Petri Porvari

The Effects of Forestry on Hg Bioaccumulation in Nemoral/Boreal Waters and Recommendations for Good Silvicultural Practice

Abstract Mercury (Hg) levels are alarmingly high in fish from lakes across Fennoscandia and northern North America. The few published studies on the ways in which silviculture practices influence this problem indicate that forest operations increase Hg in downstream aquatic ecosystems. From these studies, we estimate that between one-tenth and one-quarter of the Hg in the fish of high-latitude, managed forest landscapes can be attributed to harvesting. Forestry, however, did not create the elevated Hg levels in the soils, and waterborne Hg/MeHg concentrations downstream from harvested areas are similar to those from wetlands. Given the current understanding of the way in which silviculture impacts Hg cycling, most of the recommendations for good forest practice in Sweden appear to be appropriate for high-latitude regions, e.g., leaving riparian buffer zones, as well as reducing disturbance at stream crossings and in moist areas. The recommendation to restore wetlands and reduce drainage, however, will likely increase Hg/MeHg loadings to aquatic ecosystems.

Total and methyl mercury concentrations and fluxes from small boreal forest catchments in Finland.

Total mercury (TotHg) and methyl mercury (MeHg) concentrations were studied in runoff from eight small (0.02-1.3 km2) boreal forest catchments (mineral soil and peatland) during 1990-1995. Runoff waters were extremely humic (TOC 7-70 mg l-1). TotHg concentrations varied between 0.84 and 24 ng l-1 and MeHg between 0.03 and 3.8 ng l-1. TotHg fluxes from catchments ranged from 0.92 to 1.8 g km-2 a-1, and MeHg fluxes from 0.03 to 0.33 g km-2 a-1. TotHg concentrations and output fluxes measured in runoff water from small forest catchments in Finland were comparable with those measured in other boreal regions. By contrast, MeHg concentrations were generally higher. Estimates for MeHg output fluxes in this study were comparable at sites with forests and wetlands in Sweden and North America, but clearly higher than those measured at upland or agricultural sites in other studies. Peatland catchments released more MeHg than pure mineral soil or mineral soil catchments with minor area of peatland.

Development of fish mercury concentrations in Finnish reservoirs from 1979 to 1994

Mercury (Hg) concentrations in burbot (Lota lota), perch (Perca fluviatilis), northern pike (Esox lucius), roach (Rutilus rutilus), whitefish (Coregonus lavaretus) and peled (C. peled) were monitored in 18 reservoirs situated in western and northern Finland over a period of 16 years (1979-1994). The reservoirs were impounded from 1964 to 1980. The surface areas ranged between 1 and 417 km2. Data from downstream sites and reference lakes were compared to reservoir mercury data. Generally, fish Hg concentrations in reservoirs exceeded those in natural lakes. Shortly after inundation, fish mercury concentrations clearly increased and remain 15-25 years above background concentrations. The mean Hg concentrations in standardized 1 kg pike from 1989 to 1993 was 0.45 mg/kg in natural lakes and 0.58 mg/kg in reservoirs. The 1 mg/kg fish Hg level recommended by health authorities as the upper limit for human consumption was still exceeded in two reservoirs 20 years old or more. The two largest reservoirs (24 and 27 years old in 1994) with significant commercial fishing had Hg levels below 0.5 mg/kg in all fish species studied. Statistical models for predicting the Hg concentrations in the weight standardized pike were verified with the observed data. For most of the reservoirs, predictions from models based on pH, organic matter (COD) in water, the extent of water level regulation and reservoir age agreed well with observed data. In the most southern, new (under 20 years old) and heavily regulated reservoirs, pike Hg concentration followed a logarithmic function of reservoir age. Model predictions of mercury in pike in the planned Vuotos reservoir (in northern Finland) predicted that Hg would exceed 1 mg/kg Hg for the first 12 years after the flooding.

Climate induced thermocline change has an effect on the methyl mercury cycle in small boreal lakes.

We conducted a whole-lake experiment by manipulating the stratification pattern (thermocline depth) of a small polyhumic, boreal lake (Halsjärvi) in southern Finland and studying the impacts on lake mercury chemistry. The experimental lake was compared to a nearby reference site (Valkea-Kotinen Lake). During the first phase of the experiment the thermocline of Halsjärvi was lowered in order to simulate the estimated increase in wind speed and in total lake heat content (high-change climate scenario). The rate of methyl mercury (MeHg) production during summer stagnation (May-August) was calculated from water profiles before the treatment (2004), during treatment (2005, 2006) and after treatment (2007). We also calculated fluxes of MeHg from the epilimnion and from the hypolimnion to the sediments using sediment traps. Experimental mixing with a submerged propeller caused a 1.5-2 m deepening of the thermocline and oxycline. Methyl mercury production occurred mostly in the oxygen free layers in both lakes. In the experimental lake there was no net increase in MeHg during the experiment and following year; whereas the reference lake showed net production for all years. We conclude that the new exposed epilimnetic sediments caused by a lowering of the thermocline were a major sink for MeHg in the epilimnion. The results demonstrate that in-lake MeHg production can be manipulated in small lakes with anoxic hypolimnia during summer. The climate change induced changes in small boreal lakes most probably affect methyl mercury production and depend on the lake characteristics and stratification pattern. The results support the hypothesis that possible oxygen related changes caused by climate change are more important than possible temperature changes in small polyhumic lakes with regularly occurring oxygen deficiency in the hypolimnion.

Mobility and Methylation of Mercury in Forest Soils Development of an In-Situ Stable Isotope Tracer Technique and Initial Results

A method for investigating in-situmethylation and mobility of mercury has beendeveloped and applied to forest soil. The methodis based on additions of stable 199Hgisotopes to forest soil plots followed bysampling and analysis of isotope ratios of totalmercury and MeHg using ICP-MS. Initial resultsindicate that a fraction (30%) of the mercuryadded to the forest soils is immobilised over atime scale of 16 months. A small but significantfraction of the mercury is methylated during thesame time scale. Higher methylation rates werefound in wetland soil than in dry forest podzolsoil.

Concentrations of selected PCB congeners in pike (Esox lucius, L.) and arctic char (Salvelinus alpinus, L.) in Finland

PCBs were widely used in the 1970s in Finland. PCB usage was regulated the following decade and is presently strongly restricted. The reduction of PCB loading is generally observed as decreasing concentrations in environmental indicator species. The decreasing trend is seen clearly in the 25-year-long monitoring of northern pike in large lakes. The level of PCBs was approximately 5 to 15 μg g−1 (lipid weight) in the early 1970s, while present concentrations generally range from 0.5 to 3 μg g−1(l.w.). In recent years, increasing concern about the pollution of Arctic areas by airborne contaminants has led to an expansion of monitoring of persistent contaminants in northern areas. Arctic char (Salvelinus alpinus, L.) has been used as a circumpolar indicator species in the fresh water of the Arctic Monitoring and Assessment Programme. The level of PCBs in char was determined in two small subarctic lakes in northern Finland and were 0.1 and 0.5 μg g−1(l.w.). There are several possible reasons for the variation between lakes: population density; growth conditions; nutrient level, and catching time. The PCB congener pattern is very similar in the fish samples of all large lakes and one small subarctic lake in spite of wide geographical coverage. The dominant congeners in these samples are IUPAC nos., 153, 138, 180, 118, 101.