Lake sediments are important archives of past climate variability and lake responses to climate. In order to accurately infer past climates, it is necessary to understand, and account for, the ecological processes that affect the record of indicators preserved in lake sediment. This is particularly the case with respect to the concentration of carbon and nitrogen (TOC, TN, and calculated C:N), and the stable isotope composition of organic matter preserved in lake sediments. These are common, yet ambiguous, tracers of environmental change. Ideally, palaeoenvironmental reconstructions using the concentration and isotope composition of organic matter should be grounded in a detailed understanding of the sources of the organic material. This study documents the history and evolution of Blue Lake, an environmentally and culturally important oligotrophic, groundwater window lake on North Stradbroke Island, Queensland, Australia. We utilise organic matter δ 13 C, TOC, TN, and C:N from a 2.4 metre sediment core with a basal age of 7.5 cal kyr BP, to investigate changing organic matter sources as a measure of the climate sensitivity of Blue Lake. This interpretation is supported by data from contemporary algae, aquatic and terrestrial plants, and catchment soils. We show that lake nutrient dynamics drove an increase in algal biomass at 4.2 cal kyr BP. This change coincides with a widely documented intensification of the El Niño-Southern Oscillation, which we infer to have influenced lake nutrient concentrations by reducing groundwater throughflow. Climatic changes resulted in marked changes in lake primary productivity, despite relatively little turnover of the lake diatom flora and catchment vegetation. This suggests that south-east Queensland dune lakes are sensitive to climate changes and helps to refine past and future palaeoclimate research using sediments from these lakes. It also indicates that increased nutrient concentrations in Blue Lake may result from projected changes in 21 st Century climate.

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Carbon stable isotopes (δ13C) in modern seawater samples and planktic foraminifera Globigerina bulloides from core top and downcore sediments are used to estimate the distribution of δ13C of dissolved inorganic carbon (DIC) in the surface waters of the southwest Pacific in the modern, preindustrial (PI), and over the last 25 kyr. The predicted δ13C distribution in the modern (δ13CDIC), PI (δ13CPI), and late Holocene (from planktic foraminifera Globigerina bulloides [temperature corrected δ13CG.bulloidesTC] from core tops) displays a broad peak at the subtropical front) and subantarctic surface waters due to the combination of high biological productivity and thermodynamic air‐sea gas exchange of CO2 in this region. The estimated δ13CPI values and measured δ13CG.bulloidesTC values from the core tops are higher than the modern values due to the Suess Effect. However, there is poor agreement between the δ13CPI values and core top δ13CG.bulloidesTC values south of 40°S as the back‐calculation approach using chlorofluorocarbon‐11 (CFC‐11) method for removing the anthropogenic δ13C is not effective at these higher southern latitudes. The δ13CG.bulloidesTC from a latitudinal transect of cores in the southwest Pacific were compiled by region using a Monte Carlo approach to determine the long‐term trends in δ13C over the last 25 kyr. Glacial subantarctic δ13CG.bulloidesTC values are low, while subtropical δ13C G.bulloidesTC are high. The peak in δ13CG.bulloidesTC values shifts south in the early Holocene. These latitudinal variations in δ13C G.bulloidesTC are linked to changes in ocean circulation, biological productivity (associated with the shifts in the subtropical front), and air‐sea CO2 exchange, likely related to the structure and position of the Southern Hemisphere Westerly Wind in the South Pacific region.