Lake Ecosystems

Lake ecosystems include living organisms and their interactions with non-living physical and chemical characteristics of the lake. Important physical features include light, temperature and wind, and important chemical features of the lake include dissolved oxygen, and nutrient concentrations like phosphorus and nitrogen.

Lakes are home to a plethora of life, large and small that live in the water column, in the sediments (benthos), on the surface (insects, birds, and waterfowl), and near the shoreline (littoral zone). Most of the lake ecosystem at Elk/Beaver Lake has been altered to some degree by people, when farms, roads, residences and other structures were built in and around the lake.

The ecosystem is composed of primary producers like plants and algae that produce their own energy from the sun), consumers like fish and birds that eat other organisms, and decomposers like bacteria that break down dead organisms. All organisms play a different role in the ecosystems and occupy various habitat zones throughout the lake.

Shallow Water/Littoral Zone

The littoral zone is the shallow water area (< 3 m deep) of the lake directly adjacent to the shoreline. Lake littoral zones provide essential habitat for emergent aquatic plants, juvenile fish, frogs, turtles, and bird/waterfowl feeding and refuge. The littoral zone covers 45% of the Elk Lake and 60% of the Beaver Lake surface areas.

There are some areas around Elk Lake where the littoral zone is not vegetated with aquatic plants. This is problematic because the littoral zone vegetation protects against shoreline erosion. Shoreline erosion is evident as bare dirt and exposed tree roots. Vegetated areas in shallow water around Elk and Beaver Lakes mostly contain water-milfoil and giant bulrush (Scirpus lacustris).

Limnetic Zone (Open Water)

The limnetic zone is the well-lit, open surface waters of Elk/Beaver Lake, away from shoreline. This area is the main photosynthetic zone of the lake, where the majority of oxygen and food is produced to support other life. This area is occupied largely by a variety of phytoplankton (including blue-green algae) that use sunlight to photosynthesize. This area also includes zooplankton, small crustaceans and fish.

Profundal Zone

The profundal zone is a deep zone of the lake, below the limnetic zone where light can no longer effectively penetrate. This is zone of the lake is typically much cooler than surface temperatures. This temperature difference may be significant enough during at certain times of the year, particularly spring and summer where mixing with the above surface waters cannot occur. This lack of water mixing means that oxygenated waters at the surface don’t reach deeper waters, causing a decrease in oxygen concentrations in this deep profundal zone.

In Elk Lake, this thermocline generally forms at 8 meters below the surface waters from Spring to late summer, causing the profundal zone to become hypoxic (low in oxygen) or anoxic (no oxygen). In the fall, the surface temperatures cool down and result in mixing of bottom and surface waters, allowing the movement of oxygen and nutrients between the surface limnetic zone and deeper profundal zones.

The lack of light and oxygen in the profundal zone determines the type of biological community that can live in this region and this community is often distinctly different from the community in the overlying waters.

Benthic Zone

The benthic zone is the bottom zone of the lake, including the sediment surface and sub-surface layers. Organisms living in this zone are called benthos and include microorganisms like bacteria as well as larger invertebrates, such as the native crayfish and freshwater worms. Organisms here generally live in close relationship with the sediment and many are permanently attached to the bottom. This zone of Elk/Beaver Lake is seasonally low in oxygen, similar to the profundal zone.

Wetlands and Riparian Zones

Wetlands are areas where the soils are saturated or inundated long enough during the growing season to support wetland vegetation. Riparian zones are areas that surround water bodies and have moist soils and water-loving plant species. Plants such as flowering rush (Juncus densiflorus), snowberry (Symphoricarpos albus), willows (Salix sp.), red alder (Alnus rubra), and Western red cedar (Thuja plicata) have adapted to year-round saturation. These species are important to the lake ecosystem because they absorb nutrients and filter toxins and sediment from stormwater.