Supplementary Materials:
S1. Brief Description of Quantitative Mixing Models and Mass Balance Calculations (Text S1)
Conservative mixing curves were calculated for δ13C-DIC, Δ14C-DIC, δ13C-DOC, and Δ14C-DOC based on a simple two end-member mixing equation [Spiker, 1980; Raymond and Bauer, 2001]:
Is = Fr Ir [X]r + Fm Im [X]m (Equation 1)
[X]s
where Is, Ir and Im are isotope values corresponding to an intermediate salinity, the riverine end-member, and the marine end-member, respectively. Fr and Fm represent the riverine and marine fraction which can be calculated based on salinity (Fr + Fm = 1); and [X]s, [X]r, and [X]m are the concentrations of DIC or DOC at a given salinity, the river end-member, and the marine end-member, respectively. [X]s can be calculated as a function of Fr or Fm. Conservative mixing lines were also generated for the concentration of DIC and DOC by fitting a linear trend between the freshwater and marine end-member concentrations for each study area.
In order to establish first-order estimates of the relative contributions of the major presumed sources of DIC, DOC, and POC to riverine, estuarine, and marine surface waters, a three-source isotopic mixing model similar to those of Fry and Sherr [1984],Kwak and Zedler [1997],and McCallister et al. [2004] was used. The generalized mixing mass balance equation is:
XSAMPLE = f1XSOURCE1 + f2XSOURCE2 + (1 - f1 - f2)XSOURCE3(Equation 2)
where XSAMPLE is the isotopic composition (δC or ∆14C) of DIC, DOC, or POC from the river, estuary, and marine samples, and XSOURCE1, XSOURCE2, and XSOURCE3 are the isotopic compositions (δC or ∆14C) for each of the identified potential sources. The values f1, f2, and f3 are the relative contribution of each of the three potential sources to the total DIC, DOC, POC in each system, and f1 + f2 + f3 = 1.0. Since there are two unknowns (f1 and f2) in equation 2, the linear equations for δC and ∆14C of each carbon pool (DIC, DOC, or POC) must be solved simultaneously.
Appropriate isotope values for end-members were chosen from the literature or results of this study (Table S3, also see main text). To test the sensitivity of the three-source model to the choice of isotopic values for each end member the model was iteratively run using the average, lower, and upper δ13C and ∆14C for each source published end-member values listed in Table S3. , Although a greater number of end-member sources is possible in the complex natural landscape, a three-source mixing model was selected as a reasonable approximation of potential sources to the two SMRs, estuaries, and coasts studied here[Fry and Sherr, 1984; Kwak and Zedler, 1997; McCallister et al., 2004], Given the wide range in uncertainties in assigning isotopic values for three sources, and determining the relative percent contribution of each source (see Table 2 in the main text), the use of a mixing model that includes more end-members isnot easily justified.
References
Bauer JE (2002) Carbon isotopic composition of DOM. In: Hansell DA,CarlsonCA (Eds.). Biogeochemistry of Marine Dissolved Organic Matter. Elsevier. Pp. 403-451
Fry B,Sherr E (1984) δ13C measurements as indicators of carbon flow in marine and freshwater ecosystems. Contrib. Mar. Sci. 27:15-47
Hoefs J (2004) Stable Isotope Geochemistry, 5th Ed. Springer, Berlin. 244 pp.
Kwak TJ, Zedler JB (1997) Food web analysis of southern California coastal wetlands using multiple stable isotopes. Oecologia 110:262-277
Lynch-Stieglitz, J, Stocker TF, Broecker WS (1995) The influence of air-sea exchange on the isotopic composition of oceanic carbon: Observations and modeling. Global Biogeochem. Cycles 9:652-665
Marín-Spiotta E, Silver WL, Swanston CW, Ostertag R (2009) Soil organic matter dynamics during 80 years of reforestation of tropical pastures. Global Change Biol. 15:1584-1597
McCallister SL, Bauer JE, Cherrier JE, Ducklow HW (2004) Assessing sources and ages of organic matter supporting river and estuarine bacterial production: A multiple-isotope (Δ14C, δ13C, and δ15N) approach. Limnology and Oceanography 49:1687-1702
Mook WG, Bommerson JC, Staverman WH (1974) Carbon isotope fractionation between dissolved bicarbonate and gaseous carbon dioxide. Earth Planet. Sc. Lett. 22:169-176
Raymond PA,Bauer JE(2001) Use of 14C and 13C natural abundances for evaluating riverine, estuarine, and coastal DOC and POC sources and cycling: a review and synthesis.Org. Geochem. 32:469-48
Raymond PA, Hopkinson CS (2003) Ecosystem modulation of dissolved carbon age in a temperate marsh-dominated estuary.Ecosystems 6:694-705
Siegenthaler U, Sarmiento JL (1993) Atmospheric carbon dioxide and the ocean.Nature 365:119-125
Spiker EC (1980) The behavior of 14C and 13C in estuarine water: Effects of in situ CO2 production and atmospheric exchange. Radiocarbon 22:647-654
Telmer K, Veizer J (1999) Carbon fluxes, pCO2 and substrate weathering in a large northern river basin, Canada: Carbon isotope perspectives. Chem. Geol. 159:61-86
von Fischer JC, Tieszen LL (1995) Carbon isotope characterization of vegetation and soil organic matter in Subtropical forests in Luquillo, Puerto Rico. Biotropica 27:138-148
Table S1. Physical characteristics of the Rio Loco and Rio Fajardo Study Areas.
Physical Characteristics / Rio Loco Study Area / Rio Fajardo Study AreaCatchment area (km2) / 175 / 70
Maximum elevation (m) / 786 / 1075
Average slope (m m-1) / 0.029 / 0.045a
Mean annual precipitation (mm yr-1) / 860 / 4500
Mean annual discharge (m3 s-1) / 0.985b / 1.926
Dominant land-use categoryc / Active cropland/
pasture (60%) / Abandoned lands (Reforested) (46%)
aPike et al. 2010
bRio Loco mean annual discharge estimated from 2000-2004 when a working stream gauge was present.
cUSGS 2001
Table S2. Names, sampling dates, and geographic locations of sampling sites in the Rio Loco and Rio Fajardo study areas.
Study Area / Site Name / Sampling Date / Longitude (°N) / Latitude (°W)Rio Loco / Presado Loco / 21 March 2008 / 18.0446 / 66.8849
Rio Loco Upriver / 21 March 2008 / 18.0359 / 66.8874
LajasCanal / 21 March 2008 / 18.0091 / 66.9728
Lajas Drainage Ditch / 21 March 2008 / 18.0091 / 66.9728
RioLocoBridge / 26 Sept. 2004 / 17.9745 / 66.9144
1 March 2005
22 Oct. 2007
7 March 2008
Rio Loco Mouth / 23 Oct. 2007 / 17.9707 / 66.9221
8 March 2008
Bahia Noroeste / 27 Sept. 2004 / 17.9675 / 66.9230
1 March 2005
23 Oct. 2007
8 March 2008
GuanicaBay Mouth / 27 Sept. 2004 / 17.9549 / 66.9082
1 March 2005
23 Oct. 2007
8 March 2008
Cayo Coral / 27 Sept. 2004 / 17.9397 / 66.8921
1 March 2005
23 Oct. 2007
8 March 2008
Rio Fajardo / Mt. Britton Trail / 22 March 2008 / 18.2986 / 65.7915
Fajardo Upriver / 20 March 2008 / 18.3279 / 65.6265
FajardoBridge / 6 Oct. 2004 / 18.3219 / 65.6503
12 March 2005
24 Oct. 2007
19 March 2008
Fajardo Mouth FW / 6 Oct. 2004 / 18.3281 / 65.6278
12 March 2005
24 Oct. 2007
19 March 2008
Fajardo Mouth SW / 24 Oct. 2007 / 18.3275 / 65.6251
19 March 2008
FajardoBay / 7 Oct. 2004 / 18.3270 / 65.6219
12 March 2005
24 Oct. 2007
19 March 2008
Cayo Ahogado / 7 Oct. 2004 / 18.3245 / 65.6181
12 March 2005
24 Oct. 2007
19 March 2008
Table S3. Results of a fully factorial model III analysis of variance (ANOVA) testing for significant differences between study areas (Rio Loco vs Rio Fajardo catchments), seasons (wet vs dry), and salinity (freshwater vs marine) in dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), particulate organic carbon (POC) concentrations, and stable (δ13C) and radiocarbon (Δ14C) values. df = degrees of freedom. SS = sum of squares. F = calculated F statistic. p = probability. Sources of variability are significant when p ≤ 0.05.
Source / df / SS / F / p / Source / df / SS / F / p / Source / df / SS / F / pDIC / DOC / POC
Model / 7 / 2.01E7 / 13.01 / <0.001 / Model / 7 / 1.25E5 / 4.88 / 0.002 / Model / 7 / 4.97E6 / 1.44 / 0.238
Area / 1 / 5.75E6 / 26.06 / <0.001 / Area / 1 / 3.49E4 / 9.52 / 0.005 / Area / 1 / 1.21E6 / 2.47 / 0.130
Season / 1 / 3.65E6 / 16.53 / <0.001 / Season / 1 / 1018.24 / 0.28 / 0.603 / Season / 1 / 8.76E4 / 0.18 / 0.678
Salinity / 1 / 5582.77 / 0.03 / 0.875 / Salinity / 1 / 5.14E4 / 14.05 / 0.001 / Salinity / 1 / 8.86E5 / 1.80 / 0.193
Area*Season / 1 / 5.72E6 / 25.89 / <0.001 / Area*Season / 1 / 1.70E4 / 4.65 / 0.042 / Area*Season / 1 / 2135.58 / 0.00 / 0.948
Area*Salinity / 1 / 8.16E6 / 36.93 / <0.001 / Area*Salinity / 1 / 1.59E4 / 4.33 / 0.049 / Area*Salinity / 1 / 3.09E5 / 0.63 / 0.436
Season*Salinity / 1 / 2.22E6 / 10.06 / 0.004 / Season*Salinity / 1 / 3315.56 / 0.90 / 0.352 / Season*Salinity / 1 / 1.89E6 / 3.85 / 0.062
Area*Season*Salinity / 1 / 3.46E6 / 15.67 / <0.001 / Area*Season*Salinity / 1 / 2.69E4 / 7.35 / 0.013 / Area*Season*Salinity / 1 / 4.54E5 / 0.92 / 0.347
δ13C-DIC / δ13C-DOC / δ13C-POC
Model / 7 / 602.05 / 28.10 / <0.001 / Model / 7 / 37.42 / 1.33 / 0.282 / Model / 7 / 269.13 / 8.26 / <0.001
Area / 1 / 7.74 / 2.53 / 0.126 / Area / 1 / 0.81 / 0.20 / 0.657 / Area / 1 / 0.68 / 0.15 / 0.706
Season / 1 / 7.06 / 2.31 / 0.143 / Season / 1 / 7.69 / 1.92 / 0.180 / Season / 1 / 53.89 / 11.58 / 0.003
Salinity / 1 / 578.83 / 189.15 / <0.001 / Salinity / 1 / 11.44 / 2.85 / 0.106 / Salinity / 1 / 207.57 / 44.58 / <0.001
Area*Season / 1 / 1.92 / 0.63 / 0.436 / Area*Season / 1 / 1.63 / 0.41 / 0.530 / Area*Season / 1 / 0.38 / 0.08 / 0.778
Area*Salinity / 1 / 2.64 / 0.86 / 0.363 / Area*Salinity / 1 / 1.84 / 0.46 / 0.505 / Area*Salinity / 1 / 20.27 / 4.35 / 0.049
Season*Salinity / 1 / 2.93 / 0.96 / 0.339 / Season*Salinity / 1 / 5.96 / 1.48 / 0.236 / Season*Salinity / 1 / 32.43 / 6.97 / 0.015
Area*Season*Salinity / 1 / 0.06 / 0.02 / 0.891 / Area*Season*Salinity / 1 / 2.01 / 0.50 / 0.487 / Area*Season*Salinity / 1 / 3.23 / 0.69 / 0.414
Δ14C-DIC / Δ14C-DOC / Δ14C-POC
Model / 7 / 7800.32 / 12.10 / <0.001 / Model / 7 / 8.49E4 / 2.37 / 0.091 / Model / 7 / 2.12E4 / 1.08 / 0.444
Area / 1 / 485.19 / 5.27 / 0.032 / Area / 1 / 0.19 / 0.00 / 0.995 / Area / 1 / 1322.10 / 0.41 / 0.542
Season / 1 / 134.43 / 1.46 / 0.239 / Season / 1 / 7951.97 / 1.55 / 0.237 / Season / 1 / 1.07E4 / 3.30 / 0.107
Salinity / 1 / 6413.53 / 69.64 / <0.001 / Salinity / 1 / 7.01E4 / 13.67 / 0.003 / Salinity / 1 / 68.40 / 0.02 / 0.888
Area*Season / 1 / 925.98 / 10.05 / 0.004 / Area*Season / 1 / 387.00 / 0.08 / 0.788 / Area*Season / 1 / 658.60 / 0.20 / 0.665
Area*Salinity / 1 / 6.94 / 0.08 / 0.786 / Area*Salinity / 1 / 1970.27 / 0.38 / 0.547 / Area*Salinity / 1 / 749.96 / 0.23 / 0.644
Season*Salinity / 1 / 178.51 / 1.94 / 0.178 / Season*Salinity / 1 / 975.52 / 0.19 / 0.671 / Season*Salinity / 1 / 33.83 / 0.01 / 0.921
Area*Season*Salinity / 1 / 132.88 / 1.44 / 0.243 / Area*Season*Salinity / 1 / 5691.65 / 1.11 / 0.313 / Area*Season*Salinity / 0 / --- / --- / ---
Table S4. Average, minimum, and maximum carbon isotope (δ13C and Δ14C) values of potential end-member sources to carbon pools in the Rio Loco and Rio Fajardo study areas.
Average / Minima / MaximaSource / δ13C (‰) / Δ14C (‰) / δ13C (‰) / Δ14C (‰) / δ13C (‰) / Δ14C (‰) / References
Equilibrated Atmospheric CO2 / 0.0 / 80 / -2.0 / 50 / 2 / 100 / Mook et al. [1974]; Siegenthaler and Sarmiento [1993]; Hoefs [2004]
Riverine DOC and POC / -26.0 / 10 / -32.0 / -258 / -19.3 / 95 / This study
Oceanic surface DOC / -20.8 / -250 / -22.0 / -300 / -20.0 / -200 / Bauer [2002]
Oceanic surface POC / -17.0 / -200 / -18.0 / -300 / -16.0 / -100 / Bauer [2002]
Soil CO2 / -16.0 / -125 / -13.0 / -360 / -30.0 / 100 / Telmer and Veizer [1999], Raymond and Hopkinson [2004]
Riverine DIC / -10.5 / 42 / -12.2 / 6 / -8.2 / 81 / This study
Carbonate sediments / 0.0 / -1000 / -5.0 / -1000 / 5.0 / -1000 / Hoefs [2004]
Terrestrial C3 plants / -28.0 / 80 / -30.0 / 50 / -24.0 / 100 / von Fischer and Tiezen [1995], Marín-Spiotta et al. [2009]
Agricultural soils / -19.0 / -100 / -24.0 / -125 / -16.0 / 0 / Marín-Spiotta et al. [2009], Moyer, Bauer, and Grottoli, unpubl.
Pre-agricultural soils / -27.5 / -300 / -29.0 / -400 / -24.0 / -50 / Marín-Spiotta et al. [2009], Moyer, Bauer, and Grottoli, unpubl.
Marine primary productivity / -20.5 / 60 / -22.0 / 40 / -19.0 / 80 / Lynch-Stieglitz et al. [1995]; Hoefs [2004]