Studies on phytoplankton in the near shore waters of Zanzibar.

The phytoplankton biomass estimated by chlorophyll a' and the phytoplakton community composition were determined for over a one year period in four stations representing a range of ecosystems (namely: station I = Mangrove area, station II = Seagrass bed area, station III = Coral reef area and station IV = Open water area) of the coast of Unguja Island, Zanzibar. Some meteorological and oceanographical parameters were collected from various sources. In addition, photosynthesis experiments were done to assess the Trichodesmium photosynthetic efficiency under varying irradiances using a "light pipette". During the period of March 1994 to April 1995 samples were regularly collected twice a month from each station. Ihiplicate samples were collected during high tide at around noon for chlorophyll a', phytoplankton cell / trichome count and phosphate analysis. Temperature, salinity, pH, turbidity and light intensity measurements were also regularly taken during this period. At station I, II and III only surface samples (1 m depth) were collected while at station N subsurface samples (20 m depth) were also collected (using a 1.51 water sampler) in addition to surface samples. In May 1995 samples for Trichodesmium photosynthesis experiments were collected from station III at around noon and analyzed within two hours. A total of 192 phytoplankton species distributed over 76 genera were identified. Seven phytoplankton species were recorded for the first time in the Tanzania coastal waters and some potentially harmful microalgae were found. Also three types of symbiotic consortia in the microplankton were observed and discussed. The phytoplankton species composition was noted to vary among stations. Phytoplankton composition at stations I and II was found to be predominantly composed of the smaller pennate diatoms such as Tropidoneis spp., Nitzschia spp., Pleurosigma spp., Climacasphenia spp. and Amphora spp. At stations III and IV the phytoplankton community composition was predominantly composed of the larger centric diatoms such as Chaetoceros spp., Rhizosolenia spp., Coscinodiscus spp., Bacteriastrum spp. and Leptocylindrus spp. Smaller dinoflagellates such as Prorocentrum spp. were found mainly at stations I and II while the larger dinoflagellates such as Ceratium spp. and Protoperidinium spp. were found mainly at stations III and ICI. The common cyanobacteria species at station I were the Oscillatoria spp., Schizothrix spp. and Microcoleus lyngbyaceus, while at stations II, III and IV the common cyanobacteria were the Trichodesmium species. The cyanobacterial trichome numbers were significantly more abundant during the northern monsoon at stations II, III and IV (U = 120, p = 0.005; U =140, p < 0.001 and U =120, p = 0.005, respectively). A bloom of Trichodesmium species was encountered in April at station IV. In general, diatoms and some dinoflagellates were common throughout the year but with succession of species composition. This was more evident at stations I and II which were found to have higher fluctuations of species composition due to greater fluctuations of environmental factors observed at these stations. The biomass (chlorophyll a') was significantly higher at station I when compared to stations II, III and N (Xr2 = 43.416, p = 4.47e - 07). This was attributed to higher nutrient concentrations observed at this station. The biomass at the sub-surface waters was consistently higher compared to that of the surface waters. The nanoplankton percentage biomass was highest at station I and lowest at station IV. It appeared that the nanoplankton were responsible for most of the plankton primary production in the near shore waters of Zanzibar. The phytoplankton biomes did not show significant differences between the northern and southern monsoon periods at all stations. However, there was an increase in zooplankton numbers during the northern monsoon and a slight elevation of chlorophyll a' concentration during the rainy period was observed. Also, fluctuations of chlorophyll a concentrations were observed on a short time scale. The oxygen production during the photosynthesis experiments showed typical photosynthesis versus irradiance curves. The mean (±8 SD; n = 6) light compensation point of the phytoplankton community was found to be 133.7 ± 52.6 µE m-2s-1. The mean Ik obtained in the photosynthesis experiments was 326.5 ±58.6 µEm-2s-1. The mean light intensity at which the phytoplankton were found to have maximum photosynthesis was 1078.7 ± 271.3 µEm-2 s-1. The mean maximum photosynthesis rate was found to be 0.59 µmol 02 /mg chl. 'a' / min as the light intensity was changed up and 0.82 µmol O2 / mg chl. 'a' /min as the light intensity was changed down. At higher intensities (about 1250 uEm- Zs') photo-inhibition was observed. Inter-experimental variations in light requirements for compensation and saturation and in the maximum photosynthesis were observed. Also intra-experimental variations in light requirements for compensation and saturation and in the maximum photosynthesis were observed. The thesis is presented as follows: General introduction and literature review, methods and materials, results and discussion sections. At the end there are recommendations for future work.