1. GKSS Research Centre Geesthacht, Institute for Coastal Research, Max-Planck Strasse 1, D-21502 Geesthacht, Germany, Institute of Ecology and Environmental Chemistry, University of Lueneburg, Scharmhorst Strasse 1, D-21335 Lueneburg, Germany, and Graduate School of Oceanography, University of Rhode Island, Narragansett, Rhode Island 02882-1197