A simple method for rapid estimation of Ecklonia maxima and Laminaria pallida biomass using floating surface quadrats

In South Africa two species of kelp, Ecklonia maxima and Laminaria pallida, occur in quantities sufficient for commercial use. Currently, the former species is harvested in large quantities (about 5 000 tonnes wet weight per year) as abalone feed. In order to set limits to the amount of kelp that may be harvested, it is necessary to obtain reasonably accurate measurements of kelp biomass. Biomass estimates are traditionally obtained by destructive sampling of kelp sporophytes from quadrats placed on the bottom. Scuba divers harvest the plants and weigh them ashore. This method is slow and labour-intensive, and severely limits the area of kelp bed that can be sampled. This study investigates several alternative methods to determine an easier, quicker method of estimating kelp biomass. First, morphometric data on both species of kelp were collected to investigate if there were easily measurable characters that correlate with the weights of individual sporophytes, which then could be used as proxies for destructive sampling. Second, floating quadrats were used to establish correlations between the density of kelp heads at the water surface during low spring tides and kelp biomass. Good correlations were found between (1) individual sporophyte weight and stipe weight for E. maxima (p = 0.0000; r(2) = 0.5693) and L. pallida (p = 0.0000; r(2) = 0.6175), and (2) individual sporophyte weight and stipe length for E. maxima (p = 0.0000; r(2) = 0.5828) and L. pallida (p = 0.0000; r(2) = 0.4817). Such measurements are time consuming and labour intensive, and require scuba and destructive sampling. However, using floating 1 m(2) surface quadrats, good correlations were found between the density of kelp heads at the surface and the biomass of kelp (E. maxima, p = 0.0000, r(2) = 0.3469; L. pallida, p = 0.0000, r(2) = 0.4785). Surface density measurements are more than eight times quicker than the traditional biomass determination method, are non-destructive, require fewer personnel, can be done by snorkelling and are unaffected by water clarity. Furthermore, harvesting for abalone feed is boat-based and targets only surface-reaching kelp. Sporophytes of E. maxima that reach the surface at low water spring tide comprise on average 62% of the total biomass of kelp in these beds, so scuba should still be used where observations and measurements of subcanopy biota are required.