Changes in the chemistry of sedimentary organic matter within the Coorong over space and time

Like many other coastal systems across the world, the Coorong lagoonal ecosystem (South Australia) has degraded over the last 100 years; in this case as a result of extensive regulation and diversions of water across the Murray-Darling Basin following European settlement. To evaluate whether the sources of organic matter (OM) supporting its food-web have changed since the inception of water management and barrage construction, sedimentary OM was characterised in cores spanning the Coorong’s salinity gradient at depths representative of the last 100 years over which the management alterations to river and estuarine flow were most marked. Detailed ²¹⁰Pb, ¹³⁷Cs and Pu dating in conjunction with palaeolimnological data (Pinus pollen) allowed for the reconstruction of the timing of substantial changes observed in the composition of the OM, most of which occur during the early 1950s, concurrent with management-related variations in water flow and salinity. Negative shifts in δ¹³C of up to 8.3‰ in the 2–10 and <2 μm fractions after the 1950s suggest a pronounced alteration in biogeochemical cycling or in the origin of OM. Elemental ratios and δ¹³C values of potential sources are inconclusive as to the cause of these biogeochemical changes. However, ¹³C-NMR spectra of the sediments suggest that degraded phytoplankton constitutes a large proportion of today’s OM and also reveal that an OM source rich in lignin was present prior to the 1950s. The high δ¹³C (?18.3‰) and low C/N (7.5) signatures of the lignin-bearing sediments are inconsistent with a C3 terrestrial OM source and instead suggest that the lignin-bearing seagrass Ruppia megacarpa (δ¹³C of ?13‰) contributed to a large degree to the sediment of the North Lagoon. R. megacarpa once was abundant in the North Lagoon but today has all but vanished from the system. Thus, only through a combination of isotopic and spectroscopic techniques was it possible to effectively decipher the changes in the composition of OM deposited throughout the Coorong over space and time. These results have important implications for research in estuarine OM dynamics in other geographic locations. Specifically, utilising complementary analytical techniques may sometimes be essential in reliably determining OM sources and processes in estuaries and lagoons.