Shanley, J. B., W. H. McDowell, and R. F. Stallard (2011), Long‐term patterns and short‐term dynamics of stream solutes and suspended sediment in a rapidly weathering tropical watershed, Water Resour. Res., 47, W07515, doi:10.1029/2010WR009788.
The 326 ha Río Icacos watershed in the tropical wet forest of the Luquillo Mountains,
northeastern Puerto Rico, is underlain by granodiorite bedrock with weathering rates
among the highest in the world. We pooled stream chemistry and total suspended sediment
(TSS) data sets from three discrete periods: 1983–1987, 1991–1997, and 2000–2008.
During this period three major hurricanes crossed the site: Hugo in 1989, Hortense in
1996, and Georges in 1998. Stream chemistry reflects sea salt inputs (Na, Cl, and SO4),
and high weathering rates of the granodiorite (Ca, Mg, Si, and alkalinity). During rainfall,
stream composition shifts toward that of precipitation, diluting 90% or more in the largest
storms, but maintains a biogeochemical watershed signal marked by elevated K and
dissolved organic carbon (DOC) concentration. DOC exhibits an unusual “boomerang”
pattern, initially increasing with flow but then decreasing at the highest flows as it becomes
depleted and/or vigorous overland flow minimizes contact with watershed surfaces.
TSS increased markedly with discharge (power function slope 1.54), reflecting the erosive
power of large storms in a landslide‐prone landscape. The relations of TSS and most
solute concentrations with stream discharge were stable through time, suggesting minimal
long‐term effects from repeated hurricane disturbance. Nitrate concentration, however,
increased about threefold in response to hurricanes then returned to baseline over several
years following a pseudo first‐order decay pattern. The combined data sets provide insight
about important hydrologic pathways, a long‐term perspective to assess response to
hurricanes, and a framework to evaluate future climate change in tropical ecosystems.