If you have not already, go check out the other article: “Hydrology...Oceanography...Limnology...Glaciology! Oh, too many to count! - Introduction to Limnology” before continuing with this one! That one gives an introduction to limnology while this article provides a more in-depth view of the field. The introduction article ended with an overview of the Trophic State Index which helps categorize water quality and nutrient levels. This article will start by discussing the two major sources of water nutrients: nutrient loading from the watershed and internal loading.

Large-scale nutrient loading in a watershed typically comes from agriculture, septic systems, impermeable surfaces, and wetlands. When a watershed is undeveloped, rain enters the soil and is retained for the plants. However, when rain falls on impermeable surfaces (like concrete), the water will flow off into drainage systems and take nutrients with it. Eventually, it will reach the lake.

Internal loading starts with cold water losing all of its oxygen during summer due to aquatic organisms taking that oxygen. As this water becomes anoxic, or without oxygen, it moves to the bottom of the lake. During this time, the phosphorus escapes from sediment at the bottom of the lake and enters the water. Depending on factors including the mud’s phosphorus concentration, how long phosphorus has been seeping into the water (based on how long the water has been without oxygen), and the amount of mud, the lake’s nutrient content can vary greatly. Later in the year, once temperatures cool, surface water and water from the bottom of the lake mix, so the nutrients that were unable to escape from the anoxic sector flow freely throughout the lake. This process when waters of different temperatures mix is one type of stratification. Forel (the father of limnology) studied nutrient concentration in the lake and its stratification.

Water in lakes deeper than 3 meters typically goes through seasonal stratification. At the beginning of the year, winter causes ice to form across the top of the lake, leaving the water below fully mixed and a bit cold. Not much oxygen is present during this time. During spring, the ice begins melting, but that water is still very cold compared to deeper waters in the lake, so the icy water sinks to the bottom (as cold water is heavier than warm water), and the surface water becomes warmer. Throughout summer, the surface level water becomes very warm and light, leaving the cold water to remain at the bottom. They are separated by a “line” called a thermocline, which is also where any exchange between hot and cold waters will occur. Towards the end of the year, in fall, the surface water starts cooling again and sinking to mix with the water that has stayed cold throughout the year. This is important because when the phosphorus is released from the mud through internal loading, the barrier formed between the hot and cold water prevents nutrients from mixing immediately. While this can sometimes be harmful because it can also limit phytoplankton from accessing lower nutrient sources, it is generally beneficial.

All these different branches that were originally studied by Forel still play an invaluable role in today’s society! Next time you go to a lake, share these fascinating facts with those who are with you!!