Impacts of extreme climate events - droughts and hurricanes - on carbon and nitrogen in streams draining the Luquillo Mountains in Puerto Rico

Abstract:

Climate models for the Caribbean overall project a drier future but also more variable and extreme rainfall associated with cyclonic activity and El Niño-Southern Oscillation (ENSO) cycles. Better understanding of the response of rivers to extreme climatic events (ECEs) is vital, given they are projected to occur more intensely and more frequently in the future. ECEs such as drought and extreme hydrological events impact the Luquillo Mountains of Puerto Rico. Rainfall, stream runoff, and collections of river suspended sediment (SS), particulate organic carbon (POC), particulate nitrogen (PN), and dissolved organic carbon (DOC) have been measured over storm events or on a weekly basis over the past 25 years in the Río Mameyes at Puente Roto and Río Icacos catchments. These long-term data were collected as part of multiple projects, including the Luquillo Long Term Ecological Network (LTER), the USGS Water, Energy, and Biogeochemical Cycles (WEBB) project, the Luquillo Critical Zone Observatory (LCZO), and the USDA FS International Institute of Tropical Forestry (IITF). Over 25 years, yields of riverine SS, POC, and PN, including periods of ECEs, are examined for an extreme ecosystem response – where biogeochemical yields affected by ECEs occur on the outer boundaries of the frequency distribution. Additionally, we focus on the hydrographs of a series of events to evaluate the response of the stream to drought rewetting and hurricanes. Our main findings are that: (1) concentrations of DOC, POC, and PN during ECEs and their recovery periods fall within long-term concentration-discharge (C-Q) relations; (2) rainfall intensity, not just runoff, is a strong driver of river POC and PN concentrations and yields; and (3) total annual river carbon export (POC and DOC) is driven by ECEs. Investigation into ECEs and their responses in river geochemistry provided insight into the extreme events that are predicted to be more common in the future under a changing climate.