So What Causes the Decrease in Water Quality...?
Over time, lakes undergo a natural aging process of "eutrophication," in which sediments accumulate and oxygen content drops in the water--which eventually leads to algae and weeds relacing animal life. Humans can accelerate this aging process in lakes through "cultural eutrophication." The main cause is phosphorous-a natural element in soil, fertilizer, detergents, and sewage--for Green Lake, the main contributor was from excrements of animals--mainly from Canadian geese. When natural terrain is replaced by development, these materials can be washed by storm water directly into the lake. These excessive inputs appear to cause an imbalance in the "production versus consumption" of living material in an ecosystem. The system then reacts by producing more vegetation than can be consumed by the ecosystem. This overproduction can lead to a variety of problems ranging from anoxic waters to toxic algal blooms and decrease in diversity.
So how does phosphate cause this chain of decline? Phosphate stimulates the growth of plankton and aquatic plants which provide food for larger organisms such as zooplankton, fish, and other mammals-plankton represents the base of the food chain. Initially, this increased productivity will cause an increase in the fish population and overall biological diversity of the system. But as the phosphate loading continues and there is a build-up of phosphate in the lake, the aging process of the lake ecosystem will be accelerated. This overproduction leads to an imbalance in the nutrient and material cycling process. This aging process can result in large fluctuations in the lake water quality and periodic blooms of cyanobacteria.
Under aerobic conditions (presence of oxygen), the natural cycles may be more or less in balance until an excess of phosphate enters the system. At this time the algae begin to grow more rapidly than normal. As this happens there is also an excess die off of algae as sunlight is blocked at lower levels. Bacteria try to decompose the organic waste, consuming the oxygen, and releasing more phosphate which is known as "recycling or internal cycling".
In anaerobic conditions (absence of oxygen), all of the oxygen may be used up by bacteria in trying to decompose all of the waste. Different bacteria continue to carry on decomposition reactions, but the products are drastically different. The carbon is converted to methane gas instead of carbon dioxide, and sulfur is converted to hydrogen sulfide gas-occasionally causing the odor of Green Lake. Under anaerobic conditions, the iron phosphate precipitates in the sediments are released, which is the key component of the growth and decay cycle. The lake may gradually fill with decaying and partially decomposed plant materials to make a swamp-all a natural aging process. This eutrophication process is a natural progression-but the time it takes to age may range from only decades to over thousands of years depending on how the lake is treated.
Cynobacteria, commonly known as blue-green algae, are a natural and critical part of our ecosystems. It was this cyanobacteria that first filled the earth's atmosphere with oxygen, making other life possible. Algae may become harmful if they occur in an unnaturally high abundance or if they produce a toxin. An algal bloom is caused by a combination of factors such as temperature, nutrients, sunlight, and pH of the water. A high abundance of algae can block sunlight to underwater grasses, consume oxygen in the water leading to fish kills, produce surface scum and odors, and interfere with the feeding of organisms that filter water to obtain their food. Even without additional nutrition, Green Lake currently has enough fertile muck--over 18 feet thick--to feed cyanobacteria for years to come.
Green Lake has been closed in decades past when the algae blooms grew too thick. But that was because lifeguards feared they would be unable to see swimmers in trouble in the water. Some algal species can produce chemicals that are toxic to humans and aquatic life. Such type of algae, Aphanizomenon sp., caused the closure of Green Lake in 1999. This toxic organism comes from geese excrement, and people who accidentally swallow the water may experience vomiting, diarrhea, muscle weakness, nausea, thirst, a yellow complexion, and an irregular pulse.
Eurasian milfoil is a freshwater weed that grows into thick mats preventing sunlight from getting below the lake's surface, and rotting to give more nutrients for algae--all contributing to algal growth and eutrophication. They infested the lake in the early 1980s and they are thriving, in large part, due to fertilizer runoff from lawns in the surrounding neighborhoods that drain into the lake.
The milfoil spreads rapidly, mostly by fragmentation of plant parts. In the late summer and fall, the plants become brittle and break apart naturally. Each of these new fragment is capable of growing roots and developing into a new plant. It is competitive with native species and may completely dominate a plant community within a few years.