Aquatic Sensors Reveal Ongoing Changes in Stream Nitrate Dynamics in a Tropical Montane Forest
Abstract
Hurricanes and major windstorms are common in much of the tropics and subtropics, and can have large effects on forests that persist for decades. In the tropical Luquillo Mountains of Puerto Rico, decades of past research have shown that the effects of hurricanes on stream nitrate concentrations can last for a few years to over a decade, depending on lithology. Here we build on a 35-year weekly record by using SUNA nitrate sensors to document the ongoing effects of Hurricanes Irma and Maria (September 2017) on the chemistry of two long-term study sites, Quebrada Sonadora and Rio Icacos. In both streams, following a year of pre-hurricane deployment, we found that baseline concentrations were consistently elevated and that the behavior of NO3 in response to flow was also fundamentally altered. Pre-hurricane, storms had little to no effect on NO3 concentrations measured at 15-minute intervals. Following the hurricanes, NO3 was highly sensitive to flow, with large increases in concentration associated with every storm. This responsiveness to flow declined over the course of the next year. In the Sonadora, we also found that the declining responsiveness of NO3 to flow corresponded to a steady decline in soil solution NO3 concentrations following the hurricane. Soil solution thus appears to be the source of the high nitrate observed on the declining limb of the hydrograph. This result shows the power of sensors to link watershed conditions to stream chemistry. The newly recognized sensitivity of nitrate to stream discharge that sensors provide in both streams also has important implications for future climate change scenarios. With increased frequency and intensity of hurricanes, episodic pulses of NO3 can be expected to enter the nitrogen-limited coastal waters following each major storm.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.B53D..03M
- Keywords:
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- 0414 Biogeochemical cycles;
- processes;
- and modeling;
- BIOGEOSCIENCES;
- 0439 Ecosystems;
- structure and dynamics;
- BIOGEOSCIENCES;
- 0452 Instruments and techniques;
- BIOGEOSCIENCES;
- 1895 Instruments and techniques: monitoring;
- HYDROLOGY