First Settlers

The Ho-Chunk Nation of Wisconsin, also referred to as the Winnebago, were some of the first residents in the Green Lake Area.^[3] Green Lake, also known as Daycholah, is a spiritual place for the Winnebago. One legend describes how one must bring gifts for the Water Spirit that lives under the lake in order to enter.^[4] With the increase of European settlers, the Winnebago were subject to forced relocation by the United States government several times. Once they were removed from the Green Lake area, they were moved to Iowa until the establishment of a reservation in Blue Earth County, Minnesota.^[5]

Chief Highknocker

Chief Highknocker was the last reigning chief of the Green Lake area. He was born in 1820 with the given name Henaga. He spent most of his life around the lake but was forced to move further north because of the ever-increasing number of settlers. He continued visiting Green Lake every summer with other members of the tribe. On August 12, 1911, at the age of 91, he drowned after getting a cramp while swimming in the lake. His grave at Dartford Cemetery is marked by a large boulder with an inscription from the community.^[4]

Chief Highknocker's Grave at Dartford Cemetery

European Settlement

French settlers initially called the lake Lac Verde. In 1836, Wisconsin land could be purchased through the Green Bay Land Office at $1.25 per acre.^[4] Anson Dart built a dam with Smith Fowler and his own son in an effort to raise the lake level, leading to the namesake of Dartford (now called Green Lake). The next year, Dart built a sawmill that used the mill race created by the dam. The dam's success in raising lake level allowed different types of mills to be constructed around Green Lake. Many large, wooden hotels and one short-lived casino populated the north shore during the late 19th century in the city of Green Lake, but most have burned down. One was the Pleasant Point, which opened in the early 1880s.^[6] The name "Green Lake" comes from the green color of its deep waters.^[7] The city of Green Lake is located on the northeast shore in Dartford Bay.

Drainage Basin

At 72.24 m (237.0 ft) deep and with a volume of 0.9390 km³ (761,300 acre⋅ft),^[2] Green Lake is the deepest natural inland lake in the state and second-largest by volume.^[13] Only Lake Wazee, a man-made lake, is deeper, and only Lake Winnebago has a larger volume. The deepest and steepest part of the basin is near the west end, while the eastern third is more shallow. The maximum gradient is 33% at Lucas Bluff, a large rock face on the southeast shore. There are shoals at Malcom Bay and the east end of the lake^[14] The lake has a maximum fetch of 11.75 km (7.30 mi). This allows a theoretical maximum surface wave height of 1.14 m (3.7 ft).

Green Lake is within the Big Green Lake Watershed, which has a catchment area is 277.9 km² (107.3 sq mi). The drainage basin lies mostly in Green Lake County, but extends east into Fond du Lac and Winnebago County. In addition to Green Lake, the drainage basin includes Spring Lake and the East and West Twin Lakes.^[12]

Tributaries and Outlet

The components of the Green Lake basin water budget are, in approximate percentages: direct precipitation, 51%; surface water, 41%; ground water, 8%. Eight creeks make up the surface water supply, the largest being Silver Creek, which flows into the east end of the lake. The remaining seven are Hill Creek, White Creek, Dakin Creek, Wurches Creek, Roy Creek, Spring Creek, Assembly Creek. The primary outflow is the Puchyan River, a tributary to the upper Fox River that lies in the Lake Michigan drainage basin.^[13] This makes Green Lake a drainage lake, a lake in which surface outflow dominates.^[15] A dam in the outlet keeps the water level in the lake around 1.52 m (5.0 ft) higher than the natural level.^[14] Green Lake has a retention time of 21 years, relatively long for a temperate lake of its size. This has implications for lake health because with very low rates of flushing, there is a large lag for reversion of fertility changes.^[13]

Temperature

The highest recorded average temperature in 2018 was 27.2 °C (81.0 °F).^[16] The median ice cover duration of the last 67 seasons is 87 days.^[17] Green Lake has lost over 25 days of ice cover since 1940, and at the current rate, will continue to lose one day of ice cover every three years. At this rate, Green Lake is estimated to have zero ice cover by 2087.^[13]

Oxygen

Dissolved oxygen (DO) is key to the health of aquatic ecosystems. It is essential for the metabolism of all aerobic organisms, therefore controlling their distribution, behavior, and growth within the ecosystem. If oxygen is depleted, these organisms will die or not do well. Oxygen also reacts with many other compounds, influencing their availability and chemical forms.^[15] Due to its thermal stratification, the concentration of DO in Green Lake decreases through the thermocline and into the hypolimnion.^[13] One of the most interesting characteristics of the Green Lake DO profile is a pronounced metalimnetic oxygen minimum, documented through the literature since the 1900s at a depth of 10 m (33 ft) and with a thickness of ~3 m (9.8 ft). The dissolved oxygen drops from ~9 mg/L to ~4 mg/L before rebounding and slowly decreasing into the hypolimnion. Based on other chemical characteristics such as pH and chlorophyll fluorescence at this depth, it is likely that the metalinetic oxygen minimum results from a concentration of respiring plankton.^[18] The quality of the environment can be judged by oxygen levels. Because the DO level is <5 mg/L, the lake was classified as "impaired" by the Wisconsin DNR in 2014. The low DO in this band poses a danger for aerobic organisms and has the potential to expand.^[13]

Another threat facing the lake is hypolimnetic oxygen depletion. A paleolimnological sediment analysis conducted in the deep basins of the lake showed trends of oxygen depletion since 1950. Though the deep waters are not yet devoid of oxygen, the oxygen is being depleted faster than it was in 1950. This is a result of increased nutrient (nitrogen and phosphorus) availability in the epilimnion, which increases biological productivity. As these organisms die, sink, and aerobically decompose, oxygen is depleted.^[13]

Nitrogen and Phosphorus

Nitrogen is essential in many biomolecules, such as amino acids, and has roles in biosignalling. It is also a major excretory product of livestock and a key ingredient in fertilizers, both of which play a key role in the agriculturally-dense Big Green Lake drainage basin.^[15][12] Phosphorus is also a key element in living things, constituting molecules such nucleic acids and phospholipid membranes. Like nitrogen, it is a major excretory product of livestock and a key ingredient in fertilizers.^[15] Nitrogen and phosphorus are often identified as limiting nutrients in aquatic ecosystems, since they are limited in availability relative to their demand in the ecosystem.^[15] Should the availability of phosphorus increase, algal growth will increase, decreasing water quality.^[15] Increase in phosphorus concentration are the most pressing threat to the health of Green Lake^[13] Total phosphorus values in the epilimnion range from 10 μg/L to 50 μg/L. Due to variation and associated confidence limits in the average total phosphorus level of 17.2 μg/L, the lake does not meet the "impairment" designation of >15 μg/L of phosphorus. It is likely, however, that the lake is impaired.^[13] Increasing phosphorus is likely to blame for the hypolimnetic oxygen depletion and metalimnetic oxygen minimum mentioned in the previous section. Because most of the tributaries enter on the east end of the lake, phosphorus has generally been higher in the east end.^[13] Current lake management efforts focus heavily on monitoring and reducing phosphorus loading within the drainage basin by controlling erosion, runoff, and fertilizer use.^[13]

The specific conductance ranges from 502.80 - 532.88 microSiemens per centimeter.^[18]

Flora

Aquatic vegetation found in most recent survey^[19]

The most abundant native aquatic plant species found in Green Lake, based on littoral frequency of occurrence, are: Coontail, at 53%; Sago pondweed, at 15%; Wild celery, at 13%; and White water-crowfoot, at 12%.^[19]

Fishes

Green Lake is a highly productive and highly utilized fishery. Because of its depth, it supports both a warm-water and cold-water fishery, though species of the warm-water fishery predominate.^[13] 40 species of fish have been documented in Green Lake.^[20]

Northern Pike

Common Warm-water Fishery Species

• Largemouth bass
• Smallmouth bass
• Walleye
• Northern pike
• White bass
• Channel catfish
• Bluegill
• Black crappie
• White crappie
• Yellow perch
• Rock bass
• Muskellunge (stocked)
• Common carp

Lake Trout

Ecology of the Warm-water Fishery

Both small and largemouth bass reproduce naturally and maintain stable populations in the lake. Largemouth are susceptible to habitat loss due to the low amount of vegetated littoral habitat. Smallmouth bass thrive in the lake due to the rocky and steep bottom, and walleye naturally maintain a low population in Green Lake. Targeted fishing, limited littoral habitat, late spring warming, and predation on eggs, fry, and fingerlings limit the size of the population.^[13] Similarly, northern pike are also limited by spawning habitat degradation by development, boating, and carp. Angling also puts heavy pressure on the population, so size and bag limits have been implemented to help reduce the strain. Panfish maintain healthy populations, but angling and shoreline development represent main threats. The loss of woody vegetation along the shoreline is especially damaging.^[13]

Studies have also focused on the health of non-game fish populations as they related to ecosystem health. In Green Lake, species such as the blackchin shiner, Iowa darter, and mottled sculpin are all intolerant to environmental disruption and therefore serve as indicator species of lake health. In addition to carp disturbance, shoreline development, and boating disturbance, cyanobacteria blooms, zebra mussels, and filamentous algae growth may also represent limiting factors to habitat health and fish growth.^[20]

pancake ice along the south shoreline of Green Lake, near Tuleta Hills

Ecology of the Cold-water Fishery

Cisco are native to Green Lake and are regularly fished. They are very sensitive to environmental change, and their populations are in flux. Recent surveys in Green Lake have shown that the cisco population has slightly declined, possibly due to the ecological effects of eutrophication. Climate change severely threatens cisco, and their populations could be reduced by 30-70%. Lake trout are the prized species of the cold-water fishery, and the WDNR stocks 25,000 yearlings annually. These fish are raised at the Green Lake Cooperative fish rearing facility, which is owned by the Green Lake Sanitary District and operated in conjunction with the WDNR. There have been reports of scarring on adult lake trout, suggesting possible conflict with muskellunge in the boundaries between the warm and cold-water fisheries. A main threat facing the fish of the cold-water fishery is oxygen depletion, especially in the deep west basin.^[13] The Wisconsin inland lake record lake trout was caught on Big Green Lake by Joseph Gotz on June 1, 1957, and weighed 35 lb 4 oz (16.0 kg). The Wisconsin record cisco was caught on Big Green Lake on June 12, 1969, by Joe Miller and weighed 4 lb 10.5 oz (2.11 kg).^[21]

Aquatic Vegetation and Algae

A majority of aquatic vegetation was found growing at a depth of 14 feet (4.27 m) and a maximum depth of 23 feet (7.01 m). Of all the observed species, coontail and hybrid water milfoil were the highest frequency of occurrence. Coontail is a common aquatic species in Wisconsin and able to survive and thrive in areas with light deficiency as it receives a majority of nutrients from the water. The ability to grow in deeper areas of the littoral zone allows coontail to provide habitat for invertebrates and fish.^[13] Species like common carp and zebra mussels have contributed to the degeneration of shallow aquatic plant species. The high nutrient concentrations attribute to algal growth that may have future negative effects.^[13]

Pollution

Prior to European settlement, the lake was considered to be oligotrophic, meaning that it had low biological productivity and good water quality based on a trophic state index. Green Lake is now considered to be mesotrophic lake because of increased nutrient loading from both natural and anthropogoenic causes. Water quality has since decreased and is considered “impaired” per standards laid out in the Clean Water Act by the Environmental Protection Agency.^[7]

Invasive Species

Curly-leaf pondweed

Invasive Flora

Hybrid water milfoil and curly-leaf pondweed have been documented in Green Lake since 1971. A 2014 survey showed that hybrid water milfoil increased in dominance to make up 25% of the aquatic plant species. The most frequently found singular plant species in the lake in 2015 was Eurasian water milfoil at 20% of relative frequency. Another invasive plant species, the spiny naiad (Najas marina), was documented in a 2007 WDNR survey, but was not observed in the 2014 survey.^[19]

• Eurasian water milfoil
• Hybrid water milfoil
• Curly-leaf pondweed
• Purple loosestrife
• Reed canary grass

Zebra mussel

Green Lake Conservancy

The Green Lake Conservancy is an organization dedicated to preserving the lands surrounding Green Lake. The organization works by acquiring environmentally sensitive properties, such as woods or wetlands, and protecting them by placing them into conservancy. They are then cooperatively managed with the WDNR and Green Lake Sanitary District. To date, they have protected 15 properties around the lake.^[24]

Green Lake Association

Founded in 1951, the Green Lake Association (GLA) is a 501 (c)(3) nonprofit, membership organization. Their membership includes nearly 800 households and businesses within the drainage basin. They focus on the water quality of the Big Green Lake drainage basin.^[25]