Variations of Colored Dissolved Organic Matter in the Mandovi Estuary, Goa, During Spring Inter-Monsoon: A Comparison With COVID-19 Outbreak Imposed Lockdown Period

Albertina Dias,Suresh Thayapurath,Siby Kurian,Anil K Pratihary

doi:10.3389/fmars.2021.638583

Abstract

Colored dissolved organic matter (CDOM) is one of the important fractions of dissolved organic matter (DOM) that controls the availability of light in water and plays a crucial role in the cycling of carbon. High CDOM absorption in the Mandovi Estuary (Goa) during spring inter-monsoon (SIM) is largely driven by both in-situ production and anthropogenic activities. Here we have presented the CDOM variation in the estuary during SIM of 2014–2018 and compared it with that of 2020 when the COVID-19 outbreak imposed lockdown was implemented. During 2020, low CDOM absorption was observed at the mid-stream of the estuary as compared to the previous years, which could be attributed to low autochthonous production and less input from anthropogenic activities. On the other hand, high CDOM observed at the mouth during 2020 is linked to autochthonous production, as seen from the high concentrations of chlorophyll a. High CDOM in the upstream region could be due to both autochthonous production and terrestrially derived organic matter. Sentinel-2 satellite data was also used to look at the variations of CDOM in the study region which is consistent with in-situ observations. Apart from this, the concentration of nutrients (NO3–, NH4+, and SiO44–) in 2020 was also low compared to the previous reports. Hence, our study clearly showed the impact of anthropogenic activities on CDOM build-up and nutrients, as the COVID-19 imposed lockdown drastically controlled such activities in the estuary.

Highlights

Dissolved organic matter (DOM) is one of the key pools of organic carbon in natural waters (Hedges, 1992), and colored dissolved organic matter (DOM) (CDOM) is that fraction of DOM that interacts with light (Blough and Del Vecchio, 2002; Nelson and Siegel, 2002; Nelson et al, 2007)
Colored dissolved organic matter (CDOM) absorption ag412 (m−1) during the spring inter-monsoon (SIM) of 2020 was the highest in the upstream region (0.827 m−1) of the Mandovi Estuary, while a decrease in absorption was observed in the mid-stream region [0.263 m−1, except at a station close to Old Goa Jetty (0.713 m−1)] followed by an increase toward the mouth (0.455 m−1) (Figure 2)
CDOM showed a spatial variation in the Mandovi Estuary during the SIM of 2020 with high absorption in the upstream region and toward the mouth, and low absorption in the mid-stream region, contrasting with the previous results

Summary

Introduction

Dissolved organic matter (DOM) is one of the key pools of organic carbon in natural waters (Hedges, 1992), and colored DOM (CDOM) is that fraction of DOM that interacts with light (Blough and Del Vecchio, 2002; Nelson and Siegel, 2002; Nelson et al, 2007). The Mandovi and Zuari estuaries form the major riverine channel in the state of Goa. The estuaries of Goa have been studied extensively for various biogeochemical parameters (Shetye et al, 2007; Anand et al, 2014; Shynu et al, 2015; Araujo et al, 2018); studies on CDOM in these estuaries are limited (Menon et al, 2011; Dias et al, 2017, 2020a). Some of the anthropogenic activities observed along the bank and within the estuaries include the pleasure cruises, floating casinos, shipbuilding/repair yards, sand mining, small and large scale industries, fish processing units, fishing jetties, mine waste rejects, transportation of mineral ores by barges, discharge of treated municipal sewage, domestic waste discharge from the houses and commercial establishments, etc. (Shynu et al, 2015; Dias et al, 2020a)

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