Formation of anoxia and denitrification in the bottom waters of a tropical estuary, southwest coast of India

Abstract

Hydrographic characteristics of the southwest coast of India and its adjoining Cochin backwaters (CBW) were studied during the summer monsoon period. Anomalous formation of anoxia and denitrification were observed in the bottom layers of CBW, which have not been previously reported elsewhere in any tropical estuarine systems. The prevalent upwelling in the Arabian Sea (AS) brought cool, high saline, oxygen deficient and nutrient-rich waters towards the coastal zone and bottom layers of CBW during the high tide. High freshwater discharge in the surface layers brought high amount of nutrients and makes the CBW system highly productive. Intrusion of AS waters seems to be stronger towards the upstream end (~15 km), than had been previously reported, as a consequence of the lowering of river discharges and deepening of channels in the estuary. Time series measurements in the lower reaches of CBW indicated a low mixing zone with increased stratification, 3 h after the high tide (highest high tide) and high variation in vertical mixing during the spring and neap phases. The upwelled waters (O<sub>2</sub>≤40 μM) intruded into the estuary was found to lose more oxygen during the neap phase (suboxic O<sub>2</sub>≤4 μM) than spring phase (hypoxic O<sub>2</sub>≤10 μM). Increased stratification coupled with low ventilation and presence of high organic matter have resulted in an anoxic condition (O<sub>2</sub>=0), 2–6 km away from barmouth of the estuary and leads to the formation of hydrogen sulphide. The reduction of nitrate and formation of nitrite within the oxygen deficient waters indicated strong denitrification intensity in the estuary. The expansion of oxygen deficient zone, denitrification and formation of hydrogen sulphide may lead to a destruction of biodiversity and an increase of green house gas emissions from this region

Fate and source distribution of organic constituents in a river-dominated tropical estuary

“…The distribution pattern of TOC with texture is depicted in figure 2. During the investigation, TOC levels were high in the riverine (S1, S2, S3) and industrial zones (S12, S14, S15), on account of the transport of allochthonous material via terrestrial run-off (Martin et al 2010;Gireeshkumar et al 2012;Renjith et al 2012). Surficial sediments of the CES recorded a moderate level of TOC comparable to previous studies (Balachandran et al 2005;Joseph et al 2008;Martin et al 2010;Deepulal et al 2012;Gireeshkumar et al 2012;Renjith et al 2012;Akhil et al 2013).…”

“…Elevated levels of total lipids (range: 115.29-8450 mg/kg) reported in the present investigation reflected the productive nature and effective biological activity (Gremare et al 1997;Akhil et al 2013). A surplus of allochthonous OM input to the Cochin estuarine system has already been reported (Balachandran et al 2003;Babu et al 2006;Thottathil et al 2008;Martin et al 2010) and the observed data provided evidence for terrestrial input. Comparatively higher concentrations of dissolved and particulate organic carbon generated from sewage is delivered through various canals linked to Cochin estuary (Martin et al 2010).…”

“…During the investigation, TOC levels were high in the riverine (S1, S2, S3) and industrial zones (S12, S14, S15), on account of the transport of allochthonous material via terrestrial run-off (Martin et al 2010;Gireeshkumar et al 2012;Renjith et al 2012). Surficial sediments of the CES recorded a moderate level of TOC comparable to previous studies (Balachandran et al 2005;Joseph et al 2008;Martin et al 2010;Deepulal et al 2012;Gireeshkumar et al 2012;Renjith et al 2012;Akhil et al 2013). Organic matter constitutes an integral part of aquatic sediments and its distribution pattern in the study region was noted to be governed by the in situ primary production, supply rate of terrestrial materials, deposition rate and texture of the sediments.…”

Eutrophication induced changes in benthic community structure of a flow-restricted tropical estuary (Cochin backwaters), India

“…1). The backwaters sustained rich bio-resources in the pristine condition, but has undergone ecological degradation due to increased industrialization and urbanization (Arun 1998;Menon et al 2000;Martin et al 2010). The wetland area has reduced from 365 km 2 in the beginning of the nineteenth century to 256 km 2 due to large-scale reclamations (Gopalan et al 1983).…”

Distribution and sources of aliphatic hydrocarbons and fatty acids in surface sediments of a tropical estuary south west coast of India (Cochin estuary)

“…During the last two decades, the Cochin estuary has also received a high influx of anthropogenic nutrients, heavy metals and organic matter from increased agricultural activities, domestic sewage inputs, industrial effluents and marine fish farming (Qasim 2003;Balachandran et al 2005;Thomson 2002). There are many data sets available on nutrients and heavy metal contamination of Cochin estuary (Martin et al 2008;Martin et al 2010). Although there have been numerous works that attempted to tackle difficulties related to the source and fate of organic matter in estuarine systems, our understanding remains limited regarding the Indian estuaries.…”