The following article originally appeared in the 2002-2003 Biennial Report.

Invasive exotic species can have dramatic impacts on native species abundance, biodiversity and biogeochemical cycles within ecosystems. For the past decade, Institute scientists have been investigating the ecosystem consequences of species invasions in the Hudson River. While studying the impacts of zebra mussels, an invasive bi-valve, Drs. Nina F. Caraco and Jonathan J. Cole became interested in how aquatic vegetation in the river influenced oxygen availability.

Aquatic plants with submerged leaves, such as water celery, are referred to as submerged aquatic vegetation (SAV). Institute scientists, in collaboration with New York Department of Environmental Conservation and Cornell University, have been researching the distribution and function of SAV beds in the Hudson River. The SAV plays a vital role in aquatic systems; healthy stands oxygenate the water column and provide food and refuge to aquatic organisms.

Water celery (Vallisneria americana) and water chestnut (Trapa natans) are the two dominant forms of aquatic vegetation in the Hudson River. Water celery is native to eastern North America, while water chestnut is a Eurasian plant that was introduced into New York in the late 1800's and has been spreading throughout the state ever since. The two plant species exhibit very different growth forms. Water chestnut's leaf-growth is largely comprised of leaves that float on the water's surface, with only a few underwater feather-like leaves. In contrast, water celery's long ribbon-like leaves grow submerged throughout the water column.

Prior to water chestnut's arrival, there were no large populations of floating-leaved plants on the Hudson River. The invasive species is now the second most abundant aquatic plant in freshwater tidal areas. Given the increasing populations of water chestnut, and the growth form of the plant, Drs. Caraco and Cole questioned whether the invasive species has the same oxygenating function as native SAV. Like humans, aquatic organisms need oxygen to survive. Waters with high dissolved oxygen levels are considered healthier than waters with depressed levels.

Water chestnut and water celery occur in distinct beds in the Hudson River, with peak growth from mid-July to mid-September. Extensive dissolved oxygen measurements during this period revealed that water chestnut beds have, on average, lower oxygen levels than similar-sized water celery beds. In fact, dense water chestnut beds are often hypoxic (low oxygen), with oxygen levels low enough to stress aquatic organisms. The frequent hypoxia and anoxia measured in water chestnut beds compromise them as habitat for many fish and invertebrates. Only animals that are very tolerant of low oxygen, such as carp, may be able to live beneath water chestnut beds.

The dissolved oxygen story is tied to the morphology of the plants. A large amount of water chestnut's biomass is on the water's surface. When mats form a closed-canopy, little light penetrates the water column, and the plant's feather-like underwater leaves are unable to produce oxygen through photosynthesis. Water celery adds oxygen to the water column; water chestnut tends to deplete it.

There are no safe chemical controls for water chestnut, and manual removal requires time and labor. Future research will focus on manually manipulating water chestnut beds, with the goal of understanding how plant density affects oxygen levels. By monitoring fish during manipulations, Dr. Caraco hopes to determine if bed thinning will make water chestnut beds usable habitat for young fish.