Name:_________________________________ Date:_____________ Period:______
Fish in Hot Water1
Many estuary animals are well adapted to the normal changes of water temperature. Estuaries are
areas where mixing between fresh water and sea water occur and some temperature fluctuations
are normal. For example the Chesapeake blue crab spends the winter months buried in the soft silt
at the bottom of the deepest part of the bay. As summer approaches, crabs climb out of the mud
and do about their important business of spawning and mating. On the west coast, the Dungeness
crab (Cancer magister) buries itself in the sand to be protected from storms waves, but does not
do well in warmer waters. If temperatures rise abnormally in a given summer, however, a certain
number of less well-protected fish may suffocate. How does this happen?
Temperature and Oxygen Levels
When temperatures rise, the concentrations or levels of oxygen in the water fall because warmer
water cannot hold as much dissolved oxygen. And when the level of oxygen falls below a certain
point some fish are literally and figuratively “in hot water”. It is not the heat that threatens them
directly, but the lack of oxygen, which they need to breathe.
Scientists have been able to compare water temperatures and oxygen levels over time in order to
determine their relationship. The table on this page shows reading taken in the Chesapeake Bay
over a period of a year. The temperatures have been measured using a Celsius thermometer.
Oxygen levels have been measured in parts per million, usually abbreviated as ppm. An oxygen
reading of 10 ppm means that if a drop of water
were divided into one million parts, 10 of those
Oxygen Levels in Chesapeake Bay
parts would consist of oxygen.
Date
Temperature Oxygen Conc.
Jan 5
Jan 17
Feb 3
Feb 17
Mar 7
Mar 19
Apr 6
Apr 15
May 4
May 24
Jun 5
Jun 20
Jul 20
Aug 17
Sep 8
Sep 19
Oct 3
Oct 19
Nov 4
Nov 20
Dec 5
Dec 20
1
(ºC)
(ppm)
1
1
3
5
8
13
14
10
15
21
24
27
28
28
25
20
19
13
10
10
8
4
13
12
13
11
11
9
11
9
7
8
7
6
6
4
6
7
7
10
5
10
12
12
Oxygen Levels and Fish
By carefully watching how animals react to
temperature and oxygen changes, biologists can
determine when oxygen levels become
uncomfortable – and when levels before so low that
fish may die. Oxygen levels are best for fish when
they measure above 6 parts per million (6 ppm or 6
mg/L). Levels from 3 to 5 ppm are uncomfortable,
and levels below 3 ppm can cause death.
Effects of Industrial Wastes
At most times, fish in an estuary can survive the
changes in oxygen levels that are due to normal
rises and falls in temperature. But what if heated
water suddenly enters a bay from an outside
source? Power plants, for example are often built
near estuaries because the water can be used to
cool parts of the machinery. When the water, hot
now from contact with the machinery, is returned
to the estuary, oxygen levels may plunge,
threatening animal life. Another example of a
Adapted from “Where River Meets Sea” Prentice Hall, Simon and Schuster Education Group (1996).
thermal discharge occurs at wastewater treatment plants. After the waste water is treated it is
warmer than the river, estuary, or ocean water that receives the treated water. The City of
Sacramento discharges its treated wastewater into the Sacramento River and under its permit
must monitor temperatures so that the City does not create a situation that is harmful for the
aquatic life in the Sacramento River.
In order to use temperature as an indicator of possible dissolved oxygen concentrations, scientists
graph their results to visualize data patterns. Below are two grids in which you are to graph the
data collected from the Chesapeake Bay last year. One graph is temperature vs. dissolved
oxygen concentration to show the relationship between temperature and oxygen. One
graph is time vs. dissolved oxygen and temperature to show how dissolved oxygen and
temperature changes during a year. When graphing don’t forget to label the x axis, y axis,
and to title your graph. Once you have plotted points draw, using a ruler, a trendline
showing how data trend. You will use this trendline to make predictions.
Using your graph, predict what the dissolved oxygen concentration would be at 9ºC.
Explain how you came up with the prediction.
Using the above graph discuss how you think oxygen levels in an estuary might be likely to
rise or fall during a cold spell.
Science News Press article assignment
The former PG&E power plant in Moss Landing was purchased several years ago by Duke Energy,
who in October 2000 received permits to expand the power plant to 1,060 megawatts, making it
the largest (in terms of power output) power plant in California. The existing power plant uses
water pulled from Moss Landing Harbor to cool the turbines and discharges that "thermal cooling
water" into the Pacific Ocean, and partially into the Sanctuary, through outfall pipes offshore of
the Monterey Bay Aquarium Research Institute. The planned expansion calls for increasing the
amount of cooling water to a total of approximately 1.2 billion gallons per day at peak operating
capacity. This is by far the largest discharge into the Monterey Bay National Marine Sanctuary.2
The Ray News has begun to investigate the effects of operating this power plant. For a first story
you are to write a short (200 word) commentary article about the permitting of a power plant
adjacent to a marine sanctuary. Use the PROOF PARAGRAGH strategy. Before you start, list out
some of your thoughts for and against the project to gather talking points. Discuss your ideas
with your elbow neighbor. Write a rough draft first. Write your final using good penmanship. As
your publisher I expect you to polish your writing so that your star is shining.
2
http://montereybay.noaa.gov/educate/newsletters/2001winternewsletter/simon.html