Distribution,biomass and production of Gracilaria in Lüderitz Bay,Namibia

The extent and biomass of the Gracilaria beds in the Lüderitz Bay area, Namibia were surveyed in winter and summer. The beds covered most of the suitable sediments available (grain size 106–212 μm) and did not occur in water shallower than 1 m or deeper than 11 m. Total surface area, most of which occurred in the Bay system, changed very little from winter to summer but biomass increased by 3.5 fold. In winter, total Gracilaria biomass was 200 t and 650 t and in summer, 150 t and 2850 t respectively in the Lagoon and Bay. Maximum biomass occurred at 3–4 m in the Lagoon and 5–6 m in the Bay in both winter and summer. Gracilaria beach cast data from 1987 to 1991 for the area were analysed for seasonality and, depending on the specific site, correlated with swell, southerly wind velocity and sea temperature. Beach cast in the Lagoon system is a more continuous process than in the Bay, where beach casts at most sites follow an annual pattern. Wind driven currents and chop were the most important causative parameters of beach cast in the Lagoon whereas the depth at which the plants were growing and swell were more important in the Bay system. The two systems viz. Lagoon and Bay are independent of one another with very little or no exchange of Gracilaria between them. This was evidenced by the similar percentage each contributes to the total beach cast and the similar pattern of beach cast from year to year. The decrease in total beach cast from 1989 to the end of the study period was attributed to a trend towards increasing swell height and low sea temperatures in 1991.