The study area was located in the lower Hudson River (New York Harbor), 3 km north of the Battery and 14 km south of the George Washington Bridge. Two concrete municipal piers, Port Authority Pier A (213 x 100 m), located in New Jersey, andMarine and Aviation Pier 40 (351 x 255 m), located in Manhattan, were selected as the target study sites (Fig. 29.1). We used a comparative approach and established four representative habitats associated with piers, including underneath piers, pier edges, pile fields, and open water. Under-pier sites were established under the platform decks of piers, edge sites were established at the light-shade interface between the edge of the pier and the water beyond, open water areas were located immediately adjacent to the piers (20-40 m beyond the pier edges), and pile field sites were established in areas consisting of pilings (after the deck tops of the piers had been removed). Pile fields were situated approximately 300 m south of Pier 40 in New York and approximately 450 m south of Pier A in New Jersey.
The study area has been highly modified from its original, pre-European contours and as a result virtuallyno natural, shallow-water habitatremains. Concrete bulkheads predominate on both sides of the Hudson River and make the transition from the street to the water level abrupt; depths at the water line average 3-5 m (Duffy-Anderson, personal observation). In addition, much of the Harbor bottom is dredged so there is a marked vertical drop off toward the channel (from 3 m to 16 m). The lower Harbor is tidally flushed so that, like other estuaries with extensive freshwater input, the zone undergoes dramatic changes in salinity over a single tidal cycle, as much as 7-21%c (Duffy-Anderson, personal observation), though average ranges over one tidal cycle are approximately 5-10%e. In mid August, salinities can be as high as 28%e. Temperatures during the spring, summer, and fall sampling periods (May/June-September/October, 1993-99) generally ranged from 14-26 °C and levels of dissolved oxygen during the same period generally ranged from 3-8 mg l-1. Photic depths in the summer range from 3-6 m depending on sediment loading and phytoplankton growth (Stross and Sokol, 1989). Average light intensities in open water areas are considerably higher (10-50 |E-2 s-1, depths 2-5 m) than light levels underneath large piers with solid concrete tops (0-0.02 |E-2 s-1, depths 2-3 m) (Able et al., 1998; Duffy-Anderson and Able, 1999).
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