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.
If you wish to see them, stare into the water of a pond or lake on a
calm night with a flashlight beam shining from above.
consumers, such as planktivorous fish or predaceous invertebrates,
limits plant growth to the sunlit portions of lakes, consumers
can live and grow in all lake zones, although the lack of oxygen
may limit their abundance in bottom waters and sediments.
courtesy of University of Minnesota Limnology
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 (See Ohio Sea Grant).
organisms are easily classified as planktonic or benthic. For example,
insect larvae, remain near the sediments in daytime and migrate to upper
waters at night. These transparent predators ("phantom midges")
migrate upward to feed on zooplankton, and are, themselves, a favorite
food for fish. Mysid shrimp behave in a similar fashion and have been
shown to migrate enormous distances (>100 meters) in Lake Tahoe each
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.
View images of fish on our fish page.
which include bacteria, fungi, and other microorganisms, are the other
major group in the food
web. They feed on the remains of all aquatic organisms and in so
doing break down or decay organic matter, returning it to an inorganic
state. Some of the decayed material is subsequently recycled as nutrients,
such as phosphorus
(in the form of phosphate, PO4-3) and nitrogen
(in the form of ammonium, NH4+) which are readily
available for new plant growth. Carbon is released largely as carbon
dioxide that acts to lower the pH
of bottom waters. In anoxic
zones some carbon can be released as methane gas (CH4). Methane
gas causes the bubbles you may have observed in lake ice.
can be found in all biological zones of a lake, although they are the
dominant forms in the lower hypolimnion
where there is an abundance of dead organic matter. Oxidation of organic
matter by the decomposers (respiration) in the hypolimnion is responsible
for the depletion of dissolved
oxygen over the course of the summer, potentially leading to anoxic
conditions (no dissolved oxygen). There is no source of oxygen in the
hypolimnion to replace oxygen lost through decomposition.
prevents atmospheric oxygen from being mixed deeper than the thermocline,
and it is usually too dark for photosynthesis. Consequently, a large
volume of organic matter from a variety of sources (e.g., wastewater,
sinking algae, dying macrophytes, and organic sediment washed in from
the watershed) leads to faster oxygen depletion and often complete removal
of oxygen in the hypolimnion. The resulting anoxia
has a profound effect on both the chemistry and the biology of the lake.