Did the COVID-19 Lockdown-Induced Hydrological Residence Time Intensify the Primary Productivity in Lakes? Observational Results Based on Satellite Remote Sensing

Abstract

The novel coronavirus pandemic (COVID-19) has brought countries around the world to a standstill in the early part of 2020. Several nations and territories around the world insisted their population stay indoors for practicing social distance in order to avoid infecting the disease. Consequently, industrial activities, businesses, and all modes of traveling have halted. On the other hand, the pollution level decreased ‘temporarily’ in our living environment. As fewer pollutants are supplied in to the hydrosphere, and human recreational activities are stopped completely during the lockdown period, we hypothesize that the hydrological residence time (HRT) has increased in the semi-enclosed or closed lake bodies, which can in turn increase the primary productivity. To validate our hypothesis, and to understand the effect of lockdown on primary productivity in aquatic systems, we quantitatively estimated the chlorophyll-a (Chl-a) concentrations in different lake bodies using established Chl-a retrieval algorithm. The Chl-a monitored using Landsat-8 and Sentinel-2 sensor in the lake bodies of Wuhan, China, showed an elevated concentration of Chl-a. In contrast, no significant changes in Chl-a are observed for Vembanad Lake in India. Further analysis of different geo-environments is necessary to validate the hypothesis.

Effect of Reduced Anthropogenic Activities on Water Quality in Lake Vembanad, India

“…In this study, multi-spectral remote sensing observations were used to analyse trends in water quality parameters in Lake Vembanad, India, before, during, and after a nationally imposed lockdown (25)(26)(27)(28)(29)(30)(31) May 2020) associated with the global pandemic caused by the SARS-CoV-2. In situ samples were collected during the lockdown in May 2020 and were compared with samples collected in May 2019 as part of an ongoing study [14,26,27].…”

“…Region-specific satellite retrieval algorithms for Lake Vembanad were not available and existing algorithms were used to estimate the five different indicators of water quality. The selected algorithms have been parameterised for Sentinel-2 and Landsat-8 [16,[35][36][37] and have previously been used to study water quality in Lake Vembanad [21,25,38]. We provide further information on the comparison between the satellite and in situ observations in Section 2.4.…”

“…Other published satellite retrieval algorithms were assessed, but these did not yield a better comparison with the in situ observations of the water quality indicators. We therefore chose to use the selected satellite algorithms, considering that these algorithms have previously been used for Lake Vembanad [21,25,38] and considering that the results are presented as a relative change and separately for each observation method (remote sensing and in situ) in this study. Radiometric characterisation and cross-calibration have shown that the Sentineland Landsat-8 bands generally compared well, with the exception of the near infrared bands [47].…”

“…Yet these positive effects of reduced anthropogenic activities on the environment may locally have been countered by increased organic and inorganic domestic and medical waste and less sustainable waste management [20,23,24]. The potential effect of lockdown restrictions on marine and aquatic ecosystems in the southwest of India has recently received considerable attention [21,22,25]. Yunus et al [21] showed that total suspended matter, a key indicator of water quality, decreased during the initial phase of the lockdown (April 2020) in various zones in Lake Vembanad, yet concentrations were not necessarily the lowest recorded in April between 2013-2020.…”

“…Yunus et al [21] showed that total suspended matter, a key indicator of water quality, decreased during the initial phase of the lockdown (April 2020) in various zones in Lake Vembanad, yet concentrations were not necessarily the lowest recorded in April between 2013-2020. Avtar et al [25] showed that chlorophyll-a concentrations increased in enclosed lakes in Wuhan, China, due to lower disturbance and increased hydrological residence time, but this was not evident in Lake Vembanad. Mishra et al [22] also showed a lack of change in chlorophyll-a concentrations in the coastal waters of southwest India, in contrast to other regions along the coast of India where a decrease in chlorophyll-a was linked to lower atmospheric deposition of nitrogen during the lockdown.…”