Characterizing the photosynthetic potential of the deep chlorophyll layer (DCL) in Lake Superior using in situ fluorometry.

Abstract

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.