Lake Ontario has become increasingly oligotrophic since the 1972 Great Lakes Water Quality Agreement, which implemented measures to limit phosphorus loading. Over the past two decades, epilimnetic production has continued to decrease, raising concerns about overall ecosystem productivity and the continued health of our recreational fisheries. However, some of the decrease in epilimnetic phytoplankton growth can be offset by subsurface production in the deep chlorophyll layer (DCL). During the Cooperative Science and Monitoring Initiative (CSMI) intensive year for Lake Ontario in 2013, we completed targeted sampling of the offshore region to investigate patterns in DCL formation, maintenance, and dissipation, with the goal of improving our understanding of the contribution metalimnetic production makes to the overall productivity of Lake Ontario. Data were collected during five cruises from April through September 2013 and include in situ profiles of temperature, chlorophyll-a fluorescence, photosynthetically active radiation (PAR), dissolved oxygen, and beam attenuation coefficient. Discrete depth water samples were collected using a 12 Niskin bottle array and were analyzed for chlorophyll-a content, major nutrient concentrations, and phytoplankton community assemblage. These data were analyzed to investigate potential mechanisms of DCL formation and quantify the relative contributions of epilimnetic and metalimnetic chlorophyll-a.
Results/Conclusions
During consistently stratified months, a DCL formed between 10 – 15 meters depth and was strongly associated with the thermocline, nutricline, and subsurface maxima in both percent oxygen saturation and beam attenuation. While there was not a strong direct relationship between the 1% PAR depth and that of DCLs, the interaction between thermocline and euphotic zone depths does appear to be a critical parameter for determining DCL formation. In July, the DCL feature was uniform across the lake, but by August, the DCL began to dissipate in the eastern basin. This trend was linked to the combination of a depressed thermocline, as is common in Lake Ontario during late summer, and an abnormally shallow euphotic depth due to a whiting event. Association of the DCL with subsurface maxima in oxygen saturation and beam attenuation indicates that the DCL is caused by algal growth, rather than settling of cells along the thermocline or shade adaptation by phytoplankton. Furthermore, acoustics and stratified net tow data show high zooplankton densities associated with the DCL, indicating that metalimnetic phytoplankton growth impacts secondary production. Thus, it will be important to better integrate DCL productivity estimates into future Lakewide Management Plans (LaMPs) for Lake Ontario.