GEOL 2503 Introduction to Oceanography
Dr. David M. Bush
Department of Geosciences
University of West Georgia
Topic 21. Productivity
POWERPOINT SLIDE SHOW NOTES
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Topic 21. Productivity
Feeding strategies
Every ecosystem relies on a primary food source. Here, algae is that food. Fish eat the
algae and use that energy for metabolic functions. Anything that dies uneaten is
decomposed, and the nutrients released back into the system. This is called a food
chain.
A more complex food chain. Each step is called a trophic level. Energy is being passed
up the chain from trophic level to trophic level. This is a photic ecosystem. That is, it’s
based on photosynthesis to provide the original source of food, the first trophic level. In
marine photic ecosystems, phytoplankton is always the first trophic level (with a few
localized exceptions). Single-celled animals make up the second trophic level. Larger
animals the third, fourth, etc. In the case herring, it is at the highest trophic level in its
food chain. However, herring is only another trophic level for predators of herring. So
the number of trophic levels depends on what you are studying. Herring or predators of
herring?
In this simple food chain example, phytoplankton are the first trophic level, zooplankton
(single celled animals) the second, bony fish eat the zooplankton, seals eat the bony fish,
and killer whales eat the seals. Note the transfer of energy up the food chain. In
general, only about 10% of the energy entering a trophic level is passed to the next
higher trophic level. Incoming solar energy is one of the raw materials plants need for
photosynthesis. There is essentially a limitless supply of solar energy. Plants convert
only about 2% of that original energy into food available for heterotrophs.
Let’s look at three paths of transferring energy from phytoplankton to humans. At the
bottom sardines eat 1 kg of phytoplankton and provide 10% of that (0.1 kg) food for
humans. In the center, anchovies eat 10 kg of phytoplankton, which provides 1 kg of fish
meal for farmed salmon, providing the same 0.1 kg of food for the humans. At the top,
small zooplankton need to eat 100 kg of phytoplankton in order to provide 10 kg of food
for small fish. The wild salmon then eats 1 kg of small fish, and the wild salmon provides
0.1 kg of food for the humans. In each path, 0.1 kg of food is made available for humans,
but the sardine path only has one intervening step between phytoplankton and humans,
the farmed salmon path has two, and the wild salmon path has three. With only a 10%
efficiency in energy transfer, the fewer the steps, the less original energy
(phytoplankton) is needed to fuel the food chain.
General rules as energy moves through an ecosystem. Increasing entropy essentially
means there are fewer things to eat, and that much once-available energy has been
metabolized or decomposed and is unavailable for passing on to the next trophic level.
Trophic levels
Trophic pyramids. Simply a food chain visualized as in the following slide.
Trophic pyramids for herring, anchovy, and tuna. Each has a different number of trophic
levels
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Where does all the energy go?
Some people thought that the term food chain implied the very simple energy we have
been talking about so far. That one kind of animal eats one kind eats one kind, and so
forth. In truth, the relationships are much more complex. To reflect the complexity, the
term now used is food web.
An example of a food web. Note that it is not a simple linear progression from diatoms
to the killer whale. For example, a killer whale may eat a whale that feeds on a low
trophic level, or it may eat a seal that feeds on a higher trophic level. There are many
pathways that energy can follow to move through the system.
Productivity versus biomass
Biomass pyramid
Refer back to slides 6 and 13 which show different pathways of energy flow through an
ecosystem. There can be advantages and disadvantages to feeding at the top of or near
the bottom of the food web.
Regional productivity. In tropical oceans, productivity is constant all year because there
is so much surface heating that a strong thermocline persists and little chance for
upwelling of nutrients. In North polar seas there is no thermocline so there are plenty of
nutrients most of the year. However, only during the northern hemisphere summer is
there enough sunlight for high rates of photosynthesis. In North temperate waters, the
water column is well mixed in the winter, but there is a shallow thermocline in summer.
Light is a limiting factor in winter. In the spring there is sufficient light for
photosynthesis. So much so that productivity is high until nutrients are exhausted. In
the fall storms may mix some nutrients back into the photic zone to increase
productivity. Curves would be the opposite in the southern hemisphere.
Upwelling and nutrient supply
Primary productivity in the world ocean. Primary productivity is measured in units of
grams of carbon per square meter of ocean surface per year (gC/m2/yr). Organic
molecules contain carbon and carbon the basis of life. This diagram shows ranges of
primary productivity as different colors, dark green being the highest. It also shows the
equivalent amount of productivity in apples per m2/yr. The areas on the map colored
dark green are the areas of highest primary productivity in the oceans, over 150
gC/m2/yr, equivalent to 3-4 apples per square meter of ocean surface per year. The pale
green areas are lowest primary, less than 40 gC/m2/yr, equivalent to only an apple core
per square meter per year. Upwelling zones are the highest, although coastal areas can
have very high productivity when floods introduce nutrients or when storms mix up the
water column and resuspend sediment releasing nutrients.
Primary productivity versus area of ocean surface. Compare the two pie charts. Coastal
areas provide 49% of primary productivity in the oceans, but account for only about 18%
of surface area of the oceans. Upwelling areas also provide 49% of primary productivity
in the oceans, but account for only about 0.5% of surface area of the oceans. The open
ocean accounts for only about 1% of the primary productivity in the oceans, but
encompass 81.5% surface area.
Contribution to the total world fishery by ecosystem
Curves of sunshine, phytoplankton, nutrients, and zooplankton over the year.
The equation for photosynthesis. Glucose is stored as plant biomass.
Animal Feeding Strategies
Measuring Primary Productivity