Molecular links between whitesand ecosystems and blackwater formation in
the Rio Negro watershed
Abstract
Tropical rivers constitute a major portion of the global aquatic C flux
entering the ocean, and the Rio Negro is one of the largest single C
exporters with a particularly high export of terrestrial C. We
investigated the role of whitesand ecosystems (WSEs) in blackwater
formation in the Rio Negro basin to develop novel constraints for the
terrestrial carbon export from land to the aquatic continuum. To this
end, we used ultrahigh resolution mass spectrometry (FT-MS, Orbitrap) to
identify markers in dissolved organic carbon (DOC) from ground- and
surface waters of two contrasting WSEs feeding Rio Negro tributaries,
and compared them with known Rio Negro marker from two openly available
FT-MS datasets. Tributaries were fed by a whitesand riparian valley
connected to terra firme plateau, and a typical upland whitesand
Campina. WSE-DOC molecular composition differed by 80% from plateau
DOC, which was characterized by reworked, highly unsaturated N- and
S-containing molecules. WSE-DOC contained mainly condensed aromatics and
polyphenols. WSE samples differed by 10% in molecular DOC composition
and also by their isotopic content (14C, 18O, 2H). Upland WSE-DOC was
exported by fresh precipitation and had maximum age of 13 years, being
five years older than riparian valley WSE-DOC. Unexpectedly, only
markers from the upland WSE, which cover a small proportion of the
landscape, were identical to Negro markers. Markers of the riparian
valley WSE, which are widespread and known for high DOC export,
surprisingly showed lower coverage with Negro markers. Analysis of
robust matching WSE markers between FT-MS datasets by Pubchem suggested
well-known plant metabolites (chromenes and benzofurans) as promising
candidates for targeted approaches and calibration. Our results suggest
that terrestrial DOC from upland WSEs is a main source of specific
blackwater molecules missing in the regional ecosystem C balance,
whereas C export from the riparian valley and especially from terra
firme plateaus represents mainly recycled and transformed carbon not
directly affecting the ecosystem C balance. Our study highlights the
potential of high-resolution techniques to constrain carbon balances of
ecosystems and landscapes. Comparisons of FT-MS datasets and
complementary isotopic information shows high potential to identify
robust molecular markers that link forests, soils, aquifers and aquatic
systems, and are needed for a deeper understanding of the regional C
cycle in tropical blackwater catchments.