Hudson River Habitats: Wetlands

There are roughly 200 tidal freshwater wetlands fringing the Hudson from the Tappan Zee region to the Federal Dam in Troy. Understanding how these habitats contribute to the whole ecosystem requires simple tools to assess multiple functions (habitat for fishes, improvement in water quality) across many sites as well as insight into the many potential factors altering natural functions.

Determining relationships among ecological processes and potential controlling factors may help set restoration targets and guide practices. We have developed and validated simple indicators of twelve functions and applied this approach to 15 sites along the Hudson (Findlay, S. E. G., E. Kiviat, W. C. Nieder, and E. A. Blair. 2002. Functional assessment of a reference wetland set as a tool for science, management and restoration. Aquatic Science 64:107-117.).

This research has answered the frequently asked question, "Has the railroad embankment led to a deterioration in wetland function?" Sites with natural protection from waves, wind and strong currents had functions nearly equivalent to sites behind the RR causeway. This work has also shown that a single site can not excel in all functions, in fact the highest "score" found in our survey was only about 75% of the numerical maximum possible. Some functions are negatively correlated, probably for natural reasons, so high potential for one function may be connected to poorer scores for another.

In summer of 2005 we sampled a set of ten randomly selected wetlands spanning the range of physical attributes and sampled the effect of marsh exchange on water quality. Hudson tidal marshes consistently acted as sinks for nitrogen probably due to their generally high capacity for denitrification. Sites were also fairly consistent in showing depletion of dissolved oxygen and increased DOC concentration in ebb-tide water relative to the main channel. So, for a diverse set of wetlands they show consistency in effects on dissolved constituents in tidal waters. The degree of change was often correlated with cover of graminoid vegetation, a highly productive plant community that generally has a high standing stock of litter.

Freshwater tidal wetlands are also under considerable stress from changes occurring within their boundaries, such as the expansion of invasive species populations or human-induced changes in their upland watersheds. Common reed (Phragmites australis) has expanded rapidly in the past few decades. In some Hudson wetlands it now makes up 75% of aerial marsh coverage. By measuring how successfully microbes grew on plant litter derived from common reed, and comparing it to growth on native cattails, we were able assess food quality. Fungi dominated microbial growth, and growth rates on common reed were only somewhat lower than on cattail (Findlay, S., S. Dye and K.A. Kuehn. 2002. Microbial growth and nitrogen retention in litter of Phragmites australis and Typha angustifolia. Wetlands 22:616-625).

Because common reed attains higher biomass than cattail in most areas, this species is more effective as a site of nutrient retention and should have a beneficial effect on water quality. Work at a site in the Connecticut River showed that efforts to eradiate common reed result in several short-term negative consequences (Findlay, S., P.M. Groffman and S. Dye. 2003. Trade-offs among ecosystem functions during restoration: Phragmites removal from a tidal freshwater marsh. Wetlands Ecology and Management. 11:157-165) and these should enter decisions about how to control the spread of this plant.

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