Seagrass Biomass and Productivity in Seaweed and Non-Seaweed Farming Areas in the East Coast of Zanzibar,

Abstract

Seagrass beds are often subjected to stress resulting from natural and human activities. In this study, the shoot density, biomass and growth characteristics of Thalassia hemprichii and Enhalus acoroides were measured to assess the impact of seaweed farming activities on seagrass meadows at Marumbi, Chwaka Bay and Jambiani in the East Coast of Zanzibar. There was significantly higher T. hemprichii shoot density in non-seaweed areas compared to seaweed farmed areas. However, there were no significant differences in E. acoroides shoot density between seaweed and non-seaweed areas and between the two sites. Also, there was significantly higher total biomass of T. hemprichii in non-seaweed areas compared to seaweed areas. However, there were no significant differences in the total biomass of E. acoroides between seaweed and non-seaweed areas and among the sites. The growth and photosynthetic (ETR and Fv/Fm ratios) characteristics of both T. hemprichii and E. acoroides varied inconsistently between seaweed and non seaweed areas suggesting that there is no effect on seaweed farming to the growth rate of the seagrasses. Thus, the reduced seagrass shoot density and biomass in seaweed farms compared to non-seaweed areas observed in this particular study is most likely to be due to physical disturbances in the farms such as bioturbation or deliberate removal of seagrasses by farmers.