COMPUTER-BASED SIMULATION OF LOADS AND WATER QUALITY ...

COMPUTER-BASED SIMULATION OF LOADS AND WATER QUALITY RESPONSES WITHIN THE OWASCO LAKE WATERSHED

Barry M. Evans, Ph.D. Penn State Institutes of Energy and the Environment Penn State University University Park, PA 16802

April 2008

Introduction The relative water quality or condition of a lake is largely dictated by the quantity of critical nutrients delivered to it by the surrounding watershed. One measure commonly used to describe this condition is the “trophic state”. This trophic state, in turn, is generally determined by the relative level of phytoplankton production (or “primary productivity”). Underpinning this concept is the idea that there is a natural progression in the “life” of a lake from oligotrophy to eutrophy, which is generally measured in thousands of years. However, anthropogenic (human) activities that result in increased nutrient loads can greatly accelerate the natural “aging” process via what is termed “cultural eutrophication”. This process can lead to declines in water quality as exhibited by decreased water clarity and increased occurrence of algal blooms. A number of nutrients are needed for biomass growth, most notably nitrogen, phosphorus and carbon. The average molar ratios (Redfield ratios) of nitrogen to phosphorus to carbon (N:P:C) in algal protoplasm are approximately 15:1:105. If one of these nutrients is available in smaller proportion to the others, it tends to limit growth, while any nutrient increase will result in a direct increase of biomass. In lakes located in the northeast part of the country, phosphorus is typically the limiting nutrient so that the addition of phosphorus will spur growth while the addition of nitrogen will have minimal effects in a lentic (standing water) system. For this reason, phosphorus is typically the focus of pollution-reduction efforts for lakes in this region. Oligotrophic lakes are characterized by low phosphorus concentrations, low levels of primary productivity, excellent water clarity, and a well-oxygenated hypolimnion (i.e., water at lower depths) throughout the year. Eutrophic lakes are characterized by high phosphorus concentrations, elevated levels of primary productivity, poor water clarity, and low dissolved oxygen (DO) concentrations in the hypolimnion. Mesotrophic lakes fall between the other two categories, and are often described as having intermediate levels of phosphorus and primary productivity, moderate water clarity, and moderate levels of hypolimnetic dissolved oxygen. Table 1 illustrates typical water quality values of these three trophic designations.

Table 1. Trophic Status of Lakes Variable

Total P (ug/l) Chlorophyll-a (ug/l) Secchi disc depth (m) Hypolimnetic oxygen (% sat.)

Oligotrophic

Mesotrophic

Eutrophic

80

10-20 4-10 2-4 10-80

>20 >10