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MARINE MAMMALS

Since 2004, several hundred seals have been equipped with conductivity-temperature-depth (CTD) sensors in the Southern Ocean for both biological and physical oceanographic studies. A calibrated collection of seal-derived hydrographic data is now available from Coriolis, currently consisting of more than 300,000 temperature/salinity profiles.

Overview

The Southern Ocean plays a fundamental role in regulating the global climate. This ocean also contains a rich and highly productive ecosystem, potentially vulnerable to climate change. Very large national and international efforts are directed towards the modeling of physical oceanographic processes to predict the response of the Southern Ocean to global climate change and the role played by the large-scale ocean climate processes. However, these modeling efforts are greatly limited by the lack of in situ measurements, especially at high latitudes and during winter months. The standard data that are needed to study ocean circulation are vertical profiles of temperature and salinity, from which we can deduce the density of seawater. These are collected with CTD (Conductivity-Temperature-Depth) sensors that are usually deployed on research vessels or, more recently, on autonomous Argo profilers. The use of conventional research vessels to collect these data is very expensive, and does not guarantee access to areas where sea ice is found at the surface of the ocean during the winter months. A recent alternative is the use of autonomous Argo floats. However, this technology is not easy to use in glaciated areas.

Elephant_photo_MEMO_CGuinet
Figure 1 : Elephant seal with CTD tag© C.Guinet CEBC/CNRS

In this context, the collection of hydrographic profiles from CTDs mounted on marine mammals is very advantageous. The choice of species, gender or age can be done to selectively obtain data in particularly under-sampled areas such as under the sea ice or on continental shelves. Among marine mammals, elephant seals are particularly interesting. Indeed, they have the particularity to continuously dive to great depths (590 ± 200 m, with maxima around 2000 m) for long durations (average length of a dive 25 ± 15 min, maximum 80 min). A Conductivity-Temperature-Depth Satellite Relay Data Logger (CTD-SRDLs) has been developed in the early 2000s to sample temperature and salinity vertical profiles during marine mammal dives (Boehme et al. 2009, Fedak 2013). The CTD-SRDL is attached to the seal on land, then it records hydrographic profiles during its foraging trips, sending the data by satellite ARGOS whenever the seal goes back to the surface.

While the principle intent of seal instrumentation was to improve understanding of seal foraging strategies (Biuw et al., 2007), it has also provided as a by-product a viable and cost-effective method of sampling hydrographic properties in many regions of the Southern Ocean (Charrassin et al., 2008; Roquet et al., 2013).

International seal observing programs

After the international research program Southern Elephant seals as Oceanographic Samplers (SEaOS) initiated in 2004, the MEOP (Marine Mammals Exploring the Oceans Pole to Pole) project started as an International IPY (International Polar Year) project, aiming at coordinating the cooperation between several national programmes to provide a comprehensive, synoptic oceanographic coverage in Polar Regions. Here we provide a list of institutions that made a critical contribution to the funding of CTD-SRDL and to the logistics of seal deployments in the Southern Ocean, as part of the MEOP consortium: National Oceanographic Partnership Program, Office of Polar Program, National Science Foundation, and NOAA Antarctic Marine Living Resources Program (USA), the Norwegian Polar Institute and Research Council of Norway (NOR), the Natural Environment Research Council (UK), the Integrated Marine Observing System (AUS), the Centre National d’Etudes Spatiales and Institut Paul-Emile Victor (FR), the South African National Antarctic Programme (ZA), the Brazilian National Research Council (BRA), the International Governance and Atlantic Seal Research Program, Fisheries and Oceans, Canada and the Greenland Institute of Natural Resources (CAN), the Chinese Arctic and Antarctic Administration (China) and the Alfred Wegener Institute for Polar and Marine Research (GER).

MEOP-CTD data distribution

Distribution of T/S profiles currently present in the MEOP-CTD database. The four black lines on the map delimit natural geographical zones: (from north to south) Sub-Tropical, Sub-Antarctic, Polar, Southern ACC, and Sub Polar zones. A large majority of profiles were sampled in the subpolar zone, during fall/winter seasons.

The French MEMO Observatory "Mammifère marin Echantillonneur du Milieu Océanique" (Marine mammal, sample-taker of the marine environment) was set up in 2010 to continue acquisition of high-frequency data over the next ten years. The implementation of this program and its continuation as an ecological and oceanographic observatory of the Southern Ocean should allow scientists to know more about the secrets of elephant seals species and their environment. The French community has been strongly involved at a very early stage of the seal program, in particular for the optimization of transmitted data compression methods, the quality control of data collected (Roquet et al. 2011), and the development of fluorescence sensors with financial support of the Fondation Total (Guinet et al. 2013; Blain et al. 2013). French deployments have been carried out mainly at the Kerguelen Islands (Southern Indian Ocean). Several deployment campaigns have also been done on Weddell seals by J.-B. Charrassin (MNHN) in Adelie Land, Antarctica.

The value of these hydrographic data within the existing Southern Ocean observing system has recently been demonstrated (Roquet et al., 2013; Roquet et al., 2014). Using seal-derived data to constrain a model simulation of the ocean circulation substantially modified the estimated surface mixed- layer properties and circulation patterns in the Southern Ocean, improving the agreement of the model simulation with independent satellite observations of sea-ice concentration.

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