Cadmium enriched stable isotope uptake and addition experiments with natural phytoplankton assemblages in the Costa Rica Upwelling Dome

• Cd uptake was greatest in the upper 40 m inside the dome and decreased with depth.
• Cd uptake increased with time and trended positively with chl a concentrations.
• Cd additions (nM) reduced chl a outside-but not inside-the dome relative to controls.
• Both in and outside the dome, Cd enters the microbial loop in the upper water column.
• Higher bulk biomass and toxicity thresholds in the dome allay upwelled Cd toxicity.

Cadmium (Cd) can function as either a nutrient or toxin in the marine environment. This duality has been demonstrated in phytoplankton cultures where Cd has been shown to have toxic effects to cyanobacteria, but acts as a nutrient in the marine diatom Thalassiosira weissflogii by biochemically replacing zinc (Zn). In July of 2005, Cd bioavailability and uptake in the Costa Rica Upwelling Dome in the eastern Pacific Ocean were examined using Cd addition and enriched stable isotope uptake experiments. This dome is known to support particularly high densities of the cyanobacterium, Synechococcus. Bottle incubation experiments with Cd additions ranging from 0.5 to 5 nM resulted in reduced chlorophyll a outside and at the edge of the dome relative to control treatments, but showed no reduction in chlorophyll a inside the dome. Total dissolved Cd showed depletion of Cd in the surface waters and increased concentrations with depth. 110Cd stable isotope tracer uptake experiments were conducted at stations inside and outside the dome, in which variations with depth and time were examined. Cd uptake was greatest within the upper 40 m of water inside the dome, decreased with depth, and increased with time. Uptake trended positively with chlorophyll a concentrations. Together, these experiments demonstrate Cd uptake into the microbial loop in the upper water column both inside and outside of the dome, but show that Cd toxicity was not induced within the dome. This greater Cd tolerance within the Costa Rica Dome relative to oligotrophic waters was likely due to a combination of higher quantities of biomass, resultant greater ligand production inside the dome, metallothionein production by Synechococcus, and different toxicity thresholds and coping mechanisms of the microbial communities.