1. University of New Hampshire EXPORT OF CARBON, NUTRIENTS, AND WEATHERING PRODUCTS FROM A HIGHLY URBANIZED TROPICAL WATERSHED William H. McDowell, Jody D. Potter, Alonso Ramírez University of New Hampshire, USA, and University of Puerto Rico, USA Luquillo LTER and CZO University of New Hampshire LCZO and LUQ LTER

2. Urban Stream Syndrome Useful approach to characterizing streams in a heavily built environment Little data from tropical cities Hydrologic flow paths, carbon flux, nutrient dynamics, weathering rates can all be inferred from patterns in stream chemistry and watershed inputs and outputs Propose that we expand to think about urban watersheds; is there a distinctive urban watershed function?

4. Sampling Site Sensitive Downstream Ecosystems Upstream sources

5. Luquillo CZO and LTER, Puerto Rico Reference Sites

6. Río Piedras Watershed, San Juan, PR Area 40 km 2 Population density 2215/km 2 All drinking water imported from outside the watershed Virtually all sewage exported from watershed 67% urban land cover; 30% impervious surface Rainfall averages 1600 mm River temperature averages 27 C, air temperature 26 C River specific conductance averages 362 µSiemens DO often very low

7. No strong interannual variability (7 yrs, weekly samples)

8. Some seasonality in NO 3, not DOC or other solutes

9. Nitrate is surprisingly sensitive to flow

10. Chloride dilutes more than in forest streams

11. Si more sensitive to dilution than forest streams

12. Strong relationships among many solutes (not just Na and Cl!)

13. SoluteFWMFluxREF FluxFLUX ratio Cl 20.0 328 1721.91 NO 3 -N 0.64 10.4 1.387.57 SO 4 -S 3.20 52.4 20.52.55 Na 17.7 291 963.03 K 2.5 41.8 5.37.88 Mg 7.48 123 284.38 Ca 18.0 295 446.71 NH 4 -N 0.43 7.07 0.2726.2 PO 4 -P 72.4 1.19 0.0523.8 DOC 3.94 64.7 740.87 DON 0.35 5.66 3.71.53 SiO 2 22.9 376 1802.09 TSS108 1774 1909.34 Fluxes are generally higher than in reference watersheds! Runoff 1.6 m Rio Piedras 2.5 m reference Leaking water and sewer lines provide additional river flow Rainfall 1.6 m Rio Piedras 3.5 m reference

14. C/N of urban DOM lower and more constrained

15. Molar N:P higher in urban river and less constrained

16. Conclusions Little seasonal or annual variation in chemistry Surprisingly strong flow dependence for major solutes as well as nitrate Nitrate levels relatively low for the high TDN levels Remarkably low DOC, given the high N and P from leaking sewer lines; mineralization of sewage DOC very likely

17. Implications Stoichiometry of both DOM and N:P altered by urbanization with important implications for downstream coastal zone High silica flux suggests weathering rates are surprisingly rapid in the basin Sources of weathering products and Cl uncertain (Si from weathering of rocks or the built environment? Cl from sea salt aerosols or sewage?) Beyond “Urban Stream Syndrome”, we need to study urban watersheds to better understand basic biogeochemical cycles

18. Acknowledgements Luquillo LTER Luquillo CZO USDA Forest Service IITF and University of Puerto Rico San Juan ULTRA and Tropical Urban Biogeochemistry group