Trace metals and nutrients in tropical volcanic island rivers: Insights on chemical weathering and anthropogenic influences of Pohnpei, Micronesia

Abstract

River systems in tropical islands play a vital role in maintaining the ecosystem function of surrounding oceans by delivering land-derived materials. Researches on tropical river systems frequently focused on continents and peninsulas, while little research has been conducted in tropical islands. In the present study, we conducted field surveys in a Pacific volcanic island, Pohnpei, Micronesia, and determined the substance composition of river waters (major ions, trace metals, nutrients, etc.) to quantify terrestrial material loading and biogeochemical influence on the surrounding oceans. In Pohnpei, total dissolved solutes were observed to be 71.4 mg/L; the NH₄⁺, NO₃⁻, PO₄⁻, and SiO₃^2- concentrations were 0.46, 3.9, 0.52, 221.7 μmol/L, respectively; and NO₂⁻ was lower than the detection limit (0.14 μmol/L). The trace metals concentrations of rivers in Pohnpei Island were much lower than records in tropical rivers of the world. The variation in concentration levels of major ions and nutrients among rivers was relatively limited. A similar pattern was observed in the distribution of Al and V. Trace elements, including As, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, Se, Sr, U and Zn in rivers were most enriched in Urban Area (human activity dominated region), compared with the Mixing Area (Multiple effect region) to Natural Area (natural weathering region). Exchangeable trace metals concentrations from suspended particulate matter (SPM) were lower than in dissolved state in river waters, while water-soluble, while acid-soluble metals concentrations in the sediment were much higher than in dissolved state in rivers. In our results, trace metals in Pohnpei rivers may mainly be influenced by domestic sewage and agricultural activity—these data can be used in biogeochemical models to estimate the contribution of domestic sewage. Furthermore, we have quantified the land-derived input of trace metals from tropical volcanic islands in Pacific (TVIP) into the surrounding oceans. Though the land area of TVIP only accounts for 0.1 % of global land area, the terrigenous inputs of some trace metals from TVIP account for approximately 1 %—and even more than 2 % of those in global riverine flux. Therefore, the nutrients and trace metals inputs from TVIP would be significant for the cycle of biogenic elements in the receiving ocean.