Fig. 6. a) A lake at the unimpacted end of the spectrum in Wisconsin's Northern Highlands Lake District. Uses of the lake and its riparian areas include low-impact boating such as canoeing and activities like bird watching and cross-country skiing in the winter. Riparian areas contain coarse woody debris and macrophyte beds that allow for successful fish recruitment and productive benthic invertebrate populations. The food web is healthy, and the water quality is high (click on the food chain to see more about what influences food web structure). b) A highly affected lake representing some common human uses of and threats to lakes in the northern highlands region. The riparian areas have been deforested and replaced with concrete and buildings. Common problems include high runoff of polluted water, leaky septic tanks, and the immigration of animals that have high nutrient deposition rates through waste products. Other changes to the lake edge include the removal of macrophyte beds and coarse woody debris and the building of docks and piers. On the lake, anglers are common, as are highly polluting speedboats and personal watercraft. Exotic species within the lake are also present, including various fish species (e.g., carp and rainbow smelt), macrophytes (e.g., Eurasian water milfoil and purple loosestrife), invertebrate species (e.g., rusty crayfish, zebra mussels). Many exotic species gain access to previously uninvaded lakes by means of human activities like dumping baitfish and by the movement of boats with hitchhiking individuals attached. The many uses of the lake can lead to a less healthy food web in those areas where higher trophic levels are especially reduced or contaminated through the bioaccumulation of toxins like mercury (click on the food chain to see more about the factors that influence the structure of the food web).

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