Ecology and climate sensitivity of a groundwater-fed lake on subtropical North Stradbroke Island (Minjerribah), Queensland, Australia over the last 7500 years

Abstract

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.

Contemporary Controls on Terrestrial Carbon Characteristics in Temperate and Sub‐Tropical Australian Wetlands

“…This complexity is reflected in the remaining ∼50% of total variance which is not accounted for by canonical axes I and II. Some of the processes not accounted for in this study may be attributed to TOC/TN ratios of topsoil and leaf litter in the range of aquatic OM (<10–20, Holtvoeth et al., 2016; Forbes et al., 2021; Maxson, Tibby, Barr, et al., 2021), as well as different degrees of soil‐organic matter decomposition in the catchments across the sites studied herein. Most significantly, TOC/TN < 4 in combination with low TOC, as reported for Booberoi Lagoon (MDB‐QLD), Moira Lake (MDB‐NSW (south)), and 2 Carp Billabong (MDB‐VIC), can be attributed to decomposition controlling C‐loss and clay‐bound ammonium supply from the catchment promoting increasing N concentrations (Francke et al., 2016; Holtvoeth et al., 2016).…”

“…TOC/TN and δ 13 C from Bishops Swamp, located in the vicinity of the MDB along the east coast of NSW, suggest a more terrestrial dominated organic matter source (Figure 2). Values from the three North Stradbroke Island sites plot outside the traditionally defined range for both aquatic and C3 dominated organic matter, which may reflect a mixture of aquatic and terrestrial‐derived organic sources as well as additional processes affecting TOC/TN and δ 13 C. Previous studies demonstrated that organic matter accumulated in Blue Lake and other North Stradbroke Island sites (Cadd et al., 2018; Maxson, Tibby, Barr, et al., 2021; Maxson, Tibby, Marshal, et al., 2021) is predominantly of aquatic origin, despite an elemental and carbon isotope signature more characteristic for terrestrial sources. This has been attributed to N‐limitation artificially increasing TOC/TN, similar to the process described for Tasmania (Maxson, Tibby, Barr, et al., 2021; Maxson, Tibby, Marshal, et al., 2021).…”

“…The poor correlation may be attributed to the complex behavior of δ 15 N in general, and further amplified by the wide range of sites investigated herein. For example, wetlands on North Stradbroke Island and Tasmania are generally nitrogen limited (Cadd et al., 2018; Fletcher et al., 2014; Mariani et al., 2018; Maxson, Tibby, Barr, et al., 2021). Nitrogen limitation should result in δ 15 N values reflecting those of source nitrogen rather than isotope fractionation within the wetland linked to primary productivity (Cadd et al., 2018; Leng et al., 2005; Talbot, 2002).…”

“…Where multiple analyses were feasible for sites on North Stradbroke Island and Tasmania, low 1σ standard deviations in TOC, TN, TOC/TN, δ 15 N, and δ 13 C imply that sediment organic geochemical composition remained broadly constant with depth throughout the analyzed deposits (Table 2). The top 4 cm of sediments likely account for several centuries for sites in Tasmania and Blue Lake on North Stradbroke Island (e.g., Mariani et al., 2017; Maxson, Tibby, Barr, et al., 2021; Schneider et al., 2020), and for several decades for sites located in the MDB and Swallow Lagoon and 18 Mile Swamp on North Stradbroke Island (Barr et al., 2019; Kattel et al., 2016; Mettam et al., 2011; Reid, Chilcott, & Thoms, 2017) as inferred from previous studies on the sites studied herein and other sites from their vicinity. The homogenous nature of the sediments despite the extended time cover for the Tasmanian sites may be due to physical or biological mixing of the uppermost sediments or to their (partly remote) location in relatively protected national parks, preventing major post‐colonial anthropogenic disturbance in recent decades, both in the catchment and wetlands.…”

“…Given the complexities involved, interpretation of organic carbon and nitrogen concentrations and isotopic composition in the sedimentary archive should ideally be supported by modern calibrations and process studies. However, despite hundreds of studies, which apply these tracers to reconstruct past carbon cycling and deposition in sediments, remarkably few investigations have been conducted on the modern environment (Talbot, 1990; Teranes & Bernasconi, 2000; Lehmann et al., 2002; Teranes & Bernasconi, 2005; Moschen et al., 2009; Cadd et al., 2018; Maxson, Tibby, Barr, et al., 2021 to name a few exceptions).…”