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 10 o C). – 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? warm-water 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/ mg/L Litopenaeus stylirostris: adults/ 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

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/O 2 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.