Upwelling intensity modulates N₂O concentrations over the western Indian shelf

Abstract

Repeat measurements of dissolved nitrous oxide (N2O) along two transects of the western continental shelf of India in 2012 revealed high concentrations of 45 ± 32 nM (off Kochi) and 73 ± 63 nM (off Mangalore) during the summer monsoon (SM). N2O concentrations increased nonlinearly during the peak of the SM upwelling, when low O2 (<25 µM) conditions prevailed in the water column. Off Kochi, N2O levels fell gradually from the fall intermonsoon (20 ± 8 nM) to the winter monsoon (8.8 ± 2 nM) and remained low (9.2 ± 5.2 nM) through the spring intermonsoon season. The N2O supersaturation off Kochi (574 ± 720%) was presumably due to its high yield during sediment denitrification, whereas the higher N2O supersaturation observed off Mangalore (1046 ± 885%) was due to its production during denitrification in both the anoxic water column and the underlying sediments. Such distinctive biogeochemical behavior between these two shelf segments is at first augmented by the natural origin of intense upwelling at Mangalore relative to Kochi wherein suboxic to anoxic oxygen minimum zone waters spread from offshore to the shelf of Mangalore, over which the runoff and terrestrial nutrients supply acts in unison. Following new zonal extrapolation approach, our revised estimate of N2O effluxes from the southwestern Indian shelf (7–14°N) was four times lower (0.019–0.039 Tg y−1) than previous estimate. Nevertheless, further studies are needed to refine the N2O effluxes from the entire western Indian shelf to monitor its modification due to expansion/intensification of the coastal low‐O2 zones and also to ascertain its actual N2O contribution to the world oceans.

N2 Fixation in the Eastern Arabian Sea: Probable Role of Heterotrophic Diazotrophs

“…Low P * at NF-a could also be either due to the consumption of phosphate by diazotrophy or due to the upwelling, which starts during May as evidenced in the temperature profile at NF-a (Figure 2). Upwelling would create low oxygen just below MLD Sudheesh et al, 2016), as observed at NF-a (Figure 2). However, high P * -values would have been expected in low oxygen water but deeper water, as evidenced from the P * profiles (Table 1), has low P * -possibly because of the remineralization of diazotrophy dominated organic matter (high N:P ratio) in the deeper depths.…”

Present past and future of the OMZ in the northern Indian Ocean

“…However, this was so far the only report on the occurrence of hydrogen sulfide in Arabian Sea (Naqvi et al, 2000;Swallow, 1984) until Naqvi et al (2000) discovered an anoxic event that developed along the western Indian coast off Mumbay during the late summer and autumn in 1999. Such strong events seem to emerge not every year (Gupta et al, 2016;Sudheesh et al, 2016) but mass mortalities occur, in between, also along the eastern Indian coast in the Bay of Bengal (Altieri et al, 2017). The spreading of 'dead zones' in coastal regions is apparently a global phenomenon that does not spare the Indian coast in the Arabian Sea and the Bay of Bengal (Altieri et al, 2017;Diaz & Rosenberg, 2008).…”

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

“…The width of Cochin inlet is about 450 m, and Azhikode has a narrow width of 250 m respectively. Due to anthropogenic activity, Cochin estuary receives 1.04 × 10 5 m 3 of untreated industrial effluents and 260 m 3 of domestic or sewage wastes, which alters the bio-geochemical properties of coastal water off Cochin (Gupta et al, 2016;Sudheesh et al, 2016). The coastal waters off Cochin in SEAS is typical optically complex Case 2 water (Minu et al, 2014).…”

“…Table 1 presents the variability in water quality and optical properties during SIM, SWM, FIM and WM along the coastal waters off Cochin. The Barmouth region (B1 station) was influenced by freshwater discharge from Cochin estuary during SWM (Gupta et al, 2016;Sudheesh et al, 2016) and accordingly in our study, salinity showed a lower value at B1 station ranged between 13.43-31.62 (psu) with an average value of 22.53 ± 12.87 (psu) during SWM and similarly salinity showed a higher value at B3 station ranged between 33.62-34.53 (psu) with an average value of 33.93 ± 0.52 (psu) during WM. The value of pH was higher at B4 station varied from 8.2 to 8.3 with an average value of 8.25 ± 0.07 during FIM and showed a lower value at B1 station varied from 7.5 to 7.7 with an average value of 7.57 ± 0.12 during SIM.…”

“…During FIM a significant relationship was found between Chl_a vs a CDOM (443) with an R 2 value of 0.68, while during WM, no significant relationship was found between Chl_a, a CDOM (443) and TSM. During the entire period of the study (seasonally and spatially), a weak relationship existed between Chl_a and TSM as well as TSM and a CDOM (443) which could be due to the large contribution of inorganic particles in the total suspended matter discharged from Cochin estuary (Sudheesh et al, 2016). .…”