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

Vishnu, P.S.; Shaju, S.S.; Tiwari, Surya Prakash; Menon, Nandini; Nashad, M.; Joseph, C. Ajith; Raman, Mini; Hatha, Mohamed; Prabhakaran, M.P.; Mohandas, A.

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

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pagination_JAG_1539_3.pdf (1.84 MB)

Type

Article

Authors

Vishnu, P.S.
Shaju, S.S.
Tiwari, Surya Prakash

Menon, Nandini
Nashad, M.
Joseph, C. Ajith
Raman, Mini
Hatha, Mohamed
Prabhakaran, M.P.
Mohandas, A.

KAUST Department

Biological and Environmental Sciences and Engineering (BESE) Division
Red Sea Research Center (RSRC)

Online Publication Date

2017-12-15

Print Publication Date

2018-04

Date

2017-12-15

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 (SCDOM) 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 SCDOM with lower aCDOM (443) indicates the photo-degradation affects the absorption characteristics of CDOM. The observed nonlinearity between Chl_a and the ratio of phytoplankton absorption aph (443)/aph (670) indicating the packaging effect and changes in the intercellular composition of pigments. During the study period, aph (670) was strongly correlated with Chl_a than aph (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 (ad) 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.

Citation

Vishnu PS, Shaju SS, Tiwari SP, Menon N, Nashad M, et al. (2018) Seasonal variability in bio-optical properties along the coastal waters off Cochin. International Journal of Applied Earth Observation and Geoinformation 66: 184–195. Available: http://dx.doi.org/10.1016/j.jag.2017.12.002.

Acknowledgements

This study was funded by Space Application Centre (SAC), Indian Space Research Organization (ISRO), Ahmedabad. Second author thanks Partnership for Observation of Global Oceans (POGO) Secretariat, PML, UK for the fellowship during the study period. We thank the Department of Marine Biology, Microbiology and Biochemistry, Cochin University of Science and Technology (CUSAT) and Director, Nansen Environmental Research Centre (NERCI), India for the facilities provided. We are grateful to the staff of trawler Santa Cruz for the free help during the sampling period. We are thankful to Dominic Ricky Fernandez for the valuable assistance in current computation. We are grateful to the anonymous reviewers for their valuable comments, which helped to improve the structure and content of this paper.