1. Background

Between 2020 and 2023 the US Environmental Protection Agency (USEPA) will survey water quality and greenhouse gas (GHG) emissions from 108 reservoirs distributed across the United States (Figure 1). The objective of the research is to estimate the magnitude of GHG emissions from US reservoirs.

All reservoirs included in this study were previously sampled by the USEPA during the 2017 National Lakes Assessment (2017 NLA). Data from the 2017 NLA can be found at the EPA website. Data for Monroe Lake can be found under SITE_ID NLA17_IN-10009.

A field sensor is used to measure chlorophyll a, dissolved oxygen, pH, specific conductivity, water temperature, and turbidity near the water surface at a minimum of 15 locations within each reservoir. Water samples are collected from the deepest site for analysis of nutrients and chlorophyll a.

This preliminary report presents water quality results for Monroe Lake. These data will be included in a formal peer-reviewed publication to be submitted for publication in 2024.

Figure 1. Location of the 108 Reservoirs Included in Study.

2. Monroe Lake Survey Design

The Monroe Lake survey design included 20 sampling sites that were sampled on 2020-08-10. Water chemistry samples were collected from a 9.8m deep site nearby the dam (Figure 2). Click on any of the sites to see the site id, water temperature, pH, turbidity, and dissolved oxygen at the water surface.

Figure 2. Location of the 15 sampling sites in Monroe Lake.

3. Lake Disturbance and Trophic Status

Lakes are often classified according to their trophic state. There are four trophic state categories that reflect nutrient availability and plant growth within a lake. A eutrophic lake has high nutrients and high algal and/or macrophyte plant growth. An oligotrophic lake has low nutrient concentrations and low plant growth. Mesotrophic lakes fall somewhere in between eutrophic and oligotrophic lakes and hypereutrophic lakes have very high nutrients and plant growth. Lake trophic state is typically determined by a wide variety of natural factors that control nutrient supply, climate, and basin morphometry. A metric commonly used for defining trophic state is the concentration of chlorophyll a, an indicator of algae abundance, in the water column. Chlorophyll a concentration was 4 ug/L during the sampling, indicating the lake was mesotrophic.

Trophic State Classification
Analyte Oligotrophic Mesotrophic Eutrophic Hypereutrophic
chlorophyll a (ug/L) <=2 >2 and <=7 >7 and <=30 >30

In addition to classifying lakes by trophic status, lakes can be classified by degree of disturbance relative to undisturbed lakes (i.e. reference lakes) within the ecoregion. Degree of disturbance can be based on a wide variety of metrics, but here we use nutrients (total phosphorus (tp), total nitrogen (tn)), suspended sediment (turbidity), chlorophyll a, and dissolved oxygen (do). Lake disturbance values range from least to moderately disturbed.

Chemical Condition Indicators Measured at Water Chemistry Site
Threshold Values
Observed Values
parameter units least disturbed moderately disturbed most disturbed concentration status
do mg/l >5 >3 & <5 <3 8 least disturbed
turbidity NTU <3.32 >3.32 & <4.67 >4.67 3.60 moderately disturbed
tp ug/l <34 >34 & <56 >56 24 least disturbed
tn ug/l <657 >657 & <830 >830 200 least disturbed
chlorophyll a ug/l <6.85 >6.85 & <13.8 >13.8 4.1 least disturbed

4. Within-lake Spatial Patterns

A field sensor was used to measure water temperature, pH, dissolved oxygen, and turbidity near the water surface at all sampling sites. Data are reported in figures and tables below. Hover the curser over any point in the figures to reveal the siteID corresponding to the adjacent data table. Alternatively, click on any row in the data table to reveal the location of the sampling site on the map.

Dissolved oxygen and pH, indicators of algal activity, were highest in the upper reaches of the reservoir. This pattern often occurs when tributaries are the primary nutrient source to reservoir surface waters. Algal activity is highest in the high nutrient waters, causing dissolved oxygen and pH to be elevated (e.g. photosynthesis produces oxygen and consumes dissolved inorganic carbon, causing pH to increase). High turbidity was associated with high DO and pH, potentially reflecting high algal biomass. Closer to the dam, surface water was clearer and had lower dissolved oxygen, possibly because algal activity was limited by relatively lower nutrient availability.





















5. Depth Profiles

Dissolved oxygen is one of the most important environmental factors affecting aquatic life. The biological demand for oxygen is often greatest near the sediment where the decomposition of organic matter consumes oxygen through aerobic respiration. Near the surface of lakes, photosynthesis by phytoplankton produces oxygen, often leading to a general pattern of decreasing oxygen availability with increasing depth. This pattern can be exacerbated by thermal stratification. Thermal stratification occurs when lake surface waters are warmed by the sun, causing the water to become less dense and float on top of the deeper, cooler lake water. Since the deeper layer of water cannot exchange gases with the atmosphere, the dissolved oxygen content of the deep water cannot be replenished from the atmosphere. As a result, the deep water can become progressively depleted of oxygen as it is consumed by biological activity, sometimes causing dissolved oxygen to become sufficiently scarce to stress oxygen sensitive organisms including some fish and insects.

The deepest sampling location in Monroe Lake was 9.8 m deep. The water was strongly stratified at the sampling site, yet dissolved oxygen exceeded 4 mg/L at the lake bottom.

  1. Jake Beaulieu, United States Environmental Protection Agency, Office of Research and Development, ↩︎