Abstract: Organic matter (OM) processing in estuaries is crucial in the marine
environment as significant quantities of OM are buried or modified in these
land-ocean-interaction zones. Southeast Asia is globally important in this
regard because of high sediment inputs to the ocean and intense human
modifications in the coastal zone, as exemplarily can be observed in the
catchment of Java's second largest river, the Brantas. In order to investigate
sedimentary OM processing, surface sediments and short sediment cores were
sampled in its estuary in the rainy and dry seasons of 2008.
δ^{13}C_{org},
δ^{15}N and C/N ratios as well as amino acids and
hexosamines were used to determine the sources, transformation and fate of
estuarine sedimentary organic matter. Ranges in
δ^{13}C_{org} of −24.9 to
−20.1‰, in δ^{15}N of 3.5 to 5.4‰
and a C/N ratio of 7.9 to 16.5 in the sediments indicate a mixture of freshly
produced marine algae and degraded terrestrial soil organic matter. The relative contributions of the autochthonous and allochthonous OM
in the estuarine sediments differed according to the amount and dispersal of
the land-derived material. As the discharge of the two main river arms, the
Porong and the Wonokromo River, showed strong differences with upto five-fold
higher values in the Porong River in the rainy season, the highest proportion
of terrestrial OM was found off the Porong river mouth that received the
highest riverine runoff. Also the lowest sedimentary reactivity was detected in
this region as displayed by amino acids (AA) and hexosamines (HA). AA+HA ranged
between 0.76 and 5.25 mg g^{-1}, amino acid bound carbon
between 5.9 and 22.6% and the AA/HA ratio between 4.2 and 13.0. Furthermore, a
reduced intensity of OM degradation was observed in front of the Porong River
outlet, which has been attributed to the high quantity of deposited material
and the low reactivity of the surface sediments. In a global context, the
reactivity of sedimentary OM from the Brantas estuary was in the range of
degraded sediments from offshore regions or stations at greater water depth. It
indicates that severe OM degradation based on a strong tidally induced
resuspension of sediments in the turbid and well mixed waters of the shallow
Brantas estuary is responsible for burial of refractory carbon.
