Research

Ecosystems and Carbon Cycling

Aquatic plant beds act as hot spots of combined biological, geological and chemical (biogeochemical) activity in aquatic ecosystems. Through hydrologic exchange, these beds influence both the average chemistry of the entire ecosystem and spatial and temporal variation within it.

Dissolved oxygen (DO) levels in plant beds can regulate the degree and even direction of important biogeochemical reactions—the transformation and release of nitrogen (denitrification) and the formation of methane by decomposition (methanogensis), for example. As a result, the biogeochemistry and ecology of the entire ecosystem may be influenced indirectly by DO within beds.

Other studies suggest that characteristics of specific plant species may also be a critical factor regulating DO levels. In plant beds dominated by American wild celery (Vallisneria americana), a completely submerged aquatic plant, DO is on-average higher than that in the main river channel and hypoxia never occurs. In contrast, in beds dominated by water chestnut (Trapa natans), an introduced species both floating and submerged leaves, DO is on-average lower than in open water. In some beds very low DO, a condition known as hypoxia, is frequent and complete anoxia, or lack of oxygen, may even occur.

Considerable variation exists, however, in the dynamics and average concentration of DO between beds of a given species and spatially and temporally within some large beds. As a result, ongoing work has several objectives.

1. Understanding the variation in DO between and within beds which vary in species composition, size, shape and hydrologic exchange
2. Measuring other biogeochemical transformations in beds, including changes in nitrogen form, net retention and net trace gas formation and relating these reactions to bed characteristics including oxygen dynamics
3. Estimating the hydrologic exchanges that occur within beds as well as between beds and surrounding open channels
4. Using these exchanges along with estimates of gas exchange to develop an ecosystem model that predicts spatial-temporal variation within beds and in the main channel of the river

These objectives are being addressed through a combination of field measurements, including use of recording instruments that operate continuously, modeling that combines biogeochemistry and hydrodynamics, and field manipulation of a water chestnut bed in the Hudson to study the mechanisms that control DO and hydrologic exchange. An interdisciplinary research team that includes expertise in aquatic plant biology, nutrient cycling, trace gas production, physical gas exchange and hydrodynamics is.

This work is adding to a growing body of knowledge that suggests individual species can have very different impacts on ecosystem function, suggesting the removal or addition of species can have significant consequences. In addition, the research identifies shallow areas of aquatic ecosystems and their associated plant communities as critical “hot spots” of biogeochemical activity, similar to those already recognized in riparian areas of streams or estuarine marshes. This work is relatively unique in that ecosystem impacts will be examined in the Hudson River, and will combine physical and biogeochemical approaches with modeling and field manipulation.