lake transparency: a window into decadal variations …...lake transparency: a window into decadal...
TRANSCRIPT
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Lake transparency: a window into decadal variations in dissolved
organic carbon concentrations in Lakes of Acadia National Park, Maine
Collin Roesler
Department of Earth and Oceanographic Science, Bowdoin College
Charles Culbertson
New England Water Science Center, USGS, Augusta
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• William Gawley
• Tom Huntington
• Reviewers
• Patricia Gilbert and Todd Kana
Acknowledgements
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Outline
• Motivation
• Secchi Depth
• Building the Model
• Results
• Future Efforts
Motivation Secchi Depth Model Results Future
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Motivation • Lake properties provide a spatially and temporally
integrated record of • watershed characteristics (landscape coverage, land use,
hydrologic processes)
• Anthropogenic activities • development/recovery
• Local pollution of land and water
• Remote pollution such as acid rain
• Which vary on a variety of temporal scales • seasonal cycles
• episodic events
• climate forcing
Motivation Secchi Depth Model Results Future
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Motivation • Observations suggest that dissolved organic matter
(DOM) is increasing in surface waters (brownification) • Threatening water supplies
• Increasing carbon flux to the atmosphere
• Increasing organic carbon flux to the oceans
• Overarching Questions • How will a warming climates change the mobilization of
organic carbon from soils to aquatic systems?
• How has the reduction in sulfur emissions (e.g. acid rain) changed the mobilization of organic carbon from soils?
Motivation Secchi Depth Model Results Future
http://domqua.no/tag/brownification/
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Acid Rain and DOC in NE Watersheds • 1990-2010
• Dry years • + SO4
2- anomaly
• - DOC anomaly
• Wet years • - SO4
2- anomaly
• + DOC anomaly
• Deviations related to %wetland coverage
Twenty year record of lake DOC is too short for climate analysis
Motivation Secchi Depth Model Results Future
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We do have a much longer records of Secchi Depth in this region
Can Secchi Depth provide a useful long term proxy for biogeochemical
properties in MDI lakes?
Motivation Secchi Depth Model Results Future
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What is Secchi Depth?
• A measure of water transparency
• Low tech
• Independent of operator
• 150 yr (Angelo Secchi, 1865)
http://www.paddling.net/
http://earthobservatory.nasa.gov/Features/WaterQuality/water_quality2.php
Motivation Secchi Depth Model Results Future
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Secchi Depth Observations
• In the open ocean phytoplankton are the major drivers of variability in Secchi Depth
• Boyce et al (2010) present a global analysis of a century of Secchi Depth Observations to investigate the trends in ocean primary productivity
Motivation Secchi Depth Model Results Future
http://www.obs-vlfr.fr/Boussole/html/images/images.php
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Secchi Depth Observations
• In the open ocean phytoplankton are the major drivers of variability in Secchi Depth
• Maine Lakes are brown due to high concentrations of dissolved organic matter. • Is DOM the major driver of lake Secchi Depth variations?
• Is there a robust optical proxy between brownness and DOC?
Motivation Secchi Depth Model Results Future
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Secchi Depth: Lake to Lake Variability
• Range 0.77 to 13 m
• Coefficient of Variation 3 to 25%
Motivation Secchi Depth Model Results Future
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Secchi Depth: Seasonal Patterns
• Relatively little seasonality
• Lake to lake variations much larger
Motivation Secchi Depth Model Results Future
Lake code
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Secchi Depth: Interannual Variability
• Cyclic pattern
• ~30 years
• Range of annual means is comparable to seasonal range
• Some lakes exhibit no interannual variations
Motivation Secchi Depth Model Results Future
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What drives variations in Secchi Depth?
• Light decreases exponentially with depth according to:
E(z)= E(0) exp(-kz)
Where k is the attenuation coefficient (m-1)
• The Secchi depth, Zs, occurs from 11-22% light level
• Range kZs = 0.17 to 0.22
• So we need k
Motivation Secchi Depth Model Results Future
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The attenuation coefficient, k (m-1)
• Absorption and scattering
• Light that travels at larger angles travels a longer distance per depth
• Described mathematically
𝑘 = 𝑎𝜇
• Where 𝜇 is the cosine of the average angle
• Dominated by solar angle
• Range 𝝁 = 0.7 to 0.9
• So we need 𝑎
Motivation Secchi Depth Model Results Future
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What constituents dominate absorption? • Possibilities
• Water
• Phytoplankton
• Other particles
• Dissolved organic matter (DOM)
• Expected Patterns
• Constant
• Strong seasonal cycle
• Scatter rather than absorb light
• varies with watershed cover/use/hydrology
Motivation Secchi Depth Model Results Future
√
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Colored Dissolved Organic Matter absorption (CDOM)
• Strongly absorbs in UV, decays exponentially to red
• Described analytically as 𝑎𝐶𝐷𝑂𝑀 𝜆 = 𝑎 𝜆𝑟𝑒𝑓 ∗ exp (−𝑆𝐶𝐷𝑂𝑀 ∗ (𝜆 − 𝜆𝑟𝑒𝑓))
• Range SCDOM 0.013 to 0.018
• So we need 𝑎(𝜆𝑟𝑒𝑓)
Motivation Secchi Depth Model Results Future
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• The CDOM absorption in the UV (254 nm) is significantly related to DOC in Maine Rivers
• SUVA = aCDOM(254)/DOC (m-1 (mg/l) -1)
• Range SUVA 2.5 to 7.1
Colored Dissolved Organic Matter absorption (CDOM)
Motivation Secchi Depth Model Results Future
aC
DO
M(2
54
) (m
-1)
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Model relating Secchi depth to DOC is
• 𝑍𝑆 = Ψ ∗ 𝜇 /(𝑆𝑈𝑉𝐴 ∗ 𝐷𝑂𝐶 ∗ 𝑒−𝑆𝐶𝐷𝑂𝑀 500−254 )
• Where
Ψ describes the light level at the Secchi depth
𝜇 describes the incident solar angle below the interface
𝑆𝑈𝑉𝐴 is the ratio of the UV absorption to [DOC]
𝑒−𝑆𝐶𝐷𝑂𝑀 500−254 translates CDOM absorption from UV to visible
Motivation Secchi Depth Model Results Future
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• Observations
• 𝑍𝑆 = Ψ ∗ 𝜇 /(𝑆𝑈𝑉𝐴 ∗ 𝐷𝑂𝐶 ∗ 𝑒−𝑆𝐶𝐷𝑂𝑀 500−254 )
• Where Ψ = 2.0 (14%) 𝜇 = 0.8 𝑆𝑈𝑉𝐴 = 2.85 𝑆𝐶𝐷𝑂𝑀= 0.145
• Model fit
Using mean parameter values
Motivation Secchi Depth Model Results Future
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Now invert the equation to solve for 𝐷𝑂𝐶 , using same parameter values
• 𝐷𝑂𝐶 = Ψ ∗ 𝜇 /(𝑍𝑆 ∗ 𝑆𝑈𝑉𝐴 ∗ 𝑒−𝑆𝐶𝐷𝑂𝑀 500−254 )
• So we can use the historical observations of 𝑍𝑆 to estimate 𝐷𝑂𝐶
Motivation Secchi Depth Model Results Future
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Implications
• Moving the “DOC” record back to the 1970s provides capability for examining longer term anthropogenic and climate-scale forces
• Relating DOC to Secchi Depth provides capability for detecting DOC from Satellite (e.g. LandSat)
Motivation Secchi Depth Model Results Future
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Thank you
http://frenchhillpond.org/Acadia/Long%20Pond.htm