Net loss rates and distribution of molecular hydrogen (H 2) in mid-latitude coastal waters

Abstract

We present the first reported net loss-rate constants of molecular hydrogen, H 2, in seawater. Net loss rates and depth profiles of hydrogen were measured in coastal seawater at two mid-latitude sites in eastern Canada: the St. Lawrence Estuary and Halifax Harbour, between November 2005 and July 2006. Net loss-rate constants ranged between 0.29 d − 1 and 6.07 d − 1 in the St. Lawrence Estuary, and from 0.14 d − 1 to 8.67 d − 1 in Halifax Harbour. The 0.2 μm−5 μm particle size fraction was associated with H 2 loss, implying bacterial consumption. There was a correlation between net loss-rate constants for H 2 and CO suggesting that both substrates shared a common sink. Dissolved hydrogen profiles measured in the St. Lawrence Estuary and Bedford Basin during winter and spring generally showed H 2 levels below atmospheric equilibrium in surface waters and declining with increasing depth. Bedford Basin surface water was supersaturated (125–257%) on sunny days in June and July suggesting that net H 2 production was linked to irradiance, although the exact nature of the source is yet to be determined. A series of light and dark incubation experiments showed lower net H 2 uptake rates in irradiated samples suggesting either H 2 photo production or light inhibition of H 2 uptake.