As we learn more about the processes, refinements were made in the initial experimental design presented elsewhere. However, no changes in research directions are anticipated and we expect to meet all original objectives. For mercury photoreduction we originally stated that we would elucidate the role of DOC in controlling the availability of Hg for photoreduction and oxidation of Hg in lake and stream water by conducting a spatial survey in south-central Ontario to determine the relationship between the photochemically reducible fraction of Hg in water and DOC. We also stated that we would use the stable isotope techniques developed by Hintelmann and Evans to determine rates of Hg conversion between species. Some preliminary work has been done on this topic as part of the METAALICUS initiative. In the second phase, we proposed a more detailed examination in a few lakes of the relationship between the photochemically reducible fraction and other fractions/species of Hg in water. We also stated that we would determine the effect of photodegradation of DOC on the photochemically reducible fraction. This information was to be integrated into a model on site and time specific DGM production and volatilization from lakes and tested against observations over the ice-free season in one or two network lakes. We have indeed conducted a series of experiments linking DOC fluorescence to Hg reduction in a number of aquatic systems, and will model Hg speciation using WHAM. For a number of sites we have related DGM to reactive Hg in an attempt to prove/disprove this of mentioned relationship. We have discovered that in addition to photochemical reduction of mercury to elemental mercury that microbial reduction and oxidation can also take place. Our next objective is to determine the relative contribution of microbe vs solar radiation. We are also conduction work on photobiological reduction of Hg, since data gathered by our team have shown a good correlation between photosynthetic activity and DGM levels in some systems. In addition to biological oxidation, we have also demonstrated the importance of Hg photooxidation and the factors influencing this process, with a particular emphasis on chloride ions. More recently, we have measured the first field action spectrum for this reaction. For methyl mercury photoreduction we would identify the most important wavelengths in the photodegradation process and develop a predictive relationship between solar radiation, underwater attenuation and photochemical degradation of methyl mercury. To do this work, lakes with as wide a spectrum of properties as possible were selected. Initially, the low pH lakes of Kegimkujik Park, NS were selected (and additional funds obtained from the Toxic Substances Research Initiative). At these low pH values degradation rates were rapid. At more neutral pH levels rates were not only much slower but also in some cases MeHg was actually produced. We have discovered that most of the MeHg found in lakes is not dissolved or bound to small particulate materials as thought but in fact bound to DOC. We have developed a fractionation scheme to examine the MeHg associated with particles above 0.2 um, between 300 kD and 0.2 um as well as that between 30 and 300, 5 and 30, 1 and 5 and less than 1 kD. We have demonstrated that pH is profoundly important for rapid photodegradation and we are currently investigating degradation rates (or lack thereof) of the colloidal fractions. They test as methyl mercury but they do not behave like methyl mercury nor are they taken up in the food chain like methyl mercury. This is a significant discovery and relates to other work being done on the St. Lawrence River and elsewhere. We should recall that only methyl mercury accumulates in food webs and it is by far the most toxic form. This observation may also explain why attempts to correlate fish mercury with methyl mercury have not been successful and why fish mercury is so tightly coupled with pH. We know that most methyl mercury enters lakes associated with DOC but the mechanism of how it is released and how it moves into the food web is less well known. Our experiments will focus on the role of UV radiation on the removal of methyl mercury from DOC. In addition to the stated objectives, we conducted two additional studies related to Hg and MeHg dynamics in aquatic systems. The first study deals with daily and monthly cycling of Hg(II) in the water column. We collected year-cycle detailed information about under-ice Hg cycling, in order to supplement the available information on this significant part of the year in Canada. We also looked closely at daily changes in Hg pools in the epilimnion in order to refine current models. We did find a fast change in Hg partitioning in the epilimnion and are currently investigating the causes. A second project dealt with the fate of MeHg during fish decomposition. Fish tissues constitute a major compartment for MeHg in aquatic systems. The fate of this MeHg at fish death is unknown. In this project, for which we obtained matching funds from DFO, we demonstrated that necrophageous invertebrates, and more particularly leeches, can recycle mercury from fish carcasses into lakes and, as a consequence, the recovery time of an aquatic system after a decrease in atmospheric deposition of mercury could depend on the structure of the necrophageous invertebrate community. During the coming year, our main objectives will be: 1) to assess the water-to-land transport of Hg in Pacific Salmon stream: (a) by following the fate of Hg transferred from carcasses to terrestrial insect larvae feeding on carcasses; (b) by assessing the contribution of bear feeding in water-to-land transfer; (c) by determining the Hg profiles in tree cores taken along a transect from the river to the land; 2) to provide a mass balance calculation for two contrasting systems (high vs low fish mortality) in order to evaluate the importance of carcasses as vectors of MeHg. We are developing models for both MeHg photodegradation and photoreduction/volatilization which by themselves will be a significant contribution. As part of this investigation we will provide new values for the partitioning of both total and MeHg between particulate materials (zooplankton, algae, bacteria and detritus as well as size fractions of DOC). Currently models assume that small particulate material is the largest reservoir of methyl mercury. It is not! Most is associated with DOC and little if any is truly free. This information will be integrated in to the overall models being developed by Reed Harris and we predict that it will profoundly change the way we view the overall transport of mercury in the food webs of lakes, rivers and wetlands.