Respiration and vertical carbon flux in the Gulf of Maine water column

Abstract

The transport of carbon from ocean surface waters to the deep sea is a critical factor in calculations of planetary carbon cycling and climate change. This vertical carbon flux can be calculated by integrating the vertical profile of the seawater respiration rate but is rarely done because measuring seawater respiration is so difficult. However, seawater respiratory oxygen consumption is the product of the combined activity of all the respiratory electron transfer systems in a seawater community of bacterioplankton, phytoplankton, and zooplankton. This respiratory electron transfer system (ETS) is the membrane bound enzymatic system that controls oxygen consumption and ATP production in all eukaryots and in almost all bacteria and archaea. As such, it represents potential respiratory oxygen consumption. Exploiting this, we measured plankton-community ETS activity in water column profiles in the Gulf of Maine to give the potential-respiration of the water column. To interpret these potentials in terms of actual seawater respiration we made use of previous measurements of respiratory oxygen consumption and ETS activity in the Gulf of Maine to calculate a ratio of respiratory potential to actual respiration. Armed with this ratio we calculated seawater respiration depth profiles from the ETS activity measurements. These profiles were characterized by: (1) high oxygen consumption rates in the euphotic zone; (2) subsurface maxima near the subsurface chlorophyll maxima (SCM); (3) rapid declines associated with thermoclines; (4) low declining rates below 50 m; (5) and elevated values occasionally near the bottom. Sea surface values ranged from 229 to 489 pmol O2 min-1 L-1. Euphotic zone maximum values ranged from 457 to 682 pmol O2 min-1 L-1 while the minimum values below 70 m ranged from 10 to 27 pmol O2 min-1 L-1. A depth-normalized power function described the respiratory profiles between their maxima and minima. Integrating these respiratory oxygen consumption profiles from the respiratory maximum to the near bottom minimum, we calculated carbon flux profiles. The vertical carbon fluxes through the 30 m, 50 m, and 100 m levels were 3.09 ± 1.55, 1.76 ± 0.96, and 0.93 ± 0.68 mol C min-1 m-2, respectively.