Lakes are typically considered shallow if they have a maximum depth less than 15 feet and >10 acres in size. Due to the shallow bathymetry of these basins, shallow lakes are biogeochemically different from deep lakes. Shallow water depths increase the relative proportion of benthic (bottom) sediment area to lake volume, increasing the potential for higher sediment-water column nutrient interactions and sediment resuspension, resulting in greater system productivity. Biological communities also have a more pronounced impact on water quality of shallow lakes, an impact often referred to as a trophic cascade.
Trophic cascades occur when predators suppress the abundance or alter the behavior of their prey, thereby releasing the next lower trophic level (their prey’s prey) from predation, resulting in altering abundances and biomass across trophic levels. Trophic cascades and high system productivity play an important role in shallow lakes and understanding their influence has important implications for water quality management.
Due to strong trophic cascade relationships in shallow lakes and high system productivity, shallow lakes typically persist in one of two alternative stable water quality states: a clear water macrophyte dominated state and a turbid water algae dominated state. The clear water state is characterized by low algal biomass, an abundant and diverse submerged aquatic vegetation community, a balanced fish community (if any), and large bodied zooplankton daphnia. Alternatively, the turbid water state is characterized by high phytoplankton biomass, little to no submerged aquatic vegetation, and has an imbalanced fish community often dominated by common carp and/or fathead minnow.
Shallow lakes often exist in an area of hysteresis with the lake flipping between the clear and turbid water states due to sudden changes in the fish community. The persistence the clear water state is often the favored outcome of management activities but can be difficult to maintain in many urban and agricultural landscapes. Understanding and identifying the potential mechanisms driving the state of water quality in a shallow lake is critical to successful and sustained management of shallow lakes.
Common carp are a non-native fish species that can uproot aquatic vegetation and increase water column nutrients through a process known as bioturbation. At high densities, common carp begin to change the ecology of a lake and ‘flip’ the lake from a clear aquatic vegetation dominated system to an algal dominated, turbid water state. A lake/s will persist in the turbid water state until there is a change in the fish community. Understanding the detrimental impact common carp can have on a lake ecosystem, the Shingle Creek Watershed Management Commission along with Wenck Associates began a carp management project in the summer of 2016. The project aims to manage the common carp population within the Twin Lake system and improve lakes’ water quality and the aquatic vegetation community.
Carp management occurs in two major phases: population estimation and seasonal migration tracking. Efforts began by determining the population densities within each lake and setting removal targets that would reduce the carp population to non-determinantal densities. A subset of the population was also implanted with radio tags to allow telemetry tracking. Understanding seasonal migration patterns and where carp are spawning and over wintering are important objectives to long term project success. Understanding spring migration patterns can assist in determining where engineering controls can be used to limit access into prime spawning habitats. The loss of spawning habitat will limit the recruitment of new carp individuals into the population. Winter tracking assists with locating carp for removal through the ice. Common carp form dense school during winter months making them very susceptible to mass removal as a few radio-tagged fish will lead you to the larger community of carp.
The project efforts to date have determined population densities are causing water quality impairments (densities >100kg/ha) and a loss of vegetation within the system. Tracking efforts demonstrate a very mobile population of carp among the lakes with a strong potential for winter removal and a reduction in carp biomass within the system. Wenck will continue to determine the best locations to prevent future recruitment of common carp into the lake chain and carry out a winter removal in 2018.