Abstract
As part of the OSMOSIS project, a fleet of gliders surveyed the Porcupine Abyssal Plain site (Northeast Atlantic) from September 2012 to September 2013. Salinity, temperature, dissolved oxygen concentration and chlorophyll fluorescence were measured in the top 1000 m of the water column. Net community production (N) over an annual cycle using an oxygen-budget approach was compared to variations of several parameters (wind speed, mixing layer depth relative to euphotic depth, temperature, density, net heat flux) showing that the main theories (Critical Depth Hypothesis, Critical Turbulence Hypothesis, Heat-flux Hypothesis) can explain the switch between net heterotrophy to net autotrophy in different times of the year, The dynamics leading to an increase in productivity were related to shifts in regimes, such as the possible differences in nutrient concentration. The oxygen concentration profiles used for this study constitute a unique dataset spanning the entire productive season resulting in a data series longer than in previous studies. Net autotrophy was found at the site with a net production of (6.4 ± 1.9) mol m−2 in oxygen equivalents (or (4.3 ± 1.3) mol m−2 in carbon equivalents). The period exhibiting a deep chlorophyll maximum between 10 m and 40 m of depth contributed (1.5 ± 0.5) mol m−2 in oxygen equivalent to the total N. These results are greater than most previously published estimates.
Highlights
Marine net biological production (N) is the balance between oxygen (O2) production by phytoplankton during photosynthesis and O2 consumption during respiration by the entire marine community
Net community production (N) over an annual cycle using an oxygen-budget approach was compared to variations of several parameters showing that the main theories (Critical Depth Hypothesis, Critical Turbulence Hypothesis, Heat-flux Hypothesis) can explain the switch between net heterotrophy to net autotrophy in different times of the year, The dynamics leading to an increase in productivity were related to shifts in regimes, such as the possible differences in nutrient concentration
The distribution of oxygen measured by the gliders at Porcupine Abyssal Plain (PAP) between September 2012 and September 2013 is plotted against time and depth in Fig. 2, along with oxygen saturation (s(O2), ratio of c(O2) over csat (O2)) and Apparent Oxygen Utilization (AOU, difference csat(O2) – c (O2))
Summary
Marine net biological production (N) is the balance between oxygen (O2) production by phytoplankton during photosynthesis and O2 consumption during respiration by the entire marine community. By causing supersaturation or undersaturation of surface waters, biota is able to drive fluxes between the ocean and the atmosphere. This makes the ocean a carbon sink or a source depending on biological activity, which is important for estimating the global carbon budget and understanding how CO2 influences climate as a greenhouse gas (Falkowksi, 1998). There are challenges in separating the influence of biological and physical processes on in situ measurements (Hamme and Emerson, 2006; Emerson et al, 2008) It is not even clear what some of the methods used are measuring (Regaudie-de-Gioux et al, 2014)
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