Nitrate reducing activity pervades surface waters during upwelling

• Hydrography indicated onset of upwelling off Trivandrum (TVM) but not at Kochi (KCH).
• Upwelled nitrate off TVM intensified nitrate reducing activity (NRA).
• Nitrification/NRA was lower off TVM than at KCH indicating a shift towards reduction.
• Such bacterial activity could be in response towards restoration of homeostasis.

Nitrate reducing activity (NRA) is known to be mediated by microaerophilic to anaerobic bacteria and generally occurs in the sub-surface waters. However, we hypothesize that NRA could become prominent in the surface waters during upwelling. Hence, we examined nitrification and nitrate reduction along with hydrographic and environmental parameters off Trivandrum and Kochi, south-west-India in June 2010. Shoaling isolines of temperature, density, and nutrients revealed the onset of upwelling off Trivandrum. Shoaling of these signatures was absent in the northern transect off Kochi. The degree of nutrient consumption (DNC) was low emphasizing the presence of newly upwelled water off Trivandrum. A significant increase in NRA (df = 1, p < 0.05) was observed off Trivandrum than at Kochi. Moreover, as hypothesized, NRA at Trivandrum was pronounced at the surface with a maximum rate of 0.85 (± 0.02) μmol L1 h− 1 nearshore which was ~ 29 × higher than that at Kochi. Further, an inverse relationship between NRA and NO3− concentration (n = 34, r = − 0.415, p < 0.01) suggested transformation of the upwelled nutrient. Nitrification/NRA was ~ 10 × lower at 0.28 off Trivandrum indicating a discernible shift towards reduction. Such contribution from bacterial activity could be a response towards restoration of homeostasis.