During the spring and summer, increased levels of animal and microbial respiration and greater stratification may reduce vertical mixing, resulting in low levels of dissolved oxygen in deep water. In fact, deep parts of some tributaries like the Patuxent, Potomac and Rappahannock rivers and deep waters of the Bay's mainstem can become anoxic in summer. In the autumn when surface waters cool, vertical mixing occurs and deep waters are re-oxygenated. Carbon dioxide, another dissolved gas, is important to the well-being of the Bay's aquatic environment. It provides the carbon that plants use to produce new tissue during photosynthesis and is a byproduct of respiration.
Carbon dioxide is more soluble in water than oxygen.
Its availability also is affected by temperature and salinity in much the same fashion as oxygen. Nitrogen is essential to the production of plant and animal tissue. It is used primarily by plants and animals to synthesize protein. Nitrogen enters the ecosystem in several chemical forms and also occurs in other dissolved or particulate forms, such as in the tissues of living and dead organisms. Some bacteria and blue-green algae can extract nitrogen gas from the atmosphere and transform it into organic nitrogen compounds.