Lakes are often classified based on their “trophic state,” a measure of the amount of algal growth or primary production in the lake. Primary production is a term used in lake ecology that refers to the amount of photosynthesis occurring in a waterbody. Free-floating algae, or phytoplankton, are responsible for the majority of primary production in a lake.

There are several reasons why organizing lakes based on their primary production is beneficial. Not the least of which is that many people prefer to swim and recreate on lakes with clear water, resulting from low primary production levels. Another is that as primary production increases, the amount of energy available to the rest of the food web increases. An increase in energy in the food web can affect fish and other organisms in the lake.

There are four trophic states that a lake can belong to; oligotrophic, mesotrophic, eutrophic, and hypereutrophic.

Oligotrophic Lakes

Mesotrophic Lakes

Eutrhopic Lakes

Hypereutrophic Lakes

It is essential to recognize that a lake’s trophic state does not necessarily indicate whether that lake is polluted or has poor water quality. Lakes naturally occur in all of the trophic states described above. What is important is to determine whether a lake is moving from one trophic state to the other, and if so, why.

Factors That Influence Trophic State

Nutrient Supply

Nutrients, such as phosphorus and nitrogen, are necessary for algae and other phytoplankton to grow. Often, phosphorus is the limiting nutrient in lake ecosystems. Meaning, that if provided more phosphorus, more algae will grow. Bedrock geology, soils, and surrounding vegetation are natural sources of nutrients for lakes. Depending on the land and watershed around a lake, they will naturally fall into one of the trophic states described above.

Climate

Climate is another factor that will influence the trophic state of a lake. Temperature, cloud cover, and precipitation will all influence the trophic state of a lake. Algae and phytoplankton need sunlight and warm temperatures to grow and reproduce. In regions with warmer and sunnier climates, lakes will tend to be more productive. Precipitation also plays a role in nutrient inputs to a lake. Heavy rains can wash nutrients into a lake, while periods of drought can reduce lake flushing, trapping nutrients within the lake.

Lake & Watershed Shape

The lake’s size, shape, and depth can all influence the trophic state. Small shallow lakes tend to be more eutrophic than big deep lakes because more water is near the surface, where nutrients, sunlight, and warm temperatures feed phytoplankton. The size and shape of the watershed are also important. This determines how much bedrock, soil, vegetation, and human influences contribute nutrients to the lake relative to the size of the lake.

Lake Aging

A general concept is that lakes naturally move from an oligotrophic to a eutrophic state over time. This gross generalization likely only applies to a limited number of water bodies. Certainly, a lake’s trophic state can change over long-time scales due to changes in a lake’s watershed and natural alterations to its size, shape, and depth.

Lakes can be studied over centuries or millennia using paleolimnology. Paleolimnologists study lake sediments to reconstruct the ecological past of a lake. Many paleolimnological studies of lakes show little change in a lake’s trophic state over thousands of years. Similarly, other studies have demonstrated significant fluctuations in lake productivity due to entirely natural processes. Natural changes in climate and watershed vegetation can significantly influence the trophic state of a lake.

Cultural Eutrophication

The process of a lake becoming more productive is called eutrophication. Eutrophication can occur naturally as the lake and land around it change. Cultural eutrophication is the process of humans making a lake more productive through the addition of nutrients such as phosphorus. Lawn fertilizers, septic systems, wastewater treatment plants, and erosion from development can all add nutrients to a lake.

Excess nutrients have been a significant source of pollution to lakes worldwide over the past century. When lakes move toward a more eutrophic state, there are many undesirable consequences, such as harmful algal blooms, taste and odor issues, increased cost of water treatment, fish kills, and altered aesthetics. The Clean Water Act, passed in 1972, focused heavily on reducing point sources of pollution to waters in the United States. The federal government invested in upgrading municipal wastewater treatment facilities to reduce nutrient inputs to lakes and rivers while also regulating industrial pollution.

More recently, it has been recognized that non-point sources of nutrient pollution are also important. Agricultural runoff, lawn fertilizers, and stormwater are all examples of non-point sources of pollution. It is more challenging to regulate and manage these sources of pollution because they are difficult to quantify, come from a large number of sources, and vary depending on the landscape.