Maryland's criteria provide for a minimum level of dissolved oxygen for all aquatic plants and animals. Lower levels of dissolved oxygen reduce habitat needed for aquatic plants and animals to survive and reproduce. At very low oxygen levels, aquatic plants and animals are killed or excluded. Bacterial decomposition of organic material in the water column and sediments, and plant and animal respiration consume dissolved oxygen from the water. If too much organic material is present, or if phytoplankton populations are greatly increased by excess nutrients in the water, dissolved oxygen levels can be reduced to near zero.
Cold waters can hold much more dissolved oxygen than warm waters.
Low oxygen levels are seasonally evident during late spring to early fall. Reduce conditions contributing to low dissolved oxygen in surface waters. Continued reduction in the percentage of the State's surface waters impaired by low dissolved oxygen.