Oxygen (O2) is dissolved in the water column through diffusion from the surrounding air, by aeration of water with air through rapid movement, and through photosynthesis of underwater plants (see image below).
Submerged aquatic vegetation (SAV, shown above left) and phytoplankton (shown above right) provide oxygen for other aquatic organisms, such as fish and shellfish.
Hypoxia is the state of oxygen deficiency, which in aquatic environments means there is low dissolved oxygen in the water. Although hypoxic conditions are naturally-occuring, they have been made far more common and their effects far more widespread by human pollution of waterways
Nutrient loading or eutrification from untreated wastewater and runoff from agricultural areas can increase algal growth, causing water clarity to diminish.
Blocked by expansive algae blooms, sunlight is no-longer able to reach submerged aquatic vegetation or phytoplankton and these plants die, removing oxygen input from the system. Sudden high concentrations of algae often cannot be consumed completely and die, falling to the sediment surface where it is decomposed by bacteria.
Bacteria further consume dissolved oxygen, resulting in depleted oxygen levels in deeper waters. As oxygen diminishes to sub-optimal levels, aquatic organisms become stressed and more prone to disease and parasites and some die.
With expanding human populations, nutrient disposal increasingly becomes an issue. Many hypoxic zones in U.S. waterways have become larger (e.g., Gulf of Mexico, Chesapeake Bay), due to increased nutrient input. Prolonged hypoxic conditions can cause mass mortality of aquatic organisms, including fish.
Author:
Daniel E. Terlizzi
