A 120 year record of changes in nitrogen and carbon cycling in Lake Alexandrina, South Australia: C:N, δ15N and δ13C in sediments

Variations in δ15N and δ13C and C:N ratios of organic matter in sediment cores in Lake Alexandrina, South Australia show that there have been modifications of N and C sources due to land use changes, water diversions and irrigation since European settlement of the surrounding basin in the middle 1800s. δ15Norg and δ13Corg concentrations from three short sediment cores range from 1.8 to 3.9‰ and −26.7 to −21.1‰, respectively. All cores showed a not, vert, similar2‰ increase in δ15N over the top 100 mm with little variation below about 150 mm. δ13C values increased by about not, vert, similar3‰ from the base of the sediment cores to a maximum of not, vert, similar−21‰ at 150 mm depth, before decreasing by not, vert, similar5‰ to not, vert, similar–27‰ over the top 100 mm. C:N mass ratios <11 suggest that organic matter preserved in the sediment is composed primarily of aquatic plant material, with terrestrial plants contributing <10%. Excess 210Pb and 137Cs dating indicate a sediment accumulation rate of 3±1 mm a−1 suggesting the enrichment in 15N began in the 1950s and the δ13C maxima corresponds to approximately 1940. The increase in δ15N concentration over the past 30–50 a is attributed to the increasing agricultural expansion and development in the lower catchment, particularly intensive grazing of cattle for dairying. The authors suggest that there has been an increased flux of 15N-enriched inorganic N (derived from mineralized animal waste and sewage) to the river and lake over the past 30–40 a, which in turn is incorporated into the aquatic plants. The variation in δ13C reflects a change from terrestrial dominated input of inorganic C to increasingly marine dominated sources after water diversions commenced in the late 1800s. Once barrages were completed at the outlet of the lake in 1939 (to prevent seawater intrusion), the aquatic plants in the lake reverted to assimilation of dissolved inorganic C derived from oxidation of terrestrial organic matter.