Carbon sources and sequestration: 14C Ramped Pyroxidation in aquatic sediments.

<p>Sequestration of organic carbon in aquatic sediments can depend on its source and potential lability. Studies have shown that bulk lake and marine sediment comprises carbon of different origin but its source has been difficult to attribute. A new Ramped Pyroxidation/Combustion (RPO) system in the <sup>14</sup>CHRONO Centre has been established. RPO is a technique that incrementally heats a sample, and allows for collection of the CO<sub>2 </sub>produced for radiocarbon analyses. The results show its utility in partitioning carbon sources in lake sediment (Rostherne Mere, UK, Santa María del Orolake, Mexico), and arctic marine sediment (Chukchi Sea and Beaufort Shelf). RPO and 2-stepped combustion<sup>1</sup>. <sup>14</sup>C indicated multiple carbon sources in Rostherne Mere sediment, some of which could be attributed to the construction of a sewage treatment works (STW) on the lake shore, and subsequently inputs from this STW. RPO identified 3 carbon fractions in Mexican Lake sediment, which provided a more accurate chronology, partitioning the contemporaneous sediment date from offsets induced from volcanic activity in the area. Results from Arctic marine sediment demonstrated inputs of carbon from ancient permafrost, providing a means to refine the chronologies and a basis for future research linked with carbon loss from thawing permafrost.</p><p><sup>1</sup>Keaveney et al. 2020. Journal of Palaeolimnology 64 347-363</p>

Source and quantity of carbon influence its sequestration in Rostherne Mere (UK) sediment: a novel application of stepped combustion radiocarbon analysis

We explored the roles of phytoplankton production, carbon source, and human activity on carbon accumulation in a eutrophic lake (Rostherne Mere, UK) to understand how changes in nutrient loading, algal community structure and catchment management can influence carbon sequestration in lake sediments. Water samples (dissolved inorganic, organic and particulate carbon) were analysed to investigate contemporary carbon sources. Multiple variables in a 55-cm sediment core, which represents the last ~ 90 years of accumulation, were studied to determine historical production rates of algal communities and carbon sources. Fluctuations in net primary production, inferred from sedimentary diatom abundance and high-performance liquid chromatography (HPLC) pigment methods, were linked to nutrient input from sewage treatment works (STW) in the catchment. Stepped combustion radiocarbon (SCR) measurements established that lake sediment contains between 11% (~ 1929 CE) and 69% (~ 1978 CE) recalcitrant carbon, with changes in carbon character coinciding with peaks in accumulation rate and linked to STW inputs. Catchment disturbance was identified by radiocarbon analysis, and included STW construction in the 1930s, determined using SCR analysis, and recent nearby highway construction, determined by measurements on dissolved organic carbon from the lake and outflow river. The quantity of autochthonous carbon buried was related to diatom biovolume accumulation rate (DBAR) and decreased when diatom accumulation rate and valve size declined, despite an overall increase in net carbon production. HPLC pigment analysis indicated that changes in total C deposition and diatom accumulation were related to proliferation of non-siliceous algae. HPLC results also indicated that dominance of recalcitrant carbon in sediment organic carbon was likely caused by increased deposition rather than preservation factors. The total algal accumulation rate controlled the sediment organic carbon accumulation rate, whereas DBAR was correlated to the proportion of each carbon source buried.

The Influence of Climate Change on the Restoration Trajectory of a Nutrient-Rich Deep Lake

Nutrient reduction in impacted lowland freshwater systems is ecologically and culturally important. Gaining a greater insight into how lakes respond to lowering nutrient loads and how climate-driven physical limnology affects present and future cycling of available nutrients is important for ecosystem resource management. This study examines the nutrient decline in a hypereutrophic freshwater lake (Rostherne Mere, Cheshire, UK) 25 years after sewage effluent diversion, a uniquely long-term analysis of a recovering nutrient-rich deep lake. Using nutrient, phytoplankton, climate and catchment hydrological monitoring, the contemporary lake system is compared to previous studies from 1990 to 2002. Nutrient change since point source load diversion showed annual average and maximum phosphorus (P) concentrations decreased significantly for the first 10 years (1992: ~ 600 µg P L−1; 2002: ~ 200 µg P L−1), but have since stabilised due to a substantial legacy sediment P internal load. Dissolved inorganic nitrogen (DIN) concentrations have not substantially changed since diversion, resulting in the alteration of the DIN/SRP ratio from a system characterised by N limitation (N:P ~ 5), to one predominantly P limited (N:P > 20). Nutrient changes over this time are shown to drive ecological change, especially in the cyanobacterial and algal communities. Furthermore, very high-resolution monitoring of lake inflow and outflow (every 5 min during 2016) shows that water residence time at this lake is significantly shorter than previously estimated (~ 0.8 years compared to previous estimates of ~ 1.6–2.4 years). Together with long-term data demonstrating that the stratification period at Rostherne Mere has increased by 40 days over the last ~ 50 years (due to later autumnal mixing), we show that a rapid rate of epilimnetic flushing together with a long stratification period substantially reduces the available epilimnetic P during the summer cyanobacterial bloom. This is of growing importance for many such lakes, given widespread climate-driven lengthening of stratification and a national trend of decreasing summer rainfall (decreasing seasonal flushing) but more intense summer storm events (resulting in short-term flushing events).

Water and Nutrient Budgeting of Rostherne Mere, Cheshire, UK

This paper presents a simple model of the Rostherne Mere water catchment. The model allows calculation of water and elemental budgets on meteorological data using regressions derived from a monitoring data set. Estimates of yearly inputs and outputs of Ca, Mg, K and N suggest that the lake is currently acting as a sink for these elements. Comparison with earlier estimates shows an increase in N loading. High inputs of particulate P appear to be balanced by outputs of inorganic P, suggesting that the two may be coupled by certain physical and biogeochemical processes within the lake ecosystem, including adsorption to/desorption from particles, coprecipitation with various chemical compounds, uptake and excretion by biota, mineralisation and release from sediments.