http://hdl.handle.net/10388/11723

Cavaliere, Emily (2019) Biogeochemistry Under Ice, USASK Harvest - Theses and Dissertations, http://hdl.handle.net/10388/11723

Many lakes, ponds and reservoirs are subject to long and changing periods of ice cover. However, limited winter research has created key knowledge gaps. How does biogeochemical cycling change under ice? Are environmental variables and nutrient changes synchronous or asynchronous? Is winter a time of active nitrogen cycling? I undertook an intensive field campaign, studying prairie potholes ponds, reservoirs and lakes through winter, spring melt, and open water to understand the biogeochemical transformations that control nutrient flux and nitrous oxide (N2O) production in these systems. Surprisingly, despite lower winter temperatures, winter and summer rates of denitrification did not differ. Again, despite cold temperatures, ecologically important rates of pelagic nitrification can occur and impact NH4+, NO3– and oxygen concentrations. My work in wetland ponds in winter suggested there are two phases to winter ice cover. First, I observed declining oxygen concentrations that corresponded with the accumulation of NH4+, soluble reactive phosphorus (SRP) and the decline of NO3– concentrations during winter. During this period, N2O tended to be supersaturated. Melt conditions caused nutrient concentrations to decline, despite nutrient-rich melt water inputs. This was likely a result of increased nutrient uptake (for example, NH4+ uptake during melt was 50 times higher than winter uptake), adsorption and sedimentation. In a data rich reservoir (with nearly 40 years of semi-weekly physical, chemical and biological data), I identified two key phases of winter. Early to mid-winter conditions are characterized by decreasing oxygen, and increasing conductivity, NH4+ and SRP. Late winter conditions were characterized by increases in oxygen, corresponding to increases in chlorophyll and diatoms likely contributing to the nutrient drawdown. Together, this work supports the narrative that small ponds, lakes and reservoirs can act as hotspots for nutrient transformations despite their small size, and suggests the timing of key chemical and biological changes are driven by changes in the physical environment. Biogeochemical processes continue through winter, despite low light and temperatures. With declining ice cover, important seasonal changes are expected, although improved light during the melt period appears to be a key moment, where chemical conditions are re-set at the onset of the open water season.

https://harvest.usask.ca/bitstreams/9a1787ea-e9b3-4f85-84af-32af7926ba80/download