Tropical blackwater biogeochemistry [Elektronische Ressource] : the Siak river in Central Sumatra, Indonesia / vorgelegt von Antje Baum

Abaum

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• Biologie

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• Publié par: universitat_bremen
• Publié le: 01 janvier 2008
• Nombre de lectures: 35
• Langue: English
• Poids de l'ouvrage: 3 Mo

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












Dissertation
zur Erlangung des Doktorgrades
der Naturwissenschaften (Dr. rer. nat.)



vorgelegt von
Antje Baum






Bremen 2008





















Advisory Committee:

1. Reviewer: Dr. Tim Rixen
Center for Tropical Marine Ecology (ZMT), Bremen, Germany

2. Reviewer: Prof. Dr. Wolfgang Balzer
University of Bremen

1. Examiner: Prof. Dr. Venugopalan Ittekkot
Center for Tropical Marine Ecology (ZMT), Bremen, Germany

2. Examiner: Dr. Daniela Unger
Center for Tropical Marine Ecology (ZMT), Bremen, Germany I
Contents

Summary .................................................................................................................... III
Zusammenfassung...................................................................................................VII
1. Introduction........................................................................................................ 11
2. Published and submitted papers..................................................................... 15
2.1. Sources of dissolved inorganic nutrients in the peat-draining river Siak,
Central Sumatra, Indonesia ................................................................................... 15
2.2. The Siak, a tropical black water river in central Sumatra on the verge of
anoxia ..................................................................................................................... 31
2.3. Relevance of peat draining rivers in central Sumatra for riverine input of
dissolved organic carbon into the ocean ................................................................ 55
2.4. DOC discharges from the Indonesian blackwater river Siak and its estuary
into the Malacca Strait and their role as DOC source for the Indian Ocean .......... 69
3. General conclusions......................................................................................... 83
4. Future perspectives .......................................................................................... 85
5. References ......................................................................................................... 87
Appendix ................................................................................................................... 99







II







III
Summary
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 CO -emissons. 2

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 IV Summary
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
-1 12to be 0.38 ± 0.1 Tg C yr (Tg = 10 g) where additional DOC inputs into the Siak
Estuary derived from peatsoil leaching resulted in an overall discharge of the Siak
-1into the coastal ocean of 0.5 ± 0.3 Tg C yr . The DOC discharge of the Siak and
other peat-draining rivers increased the DOC concentration in the Malacca Strait by
-1approximately 130 μmol L , which resulted in a terrestrial DOC discharge of the
-1Malacca Strait into the Indian Ocean of ~6.4 Tg C yr . Therewith ~33% of the
-1 Indonesian DOC discharge which has been extrapolated to be ~21 Tg C yr seems Summary V
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. VI
VII
Zusammenfassung
Der wohl bekannteste tropische Schwarzwasserfluss ist der Rio Negro, einer der
größten Nebenflüsse des Amazonas in Südamerika. Zusammen mit weiteren
Nebenflüssen des Amazonas und Orinocos (Südamerika) gehört er zu den am
intensivsten untersuchten Schwarzwasserflüssen weltweit. Schwarzwasserflüsse
entwässern Einzugsgebiete, deren Böden einen hohen Anteil an organischem
Material aufweisen, dessen Auswaschung wiederum zur dunkel-braunen Färbung
des Flusswassers führt. Die Flusseinzugsgebiete der südamerikanischen Flüsse
Amazonas und Orinoco sind zu großen Teilen von mineralischen Böden
(Ferralsolen) dominiert, deren Oberböden häufig einen hohen Anteil an organischer
Substanz aufweisen. In Süd-Ost-Asien hingegen entwässern Schwarzwasserflüsse
hauptsächlich Einzugsgebiete mit einem sehr hohen Anteil an organischen
Torfböden, die als dystrische Histosole klassifiziert werden. Etwa 83% der Torfböden
Süd-Ost-Asiens liegen auf den indonesischen Inseln Sumatra, Borneo und Irian Jaya
und speichern etwa 3% des weltweit in Böden gebundenen Kohlenstoffs. Im Zuge
der Errichtung von Ölpalmplantagen hat die Abholzung von Torfwäldern und
Entwässerung von Torfböden in Indonesien in den vergangenen Jahrzehnten
drastisch zugenommen. Ein verstärkter Abbau dieser organischen Böden und eine
damit einhergehende Erhöhung von CO -Emissionen sind die Folgen. 2

Es war daher Ziel dieser Arbeit, mögliche Auswirkungen der Landnutzungs-
veränderungen auf den Fluss Siak (Zentral-Sumatra), dessen Einzugsgebiet einen
hohen Anteil solcher Torfböden aufweist und