Читать книгу Clinical Guide to Fish Medicine - Группа авторов - Страница 94

Nitrogenous wastes are produced through protein metabolism by animals and decay processes (e.g. of excess food or dead plant matter). The waste products build up rapidly in closed or recirculating systems and are toxic to fish and invertebrates. The nitrogen cycle describes the conversion of ammonia into nitrite and then nitrate (Figure A2.4).

Ammonia exists as ionized ammonium (NH₄⁺) and unionized ammonia (NH₃, known as UIA). Combined, these make up total ammonia nitrogen (TAN). The unionized form (UIA) is much more toxic to fish as it can diffuse across the gills. The proportion of each depends on temperature, pH, alkalinity, salinity, and DO. More of the ammonia is in the toxic form (UIA) at high pH, high water temperature, and low salinity (e.g. a freshwater pond after a hot day). Ammonia is converted into nitrite by ammonia‐oxidizing bacteria (AOB). These include Betaproteobacteria and Gammaproteobacteria (e.g. Nitrosomonas, Nitrosospira, Nitrosolobus, Nitrosovibrio, Nitrosococcus spp.). Nitrite can be toxic to fish in freshwater systems (Lewis and Morris 1986). Nitrite is converted into nitrate by nitrite‐oxidizing bacteria (NOB). These include Nitrobacter, Nitrococcus, Nitrospina, and Nitrospira spp., among others. Nitrate is a stressor and endocrine disruptor (Hrubec et al. 1996; Camargo et al. 2005; Morris et al. 2011). These nitrifying bacteria (AOB and NOB) exist on all surfaces within the habitat, but are concentrated where surface area is highest (e.g. sand filters and undergravel filters). The nitrification process uses oxygen and alkalinity and produces carbon dioxide via bacterial respiration. Nitrate is removed from the system through water changes, algal or vascular plant use, and denitrification systems. With immature biological filtration, increases in ammonia, nitrite, and nitrate tend to follow each other (Figure A2.5).

Figure A2.4 A simple schematic of the nitrogen cycle.

Frequency of testing: Ammonia and nitrite should be assayed routinely on all systems. This may be daily on a new system, following the addition of new fish, or following an immersion treatment in order to evaluate the efficacy of the biological filtration. It may be every one to two weeks in stable systems. Ammonia should also be assayed during or following transport or restraint. Nitrate testing is often less frequent and may be done every one to two months.

Sampling: Standard sampling is described in Box A2.1. Samples for ammonia and nitrite can be stored for a few hours at room temperature, and up to 24 hours if refrigerated or 48 hours if frozen. Nitrate is more stable and samples can be stored for longer.

Testing: Commercial test kits for ammonia use one of two approaches. The Nessler method is rapid and reliable in freshwater, but is less accurate in salt water and may be falsely elevated within 24–72 hours of treatment with formalin or ammonia‐locking compounds, as these often contain formalin. The test also includes mercury which must be disposed of as hazardous waste. The ammonium salicylate method is more accurate in salt water when the modifying reagent developed by Kingsley (2014) is used. This test produces sodium nitroferricyanide; while regulations may differ, this often requires disposal as hazardous waste. The ammonium salicylate method is not affected by the presence of formalin. The test is more expensive and slightly slower. Ammonia can also be measured using ion‐specific electrodes. It is important to know water temperature, pH, and salinity to be able to assess ammonia results, since they affect the proportion of the more toxic UIA. Nitrite and nitrate tests are colorimetric (assayed by colorimeter or more accurately by spectrophotometer); commercial test kits are readily available.

Figure A2.5 Sequential increases in ammonia, nitrite, and nitrate as the nitrifying bacteria in the biological filtration mature over time.

Units: Ammonia tests report TAN in milligrams per liter (mg/L). UIA can be calculated from TAN using a conversion factor based on temperature, pH, +/− salinity. Tables are available on most test kits and conversion calculators are available on‐line (Table A2.3). If nitrite tests only report nitrite‐nitrogen in mg/L, multiply this by 3.3 to get the nitrite ion level. If nitrate tests report only nitrate‐nitrogen in mg/L, multiply this by 4.4 to get the nitrate ion level.

Target values: Targets vary depending on species, but some potential values are listed below and in Table A2.2:

 UIA <0.02 mg/L.

 Nitrite‐nitrogen <0.1 mg/L.

 Nitrate‐nitrogen <50 mg/L for most fish and <15 mg/L for many invertebrates.

Practical considerations:

 Fish health concerns are associated with high ammonia, nitrite, or nitrate, although ammonia is the most toxic of the three, and nitrite is primarily a concern in freshwater.

 High ammonia, followed by high nitrite, is often seen in newly established recirculating systems in the first few weeks after animal additions, until there are sufficient nitrifying bacteria to handle the bioload. During this time, careful control of stocking density, monitoring, water changes, and other mitigation methods are essential to minimize fish exposure to the toxins while allowing the biological filtration to mature. Similar changes are also seen due to other animal additions, high stocking densities or organic load, inadequate water flow or low DO in biological filters, and environmental changes or medications that have damaged the bacteria in the biological filters.

 High nitrate is often seen in long‐established recirculating systems without denitrification or adequate water changes or following contamination of a system (e.g. with nitrogenous fertilizer).Table A2.3 Fraction of the total ammonia nitrogen (TAN) that is present as unionized ammonia (UIA) at various temperature–pH combinations. Multiply TAN (mg/L) by this conversion factor to obtain UIA (mg/L).Source: Modified from Emerson et al. (1975), reprinted with permission. © John Wiley & Sons.TemperaturepHCelsiusFahrenheit6.06.57.07.58.08.59.00320.00010.00030.00080.00260.00820.02550.07641340.00010.00030.00090.00280.00890.02770.08252360.00010.00030.00100.00310.00970.03000.08903370.00010.00030.00110.00340.01050.03250.09604390.00010.00040.00120.00360.01140.03520.1035410.00010.00040.00130.00400.01230.03800.1116420.00010.00040.00140.00430.01340.04110.1197450.00010.00050.00150.00460.01450.04440.1288460.00020.00050.00160.00500.01570.04790.1379480.00020.00050.00170.00540.01690.05160.14710500.00020.00060.00190.00590.01830.05560.15711520.00020.00060.00200.00630.01970.05990.16812540.00020.00070.00220.00680.02130.06440.17913550.00020.00070.00240.00740.02300.06920.19014570.00030.00080.00250.00800.02480.07430.20215590.00030.00090.00270.00860.02670.07970.21516610.00030.00090.00290.00930.02870.08540.22817630.00030.00100.00320.01000.03080.09140.24118640.00030.00110.00340.01070.03310.09780.25519660.00040.00120.00370.01150.03560.1050.27020680.00040.00130.00400.01240.03820.1120.28421700.00040.00140.00430.01330.04100.1190.29922720.00050.00150.00460.01430.04390.1270.31523730.00050.00160.00490.01540.04700.1350.33024750.00050.00170.00530.01650.05030.1440.34625770.00060.00180.00570.01770.05380.1530.36326790.00060.00190.00610.01890.05750.1620.37927810.00070.00210.00650.02030.06150.1720.39628820.00070.00220.00700.02170.06560.1820.41229840.00080.00240.00750.02320.07000.1920.42930860.00080.00250.00800.02480.07460.2030.446Values for °F are rounded off to the closest integer.

 Increases are more likely to be problematic in warm water and freshwater systems because more of the ammonia is in the unionized, toxic form.

 Low levels are never a concern for fish, although vascular plants and algae need nitrate for healthy growth.

Further discussions of ammonia, nitrite, and nitrate toxicity are available in Chapter C1.