Abstract

In heterotrophic streams the retention and export of coarse particulate organic matter and associated
elements are fundamental biogeochemical processes that influence water quality, food webs and the structural
complexity of forested headwater streams. Nevertheless, few studies have documented the quantity and quality
of exported organic matter over multiple years and under a range of conditions that includes both droughts and
hurricanes. This study quantifies the export of coarse particulate organic matter (CPOM, > 12.7 mm), over 18 y in
two headwater streams in north-east Puerto Rico. Daily exports ranged from 0 to over 170 g ha−1 d−1 and averaged
7.39 g ha−1 d−1,with similar amounts coming from leaves (3.5 g ha−1 d−1) and wood (3.2 g ha−1 d−1). Export of coarse
particulate organic carbon was 3.0 g ha−1 d−1 which constitutes only 1.32% of carbon exports.Most litter falling into
the streams was processed in place as only 2.3% of the leaf litter falling directly into these perennial channels was
exported as CPOM. On average, 6 wk y−1 had no exports while events transporting more than 10 g ha−1 d−1 occurred
every 2.8 mo. Instead of a single annual pulse as observed in deciduous systems, there were annual peaks in CPOM
exports during May and September and less export during the drier period from December to February. Ratios of C:N in
the exportedmaterial were highest in the driest month and lowest during rainymonths, while leaf fluxes for nitrogen,
phosphorus and calcium were highest in rainy months and lowest during February. Although median daily exports
and exports during low- and base-flow periods were similar before and after Hugo, after 16 y exports during moderateand
high-flow periods were still less than those in the 2 y prior to the hurricane. Our observations indicate a system
with high rates of internal processing that quickly returns to median daily conditions following hurricanes but requires
several decades for storm-flow exports to return to pre-disturbance conditions and indicates that the long-term pattern
of CPOM export is associated with the level of maturity of watershed vegetation.

Citation

Heartsill Scalley, T., Scatena, F.N., Moya S., Lugo A.E. (2012): Long-term dynamics of organic matter and elements exported as coarse particulates from two Caribbean montane watersheds. Journal of Tropical Ecology. DOI: 10.1017/S0266467411000733