The Basics of Limnology
What is Limnology?
Limnology is the study of inland waterbodies, including lakes, ponds, rivers and streams, using measurement tools to gauge environmental changes and interactions between environmental factors. Limnology encompasses components of chemistry, physics and biology.
Water Quality Characteristics
Water quality is determined by several factors, including but not limited to: nutrient concentrations, water clarity (Secchi disk depth), cyanobacteria species and density, and zooplankton species.
Trophic State Index
Lake health can be largely determined by nutrient concentrations and water clarity. The trophic state index is comprised of three major categories (Oligotrophic, Mesotrophic, and Eutrophic), which are determined by total phosphorus and total nitrogen concentrations, Secchi disk depth (water clarity) and Chlorophyll-a concentration. Oligotrophic lakes have very low nutrient concentrations, meaning that plants do not grow well. Eutrophic lakes have very high nutrient concentrations, especially nitrogen and phosphorus, and therefore high biological productivity (excessive plant and algae growth).
The Seasonal Cycle: Stratification and Mixing
In lakes and ponds that are relatively deep (generally more than about 3 meters), the water undergoes seasonal cycles of thermal stratification and mixing. Cool water is heavier than warm water meaning that, in the summer, the water at the surface heats up and becomes lighter, ‘floating’ above the heavier cool water that sinks to the bottom. In the fall, cold air and wind cools the surface water, which becomes heavier and sinks in the water column, mixing with bottom water that remained cool all summer. In the winter, when ice forms on the lake’s surface, the water below remains mixed but with slight temperature variation. Cold water sits directly below the ice, while water right around 4 degrees Celsius remains at the lake bottom. In the spring, the water from melting ice is colder than the rest of the water in the lake and so this colder, heavier water sinks to the lake bottom and the surface water heats up, leading to summer stratification of the water column.
Where Do Nutrients Come From?
There are two main sources of nutrients in lakes: nutrient loading from the watershed and internal loading.
Nutrient Loading from the Watershed
The most common sources of large-scale nutrient loading in a watershed are agriculture, septic systems, wetlands, and large-scale development of impermeable surfaces.
In an undeveloped watershed, rain infiltrates into the ground and is retained within the soil, before being utilized by terrestrial vegetation. However, when rain falls on impermeable surfaces such as pavement and roofs, the water flows over the surface and into drainage basins, picking up nutrients along the way, before flowing directly into the lake.
During the summer, organisms deplete the store of oxygen in the cold water that sits at the lake bottom, causing the water to become anoxic (devoid of oxygen). When the water at the lake bottom is anoxic (devoid of oxygen) the bottom sediment is unable to retain the accumulated phosphorus, so this phosphorus is released into the water. The rate of diffusion and the resulting concentration in the water are dependent on several factors: the concentration of available phosphorus in mud, the length of time the water over the mud has been devoid of oxygen, the intensity of the oxygen debt in the water, and the surface area of anoxic mud. In the fall, the surface water begins to cool and mixes with the bottom water and the nutrients that were trapped in the anoxic water in the summer become mixed in the water column.
Light Transmission and the Littoral Zone
Plants need light to photosynthesize and therefore can only survive in water that is shallow enough for light to penetrate to the bottom. The clearer the lake, the deeper light can penetrate.