Chemistry of bulk precipitation in southwestern Viti Levu, Fiji

The amounts and chemical composition of bulk precipitation were investigated over a continuous 21-mo period (January 1990 to September 1991) at four sites along an East-West transect perpendicular to the coast of Southwest Viti Levu, Fiji. Measured rainfall totals in 1990 ranged from 1796 mm at the coastal Korokula station to 2113 mm at the inland Tulasewa station, which is somewhat higher than the long-term average of 1707 mm at a reference weather station located in the centre of the study area. The first 9 mo of 1991 were relatively dry (range 1027-1533 mm) with a total of 1157 mm at the reference site as compared to a long-term average of 1330 mm. Concentrations of all investigated constituents in bulk rainfall were low, except during the passage of cyclone Sina due to the deposition of large amounts of, especially, chloride, sodium and sulphate in sea spray. Concentrations of sodium and magnesium could be explained fully by maritime contributions to the rainfall composition at all sites, Maritime contributions to the concentrations of calcium, sulphate and potassium accounted for 10-40% of the total, whereas bicarbonate, ammonium, nitrate, silicon, aluminium, iron and manganese were derived exclusively from terrestrial sources. The annual atmospheric nutrient deposition rates were low by pan-tropical standards, particularly when the contribution of cyclone Sina was excluded. Annual totals (in kg ha(-1)) ranged from 2.4-8.8 for nitrogen, 0.4-1.1 for phosphorus, 2.3-4.9 for potassium, 1.4-1.9 for calcium and from 1.1-1.3 for magnesium. The inclusion of the contribution by the cyclone more than doubled the deposition of potassium, calcium and magnesium, although values still remained well within the range reported for humid tropical areas. The estimated atmospheric deposition of nutrients over a typical rotation period (16 y) was sufficient to balance losses in harvested Pinus caribaea logs (stemwood plus bark) of potassium, calcium and magnesium, but not of nitrogen and, probably, phosphorus. Nutrient losses associated with the harvesting of stemwood alone were compensated entirely by the atmospheric inputs.