MERCURY CONCENTRATIONS IN LAKE SEDIMENTS – REVISITING THE PREDICTIVE POWER OF CATCHMENT MORPHOMETRY AND ORGANIC MATTER COMPOSITION MARTIN KAINZ 1,2,∗ and MARC LUCOTTE 1 1 Universit´ e du Qu´ ebec ` a Montr´ eal, Chaire de Recherche en Environnement (Hydro-Qu´ ebec/CRSNG/UQAM), C.P. 8888, Succursale Centre-Ville, Montr´ eal (PQ), H3C 3P8, Canada; 2 Department of Biology, University of Victoria, P.O. Box 3020, Stn. CSC, Victoria (British Columbia), V8W 3N5, Canada ( ∗ author for correspondence, e-mail: mkainz@uvic.ca) (Received 21 April 2005; accepted 9 September 2005) Abstract. Lake sediments are a potential source of mercury (Hg) for aquatic biota. Here, we inves- tigated the predictive power of (a) key parameters for lake catchment morphometry and (b) organic matter composition of sediments in an effort to account for observed variations of total (THg) and methyl (MeHg) mercury concentrations in lake sediments. Using regression models we demonstrate that the morphometric parameters lake depth as well as inclination of catchment soils and lake bot- toms can significantly predict variations of THg concentrations, but not MeHg, at profundal lake sediments. Although THg and MeHg concentrations in sediments could not be predicted by specific organic matter sources, as elucidated by atomic C/N ratios, our data suggest that wetland-derived to- tal organic carbon (TOC) contained less THg concentrations than TOC derived from mostly forested watersheds. However, TOC concentrations could significantly predict MeHg concentrations and the proportion of methylated Hg at all sediment stations. Finally, from an ecotoxicological point of view, we propose that concentrations of TOC at surface lake sediments, rather than parameters of catchment morphometry, may predict dietary sources of MeHg for benthic consumers and consequently perhaps for organisms at higher trophic levels. Keywords: mercury, methyl mercury, sediments, catchment morphometry, organic matter, organic carbon 1. Introduction Freshwater sediments are a major reservoir for natural and anthropogenically re- leased mercury (Hg; Lucotte et al., 1995; Munthe et al., 1995; Swain et al., 1992). Meili (1997) calculated that total Hg (THg) storage in the top three millimetres of lacustrine sediments could be equal to the total amount of Hg in the entire wa- ter column. Hg from upper sediment layers can be removed by bottom feeders (Tremblay et al., 1998; Downs, et al., 1998; Gagnon and Fisher, 1997) and conse- quently be conveyed to organisms at higher trophic levels of the aquatic food chain. Thus, it is important to understand those environmental parameters that lead to Hg accumulation in sediments. Water, Air, and Soil Pollution (2006) 170: 173–189 DOI: 10.1007/s11270-006-3009-z C  Springer 2006