The Atlantic Meridional Transect (AMT) is an annual oceanographic expedition between the UK and the South Atlantic which has been operational since 1995. For the 2017 research cruise between Southampton and the Falkland Islands a collaboration between Sercon Ltd and Andy Rees of the Plymouth Marine Laboratory saw a Sercon Integra deployed at sea for the first time.
Aims of the AMT programme include making measurements of plankton diversity and activity in the upper layers of the Atlantic Ocean. Some of these measurements, including those of bacterial and zooplankton abundance have occurred on each of the 27 cruises completed. During this voyage Rees and his team have enhanced these routinely made measurements by using 13C and 15N signatures to inform on the distribution and trophic condition of plankton communities. This has been done using two approaches 1) An investigation of the natural variability of the both isotopes 2) using tracer additions of 15N and 13C to follow the fixation of nitrogen and carbon by diazotrophic (nitrogen fixing) and photosynthetic organisms.
Experimental procedures: A) Each day zooplankton samples were collected from between 200m and the surface. 1 ml from each concentrated sample was collected onto ashed GF/F filters, dried at 50°C and analysed in the Integra using standard conditions of combustion and reduction.
Early results indicate that the nutritional status of zooplankton communities which is reflected by their isotopic signature can be seen to align with ocean provinces as indicated by the temperature and salinity of surface seawater. This work is very preliminary, with the scientific team still at sea until early November, but it would seem that this technique may prove invaluable in assessing the bio-geography of plankton communities.
B) Triplicate 2.4L bottles were filled with surface seawater and amended with 15N-N2 (daily) to determine N-fixation and with 13C-bicarbonate (alternate days) to determine C-fixation. Rates of these critical biological processes have not been calculated at this time, as supporting data required is still in a raw format. Probably the first time ever that isotopic data has been produced before the ecosystem variables needed to complete the analysis. Despite this it is possible to see positive enrichment of 15N (blue in figure) and 13C (red in figure) at most ocean stations occupied.