Abstract:
Under high light intensity, phytoplankton protect
their photosystems from bleaching through nonphotochemical
quenching processes. The consequence of
this is suppression of fluorescence emission, which must
be corrected when measuring in situ yield with fluorometers.
We present data from the Southern Ocean, collected
over five austral summers by 19 southern elephant seals
tagged with fluorometers. Conventionally, fluorescence data
collected during the day (quenched) were corrected using the
limit of the mixed layer, assuming that phytoplankton are
uniformly mixed from the surface to this depth. However,
distinct deep fluorescence maxima were measured in approximately
30% of the night (unquenched) data. To account for
the evidence that chlorophyll is not uniformly mixed in the
upper layer, we propose correcting from the limit of the euphotic
zone, defined as the depth at which photosynthetically
available radiation is 1%of the surface value. Mixed layer
depth exceeded euphotic depth over 80% of the time. Under
these conditions, quenching was corrected from the depth of
the remotely derived euphotic zone Zeu, and compared with
fluorescence corrected from the depth of the density-derived
mixed layer. Deep fluorescence maxima were evident in only
10%of the day data when correcting from mixed layer depth.
This was doubled to 21% when correcting from Zeu, more
closely matching the unquenched (night) data. Furthermore,
correcting from Zeu served to conserve non-uniform chlorophyll
features found between the 1% light level and mixed
layer depth.
