Shallow lakes are unique lacustrine systems. Shallow normally have a maximum depth of 15 feet and have 80 percent or more of the lake area shallow enough to support emergent or submergent aquatic macrophytes. Unlike deeper lakes , shallow lakes do not thermally stratify during the summer months. Additionally, shallow lakes tend to become depleted of dissolved oxygen (DO) during the winter months under natural precipitation and runoff cycles. Ice and deep snow cover prevent sunlight from penetrating into the water column, causing submerged aquatic plants to die. As plants die, oxygen is no longer produced and bacterial decomposition of plant material further consumes residual DO in the water. Low oxygen levels stress fish in wetlands and shallow lakes, and often fish in these systems will die off or winterkill.
Interestingly, shallow lakes exist in two stable ecological conditions, or "states." The clear water state is marked by low turbidty and a floral community dominated by aquatic macrophytes. The turbid water state is dominated by phytoplankton production and has high turbidity. The pristine state for most shallow lakes is the clear water state. Disturbances cause lakes to shift from one state to the other. However, once in a stable ecological state, a lake has a tendency to remain in that state.
Fish (primarily minnows, young panfish, bottom-feeding carp, buffalo and bullhead) are a leading driver of the shift of shallow lakes from clear to turbid water states. While feeding, carp and bullhead uproot desirable aquatic vegetation and stir up and suspend nutrient laden sediments. Phosphorous and nitrogen released from the disturbed sediments accelerates the growth of phytoplankton (blue-green algae). The resultant algal blooms negatively affect the submerged plant community further by increasing the lake's turbidity, thereby shading out and reducing macrophyte growth. The lake quickly becomes a soup of algae, devoid of aquatic invertebrates and beneficial plants. The disturbance caused by rough fish has flipped the lake into a turbid water state. The loss of both invertebrates (such as freshwater shrimp) and plants (such as sago pondweed) in wetlands results in a loss of food for ducks. Without aquatic plant and invertebrate food in wetlands, duck use is low, and even the survival of locally produced ducklings can be negatively affected. Especially vulnerable are lesser scaup (bluebills), whose diet consists of aquatic invertebrates exclusively and ducklings of all species.
Depicted in the picture below is a Minnesota shallow lake bisected by a road. The water body on the right of the road was stocked with minnows causing a trophic cascade sending the lake into a turbid water state. No fish are in the lake on the left of the road; this lake remains in the clear water state.
Manipulation of fish populations is a means to change a turbid water lake back to its pristine clear water state. Fish will often winterkill in shallow lakes and marshes experiencing normal precipitation and runoff regimes. However, wetland destruction and agricultural field drainage within a lake's watershed increase the amount of runoff reaching the lake and will impact the natural winterkill cycle. Installation of water control and fish exclusion structures is often required to artificially manipulate water levels and prevent fish from entering the lake from feeder ditches and streams. In addition, fish kills will occasionally be induced by applying chemical toxins (e.g. rotenone) to the lake.