Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 Impact on Upper Midwest Regional Carbon Balance Carbon Cycling in Lake Superior Ankur R Desai 1, Galen A McKinley 1, Noel Urban 2, Chin Wu 3 With support from: Nazan Atilla 1, Nobuaki Kimura 1, Val Bennington 1, and Marek Uliasz 4 Funding from NSF Carbon-Water 1 Dept of Atmospheric & Oceanic Sciences, University of Wisconsin-Madison 2 Dept of Civil and Environmental Engineering, Michigan Technological University 3 Dept of Civil and Environmental Engineering, University of Wisconsin-Madison 4 Dept of Atmospheric Sciences, Colorado State University AGU Fall Meeting 2007 B41F-03 December 13, 2007

Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 Why talk about Lake Superior?

Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 Global view Not so important at short time scales on global scale But: Great Lakes have been globally important long term sinks of carbon in sediments On short term, many lakes in general are sources of carbon (recycling of terrestrial input) Important freshwater source (Great Lakes = >20% of world’s non-frozen freshwater) More ocean-like than lake-like in physical and biogeochemical processes

Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 Regional view Small catchment, low productivity, large area A regional annual net flux of same order as terrestrial carbon sink/source? Strongly influences regional tracer concentrations –Not likely to affect flux tower footprints Role of water bodies and wetlands not well studied in observing, modeling, and predicting regional carbon exchange Lakes are indicators of long-term regional climate change and carbon cycling

Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 Regional view: CHEAS-y lake

Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 Regional view: Rel. contribution to WLEF tracer source area Land: 85.4% Lake Superior: 9.5% Lake Michigan: 1.8% Other water: 3.1%

Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 Regional view: Land-air flux Net regional land flux likely a small sink

Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 Climate change view Significant changes in temperature and precipitation expected in upper Midwest over next 100 years Recent activity to quantify climate change effects on regional land carbon cycle as part of NACP and MCI efforts Limited work on Lake Superior Trends in ice cover, lake temperature and lake levels have been noted

Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 Climate change view: Ice cover Declining trends in mean ice cover

Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 Climate Change view: Temperature Water temperature trend tracking air temperature

Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 What do we know about Lake Superior’s carbon cycle?

Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 Bottom-up scaling: Observations Atmos. flux is ~3 Tg yr -1 = gC m -2 yr -1

Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 Bottom-up scaling: Issues DOC flux too low to support net demand Urban et al (2005) JGR Implies fast pool: DOC residence time 8 years –vs. hydrologic residence time of 170 years Inputs Tg Erosion0.02 River Precip Photosynthesis Outputs Tg Outflow0.1 Resp Burial0.45

Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 Top-down scaling: Observations [CO 2 ] Air flowing over lake > [CO 2 ] over land

Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 Top-down scaling: Potential Potential exists for constraining flux with regional observations of CO 2

Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 Top-down scaling: Issues pCO 2 is supersaturated with respect to atmosphere in obs made in Apr and Aug ‘07 –Some individual measurements are below in summer, suggesting drawdown by algae –More obs needed over seasonal cycle Simple boundary layer budget tracer study suggests summer 2007 efflux: 4-14 gC m -2 d -1 –Analysis requires modeling of stable marine boundary layer –Much larger than traditional air-sea pCO 2 exchange calculation –Requires significant respiration in water column

Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 Upshot On annual and decadal timescales, Lake Superior is possibly a source of CO 2 to the atmosphere This source could be on the order of magnitude as the terrestrial regional flux Regional carbon budgets have to take lakes into account What’s missing: Full biogeochemical accounting/modeling to understand and predict variability in Lake Superior carbon cycle

Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 The Way Forward: Modeling Lake Superior

Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 An oceanic lake CyCLeS: Cycling of Carbon in Lake Superior Adapt the MIT-GCM ocean model to simulate physical and biogeochemical environment of Lake Superior –10km and 2km resolution models Physical model of temperature, circulation –Mostly implemented Biogeochemical model of trace nutrients and air-sea exchange –In progress

Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 Computation domain: 2km

Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 Progress: Thermal forcing Compares well to AVHRR SST MAY JUNE JULY AUGUST SEPTEMBER OCTOBER

Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 Progress: Circulation Beletsky et al 1999 Our model

Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 Challenges: Vertical mixing Sharp gradients at thermocline difficult to capture Obs. 10km 2km Depth [m]

Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 Challenges: Thermal bars Thermal bars typically develop in spring near shallow coastal areas Both a modeling challenge (resolution) and of interest for biogeochemical cycling

Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 Challenges: Variability Lots of interannual biogeochemical variability –e.g., Annual avg. dissolved organic carbon (DOC)

Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 Challenges: Variability Spatial, too.

Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 Challenges: Forcing/observations Many observations are sparse

Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 Conclusion In some respects, the Great Lakes are harder to model than ocean basin! –Many challenges remain, biogeochemical modeling in progress –Multiple top-down & bottom-up constraints needed Unlike small lakes, where terrestrial input dominates, in Lake Superior, internal processes dominate interannual variability in CO 2 fluxes Great Lakes are significant players in regional carbon budgets and have potential to offset land carbon uptake Regional climate changes likely to significantly affect land & water carbon cycles in Midwest

Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 CyCLeS Project (NSF)

Ankur R Desai, UW-Madison AGU Fall 2007 B41F observation June, 1973 observation by Niebauer et al 1977 (Chen et al 2001) Thermo-bar

Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03

Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03

Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03

Ankur R Desai, UW-Madison AGU Fall 2007 B41F

Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03