Effect of Mariculture on bio-optical properties and water quality of Gulf of Mannar and Palk Bay

Abstract Marine cage aquaculture is gaining importance in India, due to its contribution as an alternate livelihood to coastal communities and also because of its export value. Water quality is the most important determinant for sustainable marine cage farming. Nutrient enrichment in coastal waters results in increased occurrence of algal blooms. A mariculturepractice makes the coastal waters eutrophic due to increased input of nitrogen and phosphorous, ultimately leading to bloom. A phytoplankton bloomdominated by Trichodesmiumspecies was detected outside mariculturecages located in Gulf of Mannar during August, 2015, which possibly interfered with fish gillfunction. High nutrient and chlorophyll a (Chl-a) (29.97 mg/m 3 ) concentration were observed during peak bloomperiod. Three groups of phytoplankton were identified — diatoms (24 species with, 14 centric species and 10 pennate species), dinoflagellates (10 species) and cyanobacteria (one species). Stable salinity condition and the depletion in nutrient concentration due to higher primary production might have triggered the bloomof Trichodesmium. In-situ bio-optical measurements were also made to understand the spatial and temporal variation and effect of bloomon the optical components. Our study is a preliminary step to understanding the in-situ bio-geochemical and bio-optical characteristics of coastal waters of Gulf of Mannar and Palk Bay, which could aid in the management of cage culture sites.