Generally the information on the phytoplankton community occurring in the mangrove swamps at Sandspit backwaters (Pakistan) with considerable anthropogenic activities and influx of pollutants is lacking. The dearth of information has compelled to undertake the present study to assess the community structure of phytoplankton and its variability with respect to season and environmental conditions in Sandspit mangrove swamps. In addition, it was also aimed at the measurement of diatom growth in these polluted waters. The present study thus constitutes the first conclusive report on the ecology and chemistry of semiarid mangrove habitat of Sandspit back water at Karachi, Pakistan borders the northern Arabian Sea. In this research the distribution and diversity of marine phytoplankton are investigated. The results are presented as research chapters. The general introduction (Chapter 1) informs on the mangrove ecosystem and its importance on the coastal productivity. The research conducted in Pakistan in the planktonic community in the coastal waters is also summarized. The research summarizes that mangrove areas in the back waters of Karachi have particularly been ignored and a need to study phytoplankton and their behavior in the polluted mangrove environment is emphasized. The phytoplankton community occurring at the study site was assessed qualitatively and quantitatively (Chapter 2). Species of diatoms appeared to be more diverse and dominant as oppose to dinoflagellates which exhibited very low species diversity. Phytoplankton cell density varied seasonally and their abundance ranged from 0.25 x106-7.044x106 and 0.042x106-5.172x106 cells L‾1 during high and low tide, respectively. Pennate diatoms were highly diversified (23 taxa) as compare to centric diatoms (3 taxa). The low diversity and abundance of phytoplanktons is considered as the result of organic and inorganic pollution fluxed in through Layari River into the area. Chapter 3 presents diatom community composition and environmental variables affecting them in the mangrove ecosystem during summer and winter season, 2015. Centric diatoms were dominated by Cyclotella cf. menenghiana, whereas Cyilndrotheca closterium was the xvi most abundant species in pennate group (Chapter 2). The unfavorable conditions prevailing in the region appear to control low diversity and abundance of diatom species. As there exists no information with reference to their correlation with planktonic community, the present study was therefore carried out to assess the status of commonly polluting metals and pesticides in the sediment and water (Chapter 4) with an aim to present the current status of major components of effluent being fluxed in Sandspit backwaters through Layari and ascertain its influence on the distribution, abundance and diversity of phytoplankton. In this study seven metals (Cr, Cu, Fe, Hg, Mn, Pb, and Zn) were determined at high and low tidal conditions in water and sediments. Overall it was found that Fe and Mn had high concentrations and that Cu, Cr, Zn, Pband Hg maintained low concentrations. Generally, metal concentrations in in the area were found in the following order: Fe>Mn>Cu>Cr>Zn>Pb>Hg. The pollution/comtamination levels in the backwaters due to the Layri River input were assessed using different ecological tools, such as, Contimination factor (Cf), Ecological risk factor (Er) and Pollution load index (PLI). In water at both tidal conditions contamination factor (Cf) values of Cu and Pb were dominated over Cr and Zn and categorized as very high contamination. A comparison between stations for Cf showed preponderance of Cu at St 1 and Pb at St 2. The Cf values of all other metals indicate low contamination level at all stations. This is also evident from the data of PLI values which clearly show that the mangrove habitat in the study area is not as polluted as has been observed in the previous study from same habitat, except for St 1 where PLI was > 1 indicating polluted environment. The very high Rri mainly due to the Cu contamination in water at both high and low tides was noticed. However, no ecological risk in sediment was noted. The results thus endorse that this water body as significantly contaminated and therefore, accumulation of these metals may influence the aquatic life such as seen in case of phytoplankton abundance and diversity recorded in this study. The levels of organochlorine pesticides (OCPs) remained below detection limits though the area is generalviewed asheavilypolluted. Therefore it appears that bioaccumulation and transference of these chemicals from lower to higher trophic levels is not an issue in the area and that the low productivity may be influenced by metals and/or other environmental parameters. In Chapter V in-situ incubation experiments were conducted to assess the growth rate of the diatom community during winter at Sandspit backwater generally considered as polluted. Only a few pollution-resistant diatom species appear to grow in this habitat. A total of nine species were observed in the unfractionated (control) and fractionated samples. Growth rate of seven species were recorded ranging from -1.79 - 1.43 d-1 and 1.61 - 3.21 d-1 in unfractionated and fractionated samples, respectively. Average positive growth of diatom species, on the basis of cell abundance, was 0.8 d-1 in unfractionated (control) and 1.21 d-1 in fractionated samples.Positive growth was recorded for Coscinodiscus sp. (1.43 d-1), Navicula transitans var. derasa (1.39 d-1), C. cf. meneghiniana (0.37 d-1) and Cheaetoceros closterium (0.02 d-1) in unfractionated (control) samples, and Chaetoceros compressus (2.93 d-1), Cyclotella cf. meneghiniana (0.82 d-1), Coscinodiscus sp. (0.54 d-1), and Cylindrotheca closterium (0.53 d-1) in fractionated samples. Negative growth may be attributed to high grazing pressure or slow growth which fails to compete with grazing. The diatom diversity is low and the growth of many species is compromised in this polluted environment but the resistant species grow with the rates comparable to other species reported from cleaner waters.