Adequate dissolved oxygen (O₂) is necessary for the health and survival of fish and shellfish species. Oxygen diffuses into the water from the atmosphere, which is almost 20% oxygen, and from the photosynthesis of aquatic plants (mostly algae). Processes consuming oxygen include the respiration of plants and animals, and the decomposition of organic matter by bacteria. Estuarine water is saturated with oxygen at about 8 mg/L. If the oxygen concentration falls below about 2 mg/L estuarine fish and invertebrates can become stressed. This condition is environmental hypoxia. Some species, such as bivalves, can persist by lowering metabolic rates, and others may escape by swimming away. However, the rapid decline in oxygen concentration that can occur in the presence of large accumulations of macroalgae can result in extensive fish and shellfish deaths. Chronic hypoxia enhances the risk of environmental anoxia, the complete absence of oxygen. Anoxia is promoted when oxygen uptake exceeds supply. Under anoxic conditions, anaerobic bacterial decomposition processes release hydrogen sulfide (H₂S, with the "rotten-egg" aroma), a toxic substance to most oxygen-requiring organisms. Usually such anaerobic processes are hidden in the sediments, but can rise to the sediment surface and into the water column if anoxic conditions persist. Anoxia is especially prevalent in estuaries receiving a high load of organic matter and/or nutrients, and where water mixing and tidal exchange is diminished.