Modeling the export of DOC from large watersheds and its influence on the optical properties of coastal waters C.W. Hunt 1, W.M. Wollheim 2,3, J.S. Salisbury 1, R.J. Stewart 3, K.W. Hanley 4 and G.R. Aiken 4 ASLO Session SS54, New Orleans LA February 19, 2013

Study Motivations  River transport of DOC is a major component of global C cycle  River-borne DOC also influences the reactivity and optical properties of inland and coastal ocean aquatic systems  Recent studies* indicate that wetland abundance within small and large catchments is correlated with DOC quantity and quality at the catchment mouth *Hanley et al in review, Buffam et al. 2007

DOC Quality- SUVA 254 Little light passes through sample Aromatic DOC absorbs strongly UV light at 254 nm Image by K.W. Hanley

Study Approach  Couple a dynamic hydrological model (FrAMES, 6min) to a process-based DOC quantity/quality model using parameters found in literature.  Simulate DOC loading as a function of land cover and runoff conditions. Partition DOC quality into Hydrophobic Organic Acids (HPOA, aromatic) and non-HPOA stocks. The %HPOA can be used to derive SUVA 254.  Test model in 17 USA watersheds with processing (Respiration and photo-oxidation) turned on and off Butman et al. 2012

Chapter 1: Large Rivers

Organic Layer Mineral Layer Forest Wetlands Weakly UV-absorbing, DOC-depleted Strongly UV-absorbing, DOC-enriched How do wetlands affect DOC quantity and quality?

HPOA Non- HPOA River DOC Photo Respiration GPP* HPOA Non- HPOA Local Input HPOA Non- HPOA Downstream exports HPOA Non- HPOA Upstream inputs

St Mary's River

Conclusions and Future Work  The model shows promise for predicting bulk DOC loading and export at the catchment mouth  Adding two compartments (HPOA and nHPOA) is helpful, but…  There is information we are not capturing in DOC quality (SUVA)

Amazon River Study From Salisbury et al. 2011

Model Parameter Values- after Monte Carlo ParameterOriginalMonte Carlo test DOC vs. Wetland% Intercept at low runoff (mg/l)2.36 DOC vs. Wetland% Slope at low runoff DOC vs. Runoff asymptote at high flow (mg/l) HPOA% vs. Wetland%.0084*%wet Photo degredation constant (m-1) Resp degredation constant (at Tref=30, m/d) Resp degredation constant Q102.0

Monte Carlo Results

References Butman, D., Raymond, P.A., Butler, K. and G. Aiken Relationships between Δ14C and the molecular quality of dissolved organic carbon in rivers draining to the coast from the conterminous United States. Global Biogeochemical Cycles, 26: GB4014. Hanley, K.W., Wolheim, W.M., Salisbury, J., Huntington, T., and G. Aiken Controls on dissolved organic carbon quantity and quality in large North American rivers. Global Biogeochemical Cycles, in review. Raymond, P.A. and J.E. Saiers Event controlled DOC export from forested watersheds. Biogeochemistry dio /s Salisbury, J., Vandemark, D., Campbell, J., Hunt, C.W., Wisser, D., Reul, N., and B. Chapron Spatial and temporal coherence between Amazon River discharge, salinity, and light absorption by colored organic carbon in western tropical Atlantic surface waters. J. Geophys. Res. 116: COOHO2.

Methods The fraction of DOC as hydrophobic organic acids (HPOA%) was determined according to Hanley et a. 2012: HPOA% = ((1.19 * log10(wetlands%)) ) / Finally, the specific ultraviolet absorbance of DOC at 254 nm, an indicator of DOC aromaticity, was estimated: SUVA-254=(HPOA% * 8.792)

*from Hanley et al in press

Processing and DOC Quality DOC (HPOA) DOC (non- HPOA) Upstream DOC (HPOA) Upstream DOC (non- HPOA) Photo degradation GPP* Resp

Water Balance Model (WBM) Vorosmarty et al (Appendix B) FrAMES Water Transport Model (WTM, STN) Vorosmarty et al Other functions* “Vertical” movement of water (precip, ET, etc.) Wollheim et al Wisser et al Stewart et al “Horizontal” movement of water (river network routing using STN or Simulated Topological Network) Nitrogen, Reservoirs, Transient Storage * These are often embedded within WBM, WTM Grid Cell

Snowmelt ET Rooting Zone Shallow Groundwater Detention Pool Snowpac k Recharge Precipitation WBM Grid Cell

ParameterValue or formulaSource DOC vs. Wetland% Intercept at low runoff (mg/l)2.36Raymond and Saiers 2010 DOC vs. Wetland% Slope at low runoff0.374 Composite of Eckhardt and Moore 1990, Buffam et al 2007, and Raymond and Hopkinson 2003 DOC vs. Runoff asymptote at high flow (mg/l)6.7Raymond and Saiers 2010, at peak flow of 8 mm/d HPOA% vs. Wetland%.0084*%wet Wollheim (Ipswich-Parker river data) Photo degredation constant (m-1)0.001Literature composite Resp degredation constant (at Tref=30, m/d)1.47Literature composite Resp degredation constant Q102.0Literature composite Model Parameter Values

Soil Organic Matter Riverine DOC Sources of DOC in Rivers Image by K.W. Hanley

Without wetlands, DOC removal and fractionation can occur in the subsurface… Organic Horizon Mineral Horizon DOC added to new groundwater 1. DOC transported out of organic horizon 2. Preferential sorption of humic and fulvic acids to mineral soils and extensive microbial processing 3. Weakly UV-absorbing, DOC-depleted groundwater enters stream 4.

With wetlands, DOC depletion and fractionation are less likely… Organic Horizon Mineral Horizon DOC added to new groundwater 1. Subsurface flow through deep and often anaerobic organic horizon - little sorption or microbial processing 2. Strongly UV-absorbing, DOC-rich groundwater enters stream 3.