Seasonal variability in bio-optical properties along the coastal waters off Cochin

Abstract

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.

Light Absorption Budget in a Reservoir Cascade System with Widely Differing Optical Properties

“…The a cdom (443) from the entire cascade ranged from 0.21 m −1 to 3.17 m −1 (Table 2), which is similar to other published studies for aquatic systems [12,16,31,[50][51][52]. The highest a cdom magnitudes were found in BAR1 and IBI1 (sampled curve in Figure 5c).…”

“…Much attention has been devoted to characterizing aquatic systems and determining the spatial variability of IOPs in the ocean [5][6][7][8][9], coastal environments [10][11][12], and bays [13,14]. For inland waters, efforts have been made to characterize Australian [15], African [16], and Brazilian reservoirs [3,17] to identify the optical differences between open oceans and freshwater systems.…”

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

“…During August, a* ph (λ) values showed higher values than the bloom stations and observed a non linearity between a* ph (λ) and Chl-a. The general trend of a* ph values is to increase with decreasing Chl-a, as most attenuation occurs in the blue part of the spectrum (Bricaud and Stramski, 1990), but this relation is not always linear as it depends on the pigment packaging effect and pigment composition (Shaju et al, 2015;Vishnu et al, 2018). The pigments in the algae include Chl-a and phycoerythrin, which absorb the energy from sunlight and drives photosynthesis (Siddiqui et al, 1991).…”

“…Monitoring of mariculture sites using remote sensing could be successful only after thorough validation with in situ data, as the coastal waters are optically complex (Vishnu et al, 2018). Assessment of in-situ phytoplankton and their optical properties in the coastal waters can generate database required to develop regional bio-optical algorithms for retrieving chlorophyll a (Chl-a) from satellite remote sensing data (Lee et al, 2011).…”

“…Assessment of in-situ phytoplankton and their optical properties in the coastal waters can generate database required to develop regional bio-optical algorithms for retrieving chlorophyll a (Chl-a) from satellite remote sensing data (Lee et al, 2011). Information on bio-optical measurements from the western and eastern coastal waters of India are available (Menon et al, 2011;Tilstone et al, 2013;Shaju et al, 2015;Vishnu et al, 2018;Pandi et al, 2013Pandi et al, , 2017, but no reports from the Gulf of Mannar, where cage mariculture is very popular. In this study an attempt was made to analyse the biogeochemistry, bio-optical properties and phytoplankton composition during and after a phytoplankton bloom at a prominent mariculture site in the coastal waters of Gulf of Mannar, east coast of India.…”