Most aquatic species, like land-dwellers, need oxygen to survive: They just need it in smaller amounts. The Chesapeake Bay generally contains less than 14 parts per million of oxygen, compared with more than 200,000 ppm in the atmosphere. Amounts of dissolved oxygen along the bottom of the Bay can be much lower, though. Dying algae sinks to the bottom and decomposes in a process that consumes oxygen. When top and bottom layers become strongly stratified either because of large differences in salinity or temperature the ability of bottom water to mix with the top layer (where oxygen is replenished from the atmosphere) is limited. In that case, oxygen concentrations in the water drop and can even be eliminated.
Low oxygen levels can affect the survival and productivity of many species that live in the Bay. Eggs and larvae of many fish species are often particularly vulnerable to low oxygen levels. And even many adults that can withstand periods of low oxygen may have their growth and reproduction affected.
To help protect and improve habitat for those resources, the Bay Program has adopted a series of dissolved oxygen goals:
- Maintain dissolved oxygen concentrations of at least 1 ppm at all times throughout the Bay and its tidal tributaries.
- Dissolved oxygen concentrations between 1 ppm and 3 ppm should not occur in the Bay or its tidal tributaries for longer than 12 hours. After that, they should not dip to that level again for at least 48 hours.
- Maintain a monthly mean dissolved oxygen concentration of at least 5 ppm throughout the above-pycnocline waters of the Bay and its tidal tributaries.
- Maintain dissolved oxygen levels of at least 5 ppm at all times throughout the above-pycnocline waters of anadromous fish spawning reaches [such as shad and striped bass], spawning rivers and nursery areas of the Bay and its tidal tributaries.
- Maintain existing minimum concentrations of dissolved oxygen in areas of the Bay and its tidal tributaries where dissolved oxygen concentrations are already above those stated in the other goals.