Lakes: the ecology of freshwater systems

The biology and chemistry of a lake depends on a number of factors, including:

* how it was formed
* size and shape of the lake basin
* size, topography, and chemistry of its watershed
* regional climate
* local biological communities
* activities of humans during the past century

But as far as the organisms which inhabit lakes, the most important factors include the properties of water itself.

Thus let us start us with a primer of water, its properties and how they affect life in general, and then go on to the specifics of the life and chemistry of lakes.

I. Properties of water:
Water is the solvent, the medium and the participant in most of the chemical reactions occurring in our environment.

Although a water molecule in itself is quite simple, made up of two hydrogen and one oxygen it is its' architecture of these units taken together which produces a matrix with incredible properties

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A. Water can store tremendous quantities of heat. One must add or remove a large amount of heat energy to change the temperature of water. Energy applied to water goes into breaking hydrogen bonds and not into raising the kinetic energy (T) of the molecules

Specific heat of water : the energy required to raise the temperature of water 1C is 1 calorie. Water has a high specific heat
Biological implications?

• Contrast the environment of aquatic vs. terrestrial organisms: which is a more stable environment?
• Contrast the environmental stress of living in small vs. large water bodies------are there seasons in large bodies of water?
• Differences in temperature between lakes and rivers and the surrounding air may have a variety of effects. For example, local fog or mist is likely to occur if a lake cools the surrounding air enough to cause saturation; consequently small water droplets are suspended in the air
  • Large bodies of water, such as the oceans or the Great Lakes, have a profound influence on climate. They are the world's great heat reservoirs and heat exchangers and the source of much of the moisture that falls as rain and snow over adjacent land masses.
  • When water is colder than the air, precipitation is curbed, winds are reduced, and fog banks are formed.

  Heat of melting: the energy required to melt ice is even higher; to create 1 gm of water from ice requires 80 calories of energy. This is the same amount of energy it takes to raise water from 0 to 80C. Why?
  Biological implications? how does this protect aquatic organisms?
  
  Heat of vaporization: to evaporate 1 gram of water requires 597 calories of energy at 0 C and 536 calories at 100C
  Biological implications? your body. If this value were substantially less, you would need to loss much more water to cool down. If so, what would be the limits on organisms who live in drier climates?

  B. Thermal conductivity: flux of heat through 1 cm2

  • is .006 at 40C
  • is .02 at 0C

  Biological relevancy- water conducts heat 20x faster than air.
  Why? water is denser than air by a factor 800 Given this, what is the best way to facilitate heat gain or loss in a body? how would this impact the ambient temperature of the environment of aquatic vs terrestrial creatures?
  
  C. Water expands upon freezing - this is critical in lakes and ponds. Could fish survive if ice sunk?
  D. Water has an immense capacity to dissolve organic compounds

  A drop of rain water falling through the air dissolves atmospheric gases, often cleansing the skies. When rain reaches the earth, it affects the quality of the land, lakes and rivers through its additions - we know of acid rain and its impacts...

  E. Due to water's cohesiveness ( attraction of water molecules to each other) bodies of water have surface tension. This may act as a barrier or support for individuals sitting on or moving through water-air surfaces. This cohesiveness is critical for water movement in plants.

  Surface tension is a measure of the strength of the water's surface film. The attraction between the water molecules creates a strong film, which among other common liquids is only surpassed by that of mercury.

  • This surface tension permits water to hold up substances heavier and denser than itself. A steel needle carefully placed on the surface of a glass of water will float. Some aquatic insects such as the water strider rely on surface tension to walk on water.

  Surface tension is essential for the transfer of energy from wind to water to create waves which permit rapid oxygen diffusion in lakes and seas.

  G. Viscosity, a property of hydrogen bonding, is high in water. Not only does it cause problems in terms of eddy and laminar viscosity ( see text) but also as it is so high, forms a frictional resistance to objects that must move through water. This has a major impact on the way aquatic organisms are shaped and whether they are passive or active movers in water.

  Movement of organisms through water ( or air):

  Aquatic organisms can decide to be:

  • sessile ( nonmotile- no energy strategy)
  • passive or floaters, a low energy strategy or
  • active swimmers, a high energy strategy.

  What are the constraints?

For a floater: Stokes Law states

Velocity of free fall ( sinking rate) = [ K * r² * (p₁ - p₂)] / v
where

K is the constant determined by shape where it may be tight or expansive; some organisms go through cyclomorphosis where they will change shape expanding in warmer times with translucent projections

r is the radius or size: as r increases the organism will begin to sink so it must remain small in order to float - however must consider the consequences of being small in terms of temperature, metabolism etc.

p1 or density of the falling object: as p1 increases it will sink, so to float its value must approach 1, the density of water. It can drop p1 by including fats or oil droplets or by including air bladders ( as in shells which trap air or specific bladders which can hold gases).

p2 refers to the density of the medium itself:

• air: 1.3 x10^-3
• water = 1
• glycerin = 1.26
  
  

v refers to the viscosity of the medium i.e.. resistance of a liquid to flowing over itself.

• air = 1.8 x 10^-4
• water = 0.01
• glycerin = 15

Since water is so viscous relative to air, small organisms can stay afloat by modifying their shape and density.

TO be a swimmer, that is to move forward in a fluid, you must be able to overcome drag ( which acts against the front of the object)

Here another predictive equation is involved, the Reynolds number where:

Reynolds # = [p2 * s * length of object] / v

where p2 is the density of water = 1, and s = speed of the fluid relative to the object and v is the viscosity as above ( water = .01).

If R = 1 or less you have a situation where the fluid feels viscous and you have high drag -> felt by small objects like diatoms, zooplankton with low speeds and short lengths so it may be better to be a floater or passive

If R is high, 1000+ you feel primarily a pressure drag due to the area of the organism contacting the fluid. You must be either big & fast or slender and stream lined to compensate for the the drag.

Now back to the basics of lakes....

1.Limestone sinkholes of Florida

 Florida is dominated by the sedimentary rock known as limestone, a rock made of the mineral calcite (calcium carbonate; recall most of FL was once undersea).  The mineral calcite is very susceptible to weak acids and dissolves quite readily.  Acid rainfall (either naturally acid or artificially acidic) falls on the Florida limestone, and combined with soil acids, produces a powerful dissolving agent that eats away at the limestone bedrock. 

The acid waters percolate and infiltrate into cracks in the limestone, penetrating far underground.  Acid groundwater moves slowly through the limestone, dissolving the calcite and carrying it away as groundwater flows from high spots to low spots.

 The result (over many years) is a system of underground caves and caverns, partially or completely filled with groundwater.  The roofs of these caverns can collapse, producing sinkholes. 

Withdrawal of groundwater and lowering of the water table (the top of the water saturated ground) is one of the main causes of collapse of cave roofs, because the groundwater filling the cave partially supports the cave roof.  The water table can be lowered naturally during drought, and lowered artificially by excess groundwater withdrawal from wells (especially agricultural or industrial wells) or by draining wetlands.  The sinkholes can be dry but are usually partly filled with water, forming lakes.

2. Riverine, coastal and mountain lakes:

Riverine lakes form on wide floodplains where lakes meander across the landscape. Sediments are deposited at the slow flowing inside bend and eroded from the quick flowing outside bend. When wither the deposition becomes sufficient for the water to forge a new route or erosion is sufficient for the river to break through a narrow isthmus between two succeeding curves, a new section of river channel is created. The old cutoff section remain as a crescent shaped shallow lake fed by groundwater or seasonal flooding.

Coastal lakes are recent as sea levels only stabilized 6000 years ago after glacial meting ended. They form when a bar builds up between headlands of marine or very large freshwater bays. They come abut when an along shore current carrying sediment encounters a quiescent bay or indentation and deposits part of its load. If the resulting bar is not destroyed by storms it grows forming a lake behind it.

Mountain lakes include volcanic lakes located in craters formed after an eruption and those resulting from flowing lava damning river valleys.

3. The most common type of lake found in the US are formed by glaciers

The glaciers that covered much of Wisconsin, Minn, New York until about 12,000 years ago created their numerous lakes.

• Glaciers formed lake basins by:
  -Ice scour lakes: gouging holes in loose soil or soft bedrock,
  -Terminal moraine lakes: depositing material across stream beds, or
  -Kettle lakes: leaving buried chunks of ice that later melted to leave lake basins (Figure 1). When these natural depressions or impoundments filled with water, they became lakes.

After the glaciers retreated, sediments accumulated in the deeper parts of the lake. These sediments entered the lakes from tributaries and from decomposed organic material derived from both the watershed and aquatic from plants and algae.

Though lakes vary in many dimensions they are actually highly structured, similar to a forest ecosystem where, for example, a variety of physical variables (light, temperature, moisture) vary with depth from the canopy on down.

Light:

Light absorption and attenuation of light by the water column are major factors controlling temperature and potential photosynthesis. Photosynthesis provides the food and much of the dissolved oxygen in the water.
Solar radiation is the major source of heat to the water column and is a major factor determining wind patterns in the lake basin and water movements.

Light intensity at the lake surface varies seasonally and with cloud cover and decreases with depth down the water column. The deeper into the water column that light can penetrate, the deeper photosynthesis can occur. Photosynthetic organisms include algae suspended in the water (phytoplankton), algae attached to surfaces (periphyton), and vascular aquatic plants (macrophytes).

The rate at which light decreases with depth depends upon the amount of :

light-absorbing dissolved substances (mostly organic carbon compounds washed in from decomposing vegetation in the watershed)

absorption and scattering caused by suspended materials (soil particles from the watershed, algae and detritus).

The percentage of the surface light absorbed or scattered in a 1 meter long vertical column of water, is called the vertical extinction coefficient. This parameter is symbolized by "k".

In lakes with low k-values, light penetrates deeper than in those with high k-values. The figure below shows the light attenuation profiles from two lakes with attenuation coefficients of 0.2/m and 0.9/m.

The maximum depth at which algae and macrophytes can grow is determined by light levels. Limnologists estimate this depth to be the point at which the amount of light available is reduced to 0.5%–1% of the amount of light available at the lake surface. This is called the euphotic zone.

A general rule of thumb is that this depth is about 2 to 3 times the limit of visibility as estimated using a Secchi disk. Since photosynthesis depends fundamentally on light, significant changes in light penetration in a lake will produce a variety of direct and indirect biological and chemical effects. Significant changes in lake transparency are most often the result of human activities, usually in association with landuse activities in the watershed.

Estimated ranges of water transparency values for various lakes.

Density Stratification:

In the spring, immediately after ice-out in temperate climates, the water column is cold and nearly isothermal with depth. The intense sunlight of spring is absorbed in the water column, which also heats up as the average daily temperature of the air increases. In the absence of wind, a temperature profile with depth form in which T decreases exponentially with depth. However, density, another physical characteristic of water, plays an important role in modifying this pattern.

Water differs from most other compounds because it is less dense as a solid than as a liquid. Consequently ice floats, while water at temperatures just above freezing sinks. As most compounds change from a liquid to a solid, the molecules become more tightly packed and consequently the compound is denser as a solid than as a liquid. Water, in contrast, is most dense at 4°C and becomes less dense at both higher and lower temperatures. The density/temperature relationship of fresh water is shown below. Because of this density-temperature relationship, many lakes in temperate climates tend to stratify, that is, they separate into distinct layers.

In northern lakes, the water near a lake’s bottom will usually be at 4°C just before the lake's ice cover melts in the spring. Water above that layer will be cooler, approaching 0°C just under the ice.
As the weather warms, the ice melts. The surface water heats up and therefore it decreases in density. When the temperature (density) of the surface water equals the bottom water, very little wind energy is needed to mix the lake completely. This is called turnover.

After this spring turnover, the surface water continues to absorb heat and warms. As the temperature rises, the water becomes lighter than the water below. For a while winds may still mix the lake from bottom to top, but eventually the upper water becomes too warm and too buoyant to mix completely with the denser deeper water. The relatively large differences in density at higher temperatures are very effective at preventing mixing. It simply takes too much energy to mix the water any deeper.

As summer progresses, the temperature (and density) differences between upper and lower water layers become more distinct. Deep lakes generally become physically stratified into three identifiable layers, known as the epilimnion, metalimnion, and hypolimnion .

The epilimnion is the upper, warm layer, and is typically well mixed.
Below the epilimnion is the metalimnion or thermocline region, a layer of water in which the temperature declines rapidly with depth.

The hypolimnion is the bottom layer of colder water, isolated from the epilimnion by the metalimnion. The density change at the metalimnion acts as a physical barrier that prevents mixing of the upper and lower layers for several months during the summer.

The depth of mixing depends in part on the exposure of the lake to wind (its fetch), but is most closely related to the lake’s size. Smaller to moderately-sized lakes (50 to 1000 acres) reasonably may be expected to stratify and be well mixed to a depth of 3–7 meters in north temperate climates. Larger lakes may be well mixed to a depth of 10–15 meters in summer (e.g., Western Lake Superior near Duluth, MN).

As the weather cools during autumn, the epilimnion cools too, reducing the density difference between it and the hypolimnion. As time passes, winds mix the lake to greater depths, and the thermocline gradually deepens. When surface and bottom waters approach the same temperature and density, autumn winds can mix the entire lake; the lake is said to "turn over."

As the atmosphere cools, the surface water continues to cool until it freezes. A less distinct density stratification than that seen in summer develops under the ice during winter. Most of the water column is isothermal at a temperature of 4°C, which is denser than the colder, lighter water just below the ice. In this case the stratification is much less stable, because the density difference between 0°C and 4°C water is quite small. However, the water column is isolated from wind-induced turbulence by its cap of ice. Therefore, the layering persists throughout the winter.

Types of Mixing in Lakes:

1. This pattern (spring turnover — summer stratification — fall turnover — winter stratification) is typical for temperate lakes. Lakes with this pattern of two mixing periods are referred to as dimictic.

2. Lakes that stratify and destratify numerous times within a summer are known as polymictic lakes.

3. Much less common are lakes that circulate incompletely resulting in a layer of bottom water that remains stagnant. To distinguish them from the holomictic (mixing from top to bottom) lakes, these partially mixing lakes are referred to as meromictic. They mix partially, in the sense that they may have extensive mixing periods which go quite deeply into the hypolimnion, but they do not turn over completely, and a layer of bottom water remains stagnant and anoxic for years at a time.

The non-mixing bottom layer is known as the monimolimnion and is separated from the mixolimnion (the zone that mixes completely at least once a year) by the chemocline . The stagnant, and typically anaerobic, monimolimnion has a high concentration of dissolved solids compared to the mixolimnion.

In general, meromictic lakes have large relative depths. These lakes are typically small and sheltered from the wind by the morphology of their basin. In this case, the density differences caused by temperature are smaller than density differences due to the high dissolved solids (salts) concentration of the monimolimnion. Large lakes that rarely freeze over are also typically monomictic, mixing throughout the fall, winter and spring and stratifying in the summer.

To visualize this effect, try dissolving several tablespoons of table salt (NaCl) in hot water. Add a few drops of food coloring and then fill a mayonnaise jar half-full. Now, very gently add cool tap water with a small measuring cup to fill the glass. Set up a second jar half full with clear, cool water and then add the colored hot water to fill the glass - but don't add the salt. Compare the stability of the density stratification in the two systems by gently shaking or stirring the water columns.

Dissolved Oxygen

Biological activity peaks during the spring and summer when photosynthetic activity is driven by high solar radiation. Furthermore, during the summer most lakes in temperate climates are stratified. The combination of thermal stratification and biological activity causes characteristic patterns in water chemistry.

The figure below shows the typical seasonal changes in dissolved oxygen (DO) and temperature. The top scale in each graph is oxygen levels in mg O2/L. The bottom scale is temperature in °C. In the spring and fall, both oligotrophic and eutrophic lakes tend to have uniform, well-mixed conditions throughout the water column. During summer stratification, the conditions in each layer diverge.

The DO concentration in the epilimnion remains high throughout the summer because of photosynthesis and diffusion from the atmosphere. However, conditions in the hypolimnion vary with trophic status.

In eutrophic (more productive) lakes, hypolimnetic DO declines during the summer because it is cut-off from all sources of oxygen, while organisms continue to respire and consume oxygen. The bottom layer of the lake and even the entire hypolimnion may eventually become anoxic, that is, totally devoid of oxygen.

In oligotrophic lakes, low algal biomass allows deeper light penetration and less decomposition. Algae are able to grow relatively deeper in the water column and less oxygen is consumed by decomposition. The DO concentrations may therefore increase with depth below the thermocline where colder water is "carrying" higher DO leftover from spring mixing (recall that oxygen is more soluble in colder water).

In extremely deep, unproductive lakes such as Crater Lake, OR, Lake Tahoe, CA/NV, and Lake Superior, DO may persist at high concentrations, near 100% saturation, throughout the water column all year. These differences between eutrophic and oligotrophic lakes tend to disappear with fall turnover.

In the winter, oligotrophic lakes generally have uniform conditions. Ice-covered eutrophic lakes, however, may develop a winter stratification of dissolved oxygen.

If there is little or no snow cover to block sunlight, phytoplankton and some macrophytes may continue to photosynthesize, resulting in a small increase in DO just below the ice.

But as microorganisms continue to decompose material in the lower water column and in the sediments, they consume oxygen, and the DO is depleted. No oxygen input from the air occurs because of the ice cover, and, if snow covers the ice, it becomes too dark for photosynthesis.

This condition can cause high fish mortality during the winter, known as "winter kill." Low DO in the water overlying the sediments can exacerbate water quality deterioration, because when the DO level drops below 1 mg O2/L chemical processes at the sediment-water interface frequently cause release of phosphorus from the sediments into the water. When a lake mixes in the spring, this new phosphorus and ammonium that has built up in the bottom water fuels increased algal growth.

The watershed:

The watershed, also called the drainage basin, is all of the land and water areas that drain toward a particular river or lake. Thus, a watershed is defined in terms of the selected lake (or river). There can be subwatersheds within watersheds. For example, a tributary to a lake has its own watershed, which is part of the larger total drainage area to the lake.

A lake is a reflection of its watershed. More specifically, a lake reflects the watershed's size, topography, geology, landuse, soil fertility and erodibility, and vegetation. The impact of the watershed is evident in the relation of nutrient loading to the watershed:lake surface area ratio (Figure 7). See also the section on conductivity.

Lakes with very small watersheds that are maintained primarily by groundwater flow are known as seepage lakes.

In contrast, lakes fed primarily by inflowing streams or rivers are known as drainage lakes. In keeping with the watershed:lake area relationship, seepage lakes tend to have good water quality compared with drainage lakes. However, seepage lakes are often more susceptible to acidification from acid rain because of their low buffering capacity.

Life Span of a Lake

Lakes, like all things, change over time. Even a lake has a life span like people do - there are young lakes, middle-aged lakes and old lakes. Many of the lakes in British Columbia are still quite young because they were formed by the last ice age that carved our northern landscape.

As our lakes age, many of them will slowly shrink. Land naturally takes over at the edge of a lake. It's very slow, but it happens. This is called “lake succession,” and this is how it works.

Each year trees and shrubs along the edge of a lake shed leaves and debris into the water that then settle to the bottom.
This debris slowly decays to form new habitat for grasses, rushes and sedges. The grasses take root quickly and stabilize the new ground for dry land species. Some day shrubs and small trees will grow where lily pads once floated.
As the lake becomes smaller and smaller, it may not be able to provide enough oxygen or food for some fish. But this happens so slowly that they have plenty of time to move on to other lakes, if the lake is connected to the rest of the watershed by a stream.

Not all lakes are subject to succession, of course. If a lake has a steep, rocky shore, there is no way for the shore to creep into the lake, short of dynamite. You can tell where a lake is disappearing by the plant-life: wherever you see grasses, rushes, reeds and sedges along wet, indistinct and muddy shorelines, you know that the lake is being taken over.

Lake Chemistry

In the absence of any living organisms, a lake contains a wide array of molecules and ions from the weathering of soils in the watershed, the atmosphere, and the lake bottom. Therefore, the chemical composition of a lake is fundamentally a function of:

• its climate (which affects its hydrology) and,
• its basin geology. Each lake has an ion balance of the three major anions and four major cations

ION BALANCE FOR TYPICAL FRESH WATER

Ion balance means the sum of the negative ions equals the sum of the positive cations when expressed as equivalents.

These ions are usually present at concentrations expressed as mg/L (parts per million, or ppm) whereas other ions such as the nutrients phosphate, nitrate, and ammonium are present at µg/L (parts per billion, or ppb) levels.

Humans can have profound influences on lake chemistry.

• Excessive landscape disturbance causes higher rates of leaching and erosion by removing vegetative cover, exposing soil, and increasing water runoff velocity.
• Lawn fertilizers, wastewater and urban stormwater inputs all add micronutrients such as nitrogen and phosphorus, major ions such as chloride and potassium, and, in the case of highway and parking lot runoff, oils and heavy metals.
• Emissions from motorized vehicles, fossil fuel-burning electric utilities and industry, and other sources produce a variety of compounds that affect lake chemistry.
  

Importance of ion balances:

Acid Deposition:Perhaps the best understood ions are H+ (hydrogen ion, which indicates acidity), SO4-2 (sulfate) and NO3- (nitrate) which are associated with acid rains.

Mercury (Hg) is another significant air pollutant affecting aquatic ecosystems and can bioaccumulate in aquatic food webs, contaminating fish and causing a threat to human and wildlife health

Lakes with high concentrations of the ions calcium (Ca+2) and magnesium (Mg+2) are called hardwater lakes, while those with low concentrations of these ions are called softwater lakes.

Concentrations of other ions, especially bicarbonate, are highly correlated with the concentrations of the hardness ions, especially Ca+2.

The ionic concentrations influence the lake's ability to assimilate pollutants and maintain nutrients in solution. For example, calcium carbonate (CaCO3) in the form known as marl can precipitate phosphate from the water and thereby remove this important nutrient from the water.

The total amount of ions in the water is called the TDS (total dissolved salt, or total dissolved solids concentration). Both the concentration of TDS and the relative amounts or ratios of different ions influence the species of organisms that can best survive in the lake, in addition to affecting many important chemical reactions that occur in the water.

One example of particular interest in the Great Lakes region involves the calcium requirement of the exotic zebra mussel that is causing profound changes in Lake Erie Lake Superior appears to be relatively immune to infestation by this invader because of low calcium concentration. Its bays, however, such as the lower St. Louis River and Duluth-Superior Harbor, may not be immune to zebra mussel infestation.  

Biology of lakes

A typical lake has distinct zones of biological communities linked to the physical structure of the lake.

The littoral zone is the near shore area where sunlight penetrates all the way to the sediment and allows aquatic plants (macrophytes) to grow. Light levels of about 1% or less of surface values usually define this depth.

The 1% light level also defines the euphotic zone of the lake, which is the layer from the surface down to the depth where light levels become too low for photosynthesizers. In most lakes, the sunlit euphotic zone occurs within the epilimnion.

The higher plants in the littoral zone, in addition to being a food source and a substrate for algae and invertebrates, provide a habitat for fish and other organisms that is very different from the open water environment.

The limnetic zone is the open water area where light does not generally penetrate all the way to the bottom. The bottom sediment, known as the benthic zone, has a surface layer abundant with organisms. This upper layer of sediments may be mixed by the activity of the benthic organisms that live there, often to a depth of 2-5 cm (several inches) in rich organic sediments.

Most of the organisms in the benthic zone are invertebrates, such as Dipteran insect larvae (midges, mosquitoes, black flies, etc.) or small crustaceans. The productivity of this zone largely depends upon the organic content of the sediment, the amount of physical structure, and in some cases upon the rate of fish predation.

• Sandy substrates contain relatively little organic matter (food) for organisms and poor protection from predatory fish. Higher plant growth is typically sparse in sandy sediment, because the sand is unstable and nutrient deficient.
  
• A rocky bottom has a high diversity of potential habitats offering protection (refuge) from predators, substrate for attached algae (periphyton on rocks), and pockets of organic "ooze" (food).
  
• A flat mucky bottom offers abundant food for benthic organisms but is less protected and may have a lower diversity of structural habitats, unless it is colonized by higher plants.

Food Web:

The biological communities within lakes may be organized conceptually into food chains and food webs to help us understand how the ecosystem functions .

The simplest illustration of the organization of the organisms within an ecosystem is the ecological pyramid . The broad base of primary producers supports overlying levels of herbivores (zooplankton), planktivores and much smaller numbers of carnivores (predators) .

These individual trophic levels may be idealized as a food chain, but in fact many organisms are omnivorous and not necessarily characterized by a particular level. Further, consumers in particular often shift levels throughout their life cycle.

For example, a larval fish may initially eat fine particulate material that includes algae, bacteria and detritus. Then it may switch and graze on larger zooplankton and ultimately end up feeding on so called "forage fish" or even young game fish (i.e., top predators) when it reaches maturity

The whole interaction of photosynthesis and respiration by plants, animals, and microorganisms represents the food web. Food webs are usually very complex and, in any one lake ecosystem, hundreds of different species can be involved. Because the available energy decreases at each trophic level, a large food base of primary producers (mostly plants) is necessary to support relatively few large fish.

Microorganisms (bacteria and fungi) consume a large fraction of available oxygen in the decomposition of excreted and dead organic material.
Decomposers are sinks for plant and animal wastes, but they also recycle nutrients for photosynthesis. The amount of dead material in a lake far exceeds the living material. Detritus is the organic fraction of the dead material, and can be in the form of small fragments of plants and animals or as dissolved organic material. In recent years, scientists have recognized that zooplankton grazing on detritus and its associated bacterial community represent an additional important trophic pathway in lakes.

Primary Producers:

Much of modern limnological study revolves around the primary productivity of lakes. The ecology of plant growth is of great importance to the character and history of lakes and to all other organisms that live in lakes. The major threat to lakes involves the excessive growth of primary producers due to nutrient inputs caused by poor landuse management. Therefore, it is worth a closer look at these organisms.
The littoral zone is defined by the growth of rooted and floating aquatic plants, or macrophytes. Figure 17 provides examples of common macrophytes found in Minnesota lakes.

There may be few macrophytes in a lake when the bottom is too rocky or too sandy for the plants to anchor themselves, wave action too severe, or the water too deep. Also, sunlight may not reach the bottom even in shallow areas if the concentration of algae or silt is high.

The macrophyte community can also include large algae, such as Chara,Nitelle,orCladophora.In shallow, clear lakes, macrophytes may represent most of the green plant material present and may account for most of the photosynthesis.

Floating:

WATER LILY- Nymphaea

DUCKWEED-Spirodela

PONDWEED-Potamogeton

Submergent:

STONEWORT-Chara

COONTAIL
Ceratophyllum

BLADDERWORT
Utricularia

Emergent

CATTAIL:Typha

BULRUSHScirpus
WILD RICE: Zizania

Algae constitute the other main group of primary producers (Figure 18). They come in countless forms and live in nearly all kinds of environments. Most are microscopic, growing as single cells, small colonies, or filaments of cells.

Suspended algae are called phytoplankton, while attached algae are called periphyton. Phytoplankton grow suspended in open water by taking up nutrients from the water and energy from sunlight. If their populations are dense, the water will become noticeably green or brown and will have low transparency

Diatoms belong to a large group, classified as the golden-brown algae, which also includes chrysophytes and dinoflagellates.

The most striking characteristic of diatoms and chrysophytes is the ability to form silica (glass) cell walls. Diatoms cell walls are similar to a petri dish, having two halves that fit together. Some chrysophytes have elaborate silica scales, spines, or vase-like shells called loricas.

Diatoms are non-motile (unable to swim), so they depend on water turbulence to remain suspended. Chrysophytes have flagella (whip-like appendages) that allow them to control their position in the water column. There are other important algal groups containing motile forms.

Blue-green "algae" or the cyanobacteria are bacteria. They generally receive the greatest amount of research and management attention because of their ability to form nuisance blooms in eutrophic lakes. It is important to remember, however, that blue-green algae are very important primary producers in both freshwater and marine systems, despite often being a nuisance.

Blue-greens have several characteristics that often enable them to dominate and create nuisance or noxious conditions.

Some blue-green species have the ability to adjust their buoyancy. They can float or sink depending on light conditions and nutrient supply.

All plants, including all algae, typically satisfy their nitrogen requirement by absorbing nitrate (NO3-) and/or ammonium (NH4+) from the water. However, some blue-greens can fix molecular nitrogen (N2) derived from the atmosphere and dissolved in the water and convert it to ammonium in the cell through a process called nitrogen fixation. This allows them to maintain high rates of growth when other forms of nitrogen are sufficiently depleted to limit growth by other types of algae.

Blue-green algae typically are well-adapted to phosphorus deficiency because of their ability to absorb and store excess phosphorus when it is available -- enough to last days to weeks in some cases.

Unlike the green algae and diatoms, the blue-green algae are less suitable food for primary consumers. This is partly because some blue-greens can form large colonies of cells embedded in a gelatinous matrix which may pose handling problems for grazers. They also may produce chemicals that inhibit grazers or makes them "taste bad" to the grazers. Consequently, blue-greens have advantages over other algae at using nutrient and light resources, as well as avoiding being eaten.

Aphanizomenon flos-aquae is a common species of filamentous blue-green algae (see Figure 18) with the ability to regulate its buoyancy, fix nitrogen, form large inedible colonies, and form algal blooms. Other common bloom genera are Anabaena (N2-fixing filamentous algae) and Microcystis (colonial; not a N2-fixer). These bloom-forming algae are known to produce toxins in farm ponds that can poison cattle and, more recently, have been found to produce potent neurotoxins and hepatotoxins that may be a greater public health concern than previously realized.

Seasonal Succession

A lake’s biological characteristics are determined in large part by physical characteristics of the water column. Important physical characteristics include temperature, light transparency, and wave action, as well as the total abundance of inorganic nutrients, which is largely a watershed characteristic.

In addition, preceding populations influence successive populations by assimilating critical nutrients. Populations also have varying susceptibilities to grazing by zooplankton, which vary seasonally in type and abundance. As physical, chemical, and biological conditions in the lake change over time, some species will be effectively eliminated from a lake because they cannot tolerate the new conditions. Other species will be out-competed by organisms that are better adapted to the new environment. 

These changes represent an important ecological pattern in lakes known as algal succession. In most natural systems the seasonal succession of algae (and macrophytes) is a recurrent, if not exactly repetitive, yearly cycle.

A typical algal succession is shown above. Some species flourish for a period of time and then give way to other species more compatible with changed conditions, such as warmer water, more daylight, or lower concentrations of phosphorus or nitrogen. Short-lived plankton communities are characterized by these seasonal fluctuations; longer-lived organisms, such as fish, must be tolerant of lake conditions all year.

Zooplankton, small animals that swim about in open water (Figure 20), are primary consumers. They graze on algae, bacteria, and detritus (partially decayed organic material). Some species can be seen with the naked eye, although they are more easily observed with a hand lens or low-power microscopes.

Secondary consumers, such as planktivorous fish or predaceous invertebrates, eat zooplankton. While photosynthesis limits plant growth to the sunlit portions of lakes, consumers can live and grow in all lake zones, although the lack of oxygen (anoxia) may limit their abundance in bottom waters and sediments.
ZOOPLANKTON

daphnia	Diaptomus

Benthic organisms are major consumers and are also important recyclers of nutrients otherwise trapped in the sediments. Benthic organisms include invertebrates and bottom-feeding fish. Their feeding strategies vary widely. Some, such as clams, filter small bits of organic material from water as it flows by. Others eat detritus that has sunk to the bottom. The spread of the exotic invader, the zebra mussel, has caused dramatic changes in the water quality and ecology of Lake Erie in the past decade due to its high rates of filtration and high reproductive rate

The best known group of aquatic consumers is fish. Many small fish, such as sunfish and perch, primarily eat zooplankton.

Tertiary consumers that prey on the smaller fish include larger fish and other carnivorous animals (loons, grebes, herons, and otters). Different species exploit different habitats (niches).

Bass and pike are found in lakes that have beds of aquatic macrophytes suitable for spawning.

Walleyes, on the other hand, spawn on a gravel bottom.

Lake trout live only in very clear lakes with cold, well-oxygenated deep water. In contrast, carp are adapted to warm turbid, low oxygen lakes with mucky, high organic matter bottoms.

Images: www.npwrc.usgs.gov/resource/tools/ ndfishes/gifs/sunfish.jpg