The distribution and behaviour of Fe, Al, Si, Mn, Cu and Ni in ombrotrophic tropical peat draining blackwater estuaries on Borneo Island

Tropical peat swamps are essential ecosystems, which provide numerous services, and also serve as a rich source of dissolved organic carbon (DOC), hydrogen ions and trace elements to peat draining rivers. However, not much is known about trace element export from tropical peat swamps. We investigated trace element dynamics in rivers and estuaries draining tropical peat swamps on Borneo, and examined the influence of estuarine processes as well as dissolved organic carbon (DOC) on the distribution and concentration of trace elements. Our results indicate acidic conditions (pH = 3.3) and high DOC concentration (3500 mu mol L-1) at salinities<1. We observed an initial release of trace elements at low salinity (0.05< 0.5), followed by scavenging to particles at intermediate salinities (0.5<10) due to an increasing ionic strength and pH. Peak concentrations (mu mol kg (-1)) of Al (24.9), Si (96.2), Mn (4.9), Cu (0.035) and Ni (0.047) were observed during the dry season (July), and Fe concentrations (43.2) were highest during the wet season (December). We used the NICA-Donnan model to investigate the combined impact of DOC and pH on the formation of solid iron hydroxide (Fe(OH)(3)(s)). The Maludam river was predicted to be supersaturated for Fe hydroxides and the results affirmed our model prediction. The output showed Fe and Cu had a strong affinity for DOC and to a lesser extent Al and Ni in the conditions prevailing at the study sites. Statistical analyses also indicated strong correlation between Cu and Ni (r(2) = 0.97, 0.94 and 0.82) in Maludam, Sebuyau and Belait rivers and estuaries, respectively. The results obtained in this study are comparable to values published for southeast Asia and other continents for pristine peat draining rivers.

Automated summary

Tropical peat swamps are important ecosystems that provide various services and contribute significantly to the export of dissolved organic carbon (DOC) and trace elements to peat draining rivers. This study investigated trace element dynamics and their relationship with estuarine processes, as well as the influence of DOC and pH on their distribution in rivers and estuaries on Borneo. The results showed initial release and subsequent scavenging of trace elements at different salinities, with different peak concentrations observed during different seasons. The study also confirmed the strong affinity of certain trace elements to DOC and the correlations between certain trace elements in the study sites.