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Chemical Oceanography: Seawater Articles
How can sea mammals drink seawater? C. Chen, Silver Spring MD.
http://www.sciam.com/askexpert_question.cfm?articleID=000DBE8D-633A-1C72-9EB7809EC588F2D7
Although some marine mammals are known to drink seawater at least on occasion, it is not
well established that they routinely do so. They have other options: sea-dwelling mammals can get
water through their food, and they can produce it internally from the metabolic breakdown of food
(water is one of the by-products of carbohydrate and fat metabolism).
The salt content of the blood and other body fluids of marine mammals is not very different
from that of terrestrial mammals or any other vertebrates: it is about one third as salty as seawater.
Because a vertebrate that drinks seawater is imbibing something three times saltier than its blood, it
must get rid of the excess salt by producing very salty urine. In the seal and sea lion species, for which
measurements exist, the animals' urine contains up to two and a half times more salt than seawater
does and seven or eight times more salt than their blood.
Salt and water management in mammalian kidneys is a two-step process. First the blood
passes through a microfilter system in a part of the kidney known as the glomerulus. Most of the blood
plasma, including water and small molecules like salts, passes through the filter, but the larger
molecules, as well as the blood cells, are held back. The filtered plasma then passes through a long
tube called the loop of Henle, where the water is reabsorbed. This process concentrates the remaining
fluid, which is finally excreted as urine. One popular theory holds that a simple modification of the
standard mammalian kidney—namely, longer loops of Henle—allows marine mammals to produce a
more concentrated urine by reclaiming more of the water. Kidney anatomy in manatees and harbor
porpoises seems to support this theory, but it has not been closely studied in most marine mammal
species.
A marine mammal can minimize its salt and water balance problems by following the same
advice my doctor gave me to keep my blood pressure down: avoid salty food. With the exception of
the herbivorous manatees and dugongs, all marine mammals are carnivores. Different food types vary
in salt content. Species that subsist on plants or invertebrates (such as crustaceans and mollusks)
consume food with about the same salt content as seawater. These species thus face the same salt
removal problem they would have if they drank seawater directly. In contrast, marine mammals that
feed on fish consume food with a salt content similar to that of their own blood, thereby avoiding the
problem entirely. Indeed, a study of California sea lions showed that, on a diet of fish, these animals
can live without drinking fresh water at all.
Some species of seals and sea lions apparently do drink seawater at least occasionally, as do
common dolphins and sea otters, but the practice is very rare in some other species. When given the
choice, manatees and some pinnipeds will drink fresh water. (People who live on salt or brackish
waterways in Florida sometimes leave a garden hose flowing into the water in order to see the
manatees come to drink). Likewise, some seals will eat snow to get fresh water. For most whales and
dolphins, however, we simply do not know how they get their water, because it is difficult to observe
these animals.
Questions:
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Do marine mammals drink seawater?
What is measured in the seal?
What are the two steps in mammals for managing salt and water?
How do marine mammals minimize salt and water balance problems?
What can the sea lions eat and not need fresh water at all?
What species do drink seawater?
What is one thing seals do to get fresh water?
How do whales and dolphins get fresh water?
Alarm over acidifying oceans
19:00 24 September 03
Exclusive from New Scientist Print Edition
Climate change may be veering out of control before we understand the consequences, say scientists
studying the world's oceans. If carbon dioxide emissions keep rising, surface waters could become
more acidic than they have been for 300 million years - except perhaps during global catastrophes.
And this warning follows a report that the biological productivity of the oceans has fallen by six per
cent since the 1980s."We are changing the chemistry of the ocean and we don't know what it's going
to do," says Ken Caldeira, a climate specialist at Lawrence Livermore National Laboratory in
California.
As the amount of CO2 in the atmosphere rises, more of the gas reacts with seawater to produce
bicarbonate and hydrogen ions, increasing the acidity of the surface layer of water. Ocean pH was 8.3
after the last ice age and 8.2 before CO2 emissions took off in the industrial era. It is now 8.1.To work
out what might happen in the future, Caldeira and his colleague Michael Wickett assumed the
"business as usual" scenario, in which CO2 emissions rise with population and economic growth
throughout this century, then decline as fossil fuels are exhausted. In this scenario, atmospheric CO2
levels peak around the year 2300 at 1900 parts per million (ppm), five times as high as today. The
researchers calculate that because the ocean will soak up some of this CO2, its surface pH will drop to
7.4 by 2300 and stay that low for hundreds of years (Nature, vol 425, p 365)
Vulnerable creatures
Atmospheric CO2 has risen well above 2000 ppm several times in the past 300 million years. Caldeira
says this never pushed ocean pH below 7.5 because carbonate rocks on the seafloor act as a natural
buffer, limiting seawater's acidity. But that process takes 10,000 years or so - enough time to
neutralize acid deposited by geological processes, but not to deal with the more rapid changes caused
by human activity or natural catastrophes such as asteroid impacts.
It is not clear what such a dramatic change in acidity would do to ocean life. But acidity tends to
dissolve carbonate, so the most vulnerable creatures will be those with calcium carbonate shells or
exoskeletons, such as corals and some algae.
Experiments with double the present CO2 level in the giant, self-contained greenhouse Biosphere
showed that the rate of calcium carbonate formation in such animals fell by 40 per cent.
Meanwhile, satellite measurements of chlorophyll levels in the open ocean show that primary
productivity - the amount of new biomass being produced from carbon dioxide by photosynthesis - has
dropped sharply in the past couple of decades
Questions:
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4)
5)
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What may happen to surface water temperatures if global warming continues?
Has biological productivity in the oceans decreased? If so, by how much?
How does the acidity of the ocean water increase?
What will the pH drop to and how long will it stay there?
What will all of this do to ocean life?
What will be the most vunerable creatures? Why?
What is primary productivity?
Is primary productivity increasing or decreasing?