RADIOISOTOPIC STUDY OF MERCURY UPTAKE 285 



DISCUSSION 



A variety of Hudson River forage organisms, ranging from algae 

 to fish larvae, accumulated mercury compounds from the test 

 environment. The concentrations in organisms ranged from 10^ to 

 10^ times that in the test medium. In all cases tested, uptake of 

 mercury occurred via food, as well as directly from water. The 

 greatest relative concentrations (10^) over the external medium 

 occurred among algae; the least (10-^) occurred among the larger 

 planktonic Crustacea (e.g., Gammarus) and striped bass larvae. 

 Mercury was transferred to striped bass lairvae from copepods labeled 

 with mercury. Thus mercury concentrations in forage organisms and 

 the conditions that affect their mercury uptake are relevant to 

 predictions about mercury cycling. However, the concept of bio- 

 magnification along a food chain is misleading in aquatic systems 

 where bioaccumulation occurs from water as well as food. 



Mercury uptake in Hudson River organisms was found to vary on 

 a seasonal basis within the same species or species group, showing a 

 strong positive correlation with temperature. MacLeod and Pessah 

 (1973), in their studies of Salmo gairdneri (rainbow trout), and Heit 

 and Fingerman (1977), in their studies of Procambarus clarki and 

 Faxonella clypeata (crayfishes), also found that mercury accumula- 

 tion was strongly dependent on temperature. This suggests that 

 several possible mechanisms are operating to cause a gradual increase 

 in mercury content. First, since poikilothermic organisms generally 

 show an increase in metabolic rate as temperature increases, the 

 increased bioaccumulation during warm months may be due simply 

 to a greater rate of uptake of mercury -contaminated food. Increased 

 metabolism should also result in more rapid rates of mercury 

 clearance, however, but this is not borne out by our results. 

 Nonetheless, the potential for bioaccumulation of mercury com- 

 pounds during warm months is greater than during cool months. 

 Field studies should be carried out in mercury -contaminated environ- 

 ments to verify this hypothesis and its several ramifications: (1) that 

 the concentration of mercury compounds per gram of organism 

 increases during warm periods, (2) that uptake and retention of 

 mercury is greater during the summer, and (3) that an increase in the 

 absolute quantity of mercury compounds available for direct uptake 

 and food-chain transfer occurs in warm environments. 



The form of mercury available to organisms affects uptake 

 rates and the environmental impact of mercury on aquatic systems. 

 Experiments with Gammarus showed a greater uptake of organic 

 over inorganic forms of mercury, but only after prolonged (1-week) 



