Introduction

In the surface layer of lakes, exchange with the atmosphere and energetic mixing from wind and convection provides oxygen and light, thus enabling growth of plankton and other aquatic life. The nutrients required to sustain surface layer ecology are primarily found in the benthos, where sediment resuspension, nutrient release from the sediments, and oxygen consumption occur. In the interior of lakes, seasonal stratification of the water column suppresses vertical mixing, effectively isolating the surface layer from the sediments. However, the stratification simultaneously provides an ideal environment for internal waves, whose oscillatory currents energize a quasi-steady turbulent benthic boundary layer (TBBL) that drives vertical biogeochemical flux.

The wave motions in lakes are initiated by the surface wind stress. Waves will occur on both the free surface and internal stratifying layers (e.g., the thermo-cline) and these are referred to as surface or barotropic and internal or baroclinic motions, respectively. The waves are categorized according to their length scale. Basin-scale waves have wavelengths that are of the same order as the lake diameter and are manifest as standing wave modes - or seiches. Sub-basin-scale waves have wavelengths of 10-1000 m. These waves are progressive in nature and will break where they shoal on sloping topography at the depth of the thermocline.

a turnover event (Figure 1(a)). During the summer months, solar heating causes a lake to become stratified with a layered structure consisting of an epilim-nion, metalimnion, and hypolimnion. If the vertical density gradient is abrupt through the metalimnion, the lake may be approximated as a simple two-layer system of thickness h1 and density p1 over thickness h2 and density p2, where H = h1 + h2 is the total depth (Figure 1(b)). In lakes where a strong diurnal thermo-cline is present or the metalimnion is thick, the vertical density structure may be approximated with three contiguous fluid layers of density p1, p2, and p3 with thicknesses H = h1 + h2 + h3 (Figure 1(c,d)). The layered model for the stratification is inappropriate for shallow lakes (H< ~ 15 m), where the entire water column may be composed of weakly stratified water (e.g., western Lake Erie). In these lakes a transient diurnal thermocline may still occur. Shallow weakly stratified lakes are best characterized as having a continuous stratification (Figure 1(e)). Very deep lakes (with a thick laminar region between the metalimnion and TBBL) and those with a significant chemical (saline) component will also have a continuous stratification beneath the metalimnion (Figure 1(f)). In general, the strength of the stratification is measured according to the Brunt-Vaisala or buoyancy frequency N = \/—{g/po)dp/dz, where z is the vertical coordinate direction, g is the gravitational constant, and po = 1000 kg m-3 is the characteristic water density; in the thermocline of lakes the maximum N~ 10~2 Hz.

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