Research

Changing Hudson Project - Cary Institute & the Hudson River

For over two decades, researchers at the Cary Institute of Ecosystem Studies have been paying close attention to the Hudson River. From investigations into the aquatic food web to long-term research on invasive species, their work is generating a better understanding of how the river functions as an ecological system. This information is critical to informing environmentally-sound management and conservation efforts.

The Cary Institute Hudson River Research Program is a collaborative effort between a team of five Cary Institute scientists and several research support staff. Broad in its geographical and intellectual scope, the group has produced a rich body of knowledge about the river's ecology. Instead of looking at the river in isolated parts, projects strive to take in a "river view," with field sites located from the Battery in Manhattan to the Troy Lock.

Our staff began looking at the river in 1986, in an attempt to describe connections within the lower food web. Projects have evolved to address complex questions about submerged aquatic vegetation, invasive plants and animals, native mussels, fish populations, and the ways watershed development impacts water quality. Integrated long-term research is a hallmark of the program, as is collaboration with other organizations.

Many Hudson River Research Program projects share a common theme- how is the river responding to human generated changes? Europeans settled along the banks of the Hudson River over four centuries ago. Since that time, the river has been subjected to shoreline development, human-generated contaminants (both industrial and municipal), boating, and invasive species. In an effort to touch on all of these areas, on-going data collection includes water sampling, watershed monitoring, and the mapping of habitat and organisms.

Of particular interest is the way that non-native species are influencing the way that the river ecosystem functions. One of these invaders is the zebra mussel, a small bivalve that is native to Russia. Accidentally introduced into the Great Lakes in 1986, zebra mussels made their way into the Hudson River in 1991, when our researchers were in the process of conducting long-term food web studies. This put researchers in the unique position to record the ecological conditions of the river before and after the invasion.

Several attributes make zebra mussels a force to contend with: lack of predators, a high reproductive rate, and voracious appetites. Few native animals find them palatable and adult mussels produce 30,000 to 400,000 young a year. They adhere to hard substrate, clogging water pipes and blanketing docks and bridge abutments. Over the past fifteen years, the prolific species has become the most abundant filter-feeder in the river's freshwater reaches and have greatly reduced several types of plankton.

Plankton are microscopic plants (phytoplankton) and animals (zooplankton) that form the base of the aquatic food web. They are an essential food source for a range of animals, from larval fish to native freshwater mussels. Over the last decade, our research has documented that zebra mussels are causing a decline in plankton abundance. This has ushered in several significant ecological changes in the river, including a shift in resident fish populations and a decline in dissolved oxygen due to zebra mussel respiration.

For young fish that live in the open water, plankton are a dietary staple. Zebra mussels are reducing plankton availability throughout the river; as a result, open-water fish are declining in response to the invasion. Impacted species include striped bass, a popular sport fish, and the commercially-important American shad. When a river's ability to support young fish changes, flexible management strategies are needed to plan for long-term viability.

Zebra mussels are not the only invaders altering the Hudson River ecosystem. Water chestnut, a floating aquatic plant native to Eurasia, is also causing problems. Introduced to the river in the early 1900s, water chestnut is now the second most abundant plant in freshwater tidal areas. In slow moving parts of the river, thick mats of the plants can cover acres of open water.

Water chestnut's floating leaves blanket the water's surface, blocking out sunlight and preventing underwater photosynthesis. This is a problem because in healthy systems underwater plants add dissolved oxygen to the aquatic environment. In areas that are heavily invaded by water chestnut, oxygen levels are very low beneath beds. Low oxygen conditions diminish the quality of habitat for fish and other animals. They can also affect the way that nutrients and metals cycle within the river.

In some areas of the river, researchers have also found that water chestnut can displace beneficial submerged aquatic vegetation (SAV). These rooted underwater vascular plants, such as water celery, provide critical habitat for a range of river life, including commercial and recreational fish species. Relative to open water habitat, SAV beds are home to a greater diversity of river life. And, unlike water chestnut, plants play a critical role in supplying oxygen to the water.

Water chestnut is not the only threat to SAV beds; humans are also inadvertently harming these important plants. Because SAV beds grow in shallow areas of the river, plants are prone to being damaged by motorboat propellers. This tends to happen during high tide, when plants are not visible at the water's surface. Boater education and reliable SAV bed maps can help minimize these avoidable SAV losses. We are taking a role in advancing both of these objectives.

In addition to research on invasive species and SAV, the Hudson River Research Program is exploring questions about how watershed changes influence river function. River bacterial growth exceeds phytoplankton growth, an indicator that bacterial growth is being augmented by organic matter from the watershed. Carbon from soils and vegetation is an important part of the lower food web and it appears that the input of dissolved organic carbon is increasing for reasons that are still unclear.

The Cary Institute Hudson River Research Program is committed to sharing their findings about the Hudson River ecosystem. With over one hundred papers published in scientific journals, as well as popular magazine, radio, and television coverage, Cary Institute research helps inform sound river management strategies. To ensure that useful information is being applied to real problems, our scientists provide direct advice to a range of local, state and national management agencies. Data sets are also being used as part of the Changing Hudson Project, an education venture connecting real science with classroom learning.