1989 and has spread through the river's freshwater region, reducing photoplankton and river oxygen levels. Positively, the mussel clears suspended particles, allowing for more light to aquatic vegetation. In saltwater areas, the green crab spread in the early 20th century and the Japanese shore crab has become dominant in recent years. Habitats The Hudson has a diverse array of habitat types. Most of the river consists of deep water habitats, though its tidal wetlands of freshwater and salt marshes are among the most ecologically important. There is strong biological diversity, including intertidal vegetation like freshwater cattails and saltwater cordgrasses. Shallow coves and bays are often covered with submarine vegetation; shallower areas harbor diverse benthic fauna. Abundance of food varies over location and time, stemming from seasonal flows of nutrients. The Hudson's large volume of suspended sediments reduces light penetration in the area's water column, which reduces photoplankton photosynthesis and prevents sub-aquatic vegetation from growing beyond shallow depths. The oxygen-producing phytoplankton have also been inhibited by the relatively recent invasion of the zebra mussel species. The Hudson River estuary is the site of wetlands from New York City all the way up to Troy. It has one of the largest concentrations of freshwater wetlands in the Northeast. Even though the river can be considered brackish further south, 80 percent of the wetlands are outside the influence of the saltwater coming from the Atlantic Ocean. Currently, the river has about acres of wetlands, and rising sea levels due to climate change are expected to lead to an expansion of that area. Wetlands are expected to migrate upland as sea level (and thus the level of the river) rises. This is different from the rest of the world, where rising sea levels usually leads to a reduction in wetland areas. The expansion of the wetlands are expected to provide more habitat to the fish and birds of the region. Activities Parkland surrounds much of the Hudson River; prominent parks include Battery Park and Liberty State Park at the river's mouth, Riverside Park in Manhattan, Croton Point Park, Bear Mountain State Park, Storm King State Park and the Hudson Highlands, Moreau Lake State Park, and its source in the High Peaks Wilderness Area. The New Tappan Zee Bridge between Westchester and Rockland counties has a pedestrian and bicycling path covering a distance of about 3.6 miles. Another pedestrian and bike path exists further north,