Interactions between lithology and biology drive the long-term response of stream chemistry to major hurricanes in a tropical landscape

Humid tropical forests play a dominant role in many global biogeochemical cycles, yet long-term records of tropical stream chemistry and its response to disturbance events such as severe storms and droughts are rare. Here we document the long-term variability in chemistry of two streams in the Luquillo Mountains, Puerto Rico over a period of 27 years. Our two focal study watersheds, the Río Icacos and Quebrada Sonadora, both drain several hundred hectares of tropical wet forests, and each received direct hits from Hurricanes Hugo (1989) and Georges (1998). They differ primarily in lithology (granitic vs. volcaniclastic) and elevation. Changes in major cations, anions, silica, and dissolved organic carbon were minimal over the study period, but the concentrations of nitrate show a strong response to hurricane disturbance and the longest time to recovery. Potassium also showed a large, although less consistent, response to disturbance. In the granitic terrain, nitrate concentrations exceeded long-term pre-hurricane background levels for over a decade, but were elevated in the volcaniclastic terrain for only 1–2 years. Lithology appears to be the primary driver explaining the different response trajectories of the two watersheds. In the granitic terrain, which showed slow recovery to pre-hurricane conditions, the quartz diorite bedrock weathers to produce coarser soils, deeper groundwater flow paths, and riparian zones with sharp spatial variation in redox conditions and very high nitrogen levels immediately adjacent to the stream. Groundwater flow paths are shallow and the levels of N in streamside groundwater are much lower in the volcaniclastic terrain. The recovery of vegetation following hurricane disturbance appears similar in the two watersheds, suggesting that the extent of structural damage to canopy trees determines the magnitude of NO₃ increases, but that the duration of elevated concentrations in stream water is a function of lithology.