Foraminifera Proxy of Short Core Samples: A Retrospective Study of Ecological Development of Surabaya and Bali Semi-enclosed Bay, Indonesia

Retrospective overviews are essential in development planning and/or environmental health monitoring based on geological perspective through paleoecology. The studies are using various indicators, one of which is using recent foraminifera. Within last decades, the semi-enclosed bays of Lamong Bay in Surabaya and Benoa Bay in Bali have had enormous physical development. To ensure the continuity of existing infrastructure, it is necessary to discover the threats of natural and/or anthropogenic hazards through retrospective studies. Therefore the aim of this study is to reveal the ecological development of semi- enclosed bays using benthic foraminifera as a proxy over time periods. The research was conducted in March 2017, included analysis of absolute age and sedimentation rate using the Pb-210 isotope and recent foraminifera indexing from short core samples. The Ammonia-Elphidium index (A-E index) as a proxy of hypoxia and the planktonic/benthic ratio (P/B) as a proxy of open ocean flow were used in the study. The foraminiferal indeces revealed that both sites have experienced different ecological changes in the last few decades. Over the past 87 years, the Lamong Bay has undergone ecological changes, from the aquatic environment that was strongly influenced by the open sea and supports the benthic life, to shallow oligotrophic waters that cannot support benthic life. In contrast, Benoa Bay was shallow waters for 50 years and supports benthic life without any significant threat as the other coastal waters especially for the semi-enclosed bay. The research results showed that massive development in coastal areas will affect the ecological development/change of the area. Natural factors such as geomorphological settings and circulation patterns are secondary factors that will magnify or reduce these changes/developments.