Last summer, tropical storms Irene and Lee inflicted major damage on the Hudson River’s watershed. While the events may seem like a distant memory now, affected ecosystems are still recovering.
Not surprisingly, the storms caused large changes in the Hudson River. For example, roughly a year’s worth of water and sediment transport occurred within the span of a few short weeks. There were also less visible consequences that have taught us something new about the river’s ecology.
Ecosystems “breathe” much the same way we do, taking in oxygen and other gases, and releasing carbon dioxide. We have known for some time that because of the respiration, or “breathing,” activity of fish and other creatures, notably microbes, the Hudson River consumes a lot of oxygen. As a result, its waters are undersaturated with oxygen.
Oxygen saturation is a measure of the amount of dissolved oxygen carried in water. Cold water holds more gas than warm water — so warmer water becomes saturated with oxygen faster. Salinity and depth also play a role.
As you add in aquatic life, the scenario becomes more complex, with plants producing oxygen during the daytime and aquatic life such as fish and microbes consuming it. In the Hudson, water near the surface tends to have the most oxygen, because of the photosynthetic activity of plants. Scientists pay close attention to oxygen levels, because when they get too low, we can wind up with fish kills.
The Hudson River Environmental Conditions Observing System is a series of seven monitoring stations that spans from the Port of Albany southward to the Tappan Zee Bridge. Every 15 minutes, the stations record a number of environmental variables, including dissolved oxygen, providing scientists and resource managers with a window into the river’s health.
During the summer storm activity, the system’s data showed scientists something quite remarkable. Shortly after the rainfall started, the river drew in a large quantity of oxygen. It turns out this was because the creeks and streams that feed the river were delivering water with higher-than-normal oxygen levels.
Within about 36 hours of Irene’s arrival, oxygen levels in the Hudson River below Albany were about 20 percent greater than expected. It was as if the river had a larger than normal “lungful” of air. In the days after the storms departed, oxygen levels rapidly fell — two to three times faster than normal — as microbes and other aquatic organisms used the oxygen.
If you saw the Hudson in early September, it looked brown. This was because of the large quantities of soil and organic matter that washed into the main channel. These tiny bits of terrestrial-generated leaves and woody debris fueled microbial growth. And as microbes took advantage of this extra “food” by growing faster, they rapidly gobbled up oxygen in the process.
In the Hudson River, this was the first time scientists were able to trace such a dramatic and rapid delivery and decline in oxygen levels to organic matter delivered from the watershed. The finding is important because following large storms or hurricanes, wastewater treatment facilities can leak sewage and are often blamed for oxygen problems.
In the Irene/Lee case, the oxygen drawdown was offset by a new supply of oxygen delivered from tributaries, and we believe that watershed inputs, rather than wastewater, were the major source of organic matter.
These findings show that the organisms in the Hudson can handle and even benefit from organic matter inputs, especially when the water delivering the organic matter is also high in oxygen. It also confirms earlier research showing that while aquatic plants in the Hudson are important, organic material from the watershed also plays a large role in subsidizing the food web.
Our ability to see and interpret these rapid changes relies on the monitoring system’s network, which has the capability to reveal important things that happen quickly and under stormy conditions.