Abstract
Recently, a number of Antarctic marine environmental studies have used oceanographic parameters collected from instrumented top predators for ecological and physical information. Phytoplankton concentration is generally quantified through active measurement of chlorophyll fluorescence. In this study, light absorption coefficient (K0.75) was used as an indicator of phytoplankton concentration. This measurement, easy to obtain and requiring low electric power, allows for assessing of the fine scale horizontal structuring of phytoplankton. As part of this study, Southern elephant seals (SES) were simultaneously equipped with a fluorometer and a light logger. Along the SES tracks, variations in K0.75 were strongly correlated with chlorophyll, a concentration measured by the fluorometer within the euphotic layer. With regards to SES foraging behaviour, bottom depth of the seal’s dive was highly dependent on light intensity at 150 m, indicating that the vertical distribution of SES’s prey such as myctophids is tightly related to light level. Therefore, change in phytoplankton concentration may not only have a direct effect on SES’s prey abundance but may also determine their vertical accessibility with likely consequences on SES foraging efficiency.
Highlights
Within the context of a rapidly changing climate, it is essential to understand how both environment and species will respond to such changes over time
After removing the profiles collected over the shelf and during the night, a total of 9742 light and 181 chl-a profiles were available for the analyses
Fluorescence in itself is only a proxy of phytoplankton concentration and fluorescence response is known to vary with phytoplankton species and physiological state [33]
Summary
Within the context of a rapidly changing climate, it is essential to understand how both environment and species will respond to such changes over time. Collection of data is necessary throughout the water column, and requires adapted and often costly observation methods (cruise, Argo float, drifter, gliders...). This is true for the Southern Ocean (SO) due to its remoteness and harsh weather conditions. Data on SST or surface chl-a needs to be merged over several days or weeks to provide a synoptic view of conditions.
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