Spatial Variation in Geogenic Solute Concentration across a Tropical Stream Network and the Role of Aeolian Sources

The concentration of solutes in tropical stream ecosystems is influenced by weathering rates and inputs of aeolian dust. Dust provides critical nutrients to ecosystems contributing to elemental fluxes to the coastal zone. In the Caribbean, this dust arrives from North Africa in summer via the East Africa Jet with quantitatively significant fluxes of calcium (Ca2+), silica (Si), and phosphorus (PO43-). Here we investigate spatial and temporal variation in stream solutes across a tropical landscape where watersheds vary in lithology and landscape position. A dataset of weekly grab sample and sensor data was used to track changes in Ca2+, magnesium (Mg2+), potassium (K+), PO43-, and Si across 13 watersheds in the Luquillo Mountains of Puerto Rico. Analyses include a June 2020 dust storm, coined "Godzilla", which was a 50-year scale event. We found a surprising amount of spatial and temporal variation in the coefficient of variation (cv) of multiple solutes across study watersheds. In contrast, there is relatively little seasonal variability in rainfall and stream flow across the mountain. Stream draining the northeastern quadrant of the mountains had a cv ≥0.4 for Ca2+ and Mg+2 from weeks 10-30 of the year, while streams on the southern quadrant of the mountains had a cv ≥0.4 only during the first 10 weeks of the year. Streams on the northwestern quadrant of the mountains showed the greatest variation late in the year around week 40 with a cv ≥0.4. This spatial variability is accentuated by concentrations of K+ where streams on the northeastern and southern quadrants have little variation while streams on the northwestern quadrant show high variability. This spatial and temporal variation implicates the role of dust and marine derived solutes likely delivered during cyclonic storms and the role of orography. We found that the "Godzilla" dust storm affected variability in concentrations of Si with pulses of this dust-derived element across Luquillo streams with concentrations reaching up to 30mg/L (average is ~ 20mg/L). Contrary to our expectations, we did not find a signal of Ca+2 associated with "Godzilla". Results are important to understand inputs and exports from tropical watersheds revealing a significant amount of variation given the small geographic area (113 km2) of the watersheds and low degree of seasonality.