Natural nutrient and chemical conditions are those
that reflect local climate, bedrock, soil, vegetation type,
and topography (EPA 2000). Natural waters can range
from clear, nutrient-poor rivers and lakes on crystalline
bedrock, to much more productive and chemically enriched
freshwaters in catchments with productive soils
or limestone bedrock.
Cultural eutrophication occurs when additional nutrients
from human activities substantially increase productivity
beyond the original state (Carpenter et al.
1998). Eastern lakes demonstrate the consequences of
excess nutrients and toxic contaminants, as well as nonnative
species introductions and overfishing. Lakes
Michigan, Huron, Erie, and Ontario suffer from all of
the above (Anderson et al. 2001). Onondaga Lake, New
York, USA, which was polluted with salt brine effluent
from a soda ash industry, responded with marked
changes in plankton and fish species composition, including
invasive species (Auer et al. 1996, Hairston et
al. 1999). Nutrients contributed to 51% of the water
quality problems of U.S. lakes identified as impaired
in 1996 (EPA 1998). More than one half of agricultural
and urban streams sampled in the United States have
pesticide concentrations that exceed guidelines for the
protection of aquatic life (USGS 1999).