The interest in physical limnology as a subject of environmental fluid dynamics is threefold: (1) physical processes as an avenue for understanding lake ecosystems, (2) management of natural water resources, and (3) lakes as natural scale-up "laboratories" for stratified environmental flow studies.
First, studying aquatic ecosystems, and lakes in particular, calls for interdisciplinary approaches. Even very specific natural in situ processes can hardly ever be viewed independent of the hosting environment. As a practical example, the dynamics of an algae species can not be understood without considering the distribution of nutrients (and other biogeochemical constituents), the stratification and mixing of the water column, as well as baroclinic motions and subsequent modulations of the light regime. In this sense, physical limnology is a crucial discipline for supporting the interpretation of in situ observations of any property, which is always evolving along the fundamental balance
∂ / ∂t (property) = Rates of transformations – div(property fluxes)
(13.1)
In short, spatial and temporal changes of a property within the water column have always a transport component.
Second, on a global scale, natural water resources are intensely utilized and under increasing anthropogenic pressure. Many of the 110,000 lakes larger than 1 km²—covering an area of 2.3 million km²—are used for various purposes, including as recipients for polluted urban effluents. As habitations and infrastructure are often close to lakes, human impacts are strong and many lakes have been subject to enormous changes. To minimize detrimental effects, we have to strive for best environmental engineering practices for management of water resources (such as fisheries, water supply, irrigation, or electricity production, etc.). Therefore, we need to understand how geochemical and ecological processes are related to hydrodynamics and how anthropogenic influences, such as the currently much debated climate change (Section 13.3.2), affect lake ecosystem processes.
Third, lakes are often used as natural "laboratories" of stratified water bodies at intermediary scales for environmental fluid dynamics studies, for example, in studying small-scale turbulence, internal and surface waves, or density currents (Imberger 1998). Lakes as "laboratories" have many advantages: (1) enormous variety and variability of the physical, hydrogeochemical, and meteorological boundary conditions (Table 13.1), (2) intermediate spatial dimensions which are on geophysical scales but more easily accessible than those of ocean basins, (3) the possibility to derive budgets of tracers such as temperature, salinity, and density under natural conditions.
In this chapter, the most relevant lake-specific hydrodynamic phenomena are reviewed. Due to limited space, the review is largely incomplete and we refer to more comprehensive overviews, such as various sections in the Encyclopedia of Inland Waters (Likens 2009), textbooks (Imboden and Wüest 1995; Imberger 1998), and reviews (Imberger and Patterson 1990; Wüest and Lorke 2003).