Strong seasonal upwelling, downwelling, changes in current patterns and the volume of freshwater discharge from Cochin Estuary defines the coastal waters off Cochin. These coastal waters were investigated through monthly sampling efforts during March 2015 to February 2016 to study the seasonal and spatial variability in bio-optical properties for the four different seasons mainly Spring Inter Monsoon (SIM), South West Monsoon (SWM), Fall Inter Monsoon (FIM) and Winter Monsoon (WM). The Barmouth region is the meeting place where freshwater from Cochin Estuary directly enters to the sea through a single narrow outlet, was dominated by highly turbid waters during the entire period of study. Among the four seasons, chlorophyll a (Chl_a) concentration showed a high value during SWM, ranged from 2.90 to 11.66 mg m −3 with an average value of 6.56 ± 3.51 mg m −3. During SIM the distribution of coloured dissolved organic matter (CDOM) is controlled by decomposition of phytoplankton biomass and the river discharge, whereas during SWM the temporal distribution of CDOM is controlled only by river discharge. The highest value for CDOM spectral slope (S CDOM) was observed during SWM, ranged from 0.013 to 0.020 nm −1 with an average value of 0.015 ± 0.002 nm −1. During WM, the high S CDOM with lower a CDOM (443) indicates the photo-degradation affects the absorption characteristics of CDOM. The observed nonlinearity between Chl_a and the ratio of phytoplankton absorption a ph (443)/a ph (670) indicating the packaging effect and changes in the intercellular composition of pigments. During the study period, a ph (670) was strongly correlated with Chl_a than a ph (443), which explains the accessory pigment absorption dominating more than Chl_a in the blue part of the spectrum. Similarly, the results obtained from seasonal bio-optical data indicating that Chl_a significantly contributes light attenuation of the water column during SIM, whereas detritus (a d) significantly contributes light attenuation during SIM and WM. During the study period, the relative absorption of detritus materials dominates the relative absorption of phytoplankton and CDOM at 443, 555 and 670 nm wavelengths.

h i g h l i g h t s• This study presents the first in-situ bio-optical data for the Coastal Waters Gulf of Mannar and Palk Bay.• High temporal and spatial variability in the absorption components were observed in the present work.• The biogeochemistry and phytoplankton biomass of the effect of Trichodesmium bloom on the Coastal Waters Gulf of Mannar and Palk Bay were studied. a b s t r a c tMarine 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 mariculture practice makes the coastal waters eutrophic due to increased input of nitrogen and phosphorous, ultimately leading to bloom. A phytoplankton bloom dominated by Trichodesmium species was detected outside mariculture cages located in Gulf of Mannar during August, 2015, which possibly interfered with fish gill function. High nutrient and chlorophyll a (Chl-a) (29.97 mg/m 3 ) concentration were observed during peak bloom period. 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 bloom of Trichodesmium. In-situ bio-optical measurements were also made to understand the spatial and temporal variation and effect of bloom on 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.

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Monitoring the spatial distribution and seasonal dynamics of phytoplankton functional types (PFTs) in coastal oceans is essential for understanding fisheries production, changes in water quality, and carbon export to the deep ocean. The launch of new generation ocean color sensors such as OLCI (Ocean Land Color Instrument) onboard Sentinel 3A provides an unprecedented opportunity to study the surface dynamics of PFTs at high spatial (300 m) and temporal (daily) resolution. Here we characterize the seasonal dynamics of the major PFTs over the surface waters of the west coast of Canada using OLCI imagery and Chemical Taxonomy (CHEMTAX, v1.95) software. The satellite-based approach was adapted from a previously proven Empirical Orthogonal Function (EOF)-based algorithm by using a local matchup dataset comprising CHEMTAX model output and EOF scores derived from OLCI remote sensing reflectance. The algorithm was developed for the following PFTs: diatoms, dinoflagellates, dictyochophytes, haptophytes, green algae, cryptophytes, cyanobacteria, raphidophytes, and total chlorophyll-a (TChla) concentration. Of these PFTs, first level evaluation of the OLCI-derived retrievals showed reliable performance for diatoms and raphidophytes. The second level of validation showed that TChla had the best performance, and green algae, cryptophytes, and diatoms followed seasonal trends of a high temporal resolution in situ CHEMTAX time-series. Somewhat reduced correspondence was observed for raphidophytes. Due to their low contribution to the phytoplankton community (26%) and low range of variation, weak performance was noted for haptophytes, dictyochophytes, cyanobacteria, and dinoflagellates. The EOF-based PFT maps from daily OLCI imagery showed seasonal spring and fall diatom blooms with succession from spring blooms to high diversity flagellate dominated summer conditions. Furthermore, strong localized summer raphidophyte blooms (Heterosigma akashiwo) were observed, which are a regionally important harmful species. Overall, this study demonstrates the potential of the OLCI in deriving the surface dynamics of major PFTs of the Strait of Georgia (SoG), a critical habitat for the juvenile Pacific Salmon.

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