The coast of Pakistan (North Arabian Sea) is under the influence of seasonal cycle of the monsoons which may result in two periods of elevated phytoplankton productivity, one during the northeast monsoon (December-February) and the other during the southwest monsoon (May-September). The book deals with the qualitative and quantitative characteristics of phytoplankton community in the Miani Hor lagoon and adjacent open sea off Pakistan. During study the elevated primary productivity in the Miani Hor Lagoon and adjacent open sea off Pakistan were found during the SW monsoon (May-September) and spring inter-monsoon (March-April), while the lowest phytoplankton abundance was recorded in NE monsoon at both collection sites. Temperature and chlorophyll-a varied significantly between the seasons, with temperature being highest during SW monsoon and chlorophyll-a during spring inter-monsoon. Diatoms showed a statistically significant positive correlation with temperature and a negative significant correlation to salinity.Lower species diversity indices in the lagoon as well as in the adjacent open waters showed that the studied areas are under the influence of eutrophication.