Several aspects of the growth of the agarophyte Gelinidum sesquipedale Clemente (Thuret) were studied in sublittoral populations of the Basque coast (northern Spain) from 1981 to 1986. The standing stock of the populations varied greatly with season. During two consecutive years, 60 to 70% of summer biomass had been lost by the end of the unfavourable period of year to growth (autumn and winter). The decline in biomass was mainly due to partial losses of branches, with more branches being lost from larger fronds. Yearly variations in the standing stock were also recorded. Production estimates obtained by a destructive sampling method were not satisfactory because of the spatial heterogeneity of the populations at the three locations studied. Indirect estimation of production by measuring the growth of marked fronds in a control quadrat gave more reliable estimates. Production values of similar to 622 +/- 124, 735 +/- 147 and 615 +/- 123 g dry wt m(-2) yr(-1) were obtained at the three locations, respectively. Frond elongation rates of 7.5, 6.1, 7.5 and 7.9 cm yr(-1), respectively, were recorded during four consecutive years. In winter elongation rates of fronds were rather low. Elongation did not seem to be affected by frond length, the self-shading effect of the canopy, or changes in depth. Cuts resulting from grazing were frequent on fronds through the year, decreasing significantly with increasing depth. Sheared fronds regenerated promptly, independent of season or height at which the cut was made. For commercial purposes the best season for harvesting is summer, which is the period of maximum standing stock, infertility, and calm warm waters that provide easier working conditions for divers.
