Distribution and ecology of intertidal seagrass communities at ocean road and Kunduch areas, Dar es Salaam

Intertidal macroflora, macrofauna, water column and sediment parameters were studied at sewage polluted beach (Ocean Road) and a control site (Kunduchi beach) Dar es Salaam, to evaluate the differences in species composition and nutrient levels between the two areas. A total of six sea grass species were found at Ocean Road of which Thalassia hemprichii and Cymodocea rotundata were abundant. At Kunduchi a total of eight seagrass species were found of which Syringodium isoetifolium and Thalassodendron ciliatum were predominant. Halodule wrightii and Halophila ovalis were widely distributed in the study areas. The mean biomass of sea grasses at Ocean Road area was significantly as compared to Kunduchi area (p<0.0001). A total of 25 and 24 macro algae species were found at Ocean Road and Kunduchi areas respectively with significantly higher (t=4.838; p>0.000) biomass of green macro algae in the former beach. The study revealed higher biomass of brown macro algae in Kunduchi area compared to Ocean Road (t=2.115; p=0.039). However, there was no significant difference (t=1.986; p=0.052) in the biomass of red macro algae between the two sites. Ulva retieulata, Chaetornorpha indica and Enteromorpha ramulosa were abundant at Ocean Road area. At Kunduchi, the brown alga Cvstoseira myrica was the most abundant species. Average species richness of macrofauna was 1.75 ± 0.3 and 1.33 ± 0.66 species per quadrat for Ocean Road and Kunduchi, respectively. Average density was 213.92 17.43 and 127.32 ± 28.63 individualslm2 for Ocean Road and Kunduchi, respectively. The diversity of macro fauna at Ocean Road was significantly higher than that of Kunduchi (t = 17.944, p<<0.05). The characteristics of sediment were studied in both areas. The median particle size of the sediment between these areas was significantly different (t = 11.1044, p = 0.000) with Kunduchi area having very fine sand. These results indicate that the difference in sediment particle size is among the likely causes of the differences in macrofaunai diversity observed under this study. Detectable nutrient levels were recorded in both sites with peaks in the rainy period The levels of water nitrate recorded at Kunduchi were significantly higher than those recorded at Ocean Road (t = 4.909, p = 0.000). Ammonium levels in the water column at Ocean Road were significantly higher than those of Kunduchi (t = 3.997, p = 0.000}. The differences in water column phosphate concentration between the two study areas were not significant. Measurable nutrient levels recorded in the water column at both study areas might indicate presence of a nutrient source other than the sea itself. The results indicate that elevated levels of ammonium at Ocean Road area is a likely cause of the observed higher biomass of green macro algae.