Lake Properties

A lake is a dynamic water environment in which the chemical, physical and biological properties all interact and influence one another.  What this means is that, the biology, or life in a lake, is a product of the lake’s physical and chemical nature.  The opposite is also true since organisms within the lake can influence its chemical and physical properties.  By learning about the properties of lakes, we can better understand the influence of natural processes and our activities on lakes and make better choices to help protect them.

When looking at a lake, you may think that not much is going on underneath the water’s surface. In fact a lot is actually happening involving the lake chemistry, including oxygen and nutrient distributions which can greatly influence the lake’s health.

Oxygen

The concentration of dissolved oxygen in water plays an important role in determining the type and amount of organisms (fish, invertebrates, plants etc.) that live in a lake and is a good indicator of the lake’s overall health. 

The oxygen concentration found in lake water is influenced by the water sources entering into a lake (groundwater and surface water inputs), transfers from the air above the water’s surface and oxygen additions by aquatic plants through photosynthesis. Lake water can also lose oxygen through outflow, from losses to the atmosphere, through respiration by plant, bacteria and animals including the decomposition of organic matter, and by chemical reactions in water and lake bottom sediments.

Deep lakes may become divided into layers of water having different water density and temperatures, termed thermal stratification. The bottom layer of water, or hypolimnion, can become depleted in oxygen and can have serious implications on the water chemistry and aquatic life.

Nutrients

Nutrients are critical to the development of plant and animal life. In a healthy lake, nutrients nourish and promote growth of aquatic organisms such as algae, bacteria and aquatic plants. These form the base of the food web supporting the entire aquatic ecosystem.  Nitrogen and phosphorus are the most common nutrients in a lake. Phosphorus is usually the nutrient which limits the growth of algae and plants.  Under certain conditions when nutrients are abundant, algae and aquatic plants will continue to grow and multiply well beyond the amount needed to support the food web.  Such excess growth of algae and plants can lead to high rates of respiration and decomposition which deplete waters of oxygen and can chemically cause additional nutrients to be drawn into the water from lake sediments.

Lakes are not a uniform mass of water; in fact, they are extremely non-uniform and complex systems. The actual physical properties of a lake, such as its depth, shape and water temperature, add to this complexity through factors such as sedimentation and water circulation. 

Sedimentation

Sediments are an important carrier of nutrients and can promote plant and algae growth. Lake sediments (sand, silt, and clay) and particulate organic matter (dead plants, insects or animals) can be either suspended or deposited. In healthy lakes there is a balance between sediments entering, circulating within the lake and being used to support aquatic life.

When the quantity of sediment becomes elevated in lake water due to soil erosion or man-made activities, it can contribute to oxygen depletion in the water by promoting the growth of algae and plants. Too much suspended or free floating sediment can also decrease the amount of light able to penetrate through the lake water and can affect the health of aquatic organisms. Deposited sediments can limit habitat for aquatic life (fish, invertebrates) by infilling spaces between rocks that provide habitat supporting aquatic life.

Water Circulation

Lakes are dynamic systems which change during a year, and from year to year. Water in a lake is moving within the water column (surface to bottom), as well as through the length of the lake (inflow to outflow). During a year it is possible for lakes deeper than 5-7 meters to form layers of water having different temperature and oxygen concentrations. This is termed thermal stratification and is due to the changes in the density of water at different temperatures.  Shallow lakes however, tend to mix more readily and avoid this stratification.  From one season to the next, lake temperatures change creating a cyclical pattern that is repeated year after year. 

In a thermally stratified lake as the surface of the water cools during the fall, the cooler denser water at the surface sinks, mixing with the deeper cooler water at depth. At this time and again in the spring, the lake will mix or turnover more readily, bringing oxygen starved water up from the depths and taking oxygen down to decomposing sediments (Figure 1 - Lake seasonal thermal stratification and mixing).  As plant nutrients in the bottom sediments are stirred up, they provide fertile water for plant and algae growth. This turnover or water circulation is a key process that determines the cycling of oxygen, sediment and nutrients within a lake.

Water movement through a lake is affected by properties of the lake water such as thermal stratification, as well as the lake basin’s shape, depth and size, the location of the lake within its watershed and the size of the lake’s watershed. The time it takes for all the water within a lake to be replaced through inflow and outflow is referred to as the lake’s residence time or flushing rate. The longer it takes the water within a lake, or a substance introduced into the lake, to exit the lake, the more vulnerable the lake ecosystem will be to pollutants.

As mentioned previously, a lake is a complex system. In fact, it is an ecological system; a community of animals, plants, and microorganisms which interact and depend on each other and the environment in which they live. The biological properties of a lake affect its health and the uses of the water as much as the physical and chemical characteristics.        

Algae

Algae are photosynthetic organisms occurring in water either as single microscopic cells or visible colonies, and can be either suspended in water or attached to solid surfaces such as rocks and logs. Algae are an important living component of lakes and their presence is usually a good thing. Photosynthesizing algae and plants are the primary producers or initial converters of light energy and chemical nutrient energy into biological food energy within a lake food web. As a primary producer, algae form the basis of the food web and most other life in a lake depends on it for food, oxygen production and nutrient cycling.

Algae require light, a supply of nutrients, and specific temperature ranges in order to grow and reproduce. Of these factors, it is usually the supply of nutrients, especially phosphorus that dictates the amount of algal growth in a lake.

Aquatic plants

Like algae, aquatic plants play a vital role in a lake’s ecology.  They appear in many shapes and sizes and provide cover, habitat and food to the aquatic life in a lake.  Aquatic plants have organised structures such as roots, stems and leaves and can be grouped into four categories of plants:

• Emergent:  To about knee deep these are plants that rise well above the water surface such as cattails.
• Rooted floating-leaved: To about waist deep or deeper these are plants with leaves that rest on or slightly above the water surface such as water lilies.
• Submergent: From the shallowest lake zones out to several meters deep, these are plants where all or most of their leaves and stems are below the water surface such as pondweed or milfoil.
• Free-floating: These are plants that can be found on the lake surface, floating in the water column, or lying on the bottom.  Some species have roots freely floating in the water such as duckweed or water hyacinth.

Decomposers

Decomposers, including bacteria, fungi and other microorganisms feed on the remains of organic matter such as dead algae, plants, insects and animals that descend from the waters above.  In doing so, they break down or decay the organic matter, returning it to an inorganic state and releasing back into the water some of the basic substances the living material was composed of, such as the nutrients phosphorus and nitrogen, which then become available for new plant growth. 

When the level of organic material is excessive in a lake, available dissolved oxygen may be consumed and depleted as decomposers break down the organic plant and animal material. This process may have a negative effect on other oxygen dependant aquatic organisms such as zooplankton, fish and insects.  If levels of oxygen become too low, only a few species tolerant of low oxygen conditions will be able to decompose organic matter. Decomposers which do not need oxygen are anaerobic bacteria, and although helpful, they produce noxious gases as a by-product of their metabolism.

As you can see, there is much more to a lake than meets the eye.  The physical, biological and chemical factors that influence the workings of a lake provide the foundation needed to understand a lake, what can go wrong, and what we can do about it.

As a lake undergoes these chemical, physical and biological processes from season to season and year to year, the changes over many years result in visible signs of aging that can be seen in an overall lake’s appearance. This aging of lake systems is described in greater detail on the Lake Succession and Eutrophication page.