An assessment of MODIS products in the Southern Ocean using tagged southern elephant seals, in the context of an increasingly positive Southern Annular Mode
Abstract
The Southern Ocean maintains a complex, dynamic marine food web based on its
stock of photosynthesising phytoplankton. For the same reason, it is our most significant sink of carbon, vital to functioning of global atmospheric systems. However,
this key polar ocean is also responsive to atmospheric variability dominated by the
Southern Annular Mode (SAM), with unknown implications for phytoplankton patterns in space and over time.
MODIS surface chlorophyll-a concentration ([Chl-a]) and the depth of the 1% light
level (Z[sub]eu) were evaluated against in situ fluorescence and light data collected by
tagged southern elephant seals. Light data were processed to minimize self-shading
and environmental sources of variability, and Lee Z[sub]eu outperformed Morel Z[sub]eu when
examined relative to the resultant in situ measures of the 1% light level. Based on these
results, fluorescence data were corrected for quenching at the surface using Lee Z[sub]eu
as a reference depth; conserving deep chlorophyll maxima within hydrographically
defined mixed layers. [Chl-a] was evaluated against quenching-corrected fluorescence
and satellite products mirrored in situ trends over time.
The MODIS timeseries was interrogated for spatio-temporal shifts to phytoplankton abundance in surface waters. Trends over 13 years of austral summers point to
overall declines and perturbations to the timing of the bloom, with changes that were
significantly related to the SAM presenting as regional rather than annular. In this
context, to assess if perturbations may impact top predators, the at-sea behaviour of
tagged seals were examined relative to the fluorescence and light data they collected.
Using a cumulative sums analysis of speed, foraging phases were distinguishable
from ’outbound’ and ’homebound’ travel. When all phases were examined relative to
fluorescence and the 1% light level, foraging appeared to be associated with increased
phytoplankton abundance. This suggests that the future of these marine predators
may be linked to shifting phytoplankton patterns.
Type
Thesis, PhD Doctor of Philosophy
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