Describing the features of a Lake
The following is a brief overview of some terms relating to conditions and features that divers may encounter during their exploration and documentation of Cultus Lake.
The vertical zones of a lake.
The vertical zones of a lake can be described in a number of different ways, depending on the features or effects one is focusing on. First, one can consider a lake in terms of physical area and the areas where light penetrates and where it does not.
A lake is composed of open water Limnetic zone, and lakebed or substrate Benthic zone.
The depths where light does not penetrate (to effect photosynthesis) is called the Aphotic zone (A meaning "absence of").
The depths where light does penetrate (Photic zone) can be divided in two; the Limnetic photic zone (open water that light penetrates in a meaningful way) and the Littoral zone (plant producing lake slope and surrounding waters). Free floating plants like phytoplankton grow in the limnetic photic zone; rooted plants like milfoil grow in the littoral zone.
Next one can look at the areas of the lake where living organisms generate energy by either respiration or photosynthesis.
In the Aphotic zone, Decomposers use oxygen to break down detritus in a process called respiration. This region where oxygen is consumed is called the Tropholytic zone (feeding by decomposition). Sometimes, especially in the case where there is a high volume of detritus or little lake turnover, the oxygen of the Tropholytic zone can drop to a level that does not sustain ordinary aerobic (oxygen consuming) life and the presence of anaerobic (decomposing without oxygen) Decomposers like Cyanobacteria will predominate.
The region of the lake where light penetrates, whether limnetic or littoral, and supports photosynthesis, is known as the Trophogenic zone. Here oxygen is produced by plants , not consumed.
The region between both the Trophogenic and Tropholitic zones, where oxygen is both consumed and produced in roughly equal proportions is called the Compensation zone.
Lastly, one can look at a lake in terms of water temperature and layering.
Anyone who has dived deep enough in a lake during summer will be able to relate to the rather unpleasant experience of crossing through a Mesolimnion or "thermocline". This is the transitional zone between the upper layer of water called the Epilimnion and the lower layer known as the Hypolimnion. Whether the Epi or Hypo limnion are colder or warmer depends on the time of year and a process known as Lake turnover.
The following is a brief overview of some terms relating to conditions and features that divers may encounter during their exploration and documentation of Cultus Lake.
The vertical zones of a lake.
The vertical zones of a lake can be described in a number of different ways, depending on the features or effects one is focusing on. First, one can consider a lake in terms of physical area and the areas where light penetrates and where it does not.
A lake is composed of open water Limnetic zone, and lakebed or substrate Benthic zone.
The depths where light does not penetrate (to effect photosynthesis) is called the Aphotic zone (A meaning "absence of").
The depths where light does penetrate (Photic zone) can be divided in two; the Limnetic photic zone (open water that light penetrates in a meaningful way) and the Littoral zone (plant producing lake slope and surrounding waters). Free floating plants like phytoplankton grow in the limnetic photic zone; rooted plants like milfoil grow in the littoral zone.
Next one can look at the areas of the lake where living organisms generate energy by either respiration or photosynthesis.
In the Aphotic zone, Decomposers use oxygen to break down detritus in a process called respiration. This region where oxygen is consumed is called the Tropholytic zone (feeding by decomposition). Sometimes, especially in the case where there is a high volume of detritus or little lake turnover, the oxygen of the Tropholytic zone can drop to a level that does not sustain ordinary aerobic (oxygen consuming) life and the presence of anaerobic (decomposing without oxygen) Decomposers like Cyanobacteria will predominate.
The region of the lake where light penetrates, whether limnetic or littoral, and supports photosynthesis, is known as the Trophogenic zone. Here oxygen is produced by plants , not consumed.
The region between both the Trophogenic and Tropholitic zones, where oxygen is both consumed and produced in roughly equal proportions is called the Compensation zone.
Lastly, one can look at a lake in terms of water temperature and layering.
Anyone who has dived deep enough in a lake during summer will be able to relate to the rather unpleasant experience of crossing through a Mesolimnion or "thermocline". This is the transitional zone between the upper layer of water called the Epilimnion and the lower layer known as the Hypolimnion. Whether the Epi or Hypo limnion are colder or warmer depends on the time of year and a process known as Lake turnover.
Lake Turnover.
Fresh water has two interesting properties that effect its behavior in a lake. The first is that it becomes more bouyant as it approaches a solid state (ice) instead of denser so that colder water (4*C or less) rises instead of sinking. The other property is that waters of varying temperatures (densities) resist mixing. These two properties contribute to a process known as lake turning.
In the spring, when the ice melts, the colder water of the upper epilimnion gradually drops down through the thermocline and mingles with the warmer water of the lower hypolimnion until the water temperature of the lake is roughly equalized. This reduces the resistance to water mixing mentioned above and allows surface wind action to continue to mix the lake in a process known as Spring Turnover. Now the oxygen levels within the lake are equalized as the oxgen rich epilimnion layer mixes with the oxygen deprived hypolimnion layer for a period and aerobic life can be sustained in the tropholytic layer.
As spring continues into summer, the upper epilimnion layer of the lake warms and resists mixing with the colder hypolimnion layer and a thermocline between the two again develops. Eventually an oxygen deficit occurs in the hypolimnion layer in a process known as Summer Stagnation.
This continues into the fall until the epilimnion cools enough and sinks down through the thermocline to equalize temperatures with the hypolimnion. Once again the surface wind action can mix the lake. Fall Turnover occurs and oxygen rich epilimnion water mixes with oxygen depleted hypolimnion water.
Finally, as fall wears on, the surface of the lake nears zero degrees and the colder dense water rises and resists mixing once more by forming the familiar epilimnion-thermocline-hypolimnion layers; only now the epilimnion is colder than the lower hypolimnion layer. Once more the oxygen levels in the hypolimnion are depleted and the lake enters into a Winter Stagnation period.
The degree to which oxgen levels drop during the summer and winter stagnations is dependent on the period of mixing and stagnations and the nutrient levels in the lake. Sterile lakes may see little change in oxygen levels while lakes, with plenty of nutients and consumers to interact, may see dramatic drops. This may result in boom/bust growth cycles of growth.
Fresh water has two interesting properties that effect its behavior in a lake. The first is that it becomes more bouyant as it approaches a solid state (ice) instead of denser so that colder water (4*C or less) rises instead of sinking. The other property is that waters of varying temperatures (densities) resist mixing. These two properties contribute to a process known as lake turning.
In the spring, when the ice melts, the colder water of the upper epilimnion gradually drops down through the thermocline and mingles with the warmer water of the lower hypolimnion until the water temperature of the lake is roughly equalized. This reduces the resistance to water mixing mentioned above and allows surface wind action to continue to mix the lake in a process known as Spring Turnover. Now the oxygen levels within the lake are equalized as the oxgen rich epilimnion layer mixes with the oxygen deprived hypolimnion layer for a period and aerobic life can be sustained in the tropholytic layer.
As spring continues into summer, the upper epilimnion layer of the lake warms and resists mixing with the colder hypolimnion layer and a thermocline between the two again develops. Eventually an oxygen deficit occurs in the hypolimnion layer in a process known as Summer Stagnation.
This continues into the fall until the epilimnion cools enough and sinks down through the thermocline to equalize temperatures with the hypolimnion. Once again the surface wind action can mix the lake. Fall Turnover occurs and oxygen rich epilimnion water mixes with oxygen depleted hypolimnion water.
Finally, as fall wears on, the surface of the lake nears zero degrees and the colder dense water rises and resists mixing once more by forming the familiar epilimnion-thermocline-hypolimnion layers; only now the epilimnion is colder than the lower hypolimnion layer. Once more the oxygen levels in the hypolimnion are depleted and the lake enters into a Winter Stagnation period.
The degree to which oxgen levels drop during the summer and winter stagnations is dependent on the period of mixing and stagnations and the nutrient levels in the lake. Sterile lakes may see little change in oxygen levels while lakes, with plenty of nutients and consumers to interact, may see dramatic drops. This may result in boom/bust growth cycles of growth.
Why a lake is a lake.
There are differences between lakes, ponds and marshes though, given enough time, most lakes like Cultus will transition from lake through pond and into march or swamp stages. The difference between them is depth, vegetation and light penetration.
A lake is sufficiently deep in many places so that light cannot penetrate to its bottom and is devoid of vegetation in those deeper zones, though it may have vegetation along its borders. A pond is shallower, with light penetrating to much of its depth and therefore has submerged vegetation across most of its bottom. A marsh or swamp is shallower still, with less open water and vegetation rising above its surface in many places.
How a lake becomes a swamp or marsh.
Lakes like Cultus are created when an obstruction like a landslide blocks the flow of a creek or river or a depression is formed in a mountain valley. The water backs up and forms a lake with lake bed contours very similar to the original valley it resides in.
Over time, organic matter and vegetation in and surrounding the new lake decays and settles on the bottom along with silt and sediment from runoffs. Added to this are the effects of gravity along the lake edges; erosional forces at work above and below the waterline. This "filling in" of a lake is a very gradual process but as the bottom rises up towards the surface, more light penetrates to it and there is a subsequent increase in vegetation growth and decay. Eventually the lake fills in so much that it will take on the features of a pond and then either a marsh or swamp.
There are differences between lakes, ponds and marshes though, given enough time, most lakes like Cultus will transition from lake through pond and into march or swamp stages. The difference between them is depth, vegetation and light penetration.
A lake is sufficiently deep in many places so that light cannot penetrate to its bottom and is devoid of vegetation in those deeper zones, though it may have vegetation along its borders. A pond is shallower, with light penetrating to much of its depth and therefore has submerged vegetation across most of its bottom. A marsh or swamp is shallower still, with less open water and vegetation rising above its surface in many places.
How a lake becomes a swamp or marsh.
Lakes like Cultus are created when an obstruction like a landslide blocks the flow of a creek or river or a depression is formed in a mountain valley. The water backs up and forms a lake with lake bed contours very similar to the original valley it resides in.
Over time, organic matter and vegetation in and surrounding the new lake decays and settles on the bottom along with silt and sediment from runoffs. Added to this are the effects of gravity along the lake edges; erosional forces at work above and below the waterline. This "filling in" of a lake is a very gradual process but as the bottom rises up towards the surface, more light penetrates to it and there is a subsequent increase in vegetation growth and decay. Eventually the lake fills in so much that it will take on the features of a pond and then either a marsh or swamp.
Terms to describe nutrient levels in a lake and what they mean.
Generally speaking, there are three terms used to describe nutrient levels in a lake.
Oligotrophic means "lacking food" and describes water with little nutritional value to sustain life. New, fast flowing, very cold or deep lakes with steep sides commonly have water of this quality. As lakes fill in they both increase their load of organic matter and the capacity for plant growth and they transition from Mesotrophic "middle amount" to Eutrophic or "nutrient rich". This process is called "eutrophication" and can be seen both in terms of the aging of a lake and the increasing of its productivity.
Generally speaking, there are three terms used to describe nutrient levels in a lake.
Oligotrophic means "lacking food" and describes water with little nutritional value to sustain life. New, fast flowing, very cold or deep lakes with steep sides commonly have water of this quality. As lakes fill in they both increase their load of organic matter and the capacity for plant growth and they transition from Mesotrophic "middle amount" to Eutrophic or "nutrient rich". This process is called "eutrophication" and can be seen both in terms of the aging of a lake and the increasing of its productivity.
Eutrophication and productivity.
As noted, lakes transition over time from nutrient lacking to nutrient rich in a gradual transition called aging that is reflected by the productivity of the lake. This can be observed by both the progression and change in biodiversity. This, however, is not always a linear progression and many species can come to dominate or disappear as lake conditions change.
Newer lakes with little vegetation can be said to be in the Pioneer Stage. With little vegetation and decaying organic matter, life forms will be limited to surface algas and their consumers, insects that use water for part of their life cycle and the carnivorious fish or other animals that feed on them.
As the lake ages, plants begin to root and grow beneath the surface in an ever expanding littoral zone. This is known as the Submergent Vegetation Stage. These plants provide habitat for other lifeforms as well as food for herbivores and detritus consumers.
As eutrophication continues, animals reliant on clean water (such as trout) may die out and those more suited to nutrient rich environments (carp, suckerfish) may flourish. As the water becomes shallower in the lake, plants begin to fill in the margins and extend above the water line; this is called the Emergent Vegetation Stage. Now habitat is in place to support lake margin terrestrial life like otters, ducks and geese.
As noted, lakes transition over time from nutrient lacking to nutrient rich in a gradual transition called aging that is reflected by the productivity of the lake. This can be observed by both the progression and change in biodiversity. This, however, is not always a linear progression and many species can come to dominate or disappear as lake conditions change.
Newer lakes with little vegetation can be said to be in the Pioneer Stage. With little vegetation and decaying organic matter, life forms will be limited to surface algas and their consumers, insects that use water for part of their life cycle and the carnivorious fish or other animals that feed on them.
As the lake ages, plants begin to root and grow beneath the surface in an ever expanding littoral zone. This is known as the Submergent Vegetation Stage. These plants provide habitat for other lifeforms as well as food for herbivores and detritus consumers.
As eutrophication continues, animals reliant on clean water (such as trout) may die out and those more suited to nutrient rich environments (carp, suckerfish) may flourish. As the water becomes shallower in the lake, plants begin to fill in the margins and extend above the water line; this is called the Emergent Vegetation Stage. Now habitat is in place to support lake margin terrestrial life like otters, ducks and geese.