Seasonal mercury concentrations in plankton-net material from a contaminated coastal lagoon (Ria de Aveiro, Portugal)

Seasonal mercury accumulation was studied in material collected with 63 and 200 mu m mesh size zooplankton nets. Results indicate that zooplankton abundance is not directly affected by mercury contamination, but rather by parameters such as salinity and temperature. Mercury levels reached as high as 25 mg kg(-1), corresponding to 10(7) times the dissolved mercury concentrations and suggesting a high affinity of zooplankton and associated lithogenic particles for this metal. Two seasonal enrichment moments were observed, one in the winter period which corresponded to an increase of suspended mercury-rich particles from the contaminated area due to increased water flow, and another in summer reflecting mercury bioaccumulation by zooplankton in this high-density period. Our results suggest this fraction (>200 mu m taxocenosis) could be very important in the overall mercury cycle in estuaries, since it can be a transporting agent to other areas of the system or to the open sea, and also could play a key role in mercury bio-amplification processes along estuarine food webs.