Fundamentals of Aquatic Veterinary Medicine. Группа авторов

(Tucker 1984).

	Change
Time	Dissolved oxygen	Carbon dioxide	pH
Daylight	Increases	Decreases	Increases
Night	Decreases	Increases	Decreases

Since most test kits measure total ammonia nitrogen, it is important to determine what percentage of the total is toxic. In healthy ponds and tanks, ammonia levels should always be zero. Since the toxicity of UIA begins as low as 0.05 mg/l, a positive total ammonia nitrogen test needs to be followed by a test to find the actual concentration of UIA. Once the pH and temperature are known, the fraction of UIA present can be determined (Table 1.4), as presented in Table 1.5. Samples that cannot be submitted within a few hours should be filtered (0.45 mm) then frozen to reduce the loss of ammonia and changes in nitrate/nitrite concentrations.

      Salinity is usually not measured directly, but is instead derived from the conductivity measurement (Wagner et al., 2006). This is known as practical salinity. These derivations compare the specific conductance of the sample to a salinity standard such as seawater. Salinity measurements based on conductivity values are unitless but are often followed by the notation of practical salinity units (Nelson and Siegel, 2014).

Table 1.3 Factors for calculating carbon dioxide concentrations in water with known pH, temperature and alkalinity measurements.^a

      Source: Tucker, 1984.

pH	Temperatures (°C)
5	10	15	20	25	30	35
6.0	2.915	2.539	2.315	2.112	1.970	1.882	1.839
6.2	1.839	1.602	1.460	1.333	1.244	1.187	1.160
6.4	1.160	1.010	0.921	0.841	0.784	0.749	0.732
6.6	0.732	0.637	0.582	0.531	0.495	0.473	0.462
6.8	0.462	0.402	0.367	0.335	0.313	0.298	0.291
7.0	0.291	0.254	0.232	0.211	0.197	0.188	0.184
7.2	0.184	0.160	0.146	0.133	0.124	0.119	0.116
7.4	0.116	0.101	0.092	0.084	0.078	0.075	0.073
7.6	0.073	0.064	0.058	0.053	0.050	0.047	0.046
7.8	0.046	0.040	0.037	0.034	0.031	0.030	0.030
8.0	0.029	0.025	0.023	0.021	0.020	0.019	0.018
8.2	0.018	0.016	0.015	0.013	0.012	0.012	0.011
8.4	0.012	0.010	0.009	0.008	0.008	0.008	0.007

      ^a Factors should be multiplied by total alkalinity (mg/l) to get carbon dioxide (mg/l). For practical purposes, CO₂ concentrations are negligible above pH 8.4.

Total dissolved solids are reported in mg/l and can be measured by gravimetry or calculated by multiplying a conductivity value by an empirical factor; standard methods for the examination of water and wastewater accepts a total dissolved solids constant of 0.55–0.7 mg/l, although if the water source is known to be high in calcium or sulfate ions, a constant of 0.8 mg/l may be used (American Public Health Association et al., 2017). Depending on the ionic properties, excessive total dissolved solids can produce toxic effects on fish and fish eggs. Salmonids exposed to higher than average levels of CaSO₄ at various life stages experienced reduced survival and reproductive rates. When total dissolved solids ranged above 2200–3600 mg/l, salmonids, perch and pike all showed reduced hatching and egg survival rates (Scannel and Jacob, 2001).

      It is important for the clinician to be familiar with the types of life‐support system monitoring reports used in modern aquatic facilities.