Abstract: Increases in domestic, industrial and agricultural effluents have
nearly doubled global riverine N and P fluxes into the coastal ocean during the
last few decades. Indonesian rivers were modelled to be one global "hot spot"
with respect to N and P yields (kg km^{-2} yr^{-1}) as a consequence of large-scale
deforestation, intensive agriculture, urbanization and wastewater disposal. The
objectives of this field study were (i) to identify sources of dissolved
nutrients and (ii) to investigate the impact of anthropogenic activities on
nutrient levels in the peat-draining blackwater river Siak. During seven
expeditions between 2004 and 2009 dissolved inorganic nutrients (NO_3^-, NO_2^-,
NH_4^+ and PO_4^{3-}), chlorophyll-a (chl a), particulate and dissolved organic
matter (POM, DOM) and stable carbon isotopes (δ^{13}C_{org}) of POM were
determined along the river continuum as well as in urban sewage channels. The
results showed that the Siak is a classical blackwater river characterized by
high levels of dissolved organic matter (DOM) and low nutrient concentrations.
Nevertheless, compared to other tropical blackwater rivers, nutrient
concentrations are enriched indicating that the Siak is eutrophied.
Decomposition of DOM leached from the surrounding peat soils is one factor
controlling the DIN and the PO_4^{3-} concentration in the Siak. Wastewater
discharges increased especially the PO_4^{3-} concentrations, which exceed the
background concentration locally by a factor >4. The washout of
N-fertilizers from palm oil estates seems to be a main factor influencing the
DIN concentration and could even double the riverine DIN concentration as seen in March, 2004.
Keywords: Blackwater river, peat, dissolved inorganic nutrients, wastewater, fertilizer
