Upwelling and fish-factory waste as nitrogen sources for suspended cultivation of Gracilaria gracilis in Saldanha Bay, South Africa

In Small Bay, Saldanha, the water becomes highly stratified in summer. The cold bottom layer (of upwelling origin) is rich in nitrogen, some of which enters the surface layer by advection. However, the surface water often bec omes warm and oligotrophic leading to poor growth or death of Gracilaria gr acilis grown in experimental suspended systems. At the same time, large qua ntities of nitrogen-rich fish waste are released at a particular site in th e bay. We tested the hypothesis that Gracilaria grown close to the site of waste release (1.5 km away and in the waste plume) would grow faster than a t the control site 3.5 km away. In October and November (early summer) 1996 , all the Gracilaria at the control site died, while growth at the fish was te site was good (between 8 and 10% day(-1)). In November-December control plants grew slightly faster than those from the waste site, in February the reverse occurred, and subsequently (March-June) growth was similar at both sites as winter winds caused mixing of the water column. These results, an d analyses of the C/N ratios of the Gracilaria tissues provide some support for our hypothesis. Also, analyses of the stable N isotope ratios in the G racilaria tissues indicate that there is considerable uptake of the fish-wa ste N even at the control site. We conclude that while proximity to the was te site may sometimes benefit the Gracilaria, the fish waste would in fact provide a significant source of N for seaweed cultivated throughout the nor thern area of Small Bay, particularly when the water is highly stratified i n summer.