Seasonal variation in optically active substances at a coastal site along western Bay of Bengal

Abstract

Optically active substances (OAS) such as chlorophyll-a (chl-a), chromophoric dissolved organic matter (CDOM) and total suspended matter (TSM) play significant role in health assessment of aquatic ecosystem. Temporal variability in OAS plays important role in modulating coastal water ecology. This demand for continuous monitoring of OAS in coastal waters. The present study highlights on temporal variability of OAS off Gopalpur, a coastal site along the north-western Bay of Bengal. The OAS were found to be having strong seasonal influence apart from large variability in concentration. Chl-a concentration showed fourfold variability (0.03–12.29 mg m−3) with seasonal trend of pre-monsoon > post-monsoon > monsoon. Absorption due to CDOM at 440 nm (aCDOM440) varied between 0.02 and 4.48 m−1 following seasonal trend of pre-monsoon > monsoon > post-monsoon. TSM concentration was ranged within 0.1–28.21 mg l−1 showing maximum during monsoon and minimum during post-monsoon season. The higher concentration of chl-a and aCDOM440, during pre-monsoon season, was predominantly due to ecosystem disrupting red tide event of Noctiluca bloom during pre-monsoon season of 2014. The high load of TSM during monsoon was due to increased river influx attributed to upstream precipitation. There was no significant relation observed among the OAS indicating that multiple sources of OAS in optically complex waters of the north-western Bay of Bengal. The study provides understanding on long-term variations in OAS which is essential for development or tuning of bio-optical algorithms for accurate remote estimation of geophysical products.