Lakes and Reservoirs Lecture

Lentic System

A freshwater system or waterbody characterized by still or near still/non-moving water. Examples include lakes and ponds.

Lake

A natural basin of freshwater surrounded by land and not connected directly to the ocean.

Reservoir

A man-made basin of freshwater, typically created by impounding a Lotic System (river). Reservoirs can be created by damming a river, forming a cascade of impoundments, or as bunded/embanked systems.

Lifespan (Glaciated Lakes vs. Reservoirs)

Glaciated Lakes have a high lifespan. Reservoirs generally have a short to moderate lifespan, and a shorter overall lifespan than lakes due to increased inputs of sediment and nutrients.

Lake Lifespan Process

Over time, lakes become shallower as organic and inorganic matter settles (sedimentation), increasing productivity and accelerating the process until the lake becomes a wetland.

Water Inputs (Limnology)

Sources of water entering a lake or reservoir include precipitation, surface runoff from the basin, groundwater, and diversion.

Water Losses (Limnology)

Ways water is lost from a lake or reservoir include drainage, seepage, evaporation and evapo-transpiration (plants), and diversion.

Residence Time

The theoretical time it takes water to move through the waterbody. It is the reciprocal of the Flushing Rate. A lake with a long residence time has low inputs relative to lake volume, while a lake with a short residence time has greater inputs relative to lake volume.

Oligotrophic Lake

Characterized by low nutrient levels and low primary productivity. In these lakes, Dissolved Oxygen distribution is governed by physical processes.

Eutrophic Lake

Characterized by high nutrient levels and high productivity. In these lakes, the hypolimnion is depleted of oxygen by decomposition (biotic processes).

Mesotrophic System

Characterized by moderate nutrient levels and moderate productivity.

Autotrophs (Primary Producers)

Organisms that produce or synthesize energy, typically through photosynthesis. They form the base of the food chain.

Heterotrophs (Consumers)

Organisms that acquire energy through eating an organism lower on the food chain.

Allochthonous Material

Biomass and nutrients generated outside of the river (or lake/reservoir system), within the watershed.

Autochthonous Material (Lentic)

Biomass and nutrients generated from productivity within the river (or lake/reservoir system).

Internal Loading of Nutrients

A process that occurs in shallow lakes, which may mix sporadically throughout the summer, leading to more internal loading of nutrients.

Littoral Zone

The nearshore ecological zone of a lake. This zone supports producers like macroscopic algae, epiphytic algae & bacteria, and true plants (floating, emergent, submerged).

Limnetic Zone (Pelagic Zone)

The offshore ecological zone of a lake. This is the open water area.

Neuston

Organisms living at the water surface—the interface with the air. Examples include protozoa, springtails, water spiders, water mites, and water striders.

Photic Zone

The zone of light penetration, extending down to the compensation depth (the depth where less than 1% of light can penetrate to support primary productivity).

Profundal Zone

The deepwater zone below the compensation depth, characterized by minimal light penetration.

Epilimnion

The warm, upper layer of a stratified lake, where most primary productivity occurs.

Hypolimnion

The deep, lower layer of a stratified lake, located within the profundal zone, consisting of decreased oxygen and decreased temperature.

Metalimnion

The narrow, middle layer of a lake, consisting of decreasing temperature and often decreasing oxygen. This layer contains the thermocline.

Thermocline

The temperature transition from the warmer epilimnion to the colder hypolimnion.

Monomictic Lake

A stratifying lake that mixes once per year, often mixing over a prolonged period (e.g., fall to spring).

Dimictic Lake

A stratifying lake that mixes twice per year—usually during the fall and spring when the temperature across layers becomes uniform.

Polymictic Lake

A stratifying lake that mixes more than once per year. This is characteristic of some shallow lakes exposed to higher winds.

Hypoxic

Consisting of very low oxygen, a condition not supportive of most aerobic respiration.

Anoxic

Consisting of a lack of oxygen, a condition not supportive of aerobic respiration.

Summer Kill

Occurs when dense concentrations of macrophytes or algae die, and subsequent decomposition drives Dissolved Oxygen (DO) levels down, often associated with high temperatures and algal blooms.

Winter Kill

Occurs when a shallow lake is ice-covered and dark for a long period, causing the DO demand of plants to drive levels below the 2 mg/L required by most fish.

DO "Squeeze" (Southern Lakes)

A condition during summertime stratification in Southern lakes and ponds where the top of the epilimnion is too hot, but the hypolimnion is too hypoxic to sustain fish.

Two-Story Lake

A lake that can maintain two distinct fish communities based on thermal stratification

Hydropeaking

The process used at dams (e.g., Buford Dam on the Chattahoochee River) for energy production that results in rapid flow changes.

Hypolimnetic Release

The practice of releasing water from the cold, deep hypolimnion layer of a dam, which changes habitat characteristics (temperature and flow) downstream.

Zooplankton

Small animals, typically less than 1 mm, occupying the open water (limnetic or pelagic zone) of lakes. Key groups are Rotifers, Cladocerans, and Copepods.

Rotifers (Phylum Rotifera)

Considered super 'r' selected. They have a very short life cycle, use parthenogenesis under good conditions, and are often filter feeders or omnivorous. They serve as a link between the 'brown' (microbial) and 'green' (grazer) food webs.

Cladocerans (Phylum Arthropoda)

Considered 'r' selected. They are good swimmers using their second antennae and are capable of parthenogenesis, but have a longer lifespan (1–3 months) than rotifers. Most are non-selective filter feeders/grazers.

Copepods (Phylum Arthropoda)

Considered 'K' selected. They are slower growing, live longer than rotifers and cladocerans, and are raptorial feeders (selecting and grasping prey).

Inducible Defenses to Predators

Changes in an animal's morphology (body form or size) in response to the presence of predators. For example, cladocerans may grow helmets or spines in the presence of larger invertebrate predators to increase their size and make them harder to consume.

Diel Vertical Migration (DVM)

The daily movement of zooplankton (and often their predators) up and down in the water column. Typically, zooplankton move up into the epilimnion overnight and return to the metalimnion or profundal zone in the morning.

Effect of Fish Predation on Zooplankton

Fish predation can cause a shift in the size and composition of the zooplankton community.

Ecomorph

A specific body-form that a species may have adapted to living in different environments. An example is the European Whitefish, which has different body shapes and gill raker sizes depending on whether it is littoral benthic, profundal benthic, or pelagic.