The presence of a sub-surface deep chlorophyll layer (DCL) characterized by
chlorophyll a concentrations exceeding those in the surface mixed layer is common in
marine systems and in many lakes after thermal stratification is established during the
summer. The formation and erosion of the DCL at four stations across Lake Superior
was measured from late May to late September 2010 in response to strong seasonal
temperature changes in the water column during that period. Shifts in phytoplankton
pigment composition during the summer and with increasing depth were also observed.
In late July, the dissolved oxygen (DO) concentration of water immediately below
the surface mixed layer was the highest measured in the water column, indicating the
DCL may be a zone of active phytoplankton photosynthesis acting to increase the
concentration of DO in the water. This hypothesis was supported by high depth
resolution estimates of daily photosynthetic O2 production calculated using pigment
fluorescence-based water column profilers that showed areas of increased O2 production
within the DCL compared to the surface mixed layer. The mean fluorescence-based
estimate of daily carbon assimilation within the DCL from 10-40 m in the western arm of
the lake in early August (221 mg C m-2 d-1) was comparable to the previously measured
mean value of C assimilation using traditional 14C incubations over the same depth range
in early fall (209 mg C m-2 d-1) (Sterner, 2010). However fluorescence-based daily
production estimates made elsewhere in the lake and at different times of year were lower
than estimates made using 14C incubations. Diel variability in fluorescence-based
photosynthesis estimates and the large influence of background water fluorescence on the
estimates may make it difficult to determine a representative value for daily
photosynthetic production. Nevertheless the improved depth resolution and speed of
sampling provided by fluorescence profilers compared to 14C incubations makes them an
effective way to gain insight into the seasonal and depth variability of phytoplankton photosynthetic production across large lakes such as Lake Superior.
University of Minnesota M.S. thesis. December 2011. Major:Water resourcs science. Advisors:Stephanie J. Guildford, Robert E. Hecky. 1 computer file (PDF); xi, 121, appendix p. 121.
House, Geoffrey L..
Characterizing the photosynthetic potential of the deep chlorophyll layer (DCL) in Lake Superior using in situ fluorometry..
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