Glossary of Lake Terms

Anoxic: without oxygen.  Organisms cannot survive in prolonged periods of anoxia (<1mg/L).

Chlorophyll-a: the pigment that makes plants and algae green.  Chlorophyll-a is measured in lakes to determine algal concentration.

Dissolved oxygen: oxygen that is dissolved in the water column.  Aquatic organisms (zooplankton, aquatic invertebrates and fish) need this oxygen to survive.

Epilimnion: The top layer of a lake where the sunlight penetrates and provides energy for plants and algae to grow.

Fall turnover: when the summer stratification layers of a lake mix due to the cooling epilimnion (upper layer of the lake).  This mixing distributes all the nutrients evenly through the water column.

Fertility: the amount of plant and animal life that can be produced within a lake.  Fertility is directly related to the amount of nutrients present in the lake to "feed" plants and animals (phosphorus, nitrogen).

Hypolimnion: The deep part of a lake that is cold and dark due to no sunlight penetration.  This area of a lake can be anoxic in the summer due to stratification and decomposition.

Littoral area: the area around a lake that is shallow enough to support plant growth (usually less than 15 feet).  This part of the lake also provides the essential spawning habitat for most warm water fishes (e.g. bass, walleye, and panfish).

Nitrogen: a nutrient important for plant growth.  Nitrogen can enter a lake through groundwater, surface runoff and manure.

OP (Ortho Phosphate): the amount of inorganic phosphorus within a lake.  Inorganic phosphorus is readily usable by algae and plants for growth.

Phosphorus: a nutrient needed for plant growth.  Phosphorus can enter a lake through runoff from manure and fertilizer or through seepage from leaking septic and holding tanks.

Productivity: the amount of plant and animal life that can be produced within a lake.  Productivity is directly related to the amount of nutrients present in the lake to "feed" plants and animals (phosphorus, nitrogen).

Secchi Depth: a measure of water clarity that can indicate the overall health of a lake.  A black and white metal disc is lowered into the water on a rope until it can't be seen anymore and raised to the point it can be seen.  The depth of the disk to the surface of the water is the Secchi Depth.

Spring turnover: when the ice melts off the lake in the spring and cold water on the top of the lake sinks.  This mixing distributes all the nutrients evenly through the water column.

Stratification: The process in which most Minnesota lakes separate into three layers during the summer.  The upper layer (epilimnion) becomes warm and is penetrated by sunlight, the lower layer (hypolimnion) is cold and dark and the middle area (thermocline) separates the top and bottom layers.  Warm water is less dense than cold water, which is why the upper layer floats on top of the bottom layer and does not mix in the summer.  Minnesota lakes mix in the spring and the fall, when the top layer of the lake cools off.

Thermocline: The area between the warm top layer of a lake and the cold bottom part of the lake.  The thermocline is characterized by a sharp drop in temperature.

TP (Total Phosphorus): the total amount of organic and inorganic phosphorus within a lake.  Organic phosphorus includes detritus, feces, dead leaves and other organic matter.

TMDL (Total Maximum Daily Load): the amount of a particular pollutant that a body of water can handle without violating state water quality standards.

Trend Analysis (Mann Kendall statistic): a way to test the probability of a trend being real versus just happening by chance.  A trend probability of 90% (minimum probability used by MPCA) means that there is a 90% probability that the observed trend is real and a 10% probability that the observed trend is just from random chance.

Turbidity: refers to how clear the water is.  Cloudiness (turbidity) in the water can be due to suspended matter such as silt, clay, plankton and other organic matter.  The more turbid the water is, the less sunlight can pass through.

Watershed: the area of land that drains into a lake directly or by way of a stream that flows into the lake.  The land use practices of an entire watershed can affect the water quality of a lake.

TSI: Trophic State Index is a measurement of overall lake productivity (nutrient enrichment).  The overall TSI of a lake is the average of the TSI for phosphorus, chlorophyll-a and secchi depth.

Oligotrophic: A lake that has very clear water and very low productivity (phosphorus and chlorophyll-a).  Oligotrophic lakes have a Trophic State Index under 30, the hypolimnion contains oxygen throughout the year and can support trout.

Mesotrophic: A lake that has moderate water clarity and productivity (phosphorus and chlorophyll-a).  Mesotrophic lakes have a Trophic State Index between 30 and 50, and the hypolimnion can become anoxic during the summer.

Eutrophic: A lake that has low water clarity and high productivity (phosphorus and chlorophyll-1).  Eutrophic lakes have a Trophic State Index between 50 and 70, an anoxic hypolimnion in the summer, algal and aquatic plants are prevalent, and can only support warm water fish.