Understanding past environmental change in New Guinea: utility and problems of diatom sequences from freshwater lakes

Abstract

The rapid response of diatoms to environmental change is studied in palaeoenvironmental contexts in tropical New Guinea where diatom studies are in an embryonic stage. Previous palaeoenvironmental analyses were from Lake Waigani, near Port Moresby. In this thesis four New Guinea lakes, Aguai Ramata, Lake Kutubu, Lake Wanum and Lake Waigani, are analysed. Results show that changing water levels appear to affect the diatom assemblage responses from Aguai Ramata and Lake Kutubu. Complex ecological changes such as responses to modern pollution, nitrogen levels and aquatic vegetation cover are seen in Lake Waigani. Low diatom counts limit the palaeoenvironmental interpretatations from Lake Wanum. The controlling factors for water levels appear to be climate, (particularly precipitation and evaporation), tectonic events (earthquakes and tephra falls from volcanic eruptions) and anthropogenic input (within the lake itself and from the catchment), where land clearing (especially slash and burn in this region) can result in erosional infill and changes in water depth, water turbidity and water chemistry.

Short term climatic events such as El Nino and La Nina (ENSO) are not recorded within the sample resolution but slight hints of long-term changes in the diatom records possibly suggest regional climate variation. Further analysis is needed. Tephra is seen in the sedimentary record but there is only one example where an associated diatom assemblage change is seen. The diatom analyses did not establish disturbance of the sediments from earthquakes. Anthropogenic change has been identified in diatom response, particularly at Lake Waigani, where there has been heavy pollution of the lake since 1965, and in Lake Kutubu opposite a sago garden. Another core from this lake tentatively shows disturbance from the development of the infrastructure associated with oil and gas field developments over the past twenty years. Further observation and analyses are needed. When using a single source of data, such as diatoms, specific sources of environmental change are often difficult to identify for they may overlap and mask each other. These problems can be ameliorated by the combined use of different proxies whereby their combined distinctive responses to their environments can potentially clarify interpretations.

The diatom analyses from the four lakes have shown there are excellent prospects for the extension of future diatom analyses to a wide range of New Guinea lakes, the vast majority of which have still to be visited by scientists. Potentially, their long histories and baseline measurements derived from sedimentary cores can contribute to future interpretations of the complex New Guinea palaeoenvironments and changing ecological patterns.

In this preliminary study it is clear that no single regional influence has altered lake trajectories. All sites are individual and respond to change in the Holocene in unrelated ways. However, the histories provide a sensitive measure of local change and response that might be linked to other proxies in the future such as isotopes and chemical signatures. Show more