Hurricanes are known to drive a variety of biogeochemical responses in tropical landscapes. Stream solute response is tightly related to the terrestrial environment via riparian and groundwater inputs and this linkage can become more apparent during disturbances such as hurricanes. However, many short-term responses to hurricanes, particularly in streams, are missed because of their limited duration. High frequency sensors give us the opportunity to better characterize these responses. A suite of stream sensors was installed in a stream in the Luquillo Experimental Forest in Puerto Rico and measured a variety of physicochemical parameters such as pH, conductance, dissolved oxygen, fluorescent dissolved organic matter and nitrate at 15-min intervals for a period of a year and 8 months which included both hurricanes Irma and María. Additionally, 12 pore water samplers placed across a catena about 200m from the stream were sampled at weekly to monthly intervals before and after the hurricanes and were analyzed for major cations and anions.
Results/Conclusions
High-frequency stream sensor data showed a pattern of decreasing conductance during high-flow events before hurricanes Irma and María. In contrast, conductance after the hurricanes seemed to increase in response to increasing discharge. This inverted response lasted around a month after Hurricane María and was, interestingly, not seen during a single storm that occurred between Irma and María, perhaps as a result of differences in intensity between the hurricanes. Results from the pore water samples showed a dramatic increase in chloride concentrations a month after the hurricanes. Chloride concentrations nearly doubled for all pore water sampling sites from a range of 3-9mg/L pre-hurricane to 16-25mg/L post-hurricane. Chloride concentrations in all but two of these sites decreased after this peak but were still slightly elevated 5 months post-hurricane. These results suggest that the hurricanes, María in particular, transported and deposited a large amount of salts in the watershed as supported by the increasing chloride in the pore water samples. Rain and storm events then mobilized these salts from the soil to the stream. Frequent rain events post-hurricane possibly drove these salts out of the watershed very quickly as indicated by the return to pre-hurricane stream conductance patterns after just a month.