UC Davis

Lakes are globally relevant for socio-economic needs as they provide fresh water sources for water supply and recreational activities, can be sites of critical sustenance fisheries, are often places of great natural beauty, and have cultural importance for many groups. In addition, lakes support diverse biota. The ecosystem balance of these natural waters is linked to the lake's physical processes that drive the transport and mixing of carbon, nutrients, sediments, and oxygen. For much of the year, thermal stratification acts as a barrier to vertical transport. At the same time, the existence of a thermal stratification allows for the occurrence of the upwelling process, which can potentially promote vertical transport and mixing. The upwelling phenomenon can alter the water quality in the littoral zone, and nutrient fluxes have been reported during the active upwelling phase. Conceptual models describe the flow structures to follow either a non-rotational or a fully rotational conceptual model, with the former applied to small lakes and systems of moderate size. Despite recognizing lake hydrodynamics influenced by the Coriolis force in lakes of moderate size, the intrinsic details of the rotational influence during lake upwelling are poorly understood, and the thresholds for the validity of the conceptual models have not been defined to date.

This dissertation uses fine-resolution numerical simulations with a lake model to quantify the Coriolis force effects during the active and relaxation phases of lake upwelling and to detail the physical processes observed in lakes that exhibit partial rotational influence in the flow structures. The numerical simulations conclude that lakes that show Coriolis force effects do not follow the binary perspective of upwelling, and rotational flow structures describe lake response to wind forcing during weak stratification. Furthermore, the analysis of the flow and temperature patterns of over 300 numerical simulations of lakes resulted in the definition of a third conceptual model for lake upwelling and allowed to define the thresholds for the validity of the conceptual models for upwelling following a basin-wide parameterization. Finally, dissolved oxygen dynamics driven by the physical processes during the upwelling relaxation stage were addressed using field observations taken between 2015 and 2018 in a lake of moderate size. The analyses of the available data for a total of 18 upwellings conclude that flow structures during the upwelling relaxation stage play a relevant role in increasing dissolved oxygen levels in deep waters of lakes. Flow paths from these flow structures also indicate that the increase in oxygen may occur across the whole extent of the upwelled region instead of the effects limited to the littoral zone, as is the case for the water quality dynamics during the active phase of upwelling.

The chapters of this dissertation serve as an integral step toward a better understanding of the upwelling phenomenon and concomitant water quality dynamics in lakes, especially in those exhibiting partial rotational influence. In addition, the results from Chapter 3 served as the foundation for creating a unique warning system of physical lake conditions that has been prototyped for the Lake Tahoe community.