Life In Water

****Summary of “LIFE IN WATER”  -Chapter 3 of  Ecology: Concepts and Applications(5th edtion, by Manuel C. Molles Jr.)*****

      An astonishing 71% of the Earth’s surface is covered by water.  Even more astonishing is that only 1% of this water is fresh.  98% is contained within the world’s oceans, and 1% is frozen–found in the polar ice caps. 

     By way of the hydrolic cycle, the planet’s water is exchanged among lakes, rivers, oceans, ice, organisms, and the atmosphere.  The time it takes for the entire volume of a reservoir to be renewed is known as turnover time.  The turnover time of reservoirs varies greatly.  For example, the atmospher’s turnover time is only 9 days, whereas that of the world ocean is 3,100 years.

   Variations in physical factors such as light, temperature, water movements, and chemical factors such as salination and oxygen correlate to the biology of aquatic environments.


The largest continuous environment on earth is a body of water: the world’s ocean (Pacific, Atlantic, and Indian combined).  Oceans are divided into 5 depth zones–each of which sustain specific marine life.  The shallowest of these zones is the Epipelagic zone. Also known as the “Photic” zone, it recieves the most light and supports photosyenthetic organisms which are not supported by the deeper four zones–due to the fact that they lack light penetration.

Oceanic temperatures are by far, more stable than terrestrial temperatures.  Tropical seas are more chemically and physically stable than temperate and high-latitude seas.  Whereas the temerpate and high-latitude seas are more biologically productive.  Because oceans contribute 1/4 of the total photosynthesis in the biosphere, they greatly affect the planet’s carbon and oxygen budgets.


Kelp forests are found in temperate to subpolar regions.  Coral reefs are limited to the tropics and subtropics and include three types:  fringing reefs–which hug the shore, barrier Reefs–which are offshore, and coral attols.  Oxygen and nutrients are delivered to kelp froests and coral reefs by currents. –Coral reefs are highly biologically productive.


Waves are the most important water movements affecting the distribution and abundance of intertidal organisms.  The  gravitational pull of the sun and moon determine the size, strength, and timing of tides.  Distribution of most organisms within the intertidal zone is determined by the areas exposure to waves, bottom type, height in the intertidal zone, and biological interactions.


Salt marshes, mangroves forests, and estuaries are all zones of transition between envirionments.  Salt marshes and mangrove forests are concentrated along low-lying coasts with sandy shores (serving as a transition from sea to land), and Estuaries are found whererivers meet the sea.  Salt marcshes are found in temperate to high latitudes and mangroves in tropical and subtropical.  The diversity of species in these transitions zones is not high, however there is great biological productivity.


Rivers and streams drain most of the earth’s landscape.  Historical patterns of flooding have proven to have important influences on the processes of the river ecosystem.  Flood Pulse Concept derives that flooding of rivers allows for the exchange of nutrients and energy between the river channel and the floodplains and associated wetlands.


Most fresh water on earth is contained in a few lakes.  Salinity of lakes is highly variable–dependent upon precipitation, runoff, and evaporation.  Oligotrophic Lakes are well oxygenated, but have low biological production.  Eutrophic Lakes are depleted of oxygen, but have high biological production.  Nutrients and toxic wastes spill into lakes from cities, thankfully lakes have proven to be extremely resilient.

A good measure of the health of an aquatic system is whether it has a “balanced, integrated, adaptive community of organisms having a species composition, diversity, and functional organization comparable to that of the natural habitat of the region.” –(James Karr)

The Index of Biological Integrity is a scientific tool used to identify and classify water pollution problems.


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