Temperature variability in shallow littoral sediments of Lake Opeongo (Canada)

Near-shore sediments support the most productive and diverse biological communities in lakes and are the sites of intense biogeochemical activity. Understanding what drives temperature and short-term temperature variability in near-shore sediments is important because benthic organisms and processes are temperature-dependent. I compared sediment and near-bottom water temperatures throughout the growing season (May-September) at 5 shallow near-shore sites in Lake Opeongo (Ontario, Canada). These sampling sites were more or less exposed to hydrodynamic forces and had different sediment characteristics but were not influenced by groundwater flow. The sediments were cooler than the overlying water column, especially in spring. Sediment temperature tracked water temperature and, therefore, was variable over short time scales (h-d). Temperature changes in near-shore sediments were smaller and slightly delayed compared to those in the overlying water. Over a whole season, temperature variability measured 5 cm below the sediment surface was, on average, 40 to 60% lower than that observed in the overlying water. The reduction in temperature variability was related to sediment characteristics and differed seasonally. Thermal diffusivity of these near-shore lake sediments ranged over an order of magnitude (0.0006-0.007 cm(2)/s). Thermal diffusivity increased with decreasing sediment particle size and was lower in spring than in summer and autumn. Overall, these results suggest that wind-driven hydrodynamic forces, which determine the frequency and intensity of upwellings and drive water movements in the shallow parts of Lake Opeongo, also determine the spatial structure and temporal variability of near-shore sediment temperature.