Satellite estimates of net community production indicate predominance of net autotrophy in the Atlantic Ocean

Abstract

There is ongoing debate as to whether the oligotrophic ocean is predominantly net autotrophic and acts as a CO2 sink, or net heterotrophic and therefore acts as a CO2 source to the atmosphere. This quantification is challenging, both spatially and temporally, due to the sparseness of measurements. There has been a concerted effort to derive accurate estimates of phytoplankton photosynthesis and primary production from satellite data to fill these gaps; however there have been few satellite estimates of net community production (NCP). In this paper, we compare a number of empirical approaches to estimate NCP from satellite data with in vitro measurements of changes in dissolved O2 concentration at 295 stations in the N and S Atlantic Ocean (including the Antarctic), Greenland and Mediterranean Seas. Algorithms based on power laws between NCP and particulate organic carbon production (POC) derived from 14C uptake tend to overestimate NCP at negative values and underestimate at positive values. An algorithm that includes sea surface temperature (SST) in the power function of NCP and 14C POC has the lowest bias and root-mean square error compared with in vitro measured NCP and is the most accurate algorithm for the Atlantic Ocean. Nearly a 13 year time series of NCP was generated using this algorithm with SeaWiFS data to assess changes over time in different regions and in relation to climate variability. The North Atlantic subtropical and tropical Gyres (NATL) were predominantly net autotrophic from 1998 to 2010 except for boreal autumn/winter, suggesting that the northern hemisphere has remained a net sink for CO2 during this period. The South Atlantic sub-tropical Gyre (SATL) fluctuated from being net autotrophic in austral spring-summer, to net heterotrophic in austral autumn–winter. Recent decadal trends suggest that the SATL is becoming more of a CO2 source. Over the Atlantic basin, the percentage of satellite pixels with negative NCP was 27%, with the largest contributions from the NATL and SATL during boreal and austral autumn–winter, respectively. Variations in NCP in the northern and southern hemispheres were correlated with climate indices. Negative correlations between NCP and the multivariate ENSO index (MEI) occurred in the SATL, which explained up to 60% of the variability in NCP. Similarly there was a negative correlation between NCP and the North Atlantic Oscillation (NAO) in the Southern Sub-Tropical Convergence Zone (SSTC), which explained 90% of the variability. There were also positive correlations with NAO in the Canary Current Coastal Upwelling (CNRY) and Western Tropical Atlantic (WTRA) which explained 80% and 60% of the variability in each province, respectively. MEI and NAO seem to play a role in modifying phases of net autotrophy and heterotrophy in the Atlantic Ocean

Surface ocean carbon dioxide during the Atlantic Meridional Transect (1995–2013); evidence of ocean acidification

“…Surface Chlorophylla (Chl surf ) was lowest in the subtropical gyres whilst higher Chl surf was found in temperate waters and the equatorial region where this coincided with carbon dioxide undersaturation (negative DfCO 2 ). These distribution patterns (Chl surf and DfCO 2 ) were consistent with the distribution of productivity which is highest in mesotrophic temperate-and equatorial-waters along the AMT track Tilstone et al, 2015). In contrast, nearequilibrium DfCO 2 was observed in the subtropical gyres consistent with lower productivity.…”

Controls on Open‐Ocean North Atlantic ΔpCO2 at Seasonal and Interannual Time Scales Are Different

“…Although the carbon‐based productivity model (CbPM; Westberry et al, ) displays negative correlation between PP and Δ p CO 2 in the subpolar region at seasonal time scales (consistent with other algorithms tested), in the subtropics and at interannual time scales CbPM‐PP is positively correlated with Δ p CO 2 (differing from other algorithms tested). For NCP at the seasonal time scale, the Li and Cassar () model agrees with the Tilstone et al () results, while the Siegel et al () model shows positive, rather than negative, correlations in two midlatitude provinces. At the interannual time scale, both the Li and Cassar () and Siegel et al () algorithms have several regions where NCP and Δ p CO 2 are not significantly correlated.…”

“…At the interannual time scale, both the Li and Cassar () and Siegel et al () algorithms have several regions where NCP and Δ p CO 2 are not significantly correlated. Where they are, the sign of the correlation is not necessarily the same as for Tilstone et al (). Although results from different satellite algorithms are not always consistent, these are the only PP and NCP estimates available at the basin‐scale and multiyear time scale that are essential for our analysis.…”

Reviews and syntheses: parameter identification in marine planktonic ecosystem modelling

“…Westberry et al, 2012;Tilstone et al, 2015). These spatially resolved estimates may help to constrain parameters of plankton respiration and remineralization rates.…”