AusGeo News June 2010 Issue No. 98
Towards a more comprehensive characterisation of the seafloor
Australia's marine jurisdiction is almost 14 million square kilometres. Geoscience Australia acquires geological, geophysical and ecological data to better understand the jurisdiction. These regional pre-competitive datasets, collected as part of the Offshore Energy Security Program, are available to assist the activities of Australia's offshore oil and gas industry, and the management of the marine zone.
Seabed Mapping and Characterisation Project staff are currently working on the identification of representative seabed habitats. These studies will be used to characterise seabed environments for the purpose of defining Australia's little-known deep sea diversity. Among the variety of tools and techniques used to identify substrate type and characterise the biota of the seafloor are underwater video and still photography. The images collected to date show that most of the deep sea is characterised by soft sediment plains in which very few animals are seen.
At first glance, these habitats seem barren, but they are actually teeming with life. Previous biological sampling of the seafloor using boxcores has suggested that the deep sea may harbour greater species abundance and diversity than shallow water areas, and in some areas may even be comparable to biodiversity hotspots such as tropical coral reefs (Grassle and Maciolek 1992).
However, most of these deep sea animals are rarely seen because they are generally small and infaunal, meaning that they spend most of their lives within the sediment. During feeding and burrowing these animals move through the soft sediment and form a range of features, including starbursts, spirals, and spaghetti-like tracks (figure 1). These features are known as lebensspuren, which refers to any type of sedimentary structure produced by a living organism. Lebensspuren have been shown to be useful surrogates for the biodiversity of larger animals in subtidal systems (Widdicombe et al 2003), but no similar research has yet been conducted in deeper waters. High resolution still photographs are often collected in conjunction with video footage during surveys but, to date, have rarely been used for habitat or biodiversity analysis of deep sea habitats. They offer a means to use lebensspuren as a proxy for biodiversity assessment in deep sea habitats which are normally quite difficult to quantify with video. This facilitates a more comprehensive characterisation of the seafloor across deep sea plains.
Still images collected on the eastern and western Australian margins were analysed for lebensspuren (figure 2). The study areas were based on marine reconnaissance surveys undertaken by Geoscience Australia: Survey GA0427 (TAN0713) on the eastern margin and Survey GA2476 on the western margin (figure 2). These areas were chosen because they represent different geomorphic and geological settings; two parameters that may affect the abundance and type of biota in the sediments. The aims of the study were to:
The latter assessment was based on the detection of known biological relationships between environmental variables and biodiversity, as well as the expected biological differences between the eastern and western Australian margins.
For each region, selected still images were examined, and the different types of lebensspuren were recorded. Since very common features were difficult and time-consuming to count, only their presence or absence was recorded. The types of lebensspuren were named, classified according to the likely mechanism of formation (such as feeding, dwelling, waste), and compiled into a directory (Dundas and Przeslawski 2009).
Lebensspuren were then correlated with environmental data collected concurrently during the survey to identify any significant relationships. In both the eastern and western margins, the lebensspuren were analysed with depth and sediment properties (such as percentage mud or carbonate). At selected stations on the eastern margin, lebensspuren were also analysed with geochemical parameters (such as extractable metals, measures of organic freshness, carbon and nitrogen expressed on carbonate-free basis).
A total of 46 different types of lebensspuren were recorded from the eastern and western margins, some of which are shown in figure 1. Very few organisms were directly observed forming lebensspuren (figure 1a) which highlights a major difficulty when observing these animals in traditional video and image analysis.
Multivariate analyses confirmed significant regional differences in lebensspuren abundance and types between the eastern and western margins, with the eastern margin showing higher lebensspuren abundance and diversity. Lebensspuren were significantly correlated to depth and freshness of organic matter, with more types of lebensspuren associated with shallower depths and sediments with high organic freshness (figure 3). Depth accounted for 22 per cent of the variation in lebensspuren types among stations (locations) while organic freshness accounted for 70 per cent. Mud and carbonate content were also significantly related to lebensspuren but were confounded with depth.
Overall, the method of using still images to quantify lebensspuren as a proxy for deep seafloor biodiversity warrants further use and research. Clear geographic differences were seen in lebensspuren assemblages, and these differences were associated with environmental factors such as depth. These patterns parallel known biological patterns, thus suggesting that quantification of lebensspuren may be suitable for broad-scale comparisons of biological diversity among deep sea sites. The advantages and disadvantages of using video and still images to collect biological data indicate that optimal results are likely to be obtained when biotic and abiotic samples are collected concurrently with deep-sea video and still imagery.
Findings from this study will assist in the characterisation of deep sea areas which cover much of Australia's marine jurisdiction. This environment is one in which the biology is difficult to investigate because of the prevalence of small infauna and the logistical constraints associated with sampling in deep waters. The use of still images to quantify lebensspuren offers a cost-effective means to analyse data that are normally not assessed during deep-sea surveys. This option should increase the scope of biodiversity analyses and results from this study have been promoted through the Census of Marine Life and several other international research organisations.
For more information phone Dr Rachel Przeslawski on +61 2 6249 9101 (email firstname.lastname@example.org)
Survey of remote eastern frontier basins completed (AusGeo News 89)
Southwest Margin surveys completed (AusGeo News 94)
Revealing the Wallaby Plateau (AusGeo News 94)