Sources and Degradation of Sedimentary Organic Matter in Coastal Waters off the Brantas River,Java,Indonesia

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.