Calibration procedures and first data set of Southern Ocean chlorophyll <i>a</i> profiles collected by elephant seal equipped with a newly developed CTD-fluorescence tags

Abstract

Abstract. In-situ observation of the marine environment has traditionally relied on ship-based platforms. The obvious consequence is that physical and biogeochemical properties have been dramatically undersampled, especially in the remote Southern Ocean (SO). The difficulty in obtaining in situ data represents the major limitations to our understanding, and interpretation of the coupling between physical forcing and the biogeochemical response. Southern elephant seals (Mirounga leonina) equipped with a new generation of oceanographic sensors can measure ocean structure in regions and seasons rarely observed with traditional oceanographic platforms. Over the last few years, seals have allowed for a considerable increase in temperature and salinity profiles from the SO. However we were still lacking information on the spatio-temporal variation of phytoplankton concentration. This information is critical to assess how the biological productivity of the SO, with direct consequences on the amount of CO2 "fixed" by the biological pump, will respond to global warming. In this research program, we use an innovative sampling fluorescence approach to quantify phytoplankton concentration at sea. For the first time, a low energy consumption fluorometer was added to Argos CTD-SRDL tags, and these novel instruments were deployed on 27 southern elephant seals between 25 December 2007 and the 4 February 2011. As many as 3388 fluorescence profiles associated with temperature and salinity measurements were thereby collected from a vast sector of the Southern Indian Ocean. This paper address the calibration issue of the fluorometer before being deployed on elephant seals and present the first results obtained for the Indian Sector of the Southern Ocean. This in situ system is implemented in synergy with satellite ocean colour radiometry. Satellite-derived data is limited to the surface layer and is restricted over the SO by extensive cloud cover. However, with the addition of these new tags, we're able to assess the 3 dimension distribution of phytoplankton concentration by foraging southern elephant seals. This approach reveals that for the Indian sector of the SO, the surface chlorophyll a (chl a) concentrations provided by MODIS were underestimated by a factor of the order of 2–3 compared to in situ measurements. The scientific outcomes of this program include an improved understanding of both the present state and variability in ocean biology, and the accompanying biogeochemistry, as well as the delivery of real-time and open-access data to scientists (doi:10.7491/MEMO.1x).

Instrumented elephant seals reveal the seasonality in chlorophyll and light‐mixing regime in the iron‐fertilized Southern Ocean

“…The data were collected by 23 elephant seals ( Mirounga Leonina ) of the Kerguelen Island. The tags were deployed during five different periods between 2007 and 2011, transmitting 4498 profiles among which 3388 were usable [see Guinet et al ., , Table 1]. In our study we used a subset of this data set (1819 profiles from 21 tags; Table S1 in the supporting information) collected in high‐chlorophyll regions.…”

“…This was done before deployment by comparison of the fluorescence signal with discrete chlorophyll measurements by high‐performance liquid chromatography of samples taken at the same depth [ Xing et al ., ]. For the first two deployments, such predeployment calibration was not achieved and a posttreatment utilizing the relative variations of surface chlorophyll concentrations derived from Moderate Resolution Imaging Spectroradiometer (MODIS) was applied [ Guinet et al ., ]. For each regions defined below, the mean chlorophyll profile was calculated for periods of 15 days.…”

Calving Event Led to Changes in Phytoplankton Bloom Phenology in the Mertz Polynya, Antarctica

“…While ocean color satellite data are the optimal data set for studying trends and changes in phytoplankton phenology (Henson et al, 2018;Johnson et al, 2013), monitoring changes in polynyas could be improved by additional in situ data such as gliders (Kaufman et al, 2014) and seal-derived hydrographic data (conductivity-temperature-depth satellite relay data loggers with fluorometers; Guinet et al, 2012;Roquet et al, 2013) In particular, seal tags allow a much larger spatial and temporal coverage (Roquet et al, 2013) to the underrepresented seasons (i.e., fall and winter) and areas such as continental shelf and sea ice zones (Roquet…”

Correction of profiles of in‐situ chlorophyll fluorometry for the contribution of fluorescence originating from non‐algal matter

“…They have contributed significantly to our current understanding on the spatio‐temporal dynamics of phytoplankton (Cullen ; Boss and Behrenfeld ). To date, they have been integrated to every oceanographic research platform such as CTD rosettes (Cullen ), flow‐through systems (Platt ), moored buoys (Kinkade et al ; Pettigrew and Roesler ), Bio‐Argo floats (Boss et al ; Mignot et al ; Xing et al ), gliders (Niewiadomska et al ; Sackmann et al ; Cetinić et al ), and even elephant seals (Blain et al ; Guinet et al ).…”

“…Previously, the non‐zero deep values were considered to be due to a bias resulting from factory calibration as this was established for the sensor alone and not for the sensor mounted on the platform used for its deployment. This bias was thus simply corrected through subtracting a fixed value from the whole fluorescence profile (Schmechtig et al ), generally the deepest value from the profile (e.g., Xing et al ; Guinet et al ). However, such dark current correction often meets a problem in some regions (e.g., some sub‐tropical areas, the Arabian Sea), where the fluorescence signals display unexpected variations with depth in waters deeper than 300 m (Broenkow et al ; Lewitus and Broenkow ; Breves et al ; Claustre et al ), i.e., the so‐called “deep sea red fluorescence.” Figure illustrates this issue in the South East Pacific (Fig.…”