AME 44:31-43 (2006)  -  doi:10.3354/ame044031

ABSTRACT: A series of shipboard experiments using the radiotracer ¹⁰⁹Cd investigated the role of phytoplankton and bacteria in the uptake of dissolved Cd in the English Channel and Celtic Sea. The results demonstrate that Cd uptake is related to rates of primary production and bacterial numbers. Statistical analysis of plankton species abundance infer that Rhizosolenia, Chaetoceros and Pseudo-nitzschia diatom species are largely responsible for the higher Cd uptake observed in the >5 µm size fraction during a diatom-dominated spring bloom. Total Cd uptake rates during winter non-bloom conditions were between 0.04 and 0.29 pmol l^–1 h^–1, and increased to between 0.43 and 1.23 pmol l^–1 h^–1 during diatom bloom conditions. These uptake rates are consistent with the seasonal surface depletion of Cd reported in the Celtic Sea and attributed to uptake by phytoplankton bloom material. A calculated Cd:C ratio of 3.1 µmol mol^–1 for natural plankton samples of the Celtic Sea agrees well with results of previous culture studies, which have reported ratios between 0.1 and 5.0 µmol mol^–1 for the coastal diatoms Thalassiosira weissflogii and T. pseudonana. Cd uptake was also linearly related to bacterial numbers, which was attributed to surface adsorption of Cd ions onto bacterial particles which have relatively high specific surface areas. These results demonstrate surface adsorption of Cd onto bacterial surfaces, and other biogenic non-living particles, i.e. ‘passive Cd uptake’, which is significantly augmented during a spring diatom bloom.

KEY WORDS: Cadmium uptake · Cadmium carbon ratios · Phytoplankton · Diatom blooms · Heterotrophic bacteria · Celtic Sea