NASA Educational Workshop
SeaWiFS: Ocean Chemistry
The Basics of Ocean Chemistry:
Carbon, Circulation, and Critters
The major ions in seawater are Na+, Mg2+, Ca2+, K+, Sr2+, Cl-,
SO42- (sulfate), HCO3- (bicarbonate), Br-, B(OH)3 (boric acid),
and F-. Together, they account for almost all of the salt in
Constancy of Composition:
Ratios of major elemental ions remain constant, despite changes of salinity (i.e., the amount of water is different).
Why Calcium is More Variable
Some phytoplankton (the foraminifera, coccolithophorids, pteropods,
and heteropods, and also corals and coralline algae) form CaCO3.
The formation of CaCO3 organically or inorganically (evaporation) can locally affect Ca concentrations, particularly in shallow waters. The dissolution of CaCO3 in some regions can also affect Ca and CO32- concentrations.
A Brief Summary of Carbonate Buffer System Chemistry
Atmospheric CO2 dissolves in seawater and is hydrated to form carbonic acid, H2CO3. Carbonic acid is divalent; that is, it can undergo two de-protonation reactions to form bicarbonate (HCO3-), and carbonate (CO32-). The co-existence of these species in seawater creates a chemical buffer system, regulating the pH and the pCO2 of the oceans. Most of the inorganic carbon in the ocean exists as bicarbonate (~88%),
with the concentrations of carbonate ion and CO2 comprising about
11% and 1%, respectively.
Dissolved and Particulate Carbon
The main other type of carbon in seawater are the forms of organic carbon,
both dissolved and particulate. (It's primarily a matter of what can get
through a filter of a certain size, and what can't.) Dissolved organic
matter/carbon (DOM, DOC) can sometimes be colored (CDOM),
and a variety of semi-polymeric DOC is termed Gelbstoffe, German
for "yellow substance". Particulate organic matter/carbon
(POM, POC) is larger organic particles from a variety of sources.
SeaWiFS data is being used to calculate the amounts of CDOM (which
may be correlated with total DOM) and POM in the water column.
Though small in concentration compared to seawater's major
constituents, nutrients, primarily nitrate (N) and phosphate (P), are extremely important to the biology of the oceans. In some cases, iron (Fe) and silica (Si) may also act as limiting nutrients. The ratio of the concentration of carbon to nitrate to phosphate in phytoplankton is 106:16:1, which are the classic "Redfield Ratios". Chemical and biological oceanographers frequently analyze nutrient data with respect to the Redfield Ratios to determine which nutrient is the production-limiting nutrient. While Fe is important in some regions, particularly open ocean regions distant from land, in most productive regions the limiting nutrient is either N or P.
(N and P are actually present as dissolved nitrate ion and dissolved phosphate ion, but N and P are used for convenience.)
Marine Carbon Cycle
Carbon is produced in the upper ocean by photosynthesis, and it moves up
the trophic levels (zooplankton, nekton). Most of the carbon in the upper
ocean is recycled (the biologists can comment more on that), but some
"drops out" and sinks. In the deep ocean, organic carbon is "remineralized"
by bacterial respiration (which uses dissolved oxygen), converting it back to
inorganic carbon and also producing dissolved nutrients. You can see in the
carbon cycle diagram that there is much more inorganic carbon in deep waters
than in the surface ocean. This means that deep ocean waters also have higher
N and P concentrations than surface waters.
A brief aside: are the oceans a net source or sink of CO2?
One interesting question about the marine carbon cycle concerned whether
or not the oceans are a source of CO2, adding it to the atmosphere, or a sink,
removing it from the atmosphere. Recent research indicates that the oceans
are a net sink, though some regions (generally colder and more turbulent)
absorb CO2, and other regions (warmer and less turbulent) release CO2. The North Atlantic Ocean accounts for about 60% of the CO2 absorption by the global ocean. (CO2 is less soluble in warm water than in cold water.) For a variety of reasons, global warming could convert the oceans from
a sink to a source, which is an example of bad positive feedback.
Annual CO2 Flux
PHYSICS + CHEMISTRY = BIOLOGY
Now for the culmination of the process. In certain regions of the ocean, current interactions with the coast, or current interactions with other currents, or both, bring the cold deep nutrient-rich ocean water to the surface, a process called "upwelling". Add sunlight and plankton, and photosynthesis and productivity result. The high productivity in these regions is easily seen in SeaWiFS data.
TOO MUCH IS NOT A GOOD THING, and a demonstration
Finally, in some regions (particularly near major rivers), excess nutrients
can be added to the coastal zone. This is usually not good, because
it results in increased productivity, increased sedimentation of organic
matter, and increased bacterial respiration, resulting in a marked reduction
of dissolved oxygen, especially on the bottom, which can kill the biota
on the bottom. The term for this overabundance of carbon and lack of
oxygen is "eutrophication".
Demonstration (or experiment):
Go to a nearby stream or pond and scrape some of the algae off the
rocks. "Inoculate" a series of clean baby food jars with the algae.
Set one baby food jar aside as a control, and add increasing amounts of
Miracle-Gro or similar fertilizer. (A very small amount suffices -- the
amount you can pile on the end of a coffee stirrer is enough!) I suggest
one control and three jars with 1 portion, 2 portions, and 4 portions of
the fertilizer. Put the jars in a sunny place. Monitor daily (taking
comparison photographs each day is a good way to do this). What you'll see
is that algal growth will be enhanced in the fertilized jars compared to
the control, but in the over-fertilized jar, growth will be rapid and then
the algae will turn brown and die - that's what happens with eutrophication.
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