The effect of groundwater on macrophyte community structure in selected near-shore ecosystems.

Abstract

Results summarised here are in accordance with the second objective of the GROFLO project named” to elucidate differences in near-shore community structures and ecosystem functioning in relation to groundwater outflow”. During the last six months (mid-November-mid-May) Zanzibar (Unguja island) received part of the El-nino influenced short rains (November-December) and long rains (March-May). It was at an optimal time to observe the influence of highest groundwater output and associated anthropogenic input on the near-shore community structures and ecosystem functioning. Algal blooms that are usually associated with input of additional nutrients were observed particularly in Chwaka where the highest algal blooms (2-25 kg(FW) m-2 of Ulvaceae species, 2.5 kg(FW) m-2 Gracillaria species and 3 kg(FW) m-2 Halimeda species) were recorded in May 9a. Above ground weight of Thallasia hemprichii closest to the groundwater boreholes was also highest in Chwaka (0.25 kg(FW) m-2). As a result of blooms dominated by green algae, species diversity was reduced. In Paje the number of filter feeding bivalves were highest in the area receiving groundwater.