The current chemical and biological conditions of our lake depend on many
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
Physical Characteristics of a Lake
The amount of light within the water column is a major factor controlling
temperature and photosynthesis. It also affects:
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.
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
A pattern evolves relating to the temperature changes in a lake.
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 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 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
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 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
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.
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 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
There are two basic life-sustaining processes in lakes:
The interaction of photosynthesis and respiration by plants, animals, and
microorganisms characterize the food web.