Long-term Changes Of Coastal Ecosystem In Cygnet Bay, Western Australia

Coastal marine is one of the most important ecological and social-economic systemes in the global. The characteristic and mechanism of cosatal ecosystem degradation has been a global hot topic due to climate change and drastic human activities. However, marine ecological investigation is difficult, especially the lack of the long-term observations and reconstruction datebase. In this study, we assessed long-term environmental effect of pearl farming in Cygnet Bay, Kimberley region, norhwestern Australia, using paleo-ecological analysis e.g., δ13C, δ15N and biomarker. This work shown the ecosystem impact of long-term pearl oyster aquaculture in study area, and also improved the analysis of δ13C and δ15N for marine sidiments. The main findings are as following:（1） The analysis of carbon stable isotope ratio（δ13C） has to be decarbonated to avoid the effects from inorganic carbonate. Due to the relative high inorganic carbon content in marine sediments collected in Cygnet Bay, we compared the effects of three acid treatments on δ13C values of different marine sediments with different inorganic carbon content. The results showed that for rinse method, 6% H2SO₃ and 1 mol/L H3PO4 could not remove inorganic carbon completely for high inorganic carbon content samples, while 2 mol/L HCl could remove inorganic carbon absolutely from all marine sediments. For the fumigation method, fumigation is not suitable for sediment samples with high inorganic carbon content, but could be used for sediments with lower inorganic carbon content, and it showed that 9 h is the best fumigate time. The non-capsule method did not damage the 13 C enriched materials, and also the residual acid showed not impact the δ13C values. Therfore, the non-capsule method is confirmed as the most appropriate acid treatment for decarbonatio n of marine sediments.（2） To save costs and improve work efficiency, carbon and nitrogen and their isotopic compositions were analyzed simultaneously using ac idified sample where inorganic carbon had been completely removed, but significant bias could be produced in TN and δ15N values during acid treatment. This work investigate the bias in nitrogen contents（TN） and its isotopic compositons（δ15N） after acid treatment with 0.5 M HCl in glass tubes based on 509 marine samples with different content of CaCO₃.We found significant differences in TN and δ15N values measured with and without acid treatment, and these differences had significant effect on the environmental implication. For CaCO₃-poor sample（CaCO₃ concentration <20%）, acid treatment led to substantial losses of TN（up to 40%）, and the deviations in δ15N up to 2‰ compared to unacidified whole sample. For CaCO₃-rich sample（20-40%）, biases in TN and δ15N were on average higher in samples after acid treatment, resulting in 10-60% nitrogen loss and the depletion of δ15N values（1-14‰）. Based on these results, we suggest the analysis of nitrogen contents and its isotopic composition by unacidified whole sample, because acidification to remove carbonate in marine sediments leads to significant nitrogen losses through volatilisation and isotopic shifts, even for CaCO₃-poor marine sediments.（3） Pearl oyster aquaculture, with a 50-year history, has been regarded as a low environmental risk activity, in Cygnet Bay, Northwest Australia. Small environmental disturbances accumulated over a long period may cause a regime shift in marine ecosystems, particularly for sensitive oligotrophic waters. In this study, we applied paleo-ecological methods for tracking the long-term environmental impact of pearl oyster farming in Cygnet Bay, Kimberley coast of Western Australia. This work indicated that pearl farming caused small environmental impact, although did not reach to eutrophic level. Total organic carbon, total nitrogen, biogenic silica（BSi）, and fine sediment at farming site significantly increased with the development of stock level. We detected approximately two to three-fold increases in organic matter and two to five- fold increases in BSi concentrations after pearl farming, compare to control site. The variations in C/N, δ13C and δ15N suggested that increased organic matter was mainly contributed by the autochthonous source. The rapid development of modern long- line pearl culture since the late 1980 s was presumed to be a dominant driver. Pearl oyster farm alter the properties of the sediment, which includes reducing the grain size and increasing the organic matter produced by the reduced current speed and the deposiiton of facese and the debris fallout from the culture apparatus. Furtherly, the increasing organic matter can provide nutrients for phytoplanktone growth. This work gave us an early warning to notice accumulative effect of pearl farming in order to avoid an abrupt change causing ecological regime shift.（4） Marine ecosystem has been altered including primar y productivity, structure and function due to climate change. O ver recent decades, the sea surface temperature, annual average rainfall and El N i?o frequency all indicate an increasing trend in northwester Australia. In this study, we reconstruct ditom and dinoflagellates productivity and structure changes using biomarkers in Cygnet Bay. The concentrations of brassicasterol and dinosterol incrased gradually over the last 90 years and increased rapidly after 1990 s indicating the increasing of ditom and dinoflagellates productivity. Meanwhile, the relative contribution of diatoms increased after 1990 s accompanied with the the contribution of dinoflagellates decreased. The concentrations of n-alkanols（C28 + C30 + C32） and n-alkanes（C27+ C29+ C31） keep relative stable, and show a slight increase after the late of 1990 s indicating the slight increasing of river-derived organic matter. The growth of phytoplankton is associated with El N i?o events in Cygnet Bay, because El N i?o events typically weaken the flow of Indonesian Throughflow, and more nitrogen were concentrated in the euphic zone. After 1980 s, the frequency of El N i?o events increased, and the imput of river-derived organc matter increased owing a significant increase in annual rainfall after the late of 1990 s, with an increase of approximately 40-50 mm 10yr-1. The increased nitrogen supply promoted the growth of ditom and dinoflagellates, particularly in diatom.