Case Study Description
PME miniDOT® Loggers were used in a study of plankton activity under wintertime ice in Lake Simcoe, Ontario, Canada. The mixing and stratification of the plankton in the lake during its ice-covered winter phase can influence the dissolved oxygen level in the following spring and summer. The plankton rely on updrafts caused by convection under the ice to stay in the photic zone and continue to transform light into dissolved oxygen.
miniDOT Loggers Used to Calculate Plankton Abundancy and Dissolved Oxygen Levels
The study, conducted by the Bernard Yang, of the University of Toronto Scarborough, used a combination of miniDOT Loggers (placed every 3-5 meters down), a light sensor buoy, conductivity loggers, and temperature loggers for data. The data was collected over the winters of 2015, 2016, and 2017, as well as the springs of 2016 and 2017. Weekly phytoplankton samples were also collected in 2015 and 2016 for counting and analysis.
The researchers used the collected data to calculate the plankton abundancy as well as the dissolved oxygen levels and, in comparison to springtime levels, to come to a conclusion about how plankton and DO are affected by length of winter, temperature, and lake-ice thickness.
Stratification and Solar-Driven Convection Directly Impact Dissolved Oxygen Levels
Based on the data and calculations collected by the researchers, they found that the colder the winter, the higher the dissolved oxygen concentration in the spring. The coldest of the three recorded winters resulted in the highest supersaturation of dissolved oxygen in the lake which translated to a higher dissolved oxygen content and a greater plankton population in the spring than the other two winters caused.
Near the end of winter, as ice layers begin to thin out, more sunlight can penetrate the ice covering into the lake. This increased solar radiation raises the production of phytoplankton in the photic zone of the lake. Increased solar radiation plus increased phytoplankton population creates more convection under the ice, which in turn leads to greater dissolved oxygen levels. The stratification, or active mixing, of the water column was then determined by calculating the Thorpe displacements based on the number of density inversions.
Conclusively, this shows that dissolved oxygen levels are directly related to the degree of water stratification and solar-driven convection happening under the lake-ice in the wintertime. This study was initially conducted because no long-term studies had been done on this topic. The results of the study are important for predicting summertime dissolved oxygen levels and water stratification. With warmer temperatures from climate change, temperate lakes are predicted to experience warmer surface temperatures and shorter ice-covered seasons. This will lead to decreased dissolved oxygen levels and water quality plus increased stratification due to decreased wintertime convection.
The miniDOT Logger is a completely submersible instrument that logs dissolved oxygen and temperature measurements. The oxygen sensor is an optode that measures dissolved oxygen concentration in water through a fluorescence method. Data are recorded to an internal SD card. Operation of the miniDOT Logger such as setting the time and sample interval can be accomplished via the USB cable.