Presented dissertation consists of four chapters. First chapter debated the data obtained over a study period of four years, from 2012 to 2015, and discussed seasonal variation and distribution of seaweeds in drifted biomass. Four sites Sandspit, Kakapir, Hawksbay, and Buleji along the Karachi coast were selected. Buleji was found to be the most productive with highest diversity (52 seaweed species) while highest average biomass was recorded at Kakapir (~146 kg/m2/year). Post-monsoon season showed the highest average biomass among all sites while pre-monsoon showed maxima at Buleji only. Sargassum wightii was the most dominant species at all sites whereas Cystoseira indica was found dominating only at Buleji. To the best of our knowledge it is the first systematic report on the biomass and distribution of seaweeds drifting at Karachi coast. Highest diversity site is not necessarily showing the highest biomass. The major part of drifted seaweeds belonged to the species of sublittoral algae. As a result of survey seven major seaweeds were selected for further studies. Fresh specimens were hand-picked from drifted biomass. Chapter 2 dealt with the nutritional composition and antibacterial activity of S. wightii, S. swartzii, C. indica, Coelarthrum muelleri, Melanothamnus afaqhusainii, Solieria robusta, and Codium flabellatum. Proximate analysis covered lipids (Folch and AOAC), protein (Lowry and C/N ratio), Ash, moisture, crude fibre, pectin, and carbohydrate (difference and Dubois). Carbohydrate content was recorded high (39.01 to 118.41%) in Dubois analyses, indicating the presence of certain interfering biochemicals. Some of these were discussed to justify the false positive interferences. Hexane extract and fractions obtained from M. afaqhusainii exhibited significant antibacterial activity against MRSA and Salmonella typhi. Calorific values were also calculated and seaweeds were found as potential candidate as nutraceuticals. Chapter 3 dealt with the phycochemical analysis and pharmacological activities of selected classes of secondary metabolites including phenols, flavonoids, tannins, alkaloids, and saponins. Alkaloids were not detectable. This is the first ever report for most of the quantitative analyses of of phycochemicals e.g., saponins. Highly significant anti-inflammatory activity was recorded for the botanicals obtained from C. indica and C. flabellatum while C. muelleri and S. robusta showed delayed onset of action. All extracts exhibited significant to highly significant analgesic activity till 180 min. It can be concluded that seaweeds may possess lead molecules that can be used for therapeutic purposes. In chapter 4 antioxidant activities of six botanicals enriched with volatiles, and non-polar to moderate polar metabolites, obtained from S. wightii were performed using DPPH, ABTS, FRAP and TAC tests. Three of these fractions were prepared exclusively for fatty acid profiling. The antioxidant activity of these botanicals was found maximum for M001and H001 fractions, which showed higher activity than commonly used commercial antioxidants, BHT and BHA. These fractions were also subjected for GC-MS studies. Altogether 81 compounds were identified including 33 secondary metabolites and 48 bioaccumulated environmental pollutants. Secondary metabolites included 23 fatty acids (25, 33, 34, 37, 40, 41, 46, 47, 49, 51, 52, 54, 55, 56, 58, 59, 60, 63, 64, 65, 69, 70, and 75, structures overleaf). Of these, 37, 40, 41, 56, and 60 are identified for the first time from this species. Other natural metabolites identified included 2 fatty aldehydes; 16 and 20; 2 fatty alcohols, 68 and 73; four isoprenoids, 36, 42, 43, and 57; and 2 phytosterols, 79 and 81. All these identifications have been made for the first time from this species. S. wightii was found to bioaccumulate at least 28 petrochemicals, 14 cyclosiloxanes, and 6 phthalates. Although few hydrocarbons and phthalates are reported as natural products from seaweeds but in this study are discussed as bioaccumulated pollutants. 28 petrochemicals, included 17 aliphatic (1, 14, 15, 19, 22, 24, 27, 29, 30, 31, 38, 39, 45, 62, 74, 77, and 80); 6 aromatic (2, 7, 8, 11, 12, and 13); and 5 oxygenated hydrocarbons (4, 5, 6, 9 and 10). 9 and 10 could be the oxidative biotransformed product of ketones 3 and 4, respectively, suggesting that S. wightii is metabolising assimilated ketones. 14 organocyclosiloxanes included 3, 17, 21, 23, 28, 35, 48, 53, 61, 66, 67, 72, 76, and 78 while 6 plasticizers identified were 18, 26, 32, 44, 50, and 71.