Effect of Seagrass Cover and Mineral Content on Kappaphycus and Eucheuma Productivity in Zanzibar

The productivity of two species of red algae, namely Eucheuma denticulatum (Burman) Collins et Harvey and Kappaphycus alvarezii (Doty) Doty was investigated at Paje and Uroa, two sites in Zanzibar differing in seagrass cover. The growth rate (GR) of E. denticulatum in Paje (low seagrass cover) was 20–75% lower than in Uroa (high seagrass cover), depending on season. Growth of K. alvarezii in Paje was compromised by Polysiphonia spp. epiphytism. Manganese, iron, copper and zinc contents of both E. denticulatum and K. alvarezii from Paje were 35–99% lower than in E. denticulatum from Uroa. Culturing of epiphyte-infested or healthy K. alvarezii in seawater fortified with the mentioned minerals improved daily GR by 10–200 %. The effect of Cu, Zn and Mn in K. alvarezii with epiphytic infestation and ‘ice-ice’ was positive only when Provasoli solution was added. The GR of E. denticulatum was virtually unchanged by the addition of minerals. It is suggested that Fe, Zn, and especially Mn may play an important role in conferring resistance to physical stress to the macroalgae, stress which eventually makes them more susceptible to epiphytism and/or a bacterial infection that is responsible for a disease condition known as ‘ice-ice’. Ice-ice describes the texture and colour of the affected thalli. The possible role of seagrasses to the productivity of algae is also discussed.