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
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.
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
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.