Lake transparency: a window into decadal variations in dissolved organic carbon concentrations in Lakes of Acadia National Park, Maine

Transcription

1 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

2 Acknowledgements William Gawley Tom Huntington Reviewers Patricia Gilbert and Todd Kana

3 Outline Motivation Secchi Depth Building the Model Results Future Efforts

4 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

5 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?

6 Acid Rain and DOC in NE Watersheds Dry years + SO 4 2- anomaly - DOC anomaly Wet years - SO 4 2- anomaly + DOC anomaly Deviations related to %wetland coverage Twenty year record of lake DOC is too short for climate analysis

7 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?

8 What is Secchi Depth? A measure of water transparency Low tech Independent of operator 150 yr (Angelo Secchi, 1865)

9 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

10 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?

11 Secchi Depth: Lake to Lake Variability Range 0.77 to 13 m Coefficient of Variation 3 to 25%

12 Secchi Depth: Seasonal Patterns Relatively little seasonality Lake to lake variations much larger Lake code

13 Secchi Depth: Interannual Variability Cyclic pattern ~30 years Range of annual means is comparable to seasonal range Some lakes exhibit no interannual variations

14 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, Z s, occurs from 11-22% light level Range kz s = 0.17 to 0.22 So we need k

15 The attenuation coefficient, k (m -1 ) Absorption and scattering Light that travels at larger angles travels a longer distance per depth Described mathematically k = a μ Where μ is the cosine of the average angle Dominated by solar angle Range μ = 0.7 to 0.9 So we need a

16 What constituents dominate absorption? Possibilities Expected Patterns Water Phytoplankton Other particles Dissolved organic matter (DOM) Constant Strong seasonal cycle Scatter rather than absorb light varies with watershed cover/use/hydrology

17 Colored Dissolved Organic Matter absorption (CDOM) Strongly absorbs in UV, decays exponentially to red Described analytically as a CDOM λ = a λ ref exp ( S CDOM (λ λ ref )) Range S CDOM to So we need a(λ ref )

18 a CDOM (254) (m -1 ) Colored Dissolved Organic Matter absorption (CDOM) The CDOM absorption in the UV (254 nm) is significantly related to DOC in Maine Rivers SUVA = a CDOM (254)/DOC (m -1 (mg/l) -1 ) Range SUVA 2.5 to 7.1

19 Model relating Secchi depth to DOC is Z S = Ψ μ /(SUVA DOC e S CDOM ) 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 S CDOM translates CDOM absorption from UV to visible

20 Using mean parameter values Observations Z S = Ψ μ /(SUVA DOC e S CDOM ) Where Ψ = 2.0 (14%) μ = 0.8 SUVA = 2.85 S CDOM = Model fit

21 Now invert the equation to solve for DOC, using same parameter values DOC = Ψ μ /(Z S SUVA e S CDOM ) So we can use the historical observations of Z S to estimate DOC

22 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)

23 Thank you