Under favorable conditions of light and temperature, excess amounts of nutrients in water can increase the growth of algae and other plants. The result of this growth is an increase in the rate of eutrophication, which is a natural ecological process of change from a nutrient-poor to a nutrient-rich environment. Eutrophication is defined as the process by which a body of water becomes enriched in dissolved nutrients (as phosphates) that stimulate the growth of aquatic plant life, usually resulting in the depletion of dissolved oxygen (Merriam-Webster, Inc., 1996).

Nutrient enrichment of moving waters and lakes is a normal result of soil weathering and erosion processes. The gradual evolution of Ice Age lakes into marshes and, eventually, organic soils is a result of eutrophication. However, this process can be accelerated by the discharge of wastes containing high levels of nutrients into lakes or rivers. One example of this is Lake Erie, which is estimated to have aged the equivalent of 150 natural years in a 15-year span of accelerated eutrophication.

Excessive plant growth caused by accelerated eutrophication can lead to stagnation of the water. The stagnation is caused by an increased biological oxygen demand created by decaying plant remains. The result of this increased oxygen demand is a tendency toward anaerobic conditions and the inability of the water body to support fish and other aerobic organisms.

Nitrogen, carbon and phosphorus are essential to the growth of aquatic biota. Due to the difficulty of controlling the exchange of nitrogen and carbon between the atmosphere and water and fixation of atmospheric nitrogen by some blue-green algae, attempts to mitigate eutrophication have focused on phosphorus inputs. In fresh-water systems, phosphorus is often the limiting element. By controlling phosphorus loading, accelerated eutrophication of lake waters can be reduced.

In systems where phosphorus is the primary, controllable limiting nutrient of water body eutrophication, the amount of phosphorus present in the water body can be used to estimate the amount of eutrophication present in the water body.

A number of empirically derived equations have been developed to calculate chlorophyll a level as a function of total phosphorus concentration. SWAT+ uses an equation developed by Rast and Lee (1978) to calculate the chlorophyll a concentration in the water body.

$Chla=0.551*p^{0.76}$ 8:3.3.1

where $Chla$ is the chlorophyll $a$ concentration ($\mu$g/L) and $p$ is the total phosphorus concentration ($\mu$g/L).

The equation has been modified to include a user-defined coefficient:

$Chla=Chla_{co}*0.551*p^{0.76}$ 8:3.3.2

The user-defined coefficient, $Chla_{co}$, is included to allow the user to adjust the predicted chlorophyll $a$ concentration for limitations of nutrients other than phosphorus. When $Chla_{co}$ is set to 1.00, equation 8:3.3.2 is equivalent to equation 8:3.3.1. For most water bodies, the original equation will be adequate.

While evaluation of water quality by secchi-disk depth measurements is subjective, some general correlations between secchi-disk depth and public perception of water quality have been made. One such correlation made for Annebessacook Lake in Maine (EPA, 1980) is given in Table 8:3-3.

Table 8:3-3: Relationship between secchi-disk depth and public perception of water quality.

Table 8:3-4: SWAT+ input variables that impact eutrophication calculations in ponds, wetlands and reservoirs.