Distribution of Bacillariophyceae in the Avon-Heathcote Estuary, Christchurch, New Zealand and their use as palaeoenvironmental indicators. (2014)
AuthorsFreeman, Sean Thomasshow all
The Avon-Heathcote Estuary/Ihutai (AHE) is located on the east coast of New Zealand’s South Island, just north of Banks Peninsula. Over the last ~150 years the AHE has undergone change caused by the rapid settlement of European immigrants, extensive land use changes and the industrialisation of society. Estuaries are one of the most important ecological areas and also one of the area’s most built around areas by humans. For this reason it is important that the impacts which anthropogenic activities can have on the environment, and in particular primary producing biota are understood. This knowledge will assist with developing effective remediation processes for already damaged areas and assist in producing future communities that are environmentally friendly. While there is a moderate pool of knowledge on the current situation with pollutants in the AHE the existing knowledge of Bacillariophyceae (diatoms) is extremely limited, and prior to this study, no data existed on diatom communities pre-dating European arrival or during the industrial development of Christchurch. Modern diatom communities and environmental factors were studied in a spatial sense. Sediments collected along multiple transects around the AHE allowed a modern database to be developed containing information on trace metal concentrations, organic carbon concentration, variations in sediment size, salinity, pH and elevation changes for sample sites, as well as the structure of the AHE diatom community. Statistical analyses indicate that the modern diatom communities are relatively homogenous across the Estuary, as are the selected environmental factors. Very few correlations could be made between what little variance was recorded in diatom communities and the above environmental factors. Canonical correspondence analysis was also undertaken to determine how much of the variance seen in diatom species could be attributed to the selected environmental factors (traces metals, organic carbon, grain size, elevation). The results indicated that the above environmental factors have very little influence and the greatest influence over diatom distribution in the modern AHE is proximity to source waters of the Avon and Heathcote rivers. Following the modern study a 270cm sediment core was collected from near the Avon River mouth to determine temporal change in diatom communities in response to urban development about the AHE. Over the past 150 years of industrial activity two significant changes have occurred in the diatom community. 1: A complete change in diatom species assemblages occurred as industrial activity commenced. 2: An apparent return to a pre-industrial (natural) estuary floras after environmental remediation. All environmental factors correlated with changes in diatom community structure with a high degree of significance yet only 31% of the variance recorded by diatom species can be accounted for by trace metal, silt and clay. Of the change in diatom species recorded through the industrial period only 8% can reliably be accounted for. It is assumed that temporal variations in environmental factors such as salinity, pH and temperature is responsible some of the unaccounted variance. Finally the preservation of diatom valves has improved over the time between the industrial period and the modern depositional environment. Samples from the modern estuary and from higher sections of the core, deposited after the onset of the industrial period have a higher diversity and abundance. This has occurred as organic carbon and finer grained sediments within the AHE have increased due to industrial activity, assisting in the preservation of diatom valves. Diatoms from sediment that was deposited before the industrial period are within sands and poorly preserved and therefore likely not an accurate reflection of what species were alive at the time of burial. Uncertainty in diatom preservation has implications for correctly establishing the pre-industrial (natural) baseline in order to assess subsequent change.