The current chemical and biological conditions of our lake depend on many factors, including:

• The lake's history and how it was formed
• The size and shape of the lake
• The regional climate surrounding the lake
• Local biological communities
• The activities of humans during the past century that affect the lake

The physical, chemical and biological characteristics of lakes are extremely variable.

Physical Characteristics of a Lake

Light

The amount of light within the water column is a major factor controlling temperature and photosynthesis. It also affects:

• Wind patterns
• Water movements
• Dissolved oxygen concentrations

Light varies seasonally and with cloud cover and depth. The deeper light infiltrates a lake, the deeper photosynthesis can occur.

Photosynthesis provides the food that supports a large portion of the food web. Since photosynthesis depends greatly on light, changes in light have the potential to result in biological and chemical impacts.

Temperature

Water is less dense as a solid than as a liquid. Water is most dense at 4�C. Therefore, ice floats because it is less dense than its liquid form, water.

Because of this unique relationship between density and temperature, many lakes in temperate climates tend to separate into distinct layers (stratify). In the summer, lakes will stratify, creating a cold layer near the bottom with a warmer layer near the surface. In general, the larger the lake the deeper it will stratify.

A pattern evolves relating to the temperature changes in a lake.

• Spring turnover
• Summer stratification
• Fall turnover
• Winter stratification

Turnover

In late summer or early fall,

• Air temperatures cool the surface water
• Dense cool water falls and is replaced by lighter warm water
• The temperatures uniformly reaches approximate temperatures of 4�C (39�F)
• Wind action can then cause the lake to mix or "turnover"

The exact opposite happens in spring after ice-off as the surface water warms to 4�C. A lake can re-stratify in a few days to a couple weeks.

Winterkill

Winterkill is the dying of fish in late winter because of a lack of oxygen at the bottom of lakes (anoxia).

When ice covers a lakes surface,

• Water cannot obtain oxygen through the mixing with air because of the ice cover
• Limited sunlight penetration reduces photosynthesis
• Vegetation begins to decompose which uses up oxygen

Winterkill occurs when these conditions reduce dissolved oxygen concentrations below threshold levels and the lake is unable to support fish.

Summerkill

Summerkill is the dying of fish in the late summer because of a lack of oxygen at the bottom of lakes (anoxia).

When warmer weather occurs:

• Water cannot obtain oxygen through mixing with air because the lake has stratified
• Warmer water is able to hold less oxygen
• Large algae blooms can occur
• As algae blooms die and decompose they consume large amounts of oxygen

Summerkill occurs when these conditions reduce dissolved oxygen concentrations below threshold levels and the lake is unable to support fish.

Chemical Characteristics of a Lake

Each lake's chemistry is unique to that lake. The watershed, atmosphere and the lake bottom all affect the chemistry of a lake. Therefore the chemical make-up of a lake is affected by its climate and its basin geology. Humans also have the capability to greatly affect lakes chemistry through the input of nutrients and toxic substances that wash into the lake through stormwater runoff.

Dissolved Organic Carbon

Dissolved organic carbon (DOC) is organic material from plants and animals broken down into such a small size that it is "dissolved" into water.

DOC created by the decomposition of leaves and woody debris that have fallen around or in water are termed humic. This plant material is slowly broken down by organisms into very small particles that are dissolved into water. Yellow to black in color, this humic type of DOC is the most abundant kind found in lakes and streams and can have a great influence on water color.

Dissolved Oxygen

Dissolved oxygen is a major component of lake chemistry as it affects many aspects of a lake's ecology. The amount of dissolved oxygen in the water is an important indicator of overall lake health.

Oxygen is supplied to a lake by:

• Photosynthesis of aquatic plants including algae
• The slow diffusion of atmospheric oxygen

The concentration of dissolved oxygen determines the type of organisms that live in a lake. For example, trout need high concentrations of dissolved oxygen to survive, while other species are more tolerant of low or variable levels of dissolved oxygen.

Mixing of water, aided by wind, distributes oxygen throughout a lake's water column. Cold water can hold more oxygen than warm water. A lake will normally have the capacity to hold more oxygen during the winter than during the summer.

Nutrients

As plankton die and sink to the bottom of the lake nutrients are redistributed. The vertical movement of plankton also affects where nutrients lie.

Nutrients, such as phosphorus and nitrogen, typically increase in the spring from snow melt runoff and spring turnover. During periods with low nutrient concentrations, any additional increase in nutrients into the upper water column may trigger an algae bloom.

Phosphorus

Phosphorus is the nutrient that limits plant and algae growth in the lakes in Muskoka. Natural sources of phosphorus include wetlands and precipitation. When larger quantities of phosphorus are introduced to a lake from human sources, algae blooms may result. Human sources of phosphorus include septic systems, fertilizers, agriculture and sewage treatment systems.

Phosphorus Really Is The Key, The Muskoka Sun

Biological Characteristics of a Lake

Zones within a lake provide for different varieties of biological lake communities. There are two major water zones discussed here: the littoral zone and the limnetic zone.

The littoral zone is the nearshore area where sunlight penetrates all the way to the bottom of the lake and allows aquatic plants to grow.

The limnetic zone is the open water area where light does not usually penetrate all the way to the bottom. Floating near the surface are microscopic algae called phytoplankton and cyanobacteria. These organisms produce oxygen and are the food for zooplankton. The zooplankton in turn provides food for fish and other aquatic organisms.

These organisms are all part of a food chain or web. A food chain is a linear connection between one organism and another relying on one another to survive. A food web is an interconnected web that illustrates the many connections between organisms.

There are two basic life-sustaining processes in lakes:

• Photosynthesis
• Respiration

The interaction of photosynthesis and respiration by plants, animals, and microorganisms characterize the food web.