Tropical blackwater biogeochemistry:The Siak River in Central Sumatra, Indonesia

Abstract

The most studied tropical blackwater rivers are tributaries of the Orinoco and Amazon such as the Rio Negro in South America. The dark-brown colour of blackwater rivers results from high concentrations of dissolved organic matter that is leached from organic-rich soils within the river drainage basins. The catchment areas of the blackwater rivers in South America are mainly covered by mineral soils (ferralsols), which feature high contents of organic matter in the upper soil horizons. Blackwater rivers in South East Asia by contrast drain catchments that are dominated by organic soils (dystric histosols), commonly referred to as tropical peat. Approximately 83% of the South East Asian peatsoils are located in Indonesia, mainly on the islands Sumatra, Borneo and Irian Jaya and hold ~3% of the global carbon stored in soils. During the last few decades, deforestation and drainage of peat swamp forests have become common land-use practices in Indonesia mainly for the establishment of oil palm estates resulting in the dissolution of Indonesian peatsoils and enhanced CO2-emissons. The main objectives of this work were to investigate potential environmental impacts of land-use changes on the peat-draining Siak River (Central Sumatra) and to assess the role of Indonesian rivers as source of dissolved organic carbon (DOC) into the ocean.This work was carried out within the framework of the Indonesian/German cooperation SPICE (Science for the Protection of Indonesian Coastal Marine Ecosystems) and included four expeditions to the Siak River. Collected samples were analysed for dissolved inorganic nutrients, DOC, oxygen and amino acids (Appendix). In addition particulate carbon and nitrogen as well as their isotopic compositions were determined in river, soil and terrestrial plant samples (Appendix). DOC decomposition experiments were carried out and annual freshwater discharges of the Siak were determined by in situ measurements as well as by evaluation of precipitation and evaporation data. Based on geographical information systems (GIS) a digital terrain model was established to provide essential hydrological information on the river catchment.The low nutrient concentrations measured in the Siak relative to other rivers not only in Indonesia but world-wide may be attributed to leaching of nutrient-poor peatlands. Nevertheless, there are clear indications that nutrient concentrations in the vicinity of cities, villages and industrial sites were considerably enhanced. Furthermore, washout of fertilizers could be observed during one of the expeditions. Nutrient data measured in a peat-draining river in South Sumatra, which was sampled prior to the main cultivation of oil palms in the 1970s, revealed nutrient concentrations which are much lower than those measured in the Siak. This suggests that nutrient concentrations in the Siak doubled during the last few decades as observed also in other rivers world-wide. Contrary to the nutrient concentrations DOC in the Siak and its tributaries was mainly derived from leaching of the surrounding peatsoils. Due to massive land-use changes leaching could not be considered as natural. Although leaching is assumed to be enhanced the anthropogenic impact is not quantified yet. However, the concentrations that were measured in this study are among the highest riverine DOC concentrations reported so far. The highest concentrations were observed after dry seasons when increasing precipitation rates led to enhanced leaching from soils.The decomposition of DOC was the main factor influencing the oxygen concentrations in the Siak. According to model results an increase in the DOC concentrations of ~15% would be sufficient to produce anoxic conditions in the Siak. The average annual river discharge of the Siak into the river estuary was calculated to be 0.38 Ã ± 0.1 Tg C yr-1 (Tg = 1012 g) where additional DOC inputs into the Siak Estuary derived from peatsoil leaching resulted in an overall discharge of the Siak into the coastal ocean of 0.5 Ã ± 0.3 Tg C yr-1. The DOC discharge of the Siak and other peat-draining rivers increased the DOC concentration in the Malacca Strait by approximately 130 Ã µmol L-1, which resulted in a terrestrial DOC discharge of the Malacca Strait into the Indian Ocean of ~6.4 Tg C yr-1. Therewith ~33% of the Indonesian DOC discharge which has been extrapolated to be ~21 Tg C yr-1 seems to be exported via the Malacca Strait into the ocean. This demonstrates that the numerous small Indonesian rivers are as important as the Amazon with respect to the input of terrestrial-derived DOC into the ocean.