Calibration procedures and first dataset of Southern Ocean chlorophyll <i>a</i> profiles collected by elephant seals 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, but we were still lacking information on the spatiotemporal 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 programme, 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 addresses the calibration issue of the fluorometer before being deployed on elephant seals and presents 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 are 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 2 compared to chl a concentrations estimated from HPLC corrected in situ fluorescence measurements. The scientific outcomes of this programme 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.1).

Temporal variability of chlorophyll distribution in the Gulf of Mexico: bio-optical data from profiling floats

“…However, the values of these coefficients are rarely satisfactory and require post-processing evaluation (Boss et al, 2008;Guinet et al, 2013;Mignot et al, 2011;Xing et al, 2012). In addition, other biogeochemical processes, such as taxonomic composition and physiological acclimation mechanisms, are known to modulate the proportionality of FLUO and [CHL] (Cunningham et al, 1996;Falkowski and Kiefer, 1985;Kiefer, 1973), and must be taken into account for calibration purposes.…”

Estimates of the Southern Ocean general circulation improved by animal‐borne instruments

“…Improvements to the CTD-SRDL technology are underway, in terms of sensor accuracy, life duration, storage capacity, and satellite transmission. The challenge now is to broaden coverage by extending the methods to new sensors, such as fluorometers [Guinet et al, 2013], and using more diverse animal types. Miniaturization techniques are improving at a fast pace, and the true potential of animal-borne instruments is yet to be revealed.…”

Sustaining observations in the polar oceans

“…Recently, fluorometers have been added to seal-borne CTD tags [85], opening the possibility of measuring primary production in otherwise poorly sampled regions and times of year. And an increasing number of Argo floats are being equipped with bio-optical and chemical sensors [86,87], with miniaturized CO 2 sensors recently tested on profiling floats [88].…”