Geneva Lake Data

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Aquatic Plant Survey - 2019


Over the years the Geneva Lake Environmental Agency has conducted several aquatic plant surveys on Geneva Lake. The first one was conducted in 1976 and the most recent one was completed during the summer of 2019 with survey scheduled for 2020 and 2021. These surveys are the foundation for the aquatic plant management plan which is anticipated to be updated in 2020-21 Based upon the type and numbers of plants found in the lake, aquatic plant management strategies are approved or denied when applied for.

The early plant surveys were conducted using the transect methods, with plants identified every given feet along predetermined transects. During the summer of 2015 a comprehensive survey was conducted using the point intersect method. for the first time.   Points were identified every 50 meters and samples were collected, with plants identified and quantified. A total of 2,685 sites were located by longitude and latitude. Similar methodology was used in 2019 and will be used in 2020 and 2021.  

The data from the 2019 survey showed a healthy, rich bio-diverse aquatic plant community.  However, the presence of a new aquatic invasive species, starry stonewort was found in the lake for the first time.   s still being analyzed but some of the preliminary numbers are interesting. A total of 785 sites were sampled with 653 sites containing vegetation. Although aquatic plants may have been identified as deep as 41.6 ft., most plants were found under 30 ft. (table 1.) The remaining sites were too deep for light penetration and thus plants could not grow. A total of 31 different species were identified. Native species dominated the plant community lake wide.

Canagement efforts. Look for his results during summer of 2016.


An Aquatic Plant Management Plan

A product of the past aquatic plant survey was the preparation of an aquatic plant management plan (see map of recommended management below). This aquatic plant management plan will be reviewed and modified as needed when an assessment of the 2015 survey is completed.


In general, the lake was divided into three separate aquatic plant management areas. The “No Restriction Zone” where it is recommended that there be no restriction on the aquatic plant management strategies if they are compatible with State requirements. The “Native Zone” are areas that have a diverse and healthy plant community and thus no management is needed or recommended unless conditions change. The “Watch Zone” is the largest area in size and includes areas were the plant community may need some minor management but for the most part is relatively stable. An overlay “Riparian Zone” can be located in any one of the other three zones but is restricted to only a 30 ft. wide strip running perpendicular to the shore and extends out into deeper water. This zone is to allow for riparian management of aquatic plants in their swim, mooring or pier areas.

Zebra Muscles

To download the final report, click here

Zebra Muscle Substrate

Trophic State Index

A trophic state index assigns a numerical value to specific measurement found in the lake that are representative of the lake’s overall condition. There are several trophic status indexes that are used but for the purpose of understanding Geneva Lake’s trophic condition, a modified version referred to as the Wisconsin Trophic State Index is used. Generally, the higher the trophic value the worse the water quality.

Trophic state refers to the amount of biological activity in a lake. The more trophic a lake is the more fertile it is. A very biologically active fertile lake with a lot of plants would be considered a eutrophic lake. A lake with very little plant life and not much fertility is called an oligotrophic lake. Lakes the fall between very fertile and a not very fertile are referred to a mesotrophic lakes.

Three measurement used to evaluate a lake’s fertility are; total phosphorus. chlorophyll ”a” and secchi disk or water clarity. Phosphorus is a vital nutrient that is often the limiting nutrient for plant growth in lakes. The more phosphorus in a lake, the more plant growth. Chlorophyll ”a” is a color pigment found in green plants. A measure of the chlorophyll ”a” in the water column is an indirect measurement of plant density in the open water or plankton community. Secchi disc reading measure the depth of light penetration into the water. All three are related, the more phosphorus the more chlorophyll” a” and the more chlorophyll”a”, the more algae in the water and the less light penetrates into the water.

The figure below shows the plotted TSI values for Geneva Lake’s total phosphorus(TP), Chlorophyll (chl”a”) and secchi over the last 18 years. Geneva Lake’s secchi disk TSI is mostly in the oligotrophic range. The Chl”a” TSI value moves between mesotrophic and oligotrophic. Only the total phosphorus TSI ever reaches eutrophic and only on few occasions. It is mostly in the mesotrophic range. Based upon this information, Geneva lake would be classified as a relatively clean lake that is in transition between oligotrophic and mesotrophic states.

Chlorophyll a

Total Phosphorus

Secchi Disk Trends

A secchi disk is a round disk, 20 cm. in diameter,  that is colored alternately in black and white quadrants.  It is used to measure water clarity.  The disk is lowered in the water and the depth at which it can no longer be seen is recorded.  It is then slowly brought back up and the depth at which it reappears is recorded.  The two depths are then averaged for a Secchi depth recording. It is used to measure water clarity.


The August 9, 2018 temperature and dissolved oxygen readings show the lake is in strong summer stratification.  A strong thermocline has been established between the depths of 29.5 ft. (9.0 m) and 45.9 ft. (14 m).  The thermocline separates the upper circulating warm layer of the lake’s epilimnion  from the deep cold waters of the hypolimnion.  At the time of sampling, the epilimnion existed from the lake’s surface to a depth of 29.5 ft (9.0 m), and the hypolimnion existed from 45.9 ft (14 m) to the measured bottom at 139.8 ft(43 m). The thermocline was recorded as being 19.7 ft (6.0 m) thick. 


The summer of 2018 stared cool and wet and progressed to hot and calm.  At the time of this sampling, the surface temperature at the deepest hole was recorded as 76.6 F (24.8 C) with the bottom of the epilimnion being at a temperature of 64.7 F (18.2 C ).  Water temperature dropped rapidly in the thermocline with the bottom of the thermocline being at 49.8 F (9.9 C). Within the thermocline’s  thickness of 19.7 ft (6 m) the temperature of the water dropped 14.9 F (8.3 C).  The thermocline is defined as that part of the water column where the water temperature decreases at a rate of 1 C per meter or greater. 


The thermocline will continue to erode and sink as the circulating warmer epilimnion waters wears away at the top of the thermocline. This action will continue until the fall when cooling surface waters reach the same temperature as the thermocline.  This will cause the thermocline to disappear.  The lake will then enter into fall turnover where temperature and dissolved oxygen will be uniform from the top waters of the lake to the bottom waters. The lake experiences complete top to bottom mixing. On Geneva Lake, this generally happens in early to mid-November.


At the lake’s surface, the dissolved oxygen concentration was measured as 9.01 mg/L. At the top of the thermocline, oxygen concentration had dropped to 7.78 mg/L. Oxygen concentration slowly increased to a maximum of 9.67 mg/L at 39.4 ft

(12.0 m) and slowly declined to 5.54 mg/L at 114.8 ft (35 m).  It then decreased rapidly to 0.32 mg/L at the lake bottom at 141.0 ft (43 m).


At a depth of 121.4 ft (37 m), oxygen levels drop below 5.0 mg/L. At 131.5 ft (41 m), oxygen levels drop below 2.0 mg/L. Fish experience higher levels of stress when dissolved oxygen levels drop below 5 mg/L. Most fish species cannot survive for any period of time where dissolved oxygen is less than 2 mg/L.

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