Palaeoenvironmental applications - Lakes

Acidification

Natural acidification
Changes in land-use - decreas in pastures and acidic vegetation increases
- Draining peatlands frmo ditchs - increases load of humic substances into lakes
Acid rain from industrial sources
- SO2 and NOx emissions create sulfuric and nitric acids in the atmosphere, precipitated down and concentrates in lakes

Succession from alkaline to acidic conditions
During the early Holocene, lakes were more alkaline due to:
- Fresh moraines leaving lots of clay (base cations)
- High erosion potential (due to warming climate and less established vegetation) resulting in increased weathering of basic ions from the catchment area
Natural decrease of pH:
- in southern Finland is 0.01 - 0.03 pH / 100y
- Northern Finland 0.005 - 0.01 pH/100y
Reasons for natural acidification
- Acid bedrock
- impoverished soil
- organic acids from humic substrances (peatland vegetation)
- peatland and coniferous forest formation releases humic substances
Example from Nuuksio (Majaslampi in 1991):
- pH reconstructed from diatom fossils
- Very alkaline in Younger Dryas (10 kya)
- 8 kya it stablilizes as fairly acidic (pH 5) for the rest of its life
- This one was a natural pH change
Remote lakes in mountainous Norway have little recent pH change

Huge compared to the other sources
After industrialization we have pH decreasing by 1 (huge!)
In Finland - after WW2. England at the end of 19th c. N. America beginning of 20th c.
This is still happenning, although it gets less attention than it did in the 80s compared to climate change
Study from 1990s Sweden used qualitative method (relative frquency of diatoms from different groups) to estimate pH - not as good as modern transfer functinos, but it showed the trend of sudden pH transitions 2300 BP and 1900 AD

Montchegorsk copper/nickel smelting plant, very wastland around the factory
- Depended lots on the wind direction
Top-bottom approach
- Using a reference sample from pre-industrial times and modern samples
- Relative changes in numbr of taxa at different sites
- Increased or decreased pH? in most lakes its unchanged - seems to be correlated with wind direction because of mountains
- Some lakes as low as 3.1 pH with just a fw diatoms living there

No impact on these lakes pH from the Russian industrial activity in the area
However the buffer capacity for alkalinity is zero - this has been likely been reduced by activity

pH dropped to 5 - fish unable to reproduce
Baby Lake in 1980s had pH 4 - after building a taller smokestack it recovred nicely back to pH7
Chart shows both actual measured pH and diatom-inferred pH - diatoms don't show a pH so extremely low but the trends are the same in both

Addition of nutrients causing increased algae and other plant growth
Like throwing gasoline in the fire when combined with warming temperatures
- Can lose bottom fauna (ebnthic diatoms) and flora due to increased turbidity
- Decomposing microbes die
- increased sedimentation from dead algae
Natural process taking millenia, but by human-introduced sources it can happen in decades
Phosphorous from plants is deposited in sediment and slowly leaches back out (internal loading)
Can cause serious problems like cyanobacteria blooms creating nerve toxins

Jan's study showed 1800s had clearer water with slight eutrophication, then massive change in 1950s
at a presentation to the community a local described how he remembered massive ditching into the lake in the 1950s

Salmon bring tons of nutrients from the sea when they die in their spawning lakes
In a small lake can this massively eutrophy?
- Comparison in Alaska from nursery and control lakes
- Nitrogen levels much higher in salmon-bearing lakes
  - More productive salmon years correlated with higher nutrient level (diatom inferred)
- 1950s this trend ended with start of commercial fishing

Not uniform around the world - inland arctic areas are different than coastal, tropic, etc
Feedback mechanisms
- Decreasing albedo from melting snow and ice
- "greening" -> increase in sun energy absorbtion
- Increase permafrost melt -> increased methane emissions
- Higher air temp means higher capacity for moisture $\to$ increasing water vapor in atmosphere
Small lakes literally drying up or shrinking
- Sediment loss due to wind while dried up
- Permafrost melting affecting thermokarst lakes
- Decrease in littoral/marginal zone?
- Cladocera can be used as a proxy for water depth historically
  - Low water level also had lots of Equisetum macrofossils, which grows in shallow water, confirming this
Recent changes in diatom communities in small lakes (2005 - Smol et al. )
Dissolved Organic Carbon vs northern treeline of pine (Forest-Tundra Transition)
- In Northern Canada and Northern Fennoscandia we see the same trend - higher DOC below the treeline
Diatom inferred pH vs Air temp records in Austrian alps - clear correlation