ANTHROPOGENIC MARKER EVIDENCE FOR ACCELERATED SEDIMENTATION IN LAKE ILLAWARRA, NEW-SOUTH-WALES, AUSTRALIA

The chronology of near-surface sediments in Lake Illawarra has been investigated using radiocarbon dating and anthropogenically derived substances including trace metals, ash, and Cs-137. Sediments at depths about Im below the water-sediment interface ranged in age from Modern to 786 calendar years sp on the basis of radiocarbon dating of Notospisula trigonella valves. Multiple marker (for example ash-trace metals) depth-concentration sediment profiles yielded estimates of sedimentation rate ranging from 3 to 5 mm yr(-1) at Griffins Bay to more than 16 mm yr(-1) at Macquarie Rivulet. Sedimentation rates of approximately 10 mm yr(-1) appeared to be typical of the western and southwestern portions of the lagoon. Rates of sediment accretion, prior to catchment clearing, urbanization, and industrialization have been estimated at less than 1 mm yr(-1), thus indicating a general tenfold increase in sediment accumulation adjacent to the western foreshore caused by catchment development. Accelerated sedimentation in shallow coastal lagoons constitutes significant environmental impacts including shoaling, degradation of seagrass beds, and increased turbidity with consequent loss of aesthetic appeal. Management policy should be directed at attempts to reduce the amount of sediment input by the construction of strategically placed sediment retention ponds. Siltation within the shallow embayments of Lake Illawarra could be ameliorated by a carefully planned program of dredging.