The role of sulfate reduction in long term accumulation of organic and inorganic sulfur in lake*_sedimentsl

Sulfate reduction and the accumulation of reduced sulfur in epilimnetic sediments were studied in lakes in southern Norway, the Adirondack Mountains, and at the Experimental Lakes Area (ELA) of northwestern Ontario. In all of the lakes, in addition to the previously known formation of acid volatile sulfur, sulfate reduction also produced substantial quantities of pyrite and organic sulfur compounds. In g-month in situ experiments at ELA using 35S, there was a large loss (55%) with time of the S initially reduced and deposited in the sediments and a preferential loss of inorganic S compounds which led to a predominance of organic 35S accumulation in the sediments. An intensive study of long term accumulation of sulfur in the epilimnetic sediments of four Adirondack lakes also showed that the most important long term end product of sulfate reduction was organic S and that sulfate reduction was the major source of S to the sediments. Because of the high concentrations of iron in all of the sediments we sampled and because of the long term storage of sulfur in sediments, mostly as organic S, iron did not limit iron sulfide accumulation in these sediments. Iron limitation is unlikely to occur except in unusual circumstances. This study indicates that formation of organic S in epilimnetic sediments is primarily responsible for H+ consumption via sulfate reduction in acidified lakes. Elevated amounts of sulfate are entering lakes in regions where the acidity of atmospheric deposition has increased (e.g. Wright and Snekvik 1978; Galloway et al. 1983; Jeffries et al. 1984). This has stimulated research into sulfur cycling to improve our understanding of the fate of sulfate in lakes. A topic of special interest is the process of bacterial sulfate reduction, which consumes H+ in acidified lakes (Hongve 197 8) by producing reduced S compounds (Kelly et al. 1982). Sulfate is reduced by bacteria in both the epilimnetic and hypolimnetic sediments of lakes (Schindler et al. 1980; Kelly et al. 1982; Cook and Schindler 1983; Kelly and Rudd 1984), and rates of sulfate reduction in sediments can increase as sulfate concentrations in the overlying lake water increase (Cook and Schindler 1983; Kelly and Rudd 1984). ‘Funded by NSERC grant A2671 and by the Department of Fisheries and Oceans, Canada. Until recently, it was thought that the sole end product of sulfate reduction in lake sediments was acid volatile sulfur (AVS: 2 H2S) with later chemical conversion of FeS to FeS, (Berner 1984). This belief led to a hypothesis that the increased loading of sulfur to acidified lakes could result in the exhaustion of available reduced iron, which would in turn limit iron sulfide formation and thus consumption of H+ by sulfate reduction (Schindler 1985). During the past few years we have studied sulfate reduction and end product formation in several lakes including some which might be expected to have iron limitation. During the course of this research, in which we added 35S042to sediments, we could not account for all of the lost 35S042as [35S]AVS. This led us to explore the possibility that pyrite, which has been observed to form rapidly in salt marshes (Howarth and Merkel 1984), might be an additional important end product of sulfate reduction