UC Davis

Autonomous underwater gliders provide an opportunity for high-resolution measurements of large-scale processes in mid to large lakes. Internal waves are ubiquitous in lakes and oceans. Gliders have long been used to observe internal waves and other mesoscale processes in oceans, here they are applied to observing lakes. In stratified lakes, internal waves drive significant horizontal and vertical transport. Changing winds and stratification throughout the year lead to seasonally varying internal wavefields. In mid-sized and larger lakes, rotational effects and coastal bathymetry result in spatial heterogeneity. Glider transects, primarily from Lake Tahoe (California/Nevada, USA) but also Lake Geneva (Switzerland/France), were conducted at various times of the year along a selection of tracks. These were compared with mooring data to show that internal wave driven variability can be captured by gliders. The optimal data collection path is dependent on whether along-shore or cross-shore variability is a priority for the study. Glider transects across Lake Tahoe were used to analyze and characterize the seasonal variability of flow and transport driven by internal waves in rotationally influenced, mid-size lakes. Internal waves, including Kelvin and Poincaré waves, were observed to modify temperature structure, chlorophyll concentration and particulate matter distribution. A deep chlorophyll maximum exists in summer and spring, and its extent was deformed by the wavefield. In fall, the development of a dominant second vertical mode internal wave (period ~ 28 h) occurs with a shift from sustained high winds to strong diurnal winds. Where the metalimnion interacted with the boundary, it drove offshore and alongshore flows. These were accompanied by elevated chlorophyll concentrations in the nearshore metalimnion. Findings at Lake Tahoe are compared to results in other lakes worldwide, obtaining identifiable patterns; the work allows us to consider how changing winds and stratification may affect mid-size lakes. Spatial variation in suspended particulate matter is also observed from glider transects in Lake Tahoe which are used to assess the effectiveness of the lake’s two long term monitoring stations to resolve the spatial variability associated with suspended particulate matter. These stations under-resolve the spatial variability of vertical gradient of suspended particulate matter caused by internal waves but do capture the temporal changes in the lake. Long-term data show total volume concentration of particles increasing. In particular, particles between sizes of 40 μm and 129 μm have been increasing. There are multiple possible contributing factors to the increase and change in particle size composition. Phytoplankton cell counts show an overall increase in biomass and shift in species from smaller Cyclotella (4 – 12 µm) to larger Synedra (30 – 250 µm). Shifts in dominant phytoplankton species are timed with the 2013 – 2016 drought and high precipitation events at its conclusion. Meanwhile, increased atmospheric deposition, often attributable to wildfires, caused short-term spikes in particles. The most rapid increase in these midsized particles occurred during a period with a shallower than usual mixing depth (i.e. the maximum mixing depth for the winter). Changes in suspended particle size and total volume are markers of environmental change that may be linked to climate change.