River runoff and chemistry
River drainage networks occur in a radial fashion around the central peaks of the Luquillo Mountains, El Yunque and East Peak (Fig. 1) and drain into both the Caribbean Sea and the North Atlantic. Streams and rivers are very flashy in the Luquillo Mountains (Jones et al. 2012). Instantaneous discharge is collected by the USGS at multiple sites, with the longest record (since 1945) at Río Icacos (50075000); other past or current USGS gauges include Quebrada Guaba (50074950) in the Icacos/Blanco drainage; Río Mameyes at Puente Roto (50065500), Río Espiritu Santo at El Verde (50063800), Quebrada Sonadora (50063440) and Quebrada Toronja at El Verde (50063500), and Río Sabana at Luquillo (50067000). The University of Puerto Rico and the US Forest Service International Institute of Tropical Forestry also maintain discharge records for Quebrada Prieta (El Verde) and the Bisley Experimental Watersheds (Sabana), respectively. Smaller headwater tributaries are nested within mainstem rivers (Figure 1; Wymore et al. 2017). Critical zone structure drives flow duration, with long periods of sustained baseflow in the granitoid Icacos basin when compared to the Sonadora and Toronja basins, which are on largely volcaniclastic parent material (McDowell and Asbury 1994).
Multiple streams are sampled weekly in the Luquillo Mountains, with periodic storm sampling (e.g. Clark et al. 2017, Wymore et al. 2017). Analysis of major cations and anions, nutrients and dissolved organic matter has occurred since 1983 (Figure 3), with a focus on understanding the response of forested catchments to the frequent hurricane disturbances in the LEF (McDowell et al. 2013) and describing the role of watershed (McDowell and Liptzin 2014) versus in-channel controls on stream N dynamics (Merriam et al. 2002; Rodriguez-Cardona et al. in press). Recent deployment of high-frequency water quality sensors that measure NO3-, conductivity, temperature, dissolved oxygen, fluorescent dissolved organic matter and turbidity provides new insights into controls on stream chemistry (e.g. Wymore et al. 2019).
Stream chemistry in the Luquillo Mountains is typically circumneutral, has relatively high concentrations of sea salts due to large inputs of marine aerosols, and high concentrations of various weathering products (e.g. SiO2 and bicarbonate) due to the warm and wet environment that promotes rapid weathering in both volcaniclastic and granitoid lithologies (McDowell and Asbury 1994; Shanley et al. 2011; Murphy and Stallard 2012). Nitrogen concentrations are relatively high, with inorganic N dominated by nitrate, as is typical of many tropical forests (Lewis et al. 1999). Phosphorus concentrations are moderate, and dissolved organic matter concentrations (dissolved organic carbon and nitrogen) are low to moderate as found in many well-drained forested catchments (McDowell and Asbury 1994). Concentrations of many solutes are highly responsive to flow, with strong dilution observed for weathering products and strong flushing of dissolved organic carbon (McDowell and Asbury 1994; Shanley et al. 2011). Hurricanes result in increased concentrations and fluxes of NO3- and K+, but little change in other solutes (McDowell et al. 2013; Schaefer et al. 2000; Fig. 3). Differences in structure of the critical zone drive differences among watersheds in concentration-discharge relationships, which are particularly evident for dissolved phosphorus (Wymore et al. 2017). Weathering at the bedrock-regolith interface almost 10 m below the soil surface is a significant source of Mg2+ at low flows (Chapela Lara et al. 2017), showing that weathering products may be transported to streams along deep flow paths with little opportunity for uptake by upland vegetation (McDowell 1998). Biota can affect stream chemistry, as the assemblage of shrimp species has been shown experimentally to affect both nitrate and dissolved organic carbon concentrations (Crowl et al. 2000).
3.0 Data availability statement
Data are freely available at the LTER network EDI data portal, https://portal.edirepository.org, and the NWIS for stream gauging by USGS, https://waterdata.usgs.gov/pr/nwis/rt.
4.0 Acknowledgements
Primary support for this project comes from the US National Science Foundation, particularly the Luquillo LTER (NSF DEB 1831592). Additional support came from many specific projects such as the Luquillo CZO (NSF EAR 1331841) and the USGS WEBB program. In addition to the authors of this data note, contributors to the data sets described are authors on the papers referencing the findings. Major support is provided by the University of Puerto Rico and USDA Forest Service International Institute of Tropical Forestry. Partial funding was provided by the New Hampshire Agricultural Experiment Station. This is Scientific Contribution Number 2876. This work was supported by the USDA National Institute of Food and Agriculture McIntire-Stennis Project 1006760.
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Figure 1. Map of the Luquillo Mountains of Puerto Rico and the Luquillo Experimental Forest. Colors represent major lithology types and the primary study catchments are outlined. RI: Río Icacos; QG: Quebrada Guaba; MPR: Mameyes at Puente Roto; Q1-3: Bisley 1,2, and 3; RS: Río Sabana; RES4: Rio Espiritu Santo; QS: Quebrada Sonadora; QP: Quebrada Prieta; RG: Río Grande.