Growth, density and biomass of Ecklonia radiata at different depths and growth under artifical shading off Perth, Western Australia.

Abstract

The standing crop and density of the kelp Ecklonia radiata (C. Ag.) J. Agardh declined with depth on limestone reefs near the coast off Perth, Western Australia. Some other macroalgae became more abundant. The plant's growth rate was determined monthly at 5 m and 10 m depth over a 16-month period. Annual frond productivity at the 5-m site was 3.5 kg dry wt m^-2 year^-1, and at the 10-m site, 2 kg dry wt m^-2 year^-1. Maximum productivity occurred in spring (Sept.-Oct.) at the 5-m site and 2 months later at the 10-m site. The two sites had similar water movement and were assumed to have similar nutrient, salinity and temperature regimes. However, irradiance at 10 m was about 10% of that at the surface, while at 5 m it was about 30%. Artificial shading of the kelp canopy at the 5-m site during winter and autumn reduced productivity to levels similar to those at the 10-m site; this suggests that irradiance may limit kelp growth at these times.