Land use, soil properties and weather conditions influence nutrient fluxes into a deep oligotrophic lake

Abstract

Many streams and rivers in upland watersheds in southern New Zealand drain into deep, oligotrophic lakes with major aesthetic, recreational and conservation values. We examined the effects of increasing pasture cover and weather-related variables on nitrogen (N), phosphorus (P) and dissolved organic carbon (DOC) concentrations in eight streams draining into Lake Wanaka and hypothesised that (1) DOC and N concentrations would increase with increasing land development, but soil characteristics in the watershed would mediate P input and (2) weather-related factors that increase hydrological connectivity in the landscape would enhance the influx of N and DOC. Agricultural development correlated positively with N and DOC concentrations in stream water, but temperature and soil moisture mitigated the influence of pasture cover on surface-water DOC concentration under very dry or very wet conditions. Weather-related factors did not influence N concentrations in streams. Neither land use nor weather-related conditions correlated with concentrations of dissolved P, possibly reflecting good P-binding in soils and lack of sampling during high-flow events. Our results provided evidence that agricultural development in grassland watersheds increases the concentrations of N and DOC entering Lake Wanaka, and weather and soil conditions mediate the amount of DOC transferred from soils into streams.