Lake Water Quality Model with Focus on Cyanobacteria

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Courtesy of Water Environment Federation (WEF)

ABSTRACT
The cause of cyanobacteria’s growth and dominance in certain eutrophic lakes is explored via a water quality model application. The one-dimensional lake water quality model includes nitrogen, phosphorus, silica, three phytoplankton groups and zooplankton. Lake Washington was used as the case study due to its unique history of eutrophication during a large wastewater nutrient loading period from in the 1960s and subsequent recovery. Sampling data for a 20-year period surrounding the eutrophication includes speciation of phytoplankton. Results indicate that nutrient loading, specifically phosphorus was the main factor for the persistent dominance of Oscillatoria in Lake Washington. Other key conditions and cyanobacteria traits are analyzed and discussed.

INTRODUCTION
Eutrophication, or heightened biological production, in water bodies is a problem and concern due to nutrient-rich runoff and inflow. Eutrophication causes reduced clarity, reduced dissolved oxygen and taste and odor problems in water bodies. Efforts have been made to describe the nutrient cycles, their relation to phytoplankton and plant growth, and the overall effect of nutrient input on river and lake systems. This knowledge is important for decision-making on several levels including lake management and discharge permitting.

Blue-green algae, or cyanobacteria, are contributors to eutrophication that stand out because of their special characteristics. Technically classified under the bacteria domain, they are often grouped with phytoplankton for water quality analysis. Different species of cyanobacteria are capable of living in a wide range of climate and trophic conditions. They are prokaryotic organisms (cells containing no nucleus) that were the dominant forms of life on the Earth over 1.5 billion years ago and were the first organisms to produce oxygen and chlorophylls a and b (Graham and Wilcox 2000). Some species are used as human food sources including health foods high in vitamins and proteins. Other species are used as fertilizers in rice fields because of their ability to fix atmospheric nitrogen.

Although there are some beneficial uses to species of cyanobacteria, they are considered a nuisance group in lake ecosystems. Specifically, cyanobacteria are most commonly associated with blooms in highly eutrophic systems late in summer. They are considered a problem because, according to Paerl (1988), bloom-forming phytoplankton (blue-greens) can cause perceptible water quality deterioration, heath hazards, or losses of aesthetic and recreational value. Some cyanobacteria are toxic to animals and people when ingested. They often create an unattractive, odorous coating on the water’s surface that requires additional treatment when present in drinking water systems. When conditions are favorable for their growth, cyanobacteria tend to dominate instead of co-existing with other phytoplankton groups.

Special research interest in cyanobacteria is the result of their significantly different growth and survival characteristics. Previous work includes studies and experiments defining cyanobacteria characteristics, regression analysis correlating cyanobacteria growth with physical and chemical lake characteristics, and models that simulate cyanobacteria separately from other phytoplankton. Despite this work, agreement of the causes of cyanobacteria growth and dominance does not exist among experts.

This research combines knowledge of cyanobacteria with practical modeling techniques to simulate their growth and dominance. Specific causes of cyanobacteria growth are explored through the simulation of a large bloom. A water quality model was created including nutrient and phytoplankton groups as state variables. Cyanobacteria were modeled separately than other phytoplankton groups to distinguish their special characteristics and establish modeling techniques capable of accurate simulation.

The model was tested against a 20-year period of water quality in Lake Washington, which experienced a recurrent cyanobacteria bloom in the 1960s that disappeared and has not resurfaced since. This data set allows the model to be tested against both long-term trends and seasonal cycles of nutrients and phytoplankton during cyanobacteria’s presence and absence. Also, periods of cyanobacteria high and low growth are included to fully test the modeling techniques.

The benefit of this computer model is the ability to include several potential influences on cyanobacterial growth and dominance at once. Cyanobacteria were modeled with individual rate coefficients and nutrient needs. In this way, the factors that affect cyanobacteria growth can be quantitatively determined and tested.

This paper first summarizes current knowledge of cyanobacteria’s unique characteristics in the context of current dominance theories. These ideas form the basis of the modeling techniques used to simulate cyanobacteria differently than other phytoplankton groups. The lake water quality model is then described including mathematical equations. The model is applied to Lake Washington’s water quality history. The results are discussed in regard to the successful and not successful techniques of cyanobacteria simulation taken from the literature. The goal of this work is to identify characteristics that can be used in a modeling framework to accurately simulate cyanobacteria growth.

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