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A3 Life Support Systems
Kent J. Semmen and M. Andrew Stamper
Disney’s Animals, Science and Environment, Lake Buena Vista, FL, USA
Introduction
The condition of the water is the single most critical aspect in managing fish health. This focus on environmental factors is often the hardest learning step for veterinarians transitioning from terrestrial to aquatic animal care. It impacts all aspects of their life, including growth, reproduction, stress‐resilience, and health. The life support system (LSS) is the collection of equipment used to maintain and improve the condition of the water through managing its physical, chemical, and biological properties. Ensuring good water quality represents the ultimate preventive medicine challenge for the veterinarian working with aquatic systems, as there is both a science and an art to managing water in fish systems. This chapter will focus heavily on large, closed, recirculating systems as these are the most technically complex and are fairly common in public display aquariums. The concepts discussed can readily be “scaled down” or generalized to other systems.
Bacteria and Other Microorganisms
Aquatic systems are rich in microbes, including bacteria, archaea, protozoa, fungi, and microalgae such as diatoms. This microbiota is found within the water column and attached to all surfaces. On wet surfaces, this microbiota creates a sheltered scaffold known as the biofilm.
The bacteria can be autotrophic or heterotrophic. Autotrophic bacteria synthetize energy through photosynthesis or chemosynthesis using carbon from an inorganic source, such as carbonate. Heterotrophic bacteria get their energy from other organisms and therefore play important roles in both biodegradation and pathology. Both types have species that are anaerobic, aerobic, and facultatively anaerobic (preferring oxygen, but able to derive energy from other sources, like nitrate, when oxygen is in short supply).
The microbiota has a profound impact on animal health and water quality and should be considered part of the life support system.
System Type
Systems are often categorized as open, semi‐open, or closed. Open systems have continuous water input and discharge, usually from a natural body of water. Examples include pens in natural waterways, raceways, and aquariums where water flows through each system. Semi‐open systems also have a mechanism to recirculate some or all of the water. Closed systems do not have continuous water input or discharge. Examples of closed systems include home aquariums, intensive aquaculture systems, ponds, and most transport containers. While closed systems can be managed using a “dump and fill” approach, where some or most of the water is occasionally removed and replaced, it is more common for closed systems to be managed with recirculation through life support equipment. For this reason, the terms closed and recirculating are often used interchangeably.
Closed systems are the most complex as they are completely dependent on equipment and management practices to condition the water and sustain life. In closed systems, predictable changes occur to the water due to natural biological processes. These changes, if not countered by the LSS, will endanger fish. They include:
Dissolved oxygen is used and carbon dioxide is produced through respiration.
Nitrogenous wastes such as toxic ammonia, nitrites, and nitrates are produced from animal waste, uneaten food, and detritus.
pH fluctuates as buffers are used or added and carbon dioxide is produced.
Particulate and dissolved organic carbon (DOC) build up.
Anions and cations like carbonates and calcium are used.
Pathogen load may increase.
The LSS is used to manage and maintain appropriate levels of dissolved gases, nitrogenous wastes, and pathogens. This may involve gas exchange, mechanical, physicochemical, and biological filtration, and ultraviolet (UV) or ozone disinfection.
In all three system types, life support equipment may also be needed to process and manage incoming and discharged water to ensure suitable water condition.
The scale of LSSs varies enormously. For smaller aquariums, there are standard, off‐the‐shelf components readily available. As system size increases, LSSs often require more design and construction effort to help support fish health and system management. For the largest aquarium systems, engineers are often involved in the design and maintenance of the LSS.
Most of the filtration methods currently used for LSSs have their basis in municipal wastewater, drinking water, and swimming pool industries. These have very different goals from living systems, such as high levels of disinfection. Newer approaches are incorporating the natural stabilizing processes of wild aquatic systems. This understanding may provide more efficient and cost‐effective life support for aquatic animals in the future.
Oxygenation and Gas Exchange
Maintaining an appropriate balance of dissolved gases in fish systems is critical to support animal respiration, support microbial respiration within the biological filtration, maintain pH, and prevent gas supersaturation. The most critical dissolved gases are oxygen, nitrogen, and carbon dioxide. Based on individual gas solubility, their natural ratio in water is 1:2:4 respectively. The ratio and actual gas concentration depend on atmospheric pressure, water temperature, salinity, pH, biological activity, and gas exchange at the surface. The larger the surface area:volume ratio of an aquarium, pond, filter, or sump, the more potential there is for gas exchange. Movement at the water surface (e.g. air columns, pump returns at the surface, and surface skimmers) enhances gas exchange at the surface. Surface skimming action is particularly important, as it prevents accumulation of a film of surfactant material at the air/water interface that reduces gas exchange.
The primary components that maintain adequate gas exchange include:
Air/oxygen diffusers, e.g. air stones connected to ambient air or compressed oxygen (Figure A3.1a).
Air/oxygen exchange towers, e.g. degas towers (Figure A3.1b).
Surface skimmers that remove the surface water.
Air/oxygen venturis that suck in air or oxygen.
Supersaturation can occur whenever gas gets into a system at high pressure. The most common causes are compromised plumbing on the suction side of a pump, excessive deep‐water aeration, or supersaturated incoming water. The percentage of supersaturation (delta P) is equal to the total dissolved gases (TDGs) compared to the barometric pressure of the air at the water's surface. This is measured using TDG meters (also known as saturometers). A delta P greater than 10% can cause gas emboli in fish (see Chapter C1).
Water Flow
Water flow should be suitable for the species and provide adequate exposure to the components of the LSS. Appropriate water flow (velocity and direction) directly impacts animal health and welfare:
Promotes gas exchange at the surface and prevents anoxic or stagnant pockets of water.
Promotes healthy‐system bacteria in biofilms