A 120 year record of changes in nitrogen and carbon cycling in Lake Alexandrina, South Australia: C : N, delta N-15 and delta C-13 in sediments

Variations in delta N-15 and delta C-13 and C:N ratios of organic matter in sediment cores in Lake Alexandrina, South Australia show that there have b een modifications of N and C sources due to land use changes. water diversi ons and irrigation since European settlement of the surrounding basin in th e middle 1800s. delta N-15(org) and delta C-13(org) concentrations from thr ee short sediment cores range from 1.8 to 3.9 parts per thousand and -26.7 to -21.1 parts per thousand, respectively. All cores showed a similar to2 p arts per thousand increase in delta N-15 over the top 100 mm with little va riation below about 150 mm. delta C-13 values increased by about similar to 3 parts per thousand from the base of the sediment cores to a maximum of si milar to -21 parts per thousand at 150 mm depth, before decreasing by simil ar to5 parts per thousand to similar to -27 parts per thousand 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 Pb-210 and Cs-137 dating indicate a sedim ent accumulation rate of 3 +/- 1 mm a(-1) suggesting the enrichment in N-15 began in the 1950s and the delta C-13 maxima corresponds to approximately 1940. The increase in delta N-15 concentration over the past 30-50 a is att ributed to the increasing agricultural expansion and development in the low er catchment, particularly intensive grazing of cattle for dairying. The au thors suggest that there has been an increased flux of N-15-enriched inorga nic N (derived from mineralized animal waste and sewage) to the river and l ake over the past 30-40 a, which in turn is incorporated into the aquatic p lants. The variation in delta C-13 reflects a change from terrestrial domin ated input of inorganic C to increasingly marine dominated sources after wa ter diversions commenced in the late 1800s, Once barrages were completed at the outlet of the lake in 1939 (to prevent seawater intrusion), the aquati c plants in the lake reverted to assimilation of dissolved inorganic C deri ved from oxidation of terrestrial organic matter. (C) 2000 Elsevier Science Ltd. All rights reserved.