Nitrogen and Sulfur Deposition

Nitrogen and sulfur can cause similar and unique effects on the ecosystem.  Both contribute to acid depostion (rain).  Nitrogen is a fertilizer.  Sulfur can enhance mercury accumulation in fish. 

Acid Deposition
Acidic inputs from the atmosphere, mainly sulfate and nitrate, can negatively impact aquatic and terrestrial ecosystems. Their acidifying effects contribute to degradation of stream and lake water quality by lowering the acid neutralizing capacity (ANC) which can be thought of as the water's natural acid buffering system. As the ANC decreases, the acid levels will increase. In areas such as the central and southern Appalachians, forest streams have acidified to the point where they are no longer capable of sustaining aquatic life. The sensitivity of lakes and streams to the negative effects of acid deposition are often linked to natural watershed characteristics, most notably the bedrock geology.

Watersheds containing naturally occurring geologies/lithologies that weather (break down easily) and that are made up of minerals containing high levels of base cations (nutrients that plants need) are less susceptible to negative impacts of acid deposition. Likewise, watersheds where the soils are derived from geologies/lithologies that are resistant to weathering or that contain thin shallow soils are very susceptible to acid deposition. Correspondingly, these same susceptible areas may not only exhibit lake and stream water chemistry changes, but they may also show effects in soil chemistry such as nutrient leaching. Nutrient leaching can eventually lead to deficiencies in macro nutrients important for plant growth.  These areas that receive high levels of acidic deposition and have bedrock geology with a naturally low buffering capacity may exhibit nutrient depletion and stream acidification. 

In glaciated areas such as the Upper Midwest additional factors such as the relationship of a water body to the water table and the amount of dissolved organic matter have been shown to be important.

The Superior NF monitors acid deposition and the data can be found online

The current state of knowledge for the Superior National Forest is that while sensitive lakes do occur on the Forest (as determined by low ANC) the generally high levels of organic acids from the numerous nearby wetlands serve to buffer these lakes from acid deposition.

For more information on acid deposition in the BWCAW see the:

Nitrogen Fertilization Effects
In addition to contributing to acidic rain, nitrogen can cause other ecosystem impacts by unnaturally fertilizing land and water. These excess inputs of nitrogen termed nutrient enrichment and eutrophication can disrupt the natural flora and fauna by allowing certain species that would not naturally occur in abundance to out compete those that thrive in pristine nitrogen limited systems. The end result is an unnatural shift in species composition for sensitive species, which may have a subsequent impact on other components of the ecosystem. This effect needs further study in the BWCAW.