A study of growth and primary production in Eckloniaradiat (C.Ag.) J. Agardh (Laminariales) at a sheltered site in Port Jackson, New South Wales

Abstract

A mature closed-canopy forest of Ecklonia radiata (C. Ag.) J. Agardh at a depth of 1.5 m was studied over a period of 18 months. Growth of the primary frond was assayed using a punched-hole technique, with bimonthly sampling. Tagged plants were harvested and measurements made on age, growth, size, and dry weight (DW) of stipe and frond. Each frond was cut into 5-cm strips and the dry weight obtained. Surface area measurements were obtained in the late autumn and late spring. Extension growth of the primary frond was lowest in summer (7.6 mm · day −1) and showed a period of high activity from July (17.1 mm · day −1) to September (19.6 mm · day −1). The size and mass of the whole frond lagged behind the growth of the primary frond reaching a maximum in November or December (mean dry wt 99 g; length 46.3 cm; surface area, 7216 cm 2). It is possible, therefore, that growth of lateral meristems continued after the period of maximum growth of the primary meristem. Disregarding this possibility of error in mean growth rate and using the data on extension of the primary frond and the mean biomass of the mature regions of the frond, production varied from a minimum in May (0.27 g DW · plant −1 · day −1) to a maximum in September (0.50 g DW · plant −1 · day −1). For a mean density of mature plants in the community of 22.5 · m −2, these values lead to a primary production rate of 3.1 kg DW · m −2 · yr −1 which is similar to values found in other kelps. A second method for measuring production is suggested, which takes into account the lag in growth of secondary fronds. This method yields a similar annual mean primary production rate of 2.9 kg DW · m −2 · yr −1 but leads to higher values in spring and lower values in autumn than the first method.