Urea accounted for 30, 36, and 56%, respectively, of the water-soluble organic nitrogen (DON) compounds deposited from the atmosphere at two sites in New Zealand and one site in japan. The other DON compounds were not identified but they were all of low molecular size, apparently anionic, and did not include detectable quantities of amino acids or uric acid. In axenic culture, Chlorella spp. grew successfully using DON from New Zealand precipitation as its only source of N; Pediastrum biwae, a phytoplankton endemic to Lake Biwa in Japan, also achieved successful growth using DON separated from Japanese precipitation. Ammonium-N and DON were deposited from the atmosphere in the absence of rain or snow, and we suggest that particulate matter, possibly soil, is a major source of these N compounds in atmospheric precipitation.

Dissolved salts in lake waters of the Taupo Volcanic Zone consist predominantly of HCO 3 " salts from 'normal' weathering of rocks and soils by carbonic acid, Cl~ salts from precipitation and geothermal water, and SO 4 2~ salts from injection into the lakes or catchments of sulphuric acid from geothermal steam. Precipitation and cold spring waters are considered to be the input and final product, respectively, of normal weathering. The concentrations of major ions (Na + , K + , Ca 2+ , Mg 2+ , HCO 3 -, C1 -, SO 4 2-) in these waters and in geothermal water and steam were used to calculate the origins of the dissolved salts in each lake. Rhyolitic strata dominate most lake catchments, but in some the presence of andesite, basalt, or hydrothermal ejecta influences the major ion ratios in the fraction of the dissolved salts derived from weathering. Each of the 32 lakes studied was classified according to the origins of its lakewater dissolved salts and the effect on these salts of its catchment geology.

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