Oxygen is absolutely essential for biological and chemical processes in the Hudson River Estuary. Fish and other aquatic animals cannot survive without it and decomposition is slowed dramatically in its absence.
Unfortunately, dissolved oxygen concentrations dropped very low in the Hudson River Estuary this summer. The NYS Water Quality Standard for dissolved oxygen is 4 mg/L for most of the Hudson River Estuary excepting Haverstraw Bay which is 3mg/L. This summer, the HRECOS network recorded 2,480 dissolved oxygen observations that were below these water quality standards.
Dissolved oxygen levels were lower this year compared to 2008 and 2009. Specifically, there were 37% and 94% more observations recorded below the water quality standard this summer compared to 2008 and 2009 respectively.
Most of the low dissolved oxygen observations recorded by the HRECOS network were at the Tivoli Bays North and South. This is not surprising. Both Bays have active oxygen consuming communities including animals, plants, and microbes. Oxygen levels are often lower in Tivoli Bay South, however, because this system has fewer sources of dissolved oxygen to draw from. The predominant plant species in Tivoli Bays South, invasive water chestnut, does not provide oxygen and blocks light to other plants that could be oxygen providers. For these reasons, the Tivoli Bays are the most vulnerable to low dissolved oxygen levels within the HRECOS network.
A likely culprit for these low DO levels are the high temperatures we have been experiencing this summer. Warm water molecules are more active and push out gases including oxygen. For this reason, warmer waters hold less oxygen than cooler waters. Alternatively, human activity may have reduced dissolved oxygen concentrations. Increased nutrient discharge, suspended sediment discharge, and thermal pollution would reduce oxygen levels. There is no evidence at this time that pollution discharges have increased this year compared to 2008 and 2009.
Regardless of the cause, there is serious concern for low dissolved oxygen concentrations. Two fish kills have been reported in the Hudson River Estuary this summer. On July 13, several hundred dead Atlantic menhaden were observed in the area from the Tappan Zee Bridge down to northern Manhattan close to the eastern shore. On August 16, dead catfish were observed from Athens to Castleton-on-Hudson. The Hudson River Fisheries unit is having difficulty identifying the exact cause of these events. It is very possible, however, that the fish were exposed to unusually low dissolved oxygen concentrations.