I. Habitat Description

Calcareous fens are one of the most rare habitat types in the United States. They typically form on or near slight slopes from upwelling groundwater trapped by a layer of peat. Like bogs, fens are characterized by a peat substrate, but are fed by a supply of cold, oxygen-deprived groundwater rich in calcium and magnesium bicarbonates. As they occur on sites of cold water seepage, active springs and trout streams are often associated with fens.

II. Sensitivity to Oil Spills

Fens take millennia to form, and consequently are highly sensitive to oil spills. Poor drainage allows oil to accumulate and persist in layers of organic material. In drier hydrologic regimes, peat deposits are highly absorbent of hydrocarbons, making it difficult for clean-up without removing this valuable material, which provides vital substrate for rare plant and animal communities. Light refined oils with high amounts of water-soluble fractions can cause acute mortality to animals and plants. Heavier oils tend to coat vegetation, which may survive if oil coats only the stems or if the roots are unaffected. It is difficult for more viscous oils to penetrate densely vegetated areas.  

III. Sensitivity to Response Methods

The following text describes potential adverse impacts to this habitat resulting from various oil spill response methods and provides recommendations to reduce impact when these methods are implemented.  This is not intended to preclude the use of any particular methods, but rather to aid responders in balancing the need to remove oil with the possible adverse effects of removal.  More detail about the response methods themselves can be found in the Inland Response Tactics Manual.