1. Methyl mercury accumulation in boreal periphyton biofilms in relation to watershed disturbances and climate
Dolors Planas, Mélanie Desrosiers, Fabien Cremona, Stéphanie Hamelin & Marc Amyot

2. Context
Many lakes in the world have still advisory on human fish consumption, because of elevated concentrations of mercury (Hg).
In NE of United States and Canada Hg emissions (and other atmospheric pollutants, e. g. SO4) are regulated since the 1980’s.

3. Context
Toward the end of the last century, a downward trend in methyl mercury concentrations [MHg] in water and fish was reported; consistent with the atmospheric decrease in wet Hg concentrations [Hg] and acidifying pollutants

4. Context The question is WHY?
The trend seems to reverse in this millennium at least in several lakes in NE of North America (NA) (Brigham et al. 2014, ES&T: 48: )
The question is WHY?

5. Potential explanatory variables have been suggested:
This millennium increases
Potential explanatory variables have been suggested:
Climate change
The increase in precipitation that positively affects runoff and stream discharge.
The increase in temperature, the last decade has been the hottest in 160 years (IPCC Synthesis Report, 2014).
The alteration of the Arctic jet stream (more frequent and focal air masses, vortex) that brings higher [Hg] in NA.

6. Potential explanatory variables have been suggested:
This millennium increases
Potential explanatory variables have been suggested:
Climate change
The increase in precipitation that positively affects runoff and stream discharge.
The increase in temperature, the last decade has been the hottest in 160 years (IPCC Synthesis Report, 2014).
The alteration of the Arctic jet stream (more frequent and focal air masses, vortex) that brings higher [Hg] in NA
Increase of Hg emissions from Asia or Oceania since 1996 (Streets et al. 2009, ES&T 43: 2983-)
Alteration of land cover, such as forest harvest.

7. Potential explanatory variables
Consequence of the climate changes in
aquatic ecosystems,
Increases in precipitation:
Higher loading of: DOC-THg, H+, Ca, SO4, nutrients
2. Increases in temperature:
Longer ice-free season
Summer heat waves

8. “In lake” processes Increases in lake nutrients,
Potential explanatory variables
“In lake” processes
Increases in lake nutrients,
Higher Productivity
Increases in temperature
High Hg methylation rates and MeHg accumulation
Increases in littoral food web length,
consequently,
amplify biomagnification in the upper levels up to
top predators.

9. Objectives The aim of this study is to test some of new hypotheses, on
the causes of recent MeHg increases in freshwater,
Climate change (temperature, precipitations),
watershed perturbations (forest harvesting & wild fire)
The data analyses are from two studies done in a uniform
geological region, rather pristine, by the same team:
Using as a proxy the littoral communities at the base of
the food webs:
THE PERIPHYTON BIOFILMS

10. Periphyton biofilms Grow in the littoral of lakes and rivers
Biofilms is made of autotrophic (algae, photochimio bacteria), heterotrophic organisms (bacteria, fungi …) and micro and macro fauna.
Display strong spatial (in depth) and temporal (diurnal) redox gradient.
First receptors of runoff and subsurface water.
Absorb, process and accumulate the solutes, including contaminants..

11. Periphyton biofilms Grow in the littoral of lakes and rivers;
Biofilms of autotrophic (algae, photochimio bacteria), heterotrophic organisms (bacteria, fungus …) and micro and macro fauna;
Display strong spatial (in depth) and temporal (diurnal) redox gradient;
First receptors of runoff and subsurface water
Absorb, process and accumulate the solutes, including contaminants
It is already known that periphyton reacts faster than phytoplankton to watershed disturbances and significantly contribute to higher productivity.

12. Periphyton biofilms
Are important hotpots for Hg methylation (Desrosiers et al. 2006, ES&T. 40, ; Hamelin et al. ES&T Sci. Technol. 2011, 45, 7693–)
Accumulate MeHg (Hamelin et al Environ Poll. 197: 221-)
Supports complex food webs (Cremona et al. 2010, 647: 51-)
Fish communities depend mostly on benthic resources in small oligotrophic lakes (e.g., in Canadian. boreal regions, (Bertolo et al., 2005, Oikos 111: 543-)
in Canadian Shield lakes, published studies on the TP–chlorophyll a relationships shows that the
phytoplankton biomass per unit of TP is relatively low Planas et al 2000). This could explain how planktivory is low in some of the boreal lakes.

13. Study area
The data came from two studies covering a large area, ~30,000 km2 of the boreal Canadian Shield;
In a relatively pristine area, where the main perturbation is wildfire, forest logging and the beaver dams.
The 39 lakes used in the analysis are located in three distinct watersheds: .

14. Study site : 39 Boreal Shield Lakes (Quebec)
49°00’N
47°00’N
48°00’N
50°00’N
76°00’W
74°00’W
73°00’W
75°00’W
77°00’W
Hannah & Rupert Bay
Outaouais & Montreal
North-West St. Lawrence
Saguenay - Lac Saint-Jean
39 %
Quebec province with a total area of 1,667,000 km² has > 20% of the surface area covered with water. The highest numbers of lakes in Canada standing water (3,6 millions) & > 4,500 rivers. 62 % of the water drains towards the Hudson, James and Ungava Bays and 36% to the Saint Lawrence & 2% Fundy Bay  500 km and for a total of 355,315 km² of land is covered with water.

15. 3 set of lakes
Reference
Perturbations
Harvest
Burnt

16. Climatic oscillations: Study over 3 years: 2000, cooler and dry
Mean annual precipitation (mm)
Mean annual temperature (oC)

17. Lakes were relatively similar (watershed/lake morphometry, and lake chemistry) Max – Min (mean)
Units
Reference
Harvested
Burnt
Lakes 16 18 5 
Slope %
(11)
(8)
(12)
Wetland area
(1.8)
(4.9)
(1.1)
Perturbation area
0 – 0 (0)
(21.7)
50.1 – 100 (87)
Lake area
km2
(0.4)
(0.6)
(0.4)
DOC
mg/L
(5.6)
(7.4)
(7.8) pH
(5.8)
(6.4)
(6.2)

18. Chemical and biological variables Max-Min (mean)
PERIPHYTON
Chemical and biological variables Max-Min (mean)
Units
Reference ●
Harvest
Burnt
Chl-a
mg/m²
(3)
9 – 97 (45)
22 – 38 (29) OM %
(58)
(48)
(58)
MeHg
ng/g
(26)
(23)
(36)
Compared to reference lakes, perturbed lakes had > [TP] (2-3 x); [Chl a] 15x and 9x higher in harvested and burnt, respectively. Maximum MeHg > 2x in harvested lakes.and burnt lakes < than reference.

19. Methods
Statistical analysis
Multiple linear regression analysis with stepwise (forward /backward) variables selection technique with all pertinent explanatory variables.
Simple regression analysis  was used, e.g, to verify the importance of summer temperatures that may related to increases of net MeHg production on periphyton.

20. General model The explanatory variables used in the model comprise:
morphometric watershed and lakes parameters
mean annual and seasonal (open/ice cover) temperature
and precipitation;
watershed intensity of perturbation
(0 for reference lakes (green), < 10% (blue pale) and
>10% (navy blue) of WS/LA ratio harvest ,
and burnt (between 50 and 100% (red) burnt
% OM, an indicator of the biofilm alive and detritus mass

21. The significant variables that explain more than 60% of [MeHg] in periphyton biofilm were, % of organic matter in the biofilm, total winter precipitation, harvest area/lake area and altitude.
Adj. r² = ; p <
y = -1.02e *Pred. MeHg
0 % Perturbated
< 10 % Harvest
> 10 % Harvest
> 50 % Burnt
Organic matter (%)
r2 part = ; p =
Total winter precipitations (TWP) (mm)
r2 part = ; p =
Harvested Area/Lake Area
r2 part = ; p =
Altitude (m)
r2 part = ; p =

22. Model: Summer temperature as a predictive variable
Longer open season and higher summer temperatures influence net methylation rates. For a given amount of Hg loading to aquatic ecosystems, methylation could be enhanced.
A simple regression model was used to verify the relationship between summer temperature and mercury accumulation on periphyton biofilms.

23. No significant relationship was found between [MeHg] /Mean summer temperature (MST)
0 % Perturbated
< 10 % Harvest
> 10 % Harvest
> 50 % Burnt
The relationship was not significant but is we look closers to lakes characteristics, for a given temperature, the higher perturbed lakes had between 5 to 18 x higher Hg concentration.
Equation
ln (MeHg ng/g) = -13, , *ln (MST (°C))
Adjusted R²
0,0035; p =

24. Mean summer temperature (MST) predicts the [MeHg] in reference and low perturbed lakes.
0 % Perturbated
< 10 % Harvest
< 50 % burnt
ln (MeHg ng/g) = -85, ,466856*ln (MST(°C))
R² ; p <

25. Harvest perturbations
Take home
The main drivers of Hg accumulation in the periphyton biofilms in boreal Canadien Shield lakes are:
Winter Precipitation
Summer Temperature
and
Harvest perturbations

26. Thank you Grazie mille!