Hypoxia

Oxygen dissolved in the water is required by most aquatic species. Therefore, the measurement of dissolved oxygen provides one of the most universal indicators of overall water quality in New York Harbor.

Dissolved oxygen may be depleted during the decay of organic matter by bacteria. This depletion is most significant in near-bottom waters during warm summer months, where bacterial respiration consumes more oxygen than is replenished. When oxygen is depleted below certain levels, it produces a severe deficit called hypoxia. This may contribute to the death of many aquatic organisms which cannot escape to more oxygenated waters.

Decaying organic matter occurs directly from sewage discharges, local runoff, and rivers draining the Harbor’s watershed: and indirectly from eutrophication, the process by which an excessive supply of nutrients such as nitrogen and phosphorus stimulates the growth of algae, whose subsequent death produces organic decay.

Fortunately, the regional construction of numerous sewage treatment plants during the last few decades has eliminated raw sewage discharges, thereby reducing the largest direct source of organic matter and increasing dissolved oxygen levels in most areas of the Harbor.

During the summer, however, some local waters such as western Long Island Sound and parts of Jamaica Bay still experience significant oxygen depletion in their bottom waters. This occurs partly due to relatively poor flushing and stagnant conditions which exist in their bottom water layers. This condition is exacerbated by abnormally warm weather (which has been the case during the last several summers). In addition, the decomposition of excessive algae may consume dissolved oxygen and lead to severe hypoxic conditions.

The control of eutrophication in New York Harbor has focused on reducing nitrogen discharges from sewage treatment plants. However, because algae form the base of the aquatic food chain, and nitrogen is a key nutrient for their growth, overly astringent nitrogen controls may disrupt the existing food chain. Controlling eutrophication therefore requires a strategy, which balances the need to maintain moderate nutrient input while eliminating its adverse impact on dissolved oxygen.

This strategy relies on determining the appropriate levels of nitrogen, algae and oxygen that are needed to maximally benefit the ecosystem. New York State’s standards require that dissolved oxygen never drop below 3-5 parts per million (ppm) depending on the designated best use of the Harbor’s waters. However, impacts resulting from sub-standard oxygen levels are relatively poorly defined. New York City is committed to continuing research which develops regional criteria and minimizes the adverse impacts of low dissolved oxygen on the Harbor ecosystem.