Chapter 25 Advanced Water Quality Evaluation for Zoo Veterinarians
As indicated in the basic water quality chapter (see Chapter 23), conditioning water is much more involved than monitoring and adjusting water for the basic parameters such as temperature, pH, dissolved oxygen (DO), ammonia, nitrite, nitrate, pH, salinity (conductivity), hardness, and alkalinity. This chapter is designed to inform veterinarians, many of whom oversee the water quality laboratories in zoos and aquariums, about the more complex aspects of water conditioning. These may include oxidation-reduction potential, total organics, metals, microbial dynamics, and organic contaminants. Most of the discussion will focus on marine systems because of their complexity, but variations with freshwater will be highlighted. This chapter is intended to be a primer; for more detailed information, see Spotte24 and Clesceri and colleagues.5
Partial pressures are extremely important but are underanalyzed in an aquarium setting unless direct gas bubbles are forming on the glass of the aquarium or emboli are observed in the fish. Chronic and/or sublethal levels may cause morbidity and mortality from secondary factors. Partial pressures are measured using a gas tensionometer or saturometer. Baseline pressures are dependent on temperature and salinity and should be analyzed with methods provided in the appropriate literature.8 Because of its insolubility and biologic unavailability, nitrogen is usually the culprit, but oxygen and carbon dioxide may cause issues in extreme cases. If chronic dissolved gas levels cannot be resolved, fish should be encouraged to swim below the compensation depth, which would prevent bubble formation. This may be accomplished by feeding and shelter strategies that keep them at depth.
Nitrogen is the most abundant gas in aquatic systems, but oxygen and CO2 concentrations are the most dynamic because of manipulation by biologic activity. In an ecologically balanced system, oxygen and CO2 tend to be inversely related in terms of their activity because one is constantly being exchanged for the other between plants or photosynthetic protoctists (e.g., algae, cyanobacteria) and animals, which has resulted in many ocean symbioses; thus, this effectively using light energy to drive chemical equilibria and stability in the system.1 Probably the most important part that CO2 plays in the aquarium setting is its role in the carbonate alkalinity system, which controls the pH level and its stability; this system is crucial to maintaining organisms’ health. Other factors affecting this system include carbonic acid, carbonate, bicarbonate, and hydrogen ion levels. This is especially true in marine environments, in which animals have adapted to a very stable pH and narrow pH range.
CO2 reacts with water to form carbonic acid (H2CO3), which dissociates to bicarbonate (HCO3−) and then carbonate (CO32−) and hydrogen ions (H+), chemically proportioned by temperature, pressure, and salinity equilibria24:
Nitrifying and some denitrifying biologic water treatments use autotrophic microorganisms that metabolize the carbon from the carbonate system into biomass and hydrogen ions, driving the alkalinity and pH of the system downward. The mineralizing (biologic processes that convert organic to inorganic substances) and some denitrifying heterotrophic bacteria involved in organic decay in the system metabolize their carbon from biomass and respire CO2 along with the fish and invertebrates, which easily becomes excessive in a typical aquarium with a high bioloading. Another source of excess CO2 in systems is from rain water, which usually has at least a moderately acidic pH because of interactions with the atmosphere, even if unpolluted.
Too rapid an input of CO2 relative to photosynthetic activity and gas exchange in a system results in an accumulation of CO2 and H2CO3, increasing the H+ concentration and decreasing pH, which may be alleviated with vigorous gas exchange. A simple jar test for this in the laboratory is to stir a water sample from the system vigorously with a vortex while monitoring pH and maintaining system temperature. An increase in pH indicates inadequate gas exchange in the system relative to total CO2 input. If inadequate gas exchange cannot be easily addressed, the addition of sodium hydroxide (NaOH) to a system may help drive the conversion of excess H2CO3 from dissolved CO2 to bicarbonate:
Extreme caution must be taken not to overdose with this artificial method, which would result in too great or too rapid a pH change for the animals. We have found that adding it too rapidly to a seawater system by pipe injection may result in the precipitation of calcium carbonate, rapidly giving the exhibit water the appearance of skim milk. A more natural balancing method is to add photosynthesis to the treatment system and/or exhibit with aquatic plants and/or photosynthetic protoctists, although this approach should be in conjunction with adequate gas exchange for good pH balance.
In cases in which water changes are not feasible or at lower levels than desired, denitrification is the process of turning nitrate into nitrogen gas through anaerobic microbial action. An additional carbon source is needed through the use of methanol or sulfur, which helps facilitate the autotrophic denitrification process in which the carbon issued is from the CO2. These are complex processes and denitrification should not be attempted without experienced staff because toxic elements may be produced and released into the main water system if the procedure is not performed correctly. It is important to note that our understanding of bacteria in the nitrogen cycle is in its infancy; once more information is gained, we could significantly alter our water-conditioning capabilities. For example, as recently as 2002, it was discovered that bacteria could perform anaerobic ammonium oxidation (anammox) to N2 gas. These four genera of anammox bacteria were identified as Brocadia, Kuenenia, Scalindula, and Anammoxoblobus and are responsible for 24% to 67% of nitrogen loss in marine systems.9 They may be grown autotrophically with CO2 as the only carbon source, thus eliminating the complex dependency on several bacterial pathways, including anoxic denitrification using methanol or sulfur. It is currently being applied in industrial use and is highly experimental but could be used in zoo and aquarium settings. We would then not have to depend on complex biologic filtration systems, which are prone to failures.
Phosphate enters natural systems from dissolved inorganic phosphate (DIP) from rock, sediments, and fertilizers, dissolved organic phosphate (DOP) from sewage and organic fertilizers, and particulate organic phosphate (POP) from sediment. Algae readily take up DOP and DIP in the natural environment. Phosphate concentration is normally the primary limiting factor in the bloom of microalgae in freshwater and marine systems, but nitrogen and iron are critical nutrients as well. Sewage and fertilizer contamination may cause phytoplankton blooms in natural systems, with catastrophic toxic effects to resident animals. In aquarium systems, phosphate builds up through the introduction of food into the system. Depending on the type of system, these may be limited through biologic uptake by biologic oxidant demand (BOD), oxidizing bacteria, and precipitation with lime, lanthanum chloride, or aluminum and iron salts.
Sulfur may sometimes be an issue in aquarium settings. Hydrogen sulfide may build up in sediment and improperly maintained filter beds in which anoxic processes may take hold. Sulfur, in moderation, is beneficial for the sulfur cycle. The aim should be not to eliminate sulfur but to keep it minimal to prevent toxicity.
Total organic carbon (TOC) is an underappreciated component of aquarium chemistry. Organic compounds enter the ambient water from various sources, including influent water, animal waste products, uneaten food, and drift from the air (e.g., pollen, dust):
POC is usually efficiently removed through mechanical filtration and DOC is the fraction of the TOC that passes through a filter of a stipulated pore size of 0.45 µm. Two major components of DOC are humic and fulvic acids, which can color the water and are often discouraged in the aquarium setting. An excessively high TOC level increases oxidant demand, reduces efficacy of sterilization, and creates toxic byproducts. Biologic filtration bacteria reduce POC and DOC by incorporating the carbon when growing and reproducing but, even though this occurs, TOC often overwhelms a system. The easiest way to reduce the TOC level is to change the water, but this may be expensive or impractical, so other steps have been developed to condition the water. Ozone can be regarded as a molecular chainsaw that randomly breaks large molecular chains into smaller ones, with unpredictable ways. The combinations are dependent on water composition and are impossible to predict. Activated carbon efficiently removes TOC. We have speculated that excessive ozone and activated carbon use may lead to manipulations of the TOC level, which has adverse effects on the animal health; for example, head and lateral line disease could result.