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9 1 0 2 p o h Impacts of Freshwater Discharge Patterns on the s k Carbon Cycle in Microtidal Estuaries r o W r e m m u S Iris C. Anderson, Mark J. Brush, Virginia Institute of Marine Science, College of B William and Mary C

  1. 9 1 0 2 p o h Impacts of Freshwater Discharge Patterns on the s k Carbon Cycle in Microtidal Estuaries r o W r e m m u S Iris C. Anderson, Mark J. Brush, Virginia Institute of Marine Science, College of B William and Mary C Jennifer W. Stanhope, US Fish and Wildlife O Joseph R. Crosswell, CSIRO, AU

  2. Net Ecosystem Metabolism, which is likely to play an 9 important role in regulating FCO 2, is often not measured. 1 0 2 p o h s k r o W r e m m u S B C O

  3. 9 Potential drivers of CO 2 fluxes in estuaries 1 0 2 p • Hydrology o • FW discharge, FW age, residence time, mixing h • Temperature s k • Allochthonous inputs of carbon and nutrients r o • From marshes - DIC vs. DOC W • From industrial/urban sources – waste water treatment plants r • Forested systems – humics e • Groundwater and subterranean estuaries m • Autochthonous inputs of carbon and nutrients m • Net autotrophic systems – uptake DIC/pCO2; produce TOC; burial u • Net heterotrophic systems – mineralization of both autochthonous and allochthonous S TOC B • Alkalinity production - sulfate and nitrate reduction C O

  4. 9 We focused on identifying mechanisms responsible for observed 1 fluxes of CO 2 in two mid-Atlantic estuaries. We asked the following: 0 2 p In the York River VA (YRE) and New River NC estuaries (NRE): o h § How do air/sea CO 2 exchanges and net ecosystem metabolism vary s temporally and spatially during years with different precipitation k patterns? r o § How does FW age influence net ecosystem metabolism and air/sea CO 2 W exchanges? r § What are the direct vs. indirect regulators of CO 2 exchanges in the e m YRE? m § How do measured CO 2 fluxes in the YRE and NRE compare to other u observed and modelled fluxes in estuaries along the Atlantic Coast of S the US? B C O

  5. 9 Study sites located in the mid-Atlantic region, USA 1 0 2 p New River Estuary, NC York River Estuary, VA o (NRE) (YRE) h s k r o W r e m m u S B C O

  6. 9 Comparison of the York and New River 1 estuaries 0 2 p YRE NRE o Watershed Area, x10 6 m 2 h 6,588 1,024 s Estuary Area, x10 6 m 2 159 79 k r Watershed:Estuarine Area 41.5 13.0 o Estuary Volume, x10 6 m 3 W 809 143 Mean Depth, m 5.1 1.8 r e % Area < 2 m 38% 56% m Mean Discharge, x10 6 m 3 d -1 3.8 0.28 m Discharge:Volume, d -1 0.0048 0.0020 u Mean Flushing Time, d 67.8 67.4 S % Natural Vegetation 74.7% 69.3% B C % Agriculture 17.4% 14.0% O % Developed 6.9% 15.5%

  7. Patterns of mean annual FW discharge and flushing time differ for the 9 YRE and NRE 1 0 2 p o h s k r o W r e m m u S B C O

  8. 9 Net ecosystem metabolism was measured by the open water method 1 0 2 • Bimonthly dataflow cruises conducted at p dawn, dusk, and dawn in the YRE (2018) o and in the NRE (2013 – 14; 2014 – 2015). h s • Water pumped to YSI 6600, CDOM sensor, k r and showerhead equilibrator. o W • DO data distance weighted, averaged for each box, and interpolated over 24 h.. r e • Gas exchanges calculated (solubility m coefficient, Weiss; Schmidt number, YSI & CDOM Grab samples Equilibrator CO 2 Analyzer m Wanninkhof, 1992; gas transfer • Chl a • DIN, DON, DIP • Temp • xCO 2(ambientair) • Turbidity • DIC, DOC • Pressure • xCO 2(equilibrated parameterization, Jiang et al, 2008. • pH • TSS air) u • DO • Chl a S • Salinity • CDOM • Daily NEM calculated using average depth • Temp • 18 O • CDOM B for each box and corrected for air/sea C exchange. O

  9. CO 2 Fluxes varied with FW discharge CO 2 flux - LYRE 2018 9 200 2/6/18 CO 2 flux (mmol C m -2 d -1 ) 3/28/18 1 150 6/13/18 0 8/9/18 • In YRE highest CO 2 emissions from 100 10/9/18 2 June - October with higher than 50 p average FW discharge. In Feb and 0 o March there was net uptake of CO 2. -50 h 1 2 3 4 5 Boxes down estuary s k NRE, 13-14 200 • In NRE (2013-14) with lower than r CO 2 flux (mmol C m -2 d -1 ) 7/17/13 o 150 9/17/13 average FW discharge net emissions W 11/20/13 1/28/14 100 mainly at head of the estuary with net 4/17/14 7/16/14 r 50 uptake or balance in other boxes. e 0 m -50 1 2 3 4 5 6 7 • In NRE (2014-15) with slightly higher m Boxes down estuary than average FW discharge net u NRE, 14-15 200 emissions in most boxes during May S 7/16/14 9/11/14 150 CO 2 flux (mmol C m -2 d -1 ) and September with net or zero uptake 11/13/14 B 1/23/15 100 3/19/15 C during the rest of the year. 5/21/15 7/17/15 O 50 0 -50 1 2 3 4 5 6 7 Boxes down estuary

  10. NEM shifted from net heterotrophy to net autotrophy depending on FW Net Ecosystem Metabolism 9 2/6/18 YRE - 2018 discharge 3/28/18 1 300 6/13/18 0 NEM (mmol O 2 m -2 d -1 ) 200 8/9/18 10/9/18 2 100 • In Feb and March the YRE was net 0 p autotrophic due to low discharge and -100 o cold temperatures. From June – -200 h -300 November with high FW discharge s 1 2 3 4 5 Boxes Down Estuary k most of the estuary was net Auto NRE 2013 - 2014 r 7/17/13 o heterotrophic. 9/17/13 300 11/20/13 W 7/16/14 NEM (mM O 2 m -2 d -1 ) 200 9/11/14 100 r 0 • NEM in the NRE (2013 -14), with e -100 m lower than average discharge, -200 displayed no clear trends. m -300 1 2 3 4 5 6 7 Hetero Boxes down estuary u NRE 2014 - 2015 S 11/13/14 • In 2014 – 15 the NRE with slightly 300 1/23/15 3/19/15 greater than average FW discharge B 200 NEM (mM O 2 m -2 d -1 ) 5/21/15 C 7/17/15 100 was mainly net autotrophic. O 0 -100 -200 -300 1 2 3 4 5 6 7 Boxes down estuary

  11. CO 2 Flux vs FW Age 9 CO2 Flux vs FW Age YRE 2018 CO 2 Fluxes were highest at short FW Ages LYRE 2018 1 200 CO 2 flux (mmolC m -2 d -1 ) CO2 flux (mmolC m-2 d-1) 200 0 150 y = -45.21ln(x) + 146.72 150 y = -45.21ln(x) + 146.72 R ² = 0.788 2 R ² = 0.788 100 100 p 50 50 o 0 0 h -50 -50 0 10 20 30 40 50 60 0 10 20 30 40 50 60 s • In all sites CO 2 fluxes decreased FW Age (d) FW Age (30d average) k with increasing FW Age. NRE 2013 - 14 r o 200 W CO 2 Flux (mmolC m -2 d -1 ) 150 y = -13.53ln(x) + 47.853 R ² = 0.2048 100 r e 50 • At a FW age of approximately m 0 20 – 25 d net fluxes approached -50 m 0 20 40 60 80 zero. FW Age (d) u NRE 2014 - 15 S 200 CO 2 Flux (mmolC m -2 d -1 ) B y = -24.1ln(x) + 84.104 150 R ² = 0.4143 C 100 O 50 0 -50 0 20 40 60 80 100 FW Age (d)

  12. NEM vs FW Age YRE 2018 300 Net trophic status differed in the YRE 9 y = -0.1142x 2 + 10.89x - 174.31 NEM (mmol O 2 m -2 d -1 ) 200 R ² = 0.3617 1 and NRE and shifted with FW Age 100 0 0 2 -100 p -200 • YRE shifted from net o -300 0 10 20 30 40 50 60 h heterotrophic to autotrophic with FW Age (d) Net s increased FW age. NEM vs FW Age k Autotrophic NRE 2013 - 14 300 r o y = -22.66ln(x) + 88.455 NEM (mmol O 2 m -2 d -1 ) 200 R ² = 0.1663 W 100 • NEM in the NRE was weakly 0 r related to FW age but tended to -100 e -200 shift from net autotrophic to m -300 Net heterotrophic or balance with 0 10 20 30 40 50 60 70 m FW Age (d) Heterotrophic increasing age. NEM vs FW Age u NRE 2014-2015 300 S y = -0.4526x + 16.532 R ² = 0.0104 NEM (mmol O 2 m -2 d -1 ) 200 B 100 0 C -100 O -200 -300 -400 0 20 40 60 80 100 FW Age (d)

  13. CO 2 Flux vs NEM 9 YRE 2018 The direction of CO 2 200 1 CO 2 flux (mmolC m -2 d -1 ) 150 exchange varied with NEM 0 y = -0.171x + 18.486 100 R ² = 0.2917 2 50 p 0 o -50 -300 -200 -100 0 100 200 300 h NEM (mmol O 2 m -2 d -1 ) s • Effluxes of CO 2 when net k NRE 2013 - 14 heterotrophic; uptake when net r 200 o CO 2 Flux (mmolC m -2 d -1 ) autotrophic 150 W 100 50 r e 0 m -50 -300 -200 -100 0 100 200 NEM (mmol O 2 m -2 d -1 ) m NRE 2014 - 15 u 200 S CO 2 Flux (mmolC m -2 d -1 ) y = -0.2611x + 19.686 150 R ² = 0.3842 B 100 C 50 O 0 -50 -300 -200 -100 0 100 200 300 NEM (mmol O 2 m -2 d -1 ) Net autotrophic Net heterotrophic

  14. Other drivers that regulate CO 2 fluxes 9 1 CO 2 Flux vs DOC 0 YRE 2018 160 2 y = 0.2494x - 66.388 140 R ² = 0.8385 • In the YRE CO 2 fluxes p 120 CO 2 flux (mmolC m -2 d -1 ) o 100 strongly related to both DOC 80 h and DIN concentrations, 60 s highest at the heads of both 40 k 20 r the YRE and NRE and o 0 decreased linearly down -20 W -40 estuary 0 200 400 600 800 r DOC ( µ M) e m CO 2 Flux vs DIN YRE 2018 • In the YRE and NRE chl- a 160 m 140 was highest up estuary, y = 3.3922x - 5.4903 CO 2 Flux (mmol C m -2 d -1 ) 120 R ² = 0.7659 u weakly related to NEM but 100 S 80 unrelated to CO 2 fluxes. B 60 C 40 20 O 0 -20 0 5 10 15 20 25 30 35 DIN µ M)

  15. Structural equation models distinguished direct vs. 9 1 indirect drivers of CO 2 fluxes in the YRE 0 2 p Grey arrows represent non-significant pathways; black and red o indicate significant positive and negative relationships. The h correlation coefficient and size of each arrow corresponds to the s relative strength of the relationship. k r o W p = 0.774 Temp 0.2974 for r model - 0 . e 1 -0.5596 3 0 -0.1596 8 DIN (0.63) m - 0 . NEM (0.48) 1 7 m 5 9 1 5 7 . 3 0 Chl-a u 0 . 3 9 7 6 0 7 . 1 8 5 9 S 3 CO2 Flux 0.6140 0 3 . 0 3 5 0.0130 1 0 . (0.95) 3 0 B 4 1 - 4 . 0 - C 0.6888 DOC (0.68) O -1.3525 -0.0031 FW Age

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