Sources and fate of particulate organic matter in the sediments of the Brantas estuary, Java, Indonesia

Abstract

The important role of coastal sediments in the global carbon cycle in terms of transformation, accumulation, remineralization, and burial has been widely recognized. In this context, especially the sediments along the tropical coasts of Southeast Asia and Oceania are crucial due to their high sediment input to the ocean and the intense human activities within the coastal zone. However, most of the studies on coastal organic matter (OM) cycling were conducted in temperate regions and knowledge about tropical coastal ecosystems is still scarce. The present study aimed to investigate the spatio-temporal variations of sedimentary processes as well as their dependence on natural processes and anthropogenic influences in the coastal waters of the Brantas estuary, East Java. The Brantas River is the second largest river of Java and its catchment exemplifies a coastal zone of Southeast Asia heavily impacted by human activities. During four expeditions in the rainy and dry season of 2008 and the rainy seasons of 2010 and 2011, surface samples and sediment cores were taken and analyzed for a variety of biogeochemical parameters, such as amino acids, stable isotopes or ammonium pore water concentrations. Regional and seasonal distribution patterns of sedimentary OM characteristics were predominantly determined by the quantity and composition of the riverine material input. Regional variations mainly resulted from strongly differing discharge rates between the two main river arms, the Porong and the Wonokromo. Compared to the Wonokromo River input, the manifold higher discharge of the Porong River resulted in a much higher accumulation of terrestrial material in the adjacent estuary. The riverine organic matter was mainly derived from soil. The reactivity of the riverine OM was lower than that of the freshly produced marine OM. As their respective portions in the sediments determined the magnitude and spatial gradients of the OM reactivity in the estuarine sediments, lowest values were found off the Porong. The lower reactivity of sedimentary OM at the Porong estuary resulted in a lower OM degradation rate compared to the Wonokromo estuary. Consequently, benthic ammonium fluxes, which strongly depend on the amount of remineralized OM, were higher at the Wonokromo estuary. However, benthic ammonium supply was a considerable year-round source for the Brantas estuarine waters. Seasonal variations of OM reactivity occurred as, according to the monsoonal cycle, the discharge rates in both rivers were strongly enhanced during the rainy season. This resulted in an overall higher input of terrestrial, refractory material to the estuary and led to a lower reactivity of sedimentary OM. However, regional differences of ammonium fluxes and OM reactivity exceeded seasonal variations. Nevertheless, focusing only on the Porong estuary, material deposition and burial rates were significantly affected by seasonal river discharge variations. Due to the much higher riverine particulate organic carbon (POC) input in the rainy season that by far exceeded benthic remineralization rates, huge amounts of POC were buried. In contrast to this phase of accumulation, the dry season can be considered as a period of degradation, given that the low amounts of sedimenting POC are immediately remineralized or were even decomposed in the water column. Overall, compared to other marine environments the sediments at the Brantas estuary are characterized by a low reactivity of sedimentary OM due to a strong degradation of organic matter that is caused by intense tide-induced resuspension processes occurring in the shallow Brantas estuary. The high benthic ammonium fluxes and very high POC burial rates substantiate the role of coastal sediments as important sources and sinks in nutrient cycles.