Accumulation and physiological effects of copper and zinc on the brown macroalga 'Padina Boergesenii' and its use as a bioindicator of heavy metal contamination along the Zanzibar channel

The browm macroalga Padina boergesenii Allender & Kraft was used to assess the heavy metal contamination in the Zanzibar Channel. The algal samples were collected from 30 different stations. From Dar es Salaam to Bagamoyo on mainland Tanzania and along the West Coast of Unguja Island, Zanzibar. It was found that the mean concentrations of Mn, Zn, Cu, V, Fe and Al were higher in samples obtained from the mainland stations compared to those obtained from Zanzibar study sites. Of the mentioned metals AI followed by Fe had the highest concentration while Cd had the lowest concentration. Of the five sites with the highest metal contamination, two are found north of Dar es Salaam city, two are near the Kunduchi area and one is found near the centre of Bagamoyo town. Other elements analysed such as Cd, Cr, Ni, and As, fluctuated little and mean concentration of Cd was significantly higher in Zanzibar island side than the mainland side. Laboratory studies were conducted to investigate the ability of P. boergesenii to tolerate and accumulate different copper and zinc concentrations singly and in combination over a period of 21 days. It was found that the accumulation of both copper and zinc increased linearly with increased metal concentration in the growth media. DGR and PE inhibition increased with increase in concentration of the metals in the growth media and exposure time. From laboratory studies it is concluded that the rate of Cu and Zn accumulation in P. boergesenii thalli is directly proportional to their concentrations in growth media. However the presence of high nutrient levels in the growth media lowers metal accumulation. Similarly, Cu and Zn toxicity depends on the amount of dissolved metal in the growth media and exposure time. Generally, it is concluded that the brown algae P. boergesenii can be a useful bioindicator species for examining trends in heavy metal contamination in this region.