ACCUMULATION OF MERCURY IN ESTUARINE FOOD-CHAINS

To understand the accumulation of inorganic mercury and methylmercury at the base of the estuarine food chain, phytoplankton (Thalassiosira weissflogii) uptake and mercury speciation experiments were conducted. Complexation of methylmercury as methylmercury-bisulfide decreased th e phytoplankton uptake rate while the uptake rate of the methylmercury -cysteine and -thiourea complexes increased with increasing complexati on by these ligands. Furthermore, our results indicated that while dif ferent ligands influenced inorganic mercury/methylmercury uptake by ph ytoplankton cells, the ligand complex had no major influence on either where the mercury was sequestered within the phytoplankton cell nor t he assimilation efficiency of the mercury by copepods. The assimilatio n efficiency of inorganic mercury/methylmercury by copepods and amphip ods feeding on algal cells was compared and both organisms assimilated methylmercury much more efficiently; the relative assimilation effici ency of methylmercury to inorganic mercury was 2.0 for copepods and 2. 8 for amphipods. The relative assimilation is somewhat concentration d ependent as experiments showed that as exposure concentration increase d, a greater percentage of methylmercury was found in the cytoplasm of phytoplankton cells, resulting in a higher concentration in the copep ods feeding on these cells. Additionally, food quality influenced assi milation by invertebrates. During decay of a T. weissflogii culture, w hich served as food for the invertebrates, copepods were increasingly less able to assimilate the methylmercury from the food, while even at advanced stages of decay, amphipods were able to assimilate mercury f rom their food to a high degree. Finally, fish feeding on copepods ass imilated methylmercury more efficiently than inorganic mercury owing t o the larger fraction of methylmercury found in the soft tissues of th e copepods.