How to record observations in the polar oceans
The means available to observe this ocean in order to detect oceanographic and ecological changes affecting it remain limited. The standard oceanographic data for studying ocean circulation and the amount of heat stored in the Southern Ocean are vertical profiles of temperature and salinity. Modeling efforts are constrained by a lack of in-situ data. Indeed Argo profilers are carried from west to east by the powerful circum-Antarctic current and are ineffective for sampling the pack ice zone, as they cannot then rise to the surface because they risk being caught and crushed by the ice. Few oceanographic vessels are present in Antarctica, especially during the winter period. Moreover, these means are very expensive to implement. To make up for this lack of data, the idea was to take advantage of the work carried out on the ecology of Antarctic seals to collect oceanographic and ecological data and to complete and strengthen the observation system for this ocean.
Identifying the influence of local oceanographic conditions
At the same time, ecological data are also collected: sea trips, diving behaviour (e.g. the prey capture attempts shown opposite), but also the evaluation of the body condition, effort and fishing success of these animals according to the local oceanographic context. An important added value of SNO-MEMO is that it not only answers questions relating to physical, biogeochemical and ecological oceanography, but also makes it possible to address scientific questions at the interface of these disciplinary fields, such as determining the influence of local oceanographic conditions on the structuring of biological resource fields.
Female elephant seal equipped with a CTD-Fluorescence beacon and having travelled between Kerguelen and the Antarctic zone -(bottom left). Chlorophyll_a concentration (g.m-1) estimated from fluorescence measurements (top left) and density measurement in the first 160 meters of the water column calculated from T/S measurements obtained simultaneously. Note the sudden disappearance of phytoplankton in the water column, which corresponds to the formation of pack ice revealed by changes in density.beacon
Towards an integrated and interdisciplinary approach to oceanography and marine biology
Over the years we have supplemented physical oceanographic measurements with a whole new series of biological measurements such as, for example, the concentration of phytoplankton in the illuminated layer of the ocean. However, until very recently, we were sorely lacking information on the intermediate trophic levels, i.e. the whole zooplankton and the small fish and squid connecting phytoplankton to elephant seals. Accelerometers provide information on the number of head movements associated with attempts to capture prey during the dive and thus on the abundance and distribution of prey in the water column . These attempts are indicated by the black dots in the figure below (left side). These dots are superimposed on the temperature measured by elephant seals. Considerable progress has been made thanks to the design and realisation of a miniature sonar by Mark Johnson and Pauline Goulet  in connection with the elephant seal programme. This micro-sonar can detect zooplankton, fish and squid larger than 1 mm and estimate their abundance. These organisms can be differentiated by their acoustic size and behaviour. In the graph (in the middle) we can see organisms, some of which come close to the surface at night while others remain at the same depth whatever the time of day (Thesis M. Tournier CEBC). Bioluminescence  is another parameter that tells us about these intermediate trophic levels (right). Today, the greatest added value of SNO-MEMO is to allow us to evaluate the influence of these physical conditions (temperature, salinity, light) on the distribution, abundance and behaviour of these intermediate levels can be studied because they are difficult to observe even though they represent the largest daily biomass migration on earth. We were far from imagining the day we equipped the first elephant seal with an oceanographic beacon, that 15 years later they would provide us with so much information on the biology and food webs of the Southern Ocean. 1] Guinet et al (2014) MEPS  Goulet et al (2019) Deep Sea Research  Goulet et al (2020) J Exp BiolBeacons