The Basics of Limnology: Limnology 101
Below is an overview of some elemental information related to limnology and lake management that are important to know in understanding lakes and the big picture of lake management. For more detailed information please visit the NALMS Bookstore, which has numerous publications on general limnology and lake-related topics.
Events that occurred on or beneath the Earth’s surface thousands of years ago formed many of our lakes. As a result, lakes are usually concentrated into areas that have a large number of waterbodies. Most lakes can be found in the Northern Hemisphere, where large areas were covered by huge ice formations. On a scale of human life spans, lakes seem to be permanent features of our landscape, but they are really only geologically temporary. They are created, mature (fill-in) and eventually disappear.
The origins of the lake basins and their characteristics ultimately reflect the physical, chemical and biological events taking place within the area surrounding them. These events play an important role in how the lake responds to surrounding activities.
Glacial Lakes: By far the most important agents in the formation of lakes are the catastrophic effects of glacial ice movements that occurred 10,000 to 12,000 years ago. Gigantic sheets of ice and snow are created in climates where snow falls but does not melt. The glaciers covered an area from the Atlantic Ocean to the Rocky Mountains in ice that was more than a mile high. Although these glaciers did eventually melt, ten percent of the earth is presently covered with glaciers. Some of these glaciers can still be seen in the mountainous areas of the United States and Canada.
As a glacier moves back and forth across the land, scraping off the tops of hills and bluffs and taking rocks with it, lakes are formed. The material picked up by the glacier is later dropped off at other sites. This back and forth and stop and go movement of the glaciers permanently alters the landscape. This movement creates several important landforms. When the glacier stops, it leaves behind piles of rocks and materials that it carried over time, called moraines. These dam up rivers and smaller streams to form lakes. Sometimes, huge blocks of ice are broken off and covered by sand and gravel. When the ice melts, the sand and gravel cave in, leaving a large hole behind. These kettles may form large marshes or lakes. As the large mass of ice melts, rivers form beneath the glaciers.
Solution Lakes: Lakes can form when underground deposits of soluble rocks are dissolved by water running through the area, making a depression in the ground. Rock formations made of sodium chloride (salt), or calcium carbonate (limestone), are most likely to be dissolved by acidic waters. Once the groundwater has dissolved the rocks below the surface, the top of the land caves in, usually forming a round-shaped lake, called a solution lake. Typically, the depressions are deep enough to extend below the groundwater table and are permanently filled with water. Solution lakes are common in Michigan, Indiana, Kentucky and particularly in Florida.
Oxbow Lakes: The flow of water from rivers has a great deal of energy and erosive strength that may create lake basins. As a river winds over the earth’s surface, a greater amount of erosion occurs on the outer river bend, where the flow of water is the fastest. Materials carried by the river are deposited on the inner portion of the bend, where currents are reduced. As time passes, erosion continues and more materials are left off until the U-shaped meander of the river closes in. The main course of the river cuts a new channel to the inner end of the meander. Oxbow lakes are usually shaped like the letter C.
Man-made or Animal-made Lakes: Many small lakes in North America have been formed by the activities of the American beaver. Sticks, aquatic plants and mud are used to build dams across small streams to form an impoundment of the water. These ponds are usually very shallow and are rich in nutrients and plant life. Humans have constructed artificial lakes (reservoirs) to supply drinking water to the public, to provide power, to aid in navigation, to provide flood control and for recreational purposes. These reservoirs are usually well engineered by humans to hold back a certain quantity of water with the use of dams.
Volcanic Lakes: Sometimes, disastrous events associated with volcanic activity form lake basins. The formation of volcanic lakes can occur in different ways. As volcanic material, including magma, is discharged out of the volcano, empty depressions or cavities are formed within the volcano. Some of these depressions cannot drain and become sealed holes on top of the volcano. Rainfall and runoff eventually fill the depression with water and a new lake is formed.
Lakes that form in the craters of volcanoes, or crater lakes, are more common in areas that are subject to volcanic activity. Lakes formed by the caving in of a roof of a partially empty magmatic chamber are termed calderas. One of the most spectacular lakes formed in this way is Crater Lake in Oregon. Crater Lake is the seventh deepest lake in the world with a maximum depth of 608 m (2006 ft). Volcanic basins, like Crater Lake, are usually very round in shape. Lava flows from volcanic activity can also form lakes. The surface lava cools, and becomes solid, while the inside of the lava flow remains hot enough to continue moving. Eventually, the surface of the hardened lava collapses, forming a depression. These depressions eventually fill with water to form smaller lakes. Lava streams also flow into existing river valleys and solidify into a dam. This solid mass of rock backs up the river water into a new lake.
Landslide lakes: Large quantities of materials that fall from the sides of steep valleys into the floors of stream valleys can cause dams that create new lakes. Such landslides usually occur as a result of abnormal meteorological events, such as excessive rains acting on an unstable slope. Landslide dams may be a result of rockfalls, mudflows or even iceslides. Lakes that are formed by landslides are usually only temporary because they may be susceptible to erosion by the flow of the river or stream. If the dam is very large, the lake may become permanent.
Tectonic lakes: Tectonic basins are depressions formed by the movements of the earth’s crust deep underground. The major types of tectonic basins are formed from faulting. A depression forms when a weak section of the earth’s crust separates, resulting in an earthquake. Rainfall and groundwater may collect in this depression, forming a lake. This type of basin is referred to as a graben and is the mode of origin of a large number of the most spectacular relic lakes in the world containing a vast number of native plant and animal species. The deepest lake in the world, Lake Baikal in Siberia, was formed from tectonic activity. In the United States, Lake Tahoe in California and Nevada was formed by tectonic activity.
The formation of a lake and the structure and form, or morphology, of the lake basin affects how the lake functions throughout its life stages. Characteristics like the lake length, width, depth, area and volume are all important to how the lake water quality may be affected by changes to the land. As humans develop the land surrounding the lake, they disturb the soils, exchange trees for driveways or rooftops and replace the natural vegetation. These changes result in an increased flow of surface runoff and an increase in the amount of nutrients to the lake. The lake structure dictates how the lake will react to these cultural changes in the surround lands. Knowing the lake morphology and how the lake was formed are important tools used by scientists to help protect our lakes from pollutants that can deteriorate their health.
Mixing and Stratification
The thermal structure of lakes helps determine productivity and nutrient cycling. Lake thermal structure is determined by several factors. Lakes receive the vast majority of their heat at the surface from solar heating. Since warmer water floats, the water column must have an energy input to mix that heat deeper, and in most lakes, wind provides that energy.
A lake that is completely protected from the wind will have a very warm but shallow layer at the surface with cold water below. A lake exposed to strong winds will have a cooler but thicker upper layer overlying the colder water. Deeper lakes may form a three-layered structure that throughout the summer consists of an upper warm layer (the epilimnion), a middle transition layer (the metalimnion, within which the point of greatest vertical change in temperature is called the thermocline), and a colder bottom layer (the hypolimnion).
A lake’s thermal structure is not constant throughout the year. Beginning at ice out in early spring (provided that your lake ices over), all the lake’s water, top to bottom, is close to the same temperature; the density difference is slight and water is easily mixed by spring winds. With warmer days, the difference between the surface and bottom water temperature increases until stratification occurs if lake depth is sufficient. Eventually, solar heating declines as we move into cooler seasons, and the upper layer begins to cool and sink. Eventually in the fall, the lake has a similar temperature top to bottom. In winter, ice forms at the surface and a new, inverse stratification (cold over cool water) is created and persists until spring. The degree of stratification is important to the cycling of nutrients, variability in oxygen in deeper waters, movement of incoming water through the lake, and types of aquatic organisms that live in the lake.
The average time required to completely renew a lake’s water volume (lake volume divided by outflow rate) is called the hydraulic residence time or flushing rate. Hydraulic residence time is a function of the volume of water entering or leaving the lake relative to the volume of the lake (i.e., the water budget). The larger the lake volume and the smaller the hydraulic inputs or outputs, the longer will be the residence time.
Lake residence time may vary from a few hours or days to many years. Lake Superior, for example, has a residence time of 184 years. However, most lakes typically have residence times of days to months.
The flushing rate of a lake will determine how it responds to many inputs from the atmosphere and it’s watershed.
Lake trophic classification, or ranking of the degree of lake aging, is often classified using some type of established rating system that assigns points to a certain lake characteristics (oxygen content, algal biomass, plant material, clarity, etc). This point system allows a limnologist to assign a certain value for each of the systems’ categories. Different limnologists use different classification systems, but the categories (Oligotrophic, Mesotrophic and Eutrophic) are the same.
If a lake possessed very high levels of dissolved oxygen, a high transparency reading, had sparse vascular plant growth and relatively low levels of plankton growth, the lake would be classified as oligotrophic, or a “young” lake.
Lakes with lower dissolved oxygen levels, a shallow transparency reading, abundant vascular aquatic plants and high levels of chlorophyll-a (signifying high plankton populations) receive more points and are termed “aged" or eutrophic.
A lake that falls between the two extremes of eutrophic and oligotrophic is referred to as mesotrophic. This stage of lake development can best be termed “middle-aged.”
Lakes may suffer from many impacts of human cultural development, but it is the nutrients that end up in the lake that drive some of the critical problems in lake water quality.
All plants need an appropriate balance of the essential major nutrients, particularly phosphorus, nitrogen, and carbon. They also need light. Assuming that light is readily available, plants take up nutrients in the proportion that their cells require. The nutrient that is in shortest supply relative to the plant’s need will limit the growth of the plants. This is called the limiting nutrient concept. Some parts of the country have waters that are limited by nitrogen, while most waterbodies are limited by phosphorus. Trace elements can sometimes be limiting, but to a lesser degree.
Development of a nutrient budget (loading analysis) provides insight into the causes of lake eutrophication. Nutrient budgets depend on the determination of the amounts of a nutrient that are provided by sources such as natural surface runoff, non-point source pollution, leaking septic systems, atmospheric deposition, groundwater and wildlife. Nutrient budgets also determine the quantity of nutrients lost to the lake system by outflow and by deposition to the sediments. Quantifying nutrient loading requires assessment of the water budget and determination of the concentration of the nutrient in each source of water. Thus the quantity of nutrient provided by a tributary is the concentration times the volume of water per unit time (the flow). This is called the “load” for the nutrient and source being quantified.
Nutrient budgets are commonly determined in two primary ways: by direct measurement or by estimation from various empirical relationships determined in past studies.
Bacteria: Although never seen by most people, bacteria play a pivotal role in the life of lakes. They are the most abundant group of organisms in a lake and most of them are critical in converting any organic material to inorganic form.
Bacteria may be free-floating in the water column, attached to a substrate or in the sediments. Many are aerobic, requiring oxygen for the conversion of organic material to inorganic forms and energy. Many others are anaerobic, using other chemical pathways to derive energy.
Some bacteria create human health problems or have proven to be useful indicators of the likely presence of threats to human health. Escherichia coli (E. coli) is usually an innocuous bacterium found in our intestines, but its abundance in a lake indicates sewage, septic inputs or other fecal contaminants and the potential for the transfer of human bacterial and viral diseases.
Algae: Algae are mostly microscopic plants that may be free-floating (phytoplankton) or attached to a substrate (periphyton). They may be single-celled or have many cells. In a moderately rich lake, there could be nearly one hundred species of algae in a tablespoonful of lake water. In a eutrophic lake, there may be millions of cells in a gallon of water. Algae are divided into several major groups, including green algae (Chlorophyta), golden-brown algae (Chrysophyta), dinoflagellates (Pyrrophyta), diatoms (Bacillariophyta) and the blue-green algae (Cyanophyta).
Each of the above groups has species with characteristics that may allow them to become very abundant and troublesome. Sometimes, knowing which species is in “bloom” can help understand the cause of the bloom. For example, certain blue-green algae often bloom when phosphorus is abundant and nitrate is low because they can fix nitrogen from dissolved air. They often prefer a period of calm water because they float and consequently shade out competing species. The concurrence of these conditions will usually result in blue-greens, but the absence of one element may shift the balance to another species or another algal group. The diatoms tend to prefer times of high mixing, cooler temperatures and higher silica availability - conditions found at spring and fall turnover. Many dinoflagellates seem to prefer conditions with above average organic material.
The dynamics of the thermal, light and nutrient regimes in lakes cause a fairly predictable pattern in the seasonal succession of algal species, but there may be surprises at any time. Typically, though, spring and fall turnover favor the diatoms which may become very abundant but usually do not cause severe impacts on human use, although some species cause taste and odor problems in drinking water reservoirs and can clog filters. After thermal stratification, green algae often become dominant for most of the summer when nitrogen is available, but they may be replaced by blue-green algae at higher temperatures, lower nitrogen concentrations, and high pH.
Aquatic Macrophytes: As opposed to algae that are usually microscopic plants, these are large aquatic plants, easily visible to the naked eye. Algae and macrophytes often compete for light, so it is unusual to find both as problems in any particular lake, although it does happen. Macrophytes may be rooted or free-floating, although most are rooted. They may also be submergent, emergent, or floating-leaved. There are many taxonomic groups but the above categories are often the most useful for understanding the causes of a macrophyte problem and determining an appropriate management strategy. In fact, within each category, many species may look very similar as their growth habit responds to common lake conditions. However, even though many macrophyte species appear similar, their propensity to cause problems in lakes varies. Effective management of macrophytes usually requires species-level identification.
Plants provide the habitat and food for many forms of animal life ranging from microscopic rotifers that filter tiny algae, to zooplankton that hunt larger algae, to insects, to fish and aquatic mammals that eat even larger plants or animals. A change in any part of this food web ripples throughout the system in subtle or even dramatic ways.
A watershed can best be described as a funnel. The top edge of the funnel is the geographic features (hills, mountains) that mark the boundary of a drainage basin. The inside of the funnel, or the funnel walls, represents all the land that is within the boundary of hills or mountains that drains to the lake. The lake is at the bottom of the funnel, receiving all the water that flows from those hilltops, across the land, and into the streams.
Watershed contain your house, car, work, golf course, shopping mall and all it’s paved areas, septic system, car washes, ball fields, sand pits, and sundry other land use types. No matter how far you are from a surface water, whether you can see that lake or river or not, you are in a watershed.