Mercury pollution in fish and invertebrates in natural and wildfire impacted lakes in Norway
Year: 2014
Type: dissertation
Abstract: Mercury (Hg) contamination in fish has become a global issue and remains a major problem of concern in the Nordic countries, with atmospheric long-range transport of Hg being the main source to aquatic ecosystems. Most Hg in fish is present as monomethylmercury (CH3Hg+, MeHg), a powerful neurotoxin that biomagnifies to harmful concentrations through the aquatic food web, and is mainly obtained from the diet. The main goal of this dissertation was to investigate the Hg content in fish from lakes with different environmental characteristics, including both natural and wildfire impacted lakes, and to discuss different factors affecting the Hg content in biota. The fieldwork of this thesis was conducted in several lakes: two relatively big lakes in Telemark county (Lake Norsjø and Lake Heddalsvatn) and in nine small boreal acidic lakes located in a wildfire impacted area in Mykland, southern Norway. Hg concentrations were investigated in various fish species as well as in macroinvertebrates. In addition, stable isotope analysis of carbon and nitrogen (δ13C and δ15N) were measured in fish and macroinvertebrates. The δ15N signatures were used for estimates of trophic position, while the δ13C signatures were used to estimate the relative contribution of food derived from littoral or pelagic habitats. For the wildfire study, water samples were collected monthly over four years and analyzed for the main water chemical parameters. Fish were also sampled during 2008, 2010 and 2012 to evaluate potential year differences on Hg, δ13C and δ15N. Food web transfer of total Hg (THg) was studied using trophic magnification slopes (TMS), i.e. slopes of the linear regression between log-transformed Hg and relative trophic position (stable nitrogen isotope, δ15N) in biota, and used as indicators of the biomagnifying potential of Hg in different aquatic ecosystems. The TMS values ranged from 0.16 to 0.25 and varied depending on the ecosystem characteristics and physico-chemical parameters such as temperature related processes, total organic carbon (TOC) or pH. These values indicated trophic magnification factors of Hg ranging from 3.6 to 7.1 per trophic level. Seasonal variations of Hg concentrations were investigated in Lake Heddalsvatn during December, May and September, in three fish species. Results showed that there were seasonal differences, but the causes varied depending on the fish species. Fish caught in December (European whitefish, Coregonus lavaretus; northern pike, Esox lucius) had the highest Hg concentrations, while the concentrations in May were higher than in September for all three species, statistically significant (p < 0.0001) for European perch (Perca fluviatilis) and close to significant for pike (p = 0.07). Somatic growth dilution was the most likely explanation for the perch population, whereas for whitefish and pike this mechanism could not be the explanatory factor as there was no measurable growth from May to September. We used the δ13C signatures of primary consumers from pelagic and littoral habitats in a simple two-source mixing model to estimate the relative dietary contribution in fish from food derived from the littoral or pelagic habitats in Lake Norsjø. Perch exhibited the highest zoobenthos reliance (89 ± 14%), while European smelt (Osmerus eperlanus) exhibited the lowest (37 ± 7%). The most intermediate δ13C signatures, i.e. supported by a mixed diet of littoral and pelagic food webs, were found in piscivore species with the highest trophic position, and the subsequent highest Hg concentrations both in Lake Norsjø and Lake Heddalsvatn. The changes in water chemistry in the wildfire affected area were studied for over 4 years (2008-2012) following the fire. There were large lake-to-lake variations and hydrological conditions such as heavy rain or snowmelt periods were the main reason for the most severe water chemical episodes. There was an initial drop in the acid neutralizing capacity (ANC) of the lakes two months after the fire, due to a faster mobilization of sulfate and chloride ions compared to base cations. An initial decrease in TOC and increase in nutrient runoff (nitrogen, N, and phosphorous, P) were observed in the initial post-fire period, followed by a gradual decrease in N and P, and increases in TOC. The water chemical conditions were almost re-established one year after the wildfire and had almost returned to pre-fire levels four and a half years after the fire. A water survey analysis was conducted in June 2012, four years after the fire, and the concentrations of THg and MeHg ranged between 1.17 - 2.63 ng L-1 and 0.053 - 0.188 ng L-1, respectively. Both variables were strongly and positively correlated to TOC and TOC-related variables such as color or UV absorbance, and with total-P and total iron. MeHg was also positively correlated with total-N and chlorophyll-a. There was no conclusive evidence that the fire affected the Hg concentrations in fish, but generally higher levels were found two years after the fire. Increased nutrients and chlorophyll-a concentration in the lakes might be key factors to explain this increment. Natural factors as yearto- year variations in thermocline depth or suboxic status in lakes make it difficult to draw any strong conclusions about direct wildfire effects on Hg in the biota from the investigated lakes.
Authors: Moreno Vicente, Clara Enedina
Language: English
Subfield: Health, Toxicology and Mutagenesis
Field: Environmental Science
Domain: Physical Sciences
Open Access status: green
Funder: Norges Forskningsråd
OpenAlex ID: W2252468369