We have been working up our Net Community Production (NCP) data for some time now, and as I am about to go off to the Ocean Sciences conference to present a poster on the bulk of this work, I thought I’d add an entry on some of the NCP data along with some preliminary Continuous Plankton Recorder (CPR) data.
NCP is equivalent to the rate of organic carbon export from the surface ocean to the ocean interior. The metabolic state of a system can be defined by NCP, with autotrophic systems occurring when gross primary production is greater than respiration, and heterotrophic systems occurring when respiration is greater than primary production. Quantifying how this rate varies with plankton community structure is crucial in determining regional metabolic states and their role in the global carbon sink.
Data were collected in the North Atlantic on board MV Benguela Stream between December 2011 and March 2013. The sample area was divided in to five biogeochemical regions based on peaks in the second derivative of sea surface temperature, density and satellite derived Chl-a:
Monthly estimates of NCP for each region were determined from a simple 1D model based on abiotic parameters and the dissolved oxygen inventory. These data are presented in the figure below alongside the mean monthly abundance of 5 key phytoplankton groups obtained from the CPR survey:
Coloured bars represent monthly NCP within each biogeochemical region given in mmol O2 m-3 d-1 plotted on the left y-axis. The coloured lines represent co-located mean monthly abundance (log10(x+1)) of Diatoms (blue), Dinoflagellates (green), Rhizosolenia (red), Silicoflagellates (cyan), and Coccolithophores (purple) plotted on the right y-axis. Regions 4 and 5 are missing CPR values as the CPR is towed on this route between 40°W and 0°W.
These measurements show that all five regions are predominantly autotrophic with different phytoplankton groups influencing the metabolic state at different stages during the season. Our annual estimates of NCP agree with those derived from studies which use argon/oxygen ratios or oxygen isotopes which are often more expensive and labour intensive methods. The mean seasonal NCP was also compared with estimates of NCP calculated using dissolved inorganic carbon measurements. These two independent methods followed the same regional trend and were not significantly different in magnitude. The next steps in this study are to statistically investigate the relationships between oxygen and carbon dioxide cycling and the plankton community structure.
This study is the first to report NCP for these five regions in the North Atlantic Ocean, and shows that there was surprisingly little difference in the magnitude and seasonal variability in NCP between regions. This contrasts with global circulation models and highlights the need for improved global coverage of in situ data and an improved mechanistic understanding of why the two approaches differ. The method developed in this study is simple and cost effective (in terms of personnel time and shipboard space requirements), which is therefore applicable for use on volunteer observing ships, and ideally suited to provide the required global coverage of in situ NCP data.