Dissolved Inorganic Nitrogen and Phosphate in the Human Affected Blackwater River Siak, Central Sumatra, Indonesia
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 peatdraining blackwater river Siak. During seven expeditions between 2004 and 2009 dissolved inorganic nutrients (NO3–, NO2–, NH4
+ and PO43–), chlorophyll-a (chl a), particulate and dissolved organic matter (POM, DOM) and stable carbon isotopes (δ13Corg) 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 PO43– concentration in the Siak. Wastewater
discharges increased especially the PO43– 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.
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