If we seek some structure in our approach to surface water quality engineering, we may wish to consider water quality issues as belonging to one of two categories: those which stimulate biological activity (e.g. the discharge and internal production of organic matter) and those which depress biological activity (e.g. toxicants and chemicals which engender other biotic responses).

Stimulation of biological activity occurs when organic matter is introduced to surface waters, leading to enhanced rates of decomposition and attendant consumption of oxygen. We are examining the impact of external (allochthonous) discharges of organic matter through our stream modeling effort. Here, we will consider internal (autochthonous) production of organic matter, as stimulated by the addition of growth-limiting nutrients.

[T] organic matter production and decomposition (Mihelcic, 1999, Fig. 5-35)

The accompanying figure illustrates the process for lakes, where organic matter produced in the well-lit upper waters of lakes settles to the bottom where it decomposes. Under stratified conditions, oxygen consumed in the bottom waters through decomposition is not re-supplied. Surface waters receive oxygen through air-water exchange, but that oxygen is not transferred across the thermocline in significant amounts. If the production and decomposition of organic matter exceeds the oxygen resources of the hypolimnion, oxygen depletion will result.

[T] orthograde and clinograde DO profiles

[T] DO isopleths (Onondaga Lake)

Oxygen depletion in lakes leads to the death of fish and benthic organisms, the production of undesired chemical species (NH₃, H₂S, CH₄), and accelerated cycling of pollutants from sediments (especially P). Oxygen depletion is one of the most important and commonly observed water quality problems in lakes.

Trophy is defined as the rate at which organic matter is supplied to lakes. In contrast to river environments, the supply of organic matter in lakes is largely autochthonous, i.e. organic matter generated through primary production (algal and macrophyte photosynthesis) in the water column. Plant growth in lakes is a function of conditions of light and temperature and the supply of growth-limiting nutrients. Because regional conditions of light and temperature are more or less constant from lake to lake, trophy is primarily determined by nutrient inputs. The fundamental paradigm relating nutrient status and algal growth is termed,

The Limiting Nutrient Concept or Liebig's Law of the Minimum: "the yield of any organism will be determined by the abundance of the substance that, in relation to the needs of the organism, is least abundant in the environment".

[T] the limiting nutrient concept (Mihelcic, 1999, Fig. 5-25)

Nitrogen and phosphorus are generally found to be the nutrients limiting growth in aquatic systems, nitrogen in salt- and phosphorus in fresh-waters. Although the dissolved nitrogen compounds which support algal growth (ammonia and nitrate) are often depleted in lakes, it is difficult to say that nitrogen truly limits growth because nitrogen-fixing cyanobacteria (bluegreen algae) can proliferate, creating nuisance conditions in nitrogen-depleted environments. Thus phosphorus is the limiting nutrient for lakes.

[T] Lake 226; the divided lake (Mihelcic, 1999, Fig. 5-36)

The significance of phosphorus as a limiting nutrient stems from it's largely inert nature globally.

[T] phosphorus cycle (Wiersma, 1986, Fig. 2.10)

However, the cycling of the phosphorus is quite complex and important to lake dynamics.

[T] P cycling in lakes

Lakes are often classified according to their trophy or degree of enrichment with nutrients and organic matter - they are classified by trophic state. Five classes are recognized (ultra-oligotrophic, oligotrophic, mesotrophic, eutrophic, and hypereutrophic), however, all but two are largely degrees of one another.

[T] comparison - oligotrophic and eutrophic lakes

The process of nutrient enrichment of a water body, with attendant increases in organic matter, is termed eutrophication. This is considered to be a natural aging process for lakes, part of the succession on newly formed water bodies to dry land.

[T] lake succession (Mihelcic, 1999, Fig. 5-37)

The concept of lake aging, natural eutrophication, must be viewed with consideration of the fact that differences in (1) depth and (2) size and fertility of the drainage basin can have a major impact on the rate at which the process advances. Addition of phosphorus, and the resultant aging, which occurs through anthropogenic activity is termed cultural eutrophication. This is a focus of activity for water quality managers and surface water quality engineers.

The energy received by a lake (incident light) varies with geographical position and with the season.

[T] Incident light as a function of latitude

[T] Incident light as a function of season

Algal productivity and standing crop in lakes distinct cycles, largely in response to seasonality in meteorological conditions.

[T] seasonality in standing crop (Welch, 1992, Fig. 6.35)

[T] seasonality in productivity (Wetzel, 1983, Fig. 15.14)

Discussion: how do standing crop and production differ?

The depth to which light penetrates in lakes and the apparent color of the water are both closely tied with the public perception of water quality. Algal standing crop is one of several features which influence the color and transparency of lakes.

Light received at the lake surface is attenuated as the light penetrates the water column. The term attenuation refers to both scattering and absorption (conversion to heat). Scattering is the result of deflection of light energy by the molecular components of water and its solutes, but largely by particulate matter suspended in the water. These particulates include clays, calcium carbonate, phytoplankton and organic detritus. Absorption occurs due to water itself, dissolved organic compounds, and phytoplankton.

[T] Incident light as a function of depth

The attenuation of light with depth is well-described by first-order kinetics,

where I is light (m E· m^-2· s^-1), z is depth (m), and k_e is the extinction coefficient (m^-1). Integrating from z = 0 to z = z,

Values for k_e are determined from paired field measurements of light at depth using a log-linearization of the above equation,

Where a plot of I_z versus depth yields k_e as its slope.

[T] Extinction coefficient determination

The extinction of light may be partitioned among various factors using partial extinction coefficients,

Consideration of the relative contributions of each of these components to transparency is important when considering remediation measures.

The transparency of lakes is also conveniently estimated using a simple device called a Secchi disk. The unit of measure is meters. Secchi disk transparencies range from a few cm to over 40m.

[T] Secchi disk in Lawrence Lake

[T] Secchi disk in Onondaga Lake

As a rough approximation,

Thus Secchi disk transparency can be used to estimate the extinction coefficient and thus model light in lakes.

The distribution of colors in light as received at the earth's surface is termed the solar spectrum.

[T] Solar spectrum - Cannonsville Reservoir

UV radiation, characterized by short wavelengths (<380 nm) and high energy can cause damage to microbial life and facilitate photolysis of some organic compounds. Light of intermediate wavelengths (440 - 660 nm) is used by plants and is termed photosynthetically available radiation (PAR). The longer infrared wavelengths (>820 nm) are typically absorbed within the first meter resulting in the heating of surface waters.

Extinction coefficients vary with the wavelength (color) of the light. In distilled water, absorption is very high in the infrared, decreasing rapidly to a minimum in the blue and then increasing slightly in the violet and UV.

[T] Light penetration as a function of wavelength

The presence of dissolved organic matter, particularly humic acids (colored materials) has a marked impact on the amount and character of light in the water column. While light in the red and infrared is rapidly attenuated in all waters, the presence of even very small amounts of dissolved organic matter increases absorption in the UV and violet dramatically. Light in the blue, green, and yellow are influenced, but to a lesser extent.

The observed color of lake water results from the back-scattering of light after it has passed through water to various depths and undergone selective absorption en route. Thus the color observed at the surface will be those wavelengths which are poorly sorbed and well scattered.

Molecular scattering increases with frequency (decreases with the wavelength), i.e. light in the short wavelengths (UV, violet, and blue) are highly scattered. However, light in the UV and violet is easily absorbed and so little is actually available for scattering. Because it penetrates deeply (low absorption) and scatters well, blue light returns to the surface and results in the blue color of pristine lakes. If significant organic matter is present in the lake, the shorter wavelengths will be selectively absorbed and the emitted, scattered light will move toward the green portion of the spectrum. Highly stained waters will be even more selective, emitting light in the yellows and reds.

[T] Sunday Lake

The presence of particulate matter also has an impact on apparent color. Colloidal calcium carbonate, present in many hardwater lakes, selectively scatters light in the blues and greens, giving such systems a characteristic color. Clays can impart a red or reddish-brown color

[T] Ontonagon River

and high concentrations of algae can give the water a green or blue-green color.

[T] Cannonsville Reservoir

Lakes 'zones' may also be identified in relation to their light environment.

[T] Horne, Fig. 2.2, p. 17: structure of lakes

The free, open waters of a lake are termed the pelagial zone. The portions of the lake in contact with the sediment are the littoral zone (where enough light penetrates to support plant life) and the profundal zone (sediments free of vegetation). The lighted portion of the water column (>1% I₀) is termed the photic or euphotic zone and that below the 1% level the aphotic zone. The compensation depth, where photosynthesis and respiration are balanced is typically located near the 1% light level and thus forms a boundary between the photic and aphotic zones.