Aquatic Ecosystems

Fresh Water

Fresh water ecosystems include:

Flowing Water

Stream — Cade’s Cove, Smoky Mts., Tenn.
A stream in Cade’s Cove
Typically, rills join to form gullies, which join to form streams, which join to form a river.

Rivers can also begin as springs or seepage or outlets of ponds/lakes, for example:
Mississippi Sign — Lake Itasca, Minn.
Mississippi Sign
Lake Itasca and Mississippi River, Minn.
Wading across Mississippi River, Lake Itasca, Minn.

Ohio River, Clermont Co.
Ohio River, Clermont Co., Ohio

Eventually, many streams and small rivers join to form a large river, such as the Ohio River, which forms the border between Ohio and Kentucky...

The following areas may be found within a stream or river,

These are also known as rapids or white water. Riffles are the sites of primary production. Typically, organisms attach to the substrate to avoid being swept away by the current. Riffle areas have a high O2 content because of the churning.
These are the sites of decomposition. They serve as catch basins for organic materials and produce the CO2 used by the producers in the next riffle.
silt and debris
These substances are carried downstream and deposited in slow-moving areas.
These are those parts of the stream or river channel used at high tide or after spring rains, etc.

Green River, E. Utah
Meandering Green River in Eastern Utah

Still Water

Lake — Brown Co. State Park, Ind.
Lake, Brown Co. State Park, Indiana
Still water includes ponds and lakes. Often, these have an autumnal temperature change or “turnover” as the upper water cools, becomes more dense, and sinks. Simultaneously, the lower water (now warmer) rises. In midsummer, a lake has the typical epi-, meta-, and hypolimnion layers and other zones, as previously discussed, and the temperature in the epi- and hypolimnion are fairly constant. The temperature in the metalimnion changes with depth (there is a thermocline).


Wetlands are areas that range alone a gradient from permanently flooded to periodically saturated soil that supports “hydrophytic” plants.

Wetland Community near Clermont Campus
Wetland Community on Clermont Campus

Types of wetlands include

Marsh These are areas where there is much emergent vegetation (cattails, etc.). A marsh is a “wet prairie.”
Swamp These are areas which are wooded wetland (such as cypress or mangrove swamps).
These are areas which can range from a filled-in lake to a mat of accumulated organic material with much water. Bogs depend heavily on precipitation to supply nutrients, so they usually are low in mineral salts and are acidic.

Salt Water

Ocean — Gulf of Mexico, Panama City, Fla.
Ocean, Gulf of Mexico
Salt concentration is a limiting factor for organisms which live in salt water. While open sea contains about 35 ppt of salt, estuary areas contain much less, and the concentration is more variable. Oceans have a temperature gradient or thermocline like lakes, and overall water temperature is also based on latitude (arctic vs equatorial). Water pressure increases with depth.
Regions or zones of the ocean include:

pelagic zone
This includes all the water and may be further subdivided as with the lake, but different names are used for the same things.
benthic zone
This is divided into the continental shelf or neritic zone and deeper water or the oceanic zone.
intertidal zone
This is the shoreline area between high and low tide. Organisms living in these areas must be able to tolerate changes/fluctuations in the amount of water.
This is the area where fresh and salt water mix (at the end of a river). Organisms in an estuary must adjust to variable salt concentrations.
tidal marsh
This is an estuary with a large number of plants.

Copyright © 1999 by J. Stein Carter. All rights reserved.
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