Ecology Lab, PCB 3043L
An ecosystem is defined as the living/biotic components of a given
habitat or area (i.e. the community) and the abiotic environment of that
habitat. Ecosystem-level approaches
emphasize biotic-abiotic interactions,
and ecological study at this level often deals with the tangible emergent
properties that characterize ecosystems.
Among the most important of these emergent properties are nutrient
cycling and energy flow.
Energy flow through an ecosystem is the fundamental common denominator in
which organisms interact with each other and with their environment.
Energy “enters” the ecosystem via photosynthesis, in which autotrophs
capture and use solar energy to create organic
molecules from inorganic carbon and nutrients. This energy is passed up the food
chain, or through the food web,
every time a heterotrophic organism
consumes either a plant or another animal.
Each successive transfer of energy involves dissipation of energy,
because the Second Law of Thermodynamics
tells us that no reaction is 100% efficient.
In fact, only 10-20% of the energy at a given trophic level is actually transferred to the next higher trophic
level. This ratio of potential
energy available to actual energy transferred is known as trophic efficiency. A
simple way to think about this concept is to quantify the energy transformations in an ecosystem using units of solar energy.
If photosynthesis is 5% efficient, then it takes 100,000 units of solar
energy to “make” 5000 units of plant biomass.
If the herbivore that eats
this plant has a 10% trophic efficiency, then these 5000 units of plant biomass
will make only 500 units of herbivore biomass.
And the carnivore that eats
the herbivore will gain only 100 units of biomass from this 500 if its trophic
efficiency is 20%. The rest of the
100,000 solar units of energy flowing through this ecosystem are dissipated as
heat.
Ecosystem ecologists
frequently measure whole-system
metabolism in much the same way that an organismal biologist measures the
metabolic rate of a single organism. This
is most easily accomplished by following the transfer of either oxygen or carbon
dioxide, both of which are stoichiometrically
tied to photosynthesis (autotrophic
processes) and respiration (heterotrophic processes). Since
it is considerably easier to measure oxygen, this is most often the currency
used to quantify whole-system metabolism. Over
a given period of time, a net gain of oxygen means that photosynthesis exceeds
respiration and the system is autotrophic, while a net loss of oxygen means that
ecosystem respiration exceeds photosynthesis and the system is heterotrophic.
All systems are, by definition, heterotrophic at night.
If they fix more carbon during the day than they consume over the entire
24 hour day, the system is net
autotrophic.
If you can enclose your
ecosystem (in a bottle, or a chamber, or just in quantifiable boundaries), then
you can measure ecosystem respiration (R)
as oxygen change either at night or in a dark enclosure, and you can measure net
primary production (NPP) during the day in a clear enclosure.
The latter is measuring NPP and not gross
primary production (GPP), which is the total amount of carbon being fixed by
photosynthesizers, because both photosynthesis and respiration are occurring in
the clear chamber. If you can
assume that the respiration in your dark chamber and clear chambers are the
same, though, you can calculate GPP as clear chamber oxygen change + dark
chamber oxygen change.
In today’s lab, you will
be using this technique to look at ecosystem metabolism in two habitats of
Hennington Pond that you know very well. Your
ecosystem enclosures will be clear and black bottles that we will fill, seal,
and incubate for a set period of time. This
is known as a light-dark bottle
experiment. NPP and R are
simply:
NPP = [O2 light]final
– [O2 light]initial
R = [– [O2
dark]initial - O2 dark]final
Note that, unless something
is very wrong, the final oxygen concentration in your light bottles should be
greater than the initial concentration while the final oxygen concentration in
your dark bottles should be less than the initial. There are not many methods commonly used to measure oxygen
concentration. The two most
frequently used are the Winkler Titration
method and oxygen electrode/meters.
The Winkler titration uses chemical reactions to fix all free oxygen in a
sample, meaning that your sample must be sacrificed but also meaning that you
get an exact and real oxygen concentration.
Oxygen electrodes/meters use a gold-tipped electrode to electrically
estimate oxygen concentration based on the rate of oxygen diffusion across a
permeable membrane that is fit over the electrode.
Meters must thus be calibrated
to known oxygen concentrations, and the most reliable calibration method for a
meter is the Winkler titration. For
ease and simplicity, you will be using oxygen meters in lab today.
You will be using this
light-dark bottle experiment to compare ecosystem metabolism between the shallow
wetland areas and deeper water habitats of Hennington Pond.
You will also attempt to partition NPP, GPP, and R between two
communities of primary producers—phytoplankton and periphyton.
To do this, you will have a treatment of just water (with phytoplankton)
and a treatment of water + periphyton, from both habitats.
It is important to consider the components of the Hennington Pond
ecosystem that you are NOT including in these light-dark bottle measurements
when you analyze your results.
1.
Generate several testable hypotheses as a class that you can test with
today’s exercise.
2.
Set up field data sheets for today’s work, remembering the importance
of noting exact times of readings, oxygen concentration, water temperature,
bottle numbers, treatment type, and habitat location.
3.
Divide into groups and work as teams in the field.
Work should be divided up so that all team members get to experience each
aspect of the exercise. In other
words, don’t make one person record data for the entire lab exercise!
4.
Be sure that you have all field sampling equipment that you will need.
Read below and make a list before you leave the lab.
5.
After sampling, return to the lab and your TA will pool data from all
teams.
1.
The lab group before yours has already made the initial measurements in
the bottles and set them in the appropriate places to incubate.
Retrieve the bottles, carefully remove the ground glass caps (you do not
want to generate any bubbles in the bottles!), and insert the self-stirring
oxygen probe. Note the time, and
measure the oxygen concentration and water temperature.
Your TA will give you the initial oxygen values for all bottles when you
return to the lab.
2.
Now you have to set up an incubation for the next lab.
Empty all bottles, rinse them, refill them carefully, and organize them
by treatment (water, water+periphyton, light, dark, etc.).
Note which bottle numbers correspond to which treatment.
Carefully add a small piece of periphyton to the appropriate bottles.
Take initial oxygen and temperature readings (and the exact time of
each). Carefully replace the bottles in the appropriate
location in each habitat.
3.
Now make some measurements about the environment in which your bottles
were incubating. If possible,
measure the depth of water over your bottles and total water depth.
Measure water clarity as Secchi
Depth. Make some generalities about the gross amount of solar energy
during your incubation. Note
anything else that seems important to you.