Behaviour of Dissolved Phosphorus with the associated nutrients in relation to phytoplankton biomass of the Rajang River-South China Sea continuum

Abstract. Nutrient loads carried by large rivers and discharged into the continental shelf and coastal waters are vital to support primary production. Our knowledge of tropical river systems is still fragmented with very few seasonal studies available for Southeast Asia for example, despite estimates that these systems are among the hotspots globally for nutrient yields. The Rajang river, the longest river in Malaysia, is a tropical peat-draining river which passes through peat-domes in the estuary and has mass discharge of organic matter into the South China Sea. Three sampling campaigns (August 2016, March 2017 and September 2017) were undertaken along ~ 300 km of the Rajang river to study both spatial and seasonal distribution of nutrients and its fate in the coastal region. The analyses for nutrients encompass both inorganic (i.e Nitrate, NO₃⁻, Nitrite, NO₂⁻, Ammonium, NH₄⁺, Phosphate, PO₄⁻ (DIP) and Silicate, dSi) as well as organic (Dissolved organic nitrate, DON and Dissolved organic phosphate, DOP) fractions. It was found that DIP concentration was not seasonally influenced but was spatially different along the salinity gradient whereas DOP was both seasonally and spatially different. Both DIP and DOP exhibited non-conservative behaviour in the mixing. DIP was subjected to 57.78 % removal whereas DOP was subjected to 44.07 % addition along the salinity gradient towards the South China Sea. The bulk of the dissolved phosphate is from DOP (73.84 %), in which both DIP and DOP may have contributed to the phytoplankton biomass. Spearman’s correlations show that there was a switch in preference for DOP as compared to DIP depending on the concentrations of DIP or DOP due to seasonality. The main limitation in the Rajang River was assumed to be DIP based on the Redfield ratio. During the dry season, the NO₃-N : DIP ratios were lower, which were ideal conditions for phytoplankton proliferation while in the wet season, the increased NO₃-N : DIP ratios led to lower phytoplankton biomass. Overall, the Rajang River exports 0.12 t DIP mth⁻¹ into the South China Sea which is relatively low as compared to other major peat-draining rivers in the world. At the current pace of deforestation and the projected intensification of rainfall in the region, this finding provides an important baseline of the inventory of DIP into the South China Sea. Our results also show that local variations are important to consider for future models and that the assumption/generalization of SEA as a nutrient hotspot might not hold true for all regions and requires further investigations.