General Ecology: Lecture 23
December 2, 2005
I.
II.
Focus: The nitrogen cycle [Fig. 54.18,Campbell]
A.
Main reservoir for nitrogen is the atmosphere, but alas, the form is unusable
by most organisms…
B.
Nitrogen is “fixed” (nitrogen gas into ammonia) in three ways
1.
Biological nitrogen fixation (most N enters this way)

Accomplished by nitrogen- fixing bacteria

Mutualistic bacteria in root nodules (legumes,other plants)

Free-living bacteria in soil and water

Cyanobacteria
2.
Lightning (small amount)
3.
Production of fertilizers by humans
C.
Nitrifying bacteria further oxidize ammonia, first to nitrite and then to nitrate
D.
Primary producers use these compounds to build organic molecules
1.
In terrestrial ecosystems nitrate is used, while in aquatic ecosystems both
nitrate and ammonia are used.
E.
Cycling occurs through the food web. An individual atom of N may cycle
through a particular ecosystem many times.
1.
Bacteria are the key decomposers that convert nitrogen back to its inorganic
form (ammonia). [And then what? Continued cycling, or loss...)
F.
Nitrogen loss
1.
Runoff from terrestrial to aquatic ecosystems

Serves as nitrogen source in aquatic ecosystems

Permanent loss to ocean sediments

Once incorporated into sediments, not even upwelling will
return it
2.
Denitrification (bacteria)
G.
Review: Key roles of bacteria in the cycle
Phosphorous cycle [Fig. 54.19, Campbell]
A.
Main reservoir for phosphorous: calcium phosphate within rocks.
B.
Phosphates enter terrestrial and aquatic ecosystems as rain and wind weather
the rocks.
1.
Initial uptake of phosphates is by primary producers, and it is incorporated in
biomolecules (such as?)
2.
Normally, phosphorous is very limiting; successful plants in nonphosphorous enhanced ecosystems are efficient at uptake/conservation of
phosphorous.
C.
Cycling of phosphorous
1.
Like nitrogen, phosphorous cycles through food webs.
D.
Phosphorous loss
1.
Runoff from terrestrial to aquatic ecosystems

Serves as phosphate source in aquatic ecosystems

Permanent loss to ocean sediments

III.
IV.
V.
As for nitrogen, once incorporated into sediments, not even
upwelling will return it

Geologic uplift/rock formation could restart the loop
Anthropogenic (human-produced) sources of N and P into aquatic environments
A.
Fertilizer-rich run-off from agriculture lands
B.
Human waste and animal waste: Sewage treatment plants do not generally remove
N and P waste.
C.
Detergents (dishwashing, laundry): source of phosphates
Effects of nutrient additions (nitrogen and phosphorous) in freshwater and marine
ecosystems
A.
Increased phytoplankton biomass
1.
Sounds OK, but... when/where it sinks out, decomposition occurs at such
high levels by bacteria that they deplete oxygen and create dead zones.

A huge "dead zone" created this way exists in the Gulf of Mexico
each summer, 7,000 sq. mi.

Fish, inverts and those higher on the food chain die.
B.
Shifts in phytoplankton species composition in communities
1.
Phytoplankton which is non-competitive when nutrients are "normal" may
outcompete the types of phytoplankton which normally dominate

These species are often less edible/nutritious than those that normally
dominate. All phytoplankton species are not alike.

Some of these species are toxic

Pfiesteria gives off toxic fumes which can cause severe nausea,
respiratory problems, neurological deficits, and memory loss
resembling Alzheimer’s

The dinoflagellate causing "red tide" can lead to death.

What happens to those higher in the food web and why?
C.
Effects on benthic algae and seagrass
1.
Associated shifts in species composition similar to shifts in phytoplankton
2.
Photosynthesis by benthic macroalgae and seagrasses can be blocked with
too much phytoplankton.
3.
How would this affect organisms higher in the food web?
D.
Directly toxic to animals (major impact on macroinvertebrates and fish)
1.
Different species have different tolerances.
E.
Increased blooms of gelatinous zooplankton
1.
Can drastically reduce populations of larval invertebrates. (Why?)
F.
Death on coral reefs
1.
Overgrowth by algae
2.
Reduction of water clarity may decrease photosynthesis by symbionts
Solutions?
A.
Apply fertilizers more carefully
B.
Use crop rotation to reduce the need for fertilizer
C.
Create "buffer zones" which trap agricultural run-off, such as artificial wetlands
1.
Human depletion of wetlands has exacerbated the problem
D.
Recycle the run-off back onto fields
E.
Remove N and P from sewage prior to discharge of "gray water"
Study questions
1.
Name at least two major types of biological molecules that contain nitrogen.
2.
Sketch the nitrogen cycle without looking at question 3…
3.
Checklist for your nitrogen cycle. Make sure you have included:
a.
The main reservoir of nitrogen in the nitrogen cycle, including the form of nitrogen
in that reservoir.
b.
Conversion of nitrogen gas to ammonium (which organisms and/or processes do
this?)
c.
Conversion of ammonium into nitrates (which organisms and/or processes do this?)
d.
The movement of nitrogen through the food web.
e.
The conversion of organic nitrogen back to inorganic forms (which organisms
and/or processes do this? What form is this inorganic nitrogen?)
f.
How is nitrogen “lost” to the terrestrial cycle? How is nitrogen “lost” to the marine
cycle?
4.
Describe five major roles bacteria play in the nitrogen cycle.
5.
Which forms of nitrogen are used by primary producers? Note whether this is different in
terrestrial vs. aquatic ecosystems.
6.
Name at least two key biomolecules that contain phosphorous.
7.
Sketch the phosphorous cycle. Look at the diagram provided in class to make sure you
have included all parts (more straightforward the N cycle, so I haven’t included a
checklist).
8.
What are some of the key anthropogenic sources of nitrogen and phosphorous?
9.
How can addition of nutrients (i.e. nitrates and phosphates especially) to aquatic
ecosystems cause death of invertebrates and fish? You should be able to describe several
distinct chains of events that lead to these consequences.
10.
What are some of the solutions to the problems of excess nutrients?