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Oxygen has low water solubility and this creates an issue for aquatic animals. These animals cannot use the same means that we (land dwellers) do and that is why, we have to use artificial means of aeration when diving or staying under water for long duration. Aquatic animals have adapted to life in low oxygen containing waters by breathing through gills or skin. Fishes use gills for breathing and also for expelling carbon dioxide from their bodies. Gills provides a large surface area for gaseous exchange, thus making it possible for fishes to extract oxygen from water. Skin breathers and gills users must keep their skin and gills, respectively, moist at all times. A number of aquatic animals are capable of breathing and storing a lot of air in their body and they come back to the water surface when they run out of it. A number of other animals live close to the surface and directly breathe air from atmosphere.
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Fish need at least a 30% saturation of oxygen in the water, otherwise the water is termed "anoxic". A healthy area has 80% oxygen saturation. Pollution and eutrophication depletes oxygen. Fish kills can then occur as well as mollusks and different types of worms. Some adaptations animals have to obtain oxygen in the water is subcutaneous respiration(skin breather) or the use of gills. For example, if something is a skin breather, diffusion through their thin moist skin occurs and oxygen enters the body in that way. For fish which utilize gills, their gills must be kept moist. The operculum or gill cover opens and closes and water covers the gills. Diffusion across the thin, feathery gill filaments occurs and oxygen is absorbed into blood vessels which are located inside the gills. Gills provide tremendous surface area for the exchange of respiratory gases. Marine mammals must come to the surface to breathe. Many can stay underwater for up to an hour because they have efficient lungs that can exchange 90% of their volume with each breath. Also, some mammals can store high amounts of oxygen in their blood and muscles when diving. Marine reptiles like turtles, must eventually come to the surface to breath air into their lungs.
The water animals at least need 30% of saturated oxygen in the water otherwise they will die.
To survive, fish, crabs, oysters and other aquatic animals must have sufficient levels of dissolved oxygen (DO) in the water. The amount of dissolved oxygen in an estuary’s water is the major factor that determines the type and abundance of organisms that can live there.
Oxygen enters the water through two natural processes: (1) diffusion from the atmosphere and (2) photosynthesis by aquatic plants. The mixing of surface waters by wind and waves increases the rate at which oxygen from the air can be dissolved or absorbed into the water.
DO levels are influenced by temperature and salinity. The solubility of oxygen, or its ability to dissolve in water, decreases as the water’s temperature and salinity increase. DO levels in an estuary also vary seasonally, with the lowest levels occurring during the late summer months when temperatures are highest.
Bacteria, fungi, and other decomposer organisms reduce DO levels in estuaries because they consume oxygen while breaking down organic matter.
Oxygen depletion may occur in estuaries when many plants die and decompose, or when wastewater with large amounts of organic material enters the estuary. In some estuaries, large nutrient inputs, typically from sewage, stimulate algal blooms. When the algae die, they begin to decompose. The process of decomposition depletes the surrounding water of oxygen and, in severe cases, leads to hypoxic (very low oxygen) conditions that kill aquatic animals. Shallow, well-mixed estuaries are less susceptible to this phenomenon because wave action and circulation patterns supply the waters with plentiful oxygen.
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