This study investigated the effects of nitrogen and phosphorus on the growth and ecophysiology of a number of dominant species of macroalgae at a site in Port Phillip Bay (PPB), a large shallow water marine embayment located on the central southern coast of Victoria, Australia. This thesis investigated the physiological processes (i.e. photosynthesis, growth, nutrient uptake) of three species of macroalgae, Hincksia sordida (Harvey) Clayton (Phaeophyta), Polysiphonia decipiens Montague (Rhodophyta) and Ulva sp. (Chlorophyta) in response to a range of environmental regimes.