lamina, pneumatocyst and internode fresh weight vs node number. Morphologically, Seacliff plants resembled those in California more closely than those in the other New Zealand populations. The relative growth rate in length of stipe internodes decreased linearly with the logarithm of the distance from the apex, but was less closely related to node number. Extrapolation gave zero growth at 1.6 m at Aquarium Point and 2.9 m in California, where the internode lengths coincided with a change in their weight/length relationship. Slow relative growth proximal to these points indicated a different type (mainly elongation). It is suggested that the first, apical, type of growth, being apparently unrelated to frond length, would be a useful measure of activity over short time-periods. The relative growth rate in lamina area (and probably volume), while also related to distance from the apex, showed a very different pattern, increasing slightly to a peak at 100 to 200 mm and declining to nil by 1 m at Aquarium Point and 2 m in California. Maximal lamina size was maintained for only 10 to 15 nodes at Aquarium Point. Calculated node initiation frequencies were 2.7 d at Papanui Inlet, 2 d at Aquarium Point, and 1 d in California. At the latter sites it took about 2 mo for laminae and 2^1/2 mo for stipe internodes to mature. Deduced relative growth rates in stipe tissue and pneumatocyst fresh weight decreased from 0.24 (log_e units per day) at the apex to 0.11 at 200 mm, where basal lamina tissue had a rate of 0.29, a doubling in just over 2 d. The production of tissue per frond was estimated as 25 and 36 g fresh wt d^-1 at Aquarium Point and in California, respectively.