In a healthy bay, the nutrient levels in the water are relatively low. Low nutrient levels keep algae in check. As a result of low algae levels, the water is clear, with good sunlight penetration.
The sunlight can penetrate to the plants underwater, allowing them to perform photosynthesis and give off oxygen as a by-product. Therefore, in a healthy bay, dissolved oxygen levels are relatively high.
The high dissolved oxygen levels are required for the consumer population, with abundant fish and shellfish, to remain high.
An unhealthy bay can result from a process called eutrophication, which begins when sediment, nitrogen, and phosphorus enter the bay as runoff pollution.
These nutrients fuel plant growth, and so the extra amounts cause algal blooms, explosions of algal growth.
The algae and sediments, in turn, block sunlight, causing the underwater plants to die.
Although the algae do emit oxygen as a by-product of photosynthesis, as a whole, less oxygen is produced when the underwater plants die. The levels of dissolved oxygen in the water plummet.
The loss of dissolved oxygen can also result when the nutrients and algae sink into the deeper waters of the bay, where they are consumed by bacterial decomposers.
The decomposers increase in number, which also increases the amount of oxygen the bacteria consume for cellular respiration.
Fish and larger, mobile organisms may be able to leave the area; others cannot and will suffer in the hypoxic waters. The consumer population declines.
Sediments from the runoff also cloud the water and coat surfaces, clog gills, and smother spawning areas.
In eutrophication, excess nutrients turn a healthy bay into an unhealthy one.