Abstract

Recent studies have shown that small mountainous
rivers (SMRs) may act as sources of aged and/or
refractory carbon (C) to the coastal ocean, which may
increase organicCburial at sea and subsidize coastal food
webs and heterotrophy. However, the characteristics and
spatial and temporal variability of C and organic matter
(OM) exported from tropical SMR systems remain
poorly constrained. To address this, the abundance and
isotopic character (d13C and D14C) of the three major C
pools weremeasured in two Puerto Rico SMRs with catchments
dominated by different land uses (agricultural vs.
non-agricultural recovering forest). The abundance and
character of C pools in associated estuaries and adjacent

coastal waters were also examined. Riverine dissolved
and particulate organic C (DOC and POC, respectively)
concentrations were highly variable with respect to land
use and sampling month, while dissolved inorganic C
(DIC) was significantly higher at all times in the agricultural
catchment. In both systems, riverine DOC and POC
ranged from modern to highly aged (2,340 years before
present),while DICwas always modern. The agricultural
river and irrigation canals contained very oldDOC(1,184
and 2,340 years before present, respectively), which is
consistent with findings in temperate SMRs and indicates
that these tropical SMRsprovide a source of agedDOCto
the ocean. During months of high river discharge,OMin
estuarine and coastal waters had C isotope signatures
reflective of direct terrestrial input, indicating that
relatively unaltered OM is transported to the coastal
ocean at these times. This is also consistent with findings
in temperate SMRsand indicates thatCtransported to the
coastal ocean by SMRs may differ from that of larger
rivers because it is exported from smaller catchments that
have steeper terrains and fewer land-use types.

Citation

Moyer, R.P., Bauer, J.E., Grottoli, A.G. (2012): Carbon istope biogeochemistry of tropical small mountainous river, estuarine, and coastal systems of Puerto Rico. Biogeochemistry. DOI: 10.1007/s10533-012-9751-y