Transcript Oxygen, Temperature, Salinity

Oxygen, Temperature, Salinity
Craig Kasper
FAS 1401L
Spring 2012
Introduction
• Dissolved oxygen concentration (DO) is considered the most
important water quality variable in fish culture.
Q: What makes dissolved oxygen concentration so important in
intensive fish culture?
A: The speed with which it can change!
• Over a matter of hours, or sometimes even minutes, DO can change
from optimum to lethal levels.
• No other critical environmental variable in fish culture is so
dynamic!
How is oxygen used?
Factors affecting D.O. consumption:
– Water temperature (2-3x for every 10oC).
– Environmental (medium) D.O. concentration (determines
lower limit).
– Fish size (Respiration greater for small vs. large).
– Level of activity (resting vs. forced).
– Post-feeding period, etc. (2x, 1-6 hrs post feeding).
• What might be considered minimal levels of maintenance of
D.O.?
• Hard to determine due to compounding effects (can’t
standardize conditions).
• Major factor: exposure time
• For most species:
– long-term: 1.5 mg/L
– medium term: 1.0 mg/L
– short-term: 0.3 mg/L
Who does best?
• In general warm-water species are more
tolerant of low D.O. concentrations
• Ictalurus punctatus: adults/1.0 mg/L,
fingerlings 0.5 mg/L
• Procamberus clarkii: adults/2.0 mg/L,
juveniles/1.0 mg/L
• Litopenaeus vannamei: adults/0.5-0.8 mg/L
• Litopenaeus stylirostris: adults/1.2-1.4 mg/L
How much is enough?
• Many practical aquaculturists will recommend that
D.O. concentrations do not drop below 6.0 mg/L.
• This is an impractical guideline in that this level can
seldom be achieved at night.
• A more practical guideline might be to maintain D.O.
levels around 90% saturation.
• No lower than 25% saturation for extended periods
Oxygen Budget
Input
Photosynthesis
Inflowing water
Aeration
Diffusion
Total
Output
Overflow, drainage
Phyto respiration
Benthic respiration
Fish/shrimp resp.
Total
O2 (kg/ha)
4,130
94
99
1,050
5,373
% of total
76.9
1.7
1.8
19.6
100.0
32
3,090
1,040
1,210
5,372
0.6
57.5
19.4
22.5
100.0
Diel Oxygen Fluctuation
• Typical pattern =
oxygen max during
late afternoon.
• Difference in surface
vs. benthic for
stratified ponds.
• Dry season = faster
heating at surface and
less variation.
Influence of Sunlight on
Photosynthesis/O2 Production
Photorespiration: predictable
Oxygen dynamics
• Three items of interst here:
1. Oxygen doesn’t dissolve in water well.
(14 ppm compared to 21% in air (21,000 ppm)
2. Oxygen usage by organisms and sediments can be high.
3. Oxygen diffuses slowly from air to water.
Take these three factors together and you have a perfect
senario for rapid oxygen changes.
Therefore, measuring oxygen accurately and efficienty is
essential to any aquaculture operation.
Which method?
We discussed titration: it’s cheap and accurate, but slow!
Oxygen meters are better. We’ll look at one in lab today.
More later...
Several factors must be considered when deciding on a
method:
1) The number of ponds (tanks) to be measured
2) The level of accuracy required
3) The cost of the measurement technique.
When?
• As you can see from the graph on the previous
page, oxygen levels fluctuate widely during
the coruse of a day.
• Measuring first thing in the morning is good,
when oxygen levels will be near their lowest.
• Last thing in the day (before sunset) is also
good when oxygen levels are often highest.
Polarographic Oxygen Sensor
• A what?? It sounds complex, but it isn’t.
An oxygen meter has two components—the sensor (probe)
and the meter.
• Operations are similar between designs:
1. The sensor reacts with oxygen and an electrical
signal is produced in proportion to the oxygen
concentration.
2. The signal is then amplified, translated into
concentration units, and displayed by the meter.
What do you look for?
• Some of the desirable features of a dissolved oxygen
meter suitable for making field measurements include:
• accuracy
• rapid response
• ease of calibration
• water resistance
• sturdy, rugged construction
• automatic temperature compensation
• manual salinity compensation
• manual barometric pressure
• compensation
The critical part!
How they work
1. Most DO sensors operate as electrochemical cells with a positive electrode
(cathode) and a negative electrode (anode) connected by a “salt bridge”
consisting of a saturated electrolyte solution.
2. In most sensors, oxygen passes through a permeable membrane and is
chemically reduced within the sensor.
3. The chemical reduction of oxygen generates an electrical current that is
processed by the electronic components within the meter and displayed as
a DO concentration.
4. The current is proportional to the concentration.
5. Thus,DO meters do not measure oxygen concentration directly, but measure
a voltage that is produced by the chemical reactions of oxygen with the
various components of the sensor.