Ocean colour estimates of particulate organic carbon reservoirs in the global ocean – revisited

Abstract

We have examined 16 years (1998–2013) of particulate organic carbon (POC) concentrations derived from remotely sensed ocean colour. POC concentrations vary spatially from more than 300 mg m⁻³ in the northern North Atlantic in summer to about 20 mg m⁻³ in the oligotrophic South Pacific (16-year global average = 67.7 mg m⁻³). The seasonal variability is weak at lower latitudes and stronger at higher latitudes. The annual mean surface POC concentrations show statistically significant regional trends (p < 0.05, 95% confidence level), and are decreasing in the North Atlantic and North Pacific and increasing in the South Pacific and Southern Oceans. The global trend is not significant. The 16-year global average water column POC biomass integrated over the euphotic depth, the mixed layer depth, or based on a combination of these two depths is estimated to be about 3.97, 3.92, and 5.03 g m⁻², respectively. Water column integrated biomass shows different spatial and seasonal patterns than the surface POC concentrations, and is increasing in many ocean regions. Globally averaged POC biomass is also increasing. At the same time ocean colour data indicate a decrease in the global oceanic productivity (PP). This means that there is a negative trend in the ratio of PP to POC biomass almost everywhere in the ocean. Such a decrease could indicate that the biological pump in the ocean is weakening, but longer time series of the ocean colour data are needed to confirm this observation.

Acknowledgements

The authors are grateful to all the persons involved in the programmes providing free access to the data sets used in this study. The ocean colour data were made available through the NASA OceanColor Web (oceancolor.gsfc.nasa.gov/). The Ocean Productivity Web at the Oregon State University (www.science.oregonstate.edu/ocean.productivity/) supplied PP and mixed layer depth data. We are also grateful for mixed layer depth data made available by IFREMER/LOS Mixed Layer Depth Climatology website (www.ifremer.fr/cerweb/deboyer/mld), Argo Mixed Layers Project at the Scripps Institution of Oceanography (http://mixedlayer.ucsd.edu), and the Japan Argo Delayed-mode Data Base (www.jamstec.go.jp/ARGO/argo_web/MILAGPV/index_e.html). Global Ocean Data Assimilation System (GODAS) data were provided by the US National Oceanic and Atmospheric Administration, Earth System Research Laboratory/Physical Sciences Division from their web site at http://www.esrl.noaa.gov/psd/. We would also like to thank Sebastian Meler from the IO PAN for help with data transfers.

Additional information

Funding

Financial support for this research was provided by the National Science Centre (NCN) in Poland through [grant 2011/01/M/ST10/07728] ‘Global estimates of particulate organic carbon reservoir and export flux in the ocean based on satellite ocean color data’. Partial support for MS comes also from the statutory funds of the Institute of Oceanology of the Polish Academy of Sciences (IO PAN).