CADMIUM AND MERCURY IN SEDIMENT AND BURROWING MAYFLY NYMPHS (HEXAGENIA) IN THE UPPER MISSISSIPPI RIVER, USA

Longitudinal patterns in the cadmium and mercury content of burrowing mayflies (Hexagenia) and surrounding sediments were examined along a 5 72-km reach of the upper Mississippi River. Surficial sediments and He xagenia nymphs were sampled in 1989 at 12 sites extending from Pool 2 through Pool 16 and analyzed for total recoverable cadmium and total m ercury. In sediment and nymphs, concentrations of both metals were hig hest in Pools 2, 3, and 4, which are just downstream from the Twin Cit ies (Minneapolis-St. Paul, Minnesota) metropolitan area, the primary a nthropogenic source of metals to the studied reach of river. Concentra tions of the two metals in sediments indicated a significant anthropog enic contribution, which was most pronounced for cadmium. The cadmium concentrations in surficial sediment varied less than 3-fold (range, 1 .19-3.23 mu g/g dry weight) among the 12 sites, whereas concentrations in Hexagenia varied almost 20-fold (range, 0.13-2.35 mu g/g dry weigh t). Nymphs from Pools 2-4 had much greater concentrations of cadmium t han nymphs from sites further downstream, even though the mean concent ration of cadmium in sediment from Pools 2-4 (3.0 mu g/g) was just twi ce that(1.6 mu g/g) for the nine sites downstream. Mercury in sediment s from the 12 sites ranged from 0.038 to 0.165 mu g/g dry weight, aver aging 0.14 mu g/g in Pools 2-4 and 0.056 mu g/g in the nine sites down stream. In nymphs, mercury concentrations ranged from 0.041 to 0.134 m u g/g dry weight. The bioavailability of sediment-associated cadmium s eemed greater in Pools 2, 3, and 4 than in the sites further downstrea m, based on the relative cadmium concentrations in Hexagenia nymphs an d sediment. Moreover, it is concluded that the trapping of sediment an d associated metals in Lake Pepin (a natural riverine lake in Pool 4) significantly reduces the exposure of the ecosystem further downstream to metals from the Twin Cities and other upstream anthropogenic sourc es.