Lakeshore and littoral physical habitat structure in a national lakes assessment

Near-shore physical habitat is crucial for supporting biota and ecological processes in lakes. We used data from a statistical sample of 1101 lakes and reservoirs from the 2007 US Environmental Protection Agency National Lakes Assessment (NLA) to develop 4 indices of physical habitat condition: (1) Lakeshore Anthropogenic Disturbance, (2) Riparian Vegetation Cover Complexity, (3) Littoral Cover Complexity, and (4) Littoral-Riparian Habitat Complexity. We compared lake index values with distributions from least-disturbed lakes. Our results form the first US national assessment of lake physical habitat, inferring the condition (with known confidence) of the 49,500 lakes and reservoirs in the conterminous United States with surface areas >4 ha and depths >1m. Among the physical and chemical characteristics examined by the NLA, near-shore physical habitat was the most extensively altered relative to least-disturbed condition. Riparian Vegetation Cover Complexity was good in 46% (+/- 3%), fair in 18% (+/- 2%), and poor in 36% (+/- 2%) of lakes. Littoral-Riparian Habitat Complexity was good in 47% (+/- 3%), fair in 20% (+/- 2%), and poor in 32% (+/- 2%) of lakes. In every ecoregion, habitat condition was negatively related to anthropogenic pressures. Gradients of increased anthropogenic disturbance were accompanied by progressive simplification of littoral and riparian physical habitat. Nationwide, the proportion of lakes with degraded near-shore physical habitat was equal to or greater than that for excess nutrients and much greater than that for acidic conditions. In addition to better management of lake basins, we suggest an increased focus on protecting littoral and riparian physical habitat through better management of near-shore anthropogenic disturbance.