Naturally, more descriptive variables, such as temperature, salinity, and site observations, also have a role in characterizing the estuary. Discussions of these variables follow in Chapter 3, Setting the Stage . Both animals and plants require oxygen for respirationa process critical for basic metabolic processes. During daylight hours, plants add oxygen to the water by photosynthe sis. At night, when photosynthesis ceases, plant respiration continues to consume oxygen and produce carbon dioxide.
Animals respire around the clock, taking up oxygen continuously.
Dissolved oxygen levels in an estuary also fluctuate widely due to seasonal and climatic variations as well as human influence. In short, warm water cannot hold as much oxygen as cold water. Low DO diminishes the capability of water to support life since almost all aquatic species are dependent on oxygen for survival. Nitrogen and Phosphorus Nitrogen and phosphorus are two of the many nutrients critical for the survival of aquatic species. Phosphorus is critical for metabolic processes which involve the transfer of energy.