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A2 Water Quality
Catherine A. Hadfield
Seattle Aquarium, Seattle, WA, USA
Introduction
Fish health is dependent on the aquatic environment; appropriate management of water parameters is critical. The term “water quality” is commonly used when discussing these parameters. At a minimum, water quality management typically encompasses temperature, salinity, nitrogenous wastes, pH, hardness, alkalinity, and dissolved gases. Other critical parameters include additional anions and cations, chlorines, heavy metals, turbidity, and the microbiome. These values are interdependent and should be evaluated together, along with the history and observation of the system and animals.
The goal of this chapter is to provide a concise review of major parameters, including common sampling and testing methods, target values, and major interactions. Values should be compared to the known requirements for the species in the system; it is important to bear in mind that the preferences of different species held in the same habitat may vary. The parameters change over time and both recent values and historical trends need to be considered. Rapid changes are more likely to negatively impact health than slow changes, and are more likely in smaller volumes of water. Management options for water quality problems (e.g. low dissolved oxygen or high ammonia) are covered in Chapter C1.
Water Source
Water source can typically be divided into municipal water (tap water), surface water (ponds, streams, rivers, lakes, or seas), or ground water (wells, springs, or aquifers). Each source presents common advantages and disadvantages (Table A2.1). These impact the treatments required, e.g. incoming municipal water often requires treatment to remove chlorines and chloramines, while incoming surface water often requires filtration and disinfection to reduce particulates and pathogens. Any source water may require temperature correction.
Dissolved Oxygen
Dissolved oxygen (DO) is critical to support animal and microbial respiration. Low DO is a common cause of acute morbidity and mortality in fish, although species sensitivity varies (see Chapter C1). Adequate microbial respiration is essential for the biological filtration within a life support system. Since low DO can damage the microbial population supporting biological filtration, it can lead to increases in toxic nitrogenous wastes.
Frequency of testing: DO should be assayed routinely where possible (e.g. daily). DO may be monitored continuously in intensive aquaculture systems or with sensitive species. At a minimum, baseline values should be obtained for each system. DO can vary across the year, particularly for surface water, so baselines should be established across all seasons. DO should be checked prior to and during any immersion treatment, restraint, or transport.
Sampling: DO must be measured on site in the system. The result represents only that moment in time and values can change rapidly. DO should be measured in multiple locations, as it will vary with depth, water flow, and surface water agitation.
Testing: DO is measured using DO meters (Figure A2.1). Most consist of an oxygen‐permeable membrane and use an oxygen‐dependent chemical reaction; polarographic or galvanic electrodes measure the electric current produced. Polarographic models require the probe to be moved through the water. Fiber‐optic units measure the effect of oxygen on fluorescence. Calibration is required each time a meter is turned on. Most units are calibrated against moist room air, but the instructions in the user manual should be followed. The membrane should be replaced periodically and must be changed if there are tears or bubbles.
Table A2.1 Common advantages and disadvantages of different water sources.
| Advantages | Disadvantages | |
|---|---|---|
| Municipal water | Availability Consistency Low risk of pathogens Usually high DO Usually low H2S and CO2 | High chlorines and chloramines May have high nitrates, phosphates, or heavy metals Cost |
| Surface water | Good conditions for endemic species Usually low H2S and CO2 | Need to be physically close to the source Variable quality; changes may be seasonal High risk of pathogens High risk of toxins (e.g. harmful algal blooms, pesticides) High risk of turbidity |
| Ground water | Consistency Low risk of pathogens Low risk of environmental pollutants Low risk of organic matter | Variable availability Often low DO and supersaturated with N2 Often low pH and high CO2 Often low temperature Potentially high H2S and Fe |
CO2: carbon dioxide; DO: dissolved oxygen; Fe: iron; H2S: hydrogen sulfide; N2: nitrogen.
Figure A2.1 Dissolved oxygen meter.
Units: Oxygen is measured in milligrams per liter (mg/L), equivalent to parts per million (ppm). It is simultaneously reported as percentage saturation (%). This is because the amount required to saturate