METHYLATED ARSENIC IN THE SOUTHERN NORTH-SEA

Water samples collected in the southern North Sea in August 1988 (mid- summer), April 1989 (spring), September/October 1989 (late summer) and May 1990 were analysed for dissolved inorganic arsenic, monomethylars enic (MMA) and dimethylarsenic (DMA). In mid-summer 1988, both MMA and DMA were observed throughout the southern North Sea, with lowest conc entrations of dissolved inorganic arsenic (mean 6.48 nmol l(-1)) and t he highest proportions of methylated arsenic (29%) being found in high ly productive continental coastal waters. In April 1989, waters of the North Sea had a mean inorganic arsenic concentration of 12 nmol l(-1) and methylated species were not detected, even though phytoplankton b looms were present. Shipboard phytoplankton incubation studies (in May 1990) revealed that uptake of dissolved inorganic arsenic occurred at a rate of 0.57 nmol l(-1) day(-1), but the appearance of dissolved me thylated species was not observed. During September/October 1989, whil e MMA and DMA were present in all sectors of the North Sea, the relati ve proportion of methylated compounds (11%) in continental coastal wat ers was less than mid-summer 1988. It was shown that estuarine, porewa ter and atmospheric inputs of arsenic species were relatively small du ring the observational periods, and that almost all of the methylated compounds originated from decaying algal tissue. Demethylation of DMA and MMA throughout winter contributed to the dissolved inorganic arsen ic pool. The results are discussed in the context of the development o f a predictive model for the cycling of arsenic in the North Sea. (C) 1996 Academic Press Limited