Hong Kong shallow marine benthic ecosystem history : conservation paleoecology approach based on microfossil ostracods

Abstract

As one of the most urbanized areas in the world, Hong Kong has been under stress of anthropogenic impacts. Human activities (e.g. reclamation, eutrophication, pollution) could cause severe alteration of marine ecosystems. Ostracods (microcrustaceans) are an ideal model for long-term quantitative paleoecological analyses because of their small size, high abundnace and excellent fossil record. In this thesis, I investigate the potential of ostracods as a paleoecological proxy as well as possible model organism for long-term ecosystem monitoring in coastal and marine areas of Hong Kong. Grab sediment samples from both sediment surface (0–1 cm, representing modern fauna) and subsurface (top ~20 cm, bulk of grab sample sediment, representing past fauna) were collected and compared from 52 sites, covering all Hong Kong marine waters (Chapters 2 and 3). In total, 151 species belong to 74 genera were identified. No significant difference in abundance or diversity distribution was detected between surface and subsurface datasets. Linear regression models suggested that ostracod abundance was significantly correlated with turbidity (negative) while diversity was correlated with grain size (negative). Pairwise Bray-Curtis similarity between surface and subsurface samples showed similar (>0.6) faunal composition for most of the sites, which was significantly correlated with productivity (positive) and Cu (negative). Dominant species (e.g., Sinocytheridea impressa, Neomonoceratina delicata, Bicornucythere bisanensis s.l.) tended to show positive correlation with eutrophication and/or pollution proxies such as productivity and heavy metals (e.g., Zn) suggesting their tolerant nature. The temporal changes of ostracods aseemblages for the past hundreds of years in short sediment cores (<2 m) taken from Tolo Harbour suggested that human-induced heavy metal pollution and eutrophication were likely the main reasons for the degraded marine ecosystem in this area (Chapter 5). A Holocene sediment core (42.4 m) was retrieved offshore Lantau Island (Chapter 4). By comparing modern ostracod faunal distribution (Chapter 2) and this Holocene fossil ostracod record, the relative sea-level changes was reconstructed by Modern Analog Technique. The sea-level rose rapidly at the early Holocene (~10200 to 8500 cal. yr BP) and continued to increase with fluctuation during the middle Holocene (~8500 to 6000 cal. yr BP), thus the relative sea-level elevated from -42.5 m¬ at the beginning of Holocene to the highstand as +7.5 m at ~6200 cal. yr BP. It then gradually dropped to the present sea-level. The temporal changes of ostracods abundance, diversity and faunal composition through Holocene mainly reflect water-depth and salinity variability. A short (73 cm) sediment core was collected from Plover Cove Reservoir to examine the reservoir’s construction impacts on the local ecosystem using fossil ostracods as well as other fossils of foraminifera, molluscs and echinoderms. The results showed a rich and diverse shallow muddy inner-bay community before the reservoir construction, marginally habitable condition during the construction period, and barren benthic habitat afrer the completion of the freshwater reservoir (Chapter 6). This thesis clearly showed that ostracods could be applied as a powerful tool in palaeoecology and ecosystem monitoring, which would be valuable for Hong Kong marine conservation assessment, management and planning.