lake transparency: a window into decadal variations …...lake transparency: a window into decadal...

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

• William Gawley

• Tom Huntington

• Reviewers

• Patricia Gilbert and Todd Kana

Acknowledgements

Outline

• Motivation

• Secchi Depth

• Building the Model

• Results

• Future Efforts

Motivation Secchi Depth Model Results Future

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

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/

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

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

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

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

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

Secchi Depth: Lake to Lake Variability

• Range 0.77 to 13 m

• Coefficient of Variation 3 to 25%

Motivation Secchi Depth Model Results Future

Secchi Depth: Seasonal Patterns

• Relatively little seasonality

• Lake to lake variations much larger

Motivation Secchi Depth Model Results Future

Lake code

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

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

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

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

√

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

• 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)

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

• 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

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

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

Thank you

http://frenchhillpond.org/Acadia/Long%20Pond.htm