Grand BeachLake Winnipeg

Eutrophication processes in Alberta lakes

Alexander P. WolfeDepartment of Earth & Atmospheric SciencesUniversity of Alberta, Edmonton<awolfe@ualberta.ca>

Grand BeachLake Winnipeg

Eutrophication processes in Alberta lakes

• A general model for prairie lakes• Coupling of multiple elemental cycles• Coupling of inorganic and biological processes• An over-arching context involving climate/hydrological changes• Dramatic consequences for surface water quality

EUTROPHICATION:The state of lakes under nutrient enrichment

EUTROPHICATION

Very common in Alberta and across the prairies; Typically accompanied by:

• algal blooms• high chlorophyll• reduced biodiversity• anoxia• fish kills• esthetics and

property valuesAlberta SRD

20 µg/L

The faces of eutrophic lakes

P added

control

A key role for phosphorus (P)

Experimental Lakes Area, Ontario, 1970’s, 80’sD.W. Schindler

P concentrations >20 µg/L engender eutrophication

culprits: urban and agricultural runoff, septic failures, golf courses, etc.

Drives prim

ary production

Chemical consequences: bio-inorganic bridging• [P] drives algal production• Photosynthesis depletes CO2(aq.); pH rises

∆pH↑ = 2

CO2 + H2O CH2O + O2

What goes around comes around

• When algae die and settle on sediments, respiration of organic matter consumes dissolved O2, produces CO2, and pH drops as H2CO3 is produced:

CH2O + O2 CO2 + H2OCO2 + H2O H2CO3

Pipit Lake, Alberta

Why is this important ?

• The delicate balance between oxidizing and reducing conditions (REDOX) ultimately determines the range of chemical reactions possible in lakes

• In many Alberta lakes, the cycling of IRON (Fe) and SULFUR (P) can become critical in locking up (sequestering) or releasing (diffusing) PHOSPHORUS (P) stored in sediments.

• Internal P sources differ from external sources, and present management challenges

But, haven’t Alberta prairie lakes always been eutrophic ?

• Historical and fishery records

• Paleolimnology

Paleolimnology: recent increases in chlorophyll and cyanobacterial pigments (Lac la Nonne, Nakamun Lake)

1970

1990

Paleolimnology: recent increases in small eutrophic-indicator diatoms (Touchwood Lake)

1975

Stephanodiscus minutulus Microcystis aeruginosa

Have these lakes always been eutrophic?

• No. Primary production has increased, in a way that is entirely consistent with P enrichment

• Previously undocumented fish kills have occurred; pike and walleye fisheries have collapsed

• Microcystin toxicity has been reported (>10 µg/L)

• Eutrophication has not been synchronous in all lakes, with onsets ranging from ~1950 to the most recent years

• Key observation: even wilderness lakes show these trends, in absence of leaky septic tanks, gold courses, livestock, and fertilized crops in their catchments

What alternate sources of P exist ?

Internal loading, P recycling

• Under the right conditions, P archived in sediments can be returned to the water column by diffusion, leading to eutrophication

• Sediment P sources include organic matter, as well as mineral phases such as apatite (Ca-phosphate) and vivianite (Fe phosphate)

• A perfect biogeochemical storm ?

A small amount of chemistry

Living cells

CO2 + H2O → CH2O + O2 algal photosynthesis

Death of cells: chemical changes in the hypolimnion

CH2O + O2 → CO2 + H2O aerobic respirationCO2 + H2O → H2CO3 carbonic acid productionH2CO3→ H+ + HCO3

- protonation, pH drops

Linkage to P cycle: hydroxyl-apatite dissolution

Ca5(PO4)3(OH) + 10H+ → 5Ca2+ + 3H3PO4 + H2OH3PO4→ 3H+ + PO4

3- phosphate generation

A medium amount of chemistry

Death of cells

CH2O + O2 → CO2 + H2O aerobic respiration

As anoxic conditions evolve: (1) iron becomes much more soluble; (2) sulfide is produced by bacterial sulfate reduction; (3) Fe sulfides, including pyrite, rapidly form:

Fe2+ + nH2S → FeSn + 2n H+ iron scavenging by sulfide

In absence of available reactive Fe, production of the most stable (insoluble) sedimentary P forms is curtailed: loss of the insoluble Fe phosphate minerals vivianite (Fe3(PO4)2) and/or strengite (FePO4)

Vivianite Fe3(PO4)2

Narrow Lake• high Fe due to local geology• Fe3(PO4)2 is present in sediments• This is, not coincidentally, the lowest P lake in our regional survey, retaining reasonable water quality

Narrow Lake sediment XRD

How do we know all this ?

• Pore-water chemistry

How do we know all this ?

• Pore-water chemistry

How do we know all this ?

• Pore-water chemistry

Below N/P ~ 23, microcystin becomes abundant

1. Some degree of P enrichment from any source

2. Initial blooms of Cyanobacteria stimulated by P

3. Increased oxygen demand in the lake as the bloom dies

4. Sediments become strongly reduced by organic matter respiration

5. Climate warming maintains longer and more stable stratification

6. In Alberta, lakes have low Fe but high Ca, due to geology. Phosphate (PO4

3-) geochemistry involves hydroxyl-apatite: Ca5(PO4)3(OH) instead of vivianite Fe3(PO4)2 or strengite FePO4, as seen elsewhere (i.e., Canadian Shield lakes); sulfide also scavenges Fe effectively

7. Ca5(PO4)3(OH) solubility is highly pH-dependent

8. Organic matter decomposition lowers pore-water pH via carbonic acid production until hydroxyl-apatite becomes soluble (pH ~5); PO4

3- is then released. A P and H+ pump!

9. More P is added to the system from sediments: positive feedback

Fairly conspicuous…

Lac la Biche, Lesser Slave LakeSylvan Lake, Lake Isle, Wabamun, St. Anne, Cooking,Beaverhill, Lac la NonneLake Winnipeg, Winnipegosis

Beaverhill Lake

A synergy with climate change?

• Self-perpetuating cyanobaterial blooms

• Lowering water levels, longer lake-water residence times (less flushing), warmer water temperatures that enhance summer thermal stratification (less mixing)

• More anoxia• More internal P loading• The cycle continues

Mayatan Lake

Mayatan Lake

Mayatan Lake

23 m

7 m2 m

Surface TP ~ 20-35 µg/LEast basin peak 2012: TP = 110 µg/LPeak microcystin 2012: ~10 µg/L

Mayatan Lake: additional concerns ?

Mayatan Lake: additional concerns ?

“The power plants (3 total) near Wabamun also release a number of substances into the air, including 5882 tonnes of sulphur dioxide, 4255 tonnes of nitrogen oxides, 296 tonnes of total particulate matter and 41 tonnes of volatile organic compounds”

Current SoW Report

Bierhuzen and Prepas (1985)

Mayatan Lake west basin characteristics

Bierhuzen and Prepas (1985)

Mayatan Lake P budget

Internally-driven P repeats and even amplifies the process of lake eutrophication, even in absence of additional catchment-scale nutrient subsidies

Management solutions ?

A bleak future for cold water fisheries and recreational amenities

Climate change makes it worse: longer lake-water residence times under drought conditions

Some final thoughts: SCIENCE

• The biogeochemistry of prairie lakes is not simple

• Constant vigilance needs to be paid to the interplay between physically- and biologically-mediated geochemical phenomena, and the coupling of elemental cycles (here: P, C, Fe, and S)

• Constant vigilance needs to be paid to changing redox and pH conditions

• A holistic view does not consider biotic and abiotic chemical processes as distinct from each other, but rather as forming a continuum

Some final thoughts: PEOPLE

• Monitor your lake closely: smells, sights, and sounds; fish and birds; algae; vegetation; lake ice

• Talk and educate; compare adjacent lakes

• Minimize impacts and conservation: limit development; fisheries; agricultural point sources; motorized craft

• Identify and be vigilant as to resource development

• Fe and aeration are potential “hands-on” remediation strategies that are being investigated

Can. J. Fisheries and Aquatic Sciences 2012

Toxic levels of the potent hepatotoxin microcystin occur when N/P molar ratios decline below ~23

e – acceptorsREDUCTION

e – donorsOXIDATION

OXIC

ANOXIC

Redox chemistry in lakes

• Important sub-cycles that govern the behavior of: C, N, P, S, Fe, Mn …

• Generally involves reactions among both inorganic and organic phases

• Collectively referred to as coupled biogeochemical cycles

Kalff 2002: Limnology: the Science of Inland Waters

mean molar ratios: 106 16 1 Arthur Redfield1890-1983

1958 The biological control of chemical factors in the environment. American Scientist 46: 205-222

mean molar ratios: 106 16 1

Relatively modest increases of P can alleviate P limitation and enhance primary production