The changing face of Lake Okataina’s ‘phosphorus scape’: Who is the culprit?

Lake sediments contain a great wealth of data on biogeochemical and limnological processes of historical importance, which can be used to interpret natural ecosystem dynamics, and climatic and anthropogenic impacts. In this study, this information was used to understand the dynamics of phosphorus speciation in deep Lake Okataina, to assist with understanding how changes in the catchment influence lake productivity. A 49-cm sediment core was retrieved from Lake Okataina and its depositional history was dated using 210Pb and tephra chronology. Amongst New Zealand lakes, Okataina may be considered close to a ‘reference’ lake, with 89% native forest and without a significant change in its land use in the past c. 800 years. However, perturbations to the lake during this time may be due to expanding populations of invasive terrestrial mammals, earthquakes, volcanic eruptions and tree felling, which may alter the geochemistry of the soils in the catchment, composition of runoff and, ultimately, the composition of the bottom sediments of the lake. We hypothesised that changes in soil phosphorus geochemistry in the catchment will result in a changing phosphorus species in the lake sediments