A Citizen's Guide to Understanding and Monitoring Lakes and Streams

Chapter Two - Lakes

Lakes are great. They provide so much in the way of recreation. Visit one on a hot summer day; fishing, boating, and swimming are just a few of the activities you are likely to see. Lakes and their shorelines also provide important wildlife habitat for both aquatic and terrestrial animals. Lakes even help protect water quality. Eroded sediments, debris, and other pollutants washed from watersheds are deposited in lakes by inflowing streams so that outflowing streams often carry less of these pollutants.

Eventually lakes fill in with the material carried to them by the streams. Even without human influence, a once deep clear lake will become shallow, weed filled, and green from algae. Over time, it will become a pond, then a marsh, and finally a forest. This natural aging process in lakes – which is actually based on increased growth and productivity – is called eutrophication.

We even have terms to describe the relative age and productive state of a lake. A young lake, with low productivity, is termed oligotrophic, a middle-aged lake is mesotrophic, and an older lake that is highly enriched is called euthophic. Normally, this aging process takes hundreds to thousands of years. In lakes affected by human actions, the changes can occur more quickly – sometimes changes that would normally take centuries occur over one person’s lifetime.

Lake water quality monitoring can be used to determine the age or level of enrichment of a lake, and the degree to which it has been affected by development. This chapter provides introductory information on lake characteristics and their effects on some typical lake sampling parameters, along with guidelines on how to design a lake monitoring plan and how to analyze and interpret the data you have collected.

The Physical Character of Lakes

No two lakes are exactly alike. They may differ in size, depth, number and size of inflowing and outflowing streams, and shoreline configuration. Each of these physical factors in turn influences the lake character. Some characteristics affected include the species of fish in the lake, the likelihood the shoreline will be weed covered or that algae will turn the lake green in the summer, and whether the lake water is warm enough for swimming or suitable as a drinking water source. Physical factors also influence decisions about sampling locations, water quality monitoring parameters, and how to interpret the data collected.

Lake Depth

In a deep lake, water near the surface may be very different physically, chemically, and biologically from water near the bottom. The top portion of the lake is mixed by the wind and warmed by the sun. Because of the available light and warmer temperatures many organisms live there. The more organisms there are photosynthesizing, breathing, eating, and growing, the higher the growth rate or productivity. The bottom portion of a deep lake receives little or no light. The water is colder; it is not mixed by wind; and decay of dead organic matter, called decomposition, is the main physical biological, and chemical activity.

A shallow lake is more likely to be homogeneous – the same from top to bottom. The water is well mixed by wind, and physical characteristics such as temperature and oxygen vary little with depth. Because sunlight reaches all the way to the lake bottom, photosynthesis and growth occur throughout the water column. As in a deep lake, decomposition in a shallow lake is higher near the bottom than the top for the simple reason that when plants and animals die they sink. It also is likely that a larger portion of the water in a shallow lake is influenced by sunlight, and that photosynthesis and growth are proportionately higher.

Lake Size

Lakes range in size from little more than ponds to reservoirs many miles long. As you can imagine, a pond and a reservoir are quite different systems. Although there are few hard and fast rules that govern lake size comparisons, the size does affect a number of important relationships. Some examples are the ratio of lake surface area to miles of shoreline, the percentage of the total water volume that is influenced by sunlight, and the ratio of the size of the watershed to size of the lake. These relationships affect how lakes function. A small lake with a greater ratio of shoreline to water volume may be more susceptible to damage from shoreline or watershed activities.

Inflows and Outflows

The size and number of inflowing and outflowing streams in a lake determine how long it takes for a drop of water entering a lake to leave it – a process called flushing. Some lakes flush in days while others take years. You may know of a lake that is actually just a widening in the river, where the inflowing stream constitutes a large portion of the total lake volume. Such a lake flushes relatively rapidly. In other lakes, the inflow is not even visible; all of it comes from groundwater seeps and precipitation. In the former case, quality of the incoming water is the single most important factor influencing lake water quality. In terms of pollution, the more rapidly the lake flushes the better because pollutants are flushed from the lake before they can cause too much damage. A more rapidly flushing lake also may respond sooner to pollution control activities in the watershed.

Shoreline Configuration

Another important lake characteristic is the shape of the shoreline. Shallow bays and inlets tend to be warmer and more productive than other parts of a lake. A lake with many of these features will be different than, say a bowl-shaped lake with a smooth, round shoreline. This difference becomes important when setting up a monitoring plan. In the latter case, one mid-lake sampling station may adequately represent the lake. In a lake strongly influenced by shallow bays or inlets, water quality is likely to be greatly affected by location, and multiple sampling stations probably will be necessary.

Lake Water Quality Parameters

The parameters that are most frequently tested in lake water are discussed in this section. These include temperature, dissolved oxygen, pH, Secchi disk depth, nutrients, total suspected solids and turbidity, chlorophyll a, and fecal coliform bacteria. For each parameter, you will learn why it is important, why measured values differ over time, and how pollution could affect the measurement. Since most of these parameters are related to each other, the relationship is described twice, once under the discussion of each parameter. For example, there is a relationship between temperature and dissolved oxygen. This relationship is described in both the discussion on temperature and the discussion on dissolved oxygen. If you find it difficult to understand the discussion under one parameter, move on to the next; with luck you will find the next discussion helps clarify the first. Chapter Four describes the different methods for analysis of each parameter.

Temperature
Dissolved Oxygen
pH
Secchi Disk Depth
Nutrients
Total Suspended Solids and Turbidity
Chlorophyll a
Fecal Coliform Bacteria