Lakes and Reservoirs Terms

Allochthonous Material

Biomass and nutrients generated outside of the river, within the watershed

Autochthonous Material

Biomass and nutrients generated from productivity within the river

Residence Time

The amount of time it takes (e.g., days) for a lake or reservoir to completely recirculate all of its water

Littoral

Nearshore ecological zone of a lake

Limnetic

Offshore ecological zone of a lake

Neuston

Organisms living at the water surface—interface with the air (e.g., water striders, water spiders, water mites)

Photic Zone

Zone of light penetration, to a depth known as the compensation depth, where less light can get through to support primary productivity (~1%)

Profundal Zone

Deepwater zone below the compensation depth, with minimal light penetration

Epilimnion

Warm, upper layer of a lake, where most primary productivity occurs

Metalimnion

Narrow, middle layer of a lake consisting of decreasing temperature and often decreasing oxygen; the metalimnion is where the thermocline occurs

Hypolimnion

Deep, lower layer of a stratified lake, within the profundal zone, consisting of decreased oxygen and decreased temperature—below the thermocline

Thermocline

Temperature transition from warmer epilimnion to colder hypolimnion

Hypoxic

Consisting of very low oxygen, not supportive of most aerobic respiration

Anoxic

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

Monomictic Lake

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

Dimictic Lake

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

Polymictic Lake

A stratifying lake that mixes more than once per year; characteristic of some shallow lakes that are exposed to higher winds

Oligotrophic

A lake or system characterized by low nutrient levels and low primary productivity

Mesotrophic

A lake or aquatic system characterized by moderate nutrient levels and moderate productivity

Eutrophic

A lake or aquatic system characterized by high nutrient levels and high productivity

Autotrophs

Primary producers; organisms that produce or synthesize energy from nutrients and some outside energy source—such as photosynthesis

Heterotrophs

Consumers; organisms that acquire their energy through eating an organism lower on the food chain; herbivores are consumed by primary consumers, which are consumed by secondary consumers, and so on up to tertiary consumers

Zooplankton

Small animals, typically less than 1 mm, occupying the open water of the limnetic or pelagic zone; in lakes, zooplankton are primarily made up of Rotifers, Cladocerans, and Copepods, which can be identified by their distinguishing characteristics and phyla

Inducible Defenses to Predators

Changes in an animal’s morphology (body form or size) in response to the presence of predators; for example, cladocerans can grow helmets or spines in the presence of large invertebrate predators, and dragonfly nymphs grow larger abdominal spines when fish predators are present

Diel Vertical Migration

Daily movement of zooplankton (and often fish and other predators) up and down in the water column; typically, zooplankton move up into the epilimnion during twilight and remain there overnight, then move back to the metalimnion or profundal zone in the morning

Ecomorph

A specific body form that a species may have adapted to living in different environments; for example, European Whitefish have different body shapes and gill raker sizes depending on whether they are littoral benthic, profundal benthic, or pelagic

Two-Story Lake

A lake that can maintain two distinct fish communities based on thermal stratification; the upper warm layer supports fish adapted to warm waters, and the lower level supports fish tolerant of colder water; this occurs in deep oligotrophic or polymictic lakes where the metalimnion and hypolimnion are well oxygenated

Lotic

Freshwater system or waterbody characterized by moving water, e.g., streams & rivers

Lentic

Freshwater system or waterbody characterized by non-moving or flowing water, e.g., lakes & ponds