Cross-shelf Surface Water Distributions of Total Dissolvable Trace Metals (Cd, Cu, Fe, Ni, Zn) in the Middle Atlantic Bight During Spring and Summer 1992.

Cross-shelf distributions of total dissolvable (< 0.4 μm) trace metals were determined during March and July 1992 from the mouth of Delaware Bay to the Sargasso Sea. The region is characterized by a complicated seasonally varying hydrography, with several water masses (bay water, shelf water, slope water, Gulf Stream and Sargasso Sea water), a strong shelfbreak front, southwesterly surface flows along the coast and shelfbreak, and the northeastward flow of the Gulf Stream. Surface water (5-10 m depth) trace metal concentrations decreased with distance from shore and with increasing salinity. The highest concentrations were found at the mouth of the Delaware estuary, at salinities < 32 psu. In March, concentrations of Cu, Fe, Ni, and Zn at the bay mouth ranged from ~ 8 nM for Cu to ~ 25 nM for Zn, in keeping with previously reported values for the seawater endmember of the estuary. The Cd concentration at the bay mouth was ~ 0.7 nM. Concentrations in the shelf waters were ~ 50% lower than at the bay mouth and decreased with distance from shore. Plotting the data versus salinity showed a clear delineation between the shelf water and the slope/Gulf Stream water further offshore. Metal concentrations during the summer (July) showed a similar distribution, but concentrations were lower than in March, reflecting reduced inputs and/or greater biological removal during the summer. Several shelf/slope stations to the south of the transect had similar surface water metal concentrations. Vertical profiles of metals at stations in the Sargasso Sea will also be presented.