Like Bathurst Channel Ecosystem, South?

AQUATIC CONSERVATION: MARINE AND FRESHWATER ECOSYSTEMS Aquatic Conserv: Mar. Freshw. Ecosyst. 20: 397–406 (2010) Published online 3 December 2009 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/aqc.1085 Distribution of benthic communities in the fjord-like Bathurst Channel ecosystem, south-western Tasmania, a globally anomalous estuarine protected area NEVILLE S. BARRETTÃ and GRAHAM J. EDGAR Tasmanian Aquaculture and Fisheries Institute, University of Tasmania, GPO Box 252-49, Hobart, Tasmania 7001 ABSTRACT 1. Benthic assemblages in the fjord-like Bathurst Channel estuarine system, south-western Tasmania, vary over horizontal scales of 1–5 km and vertical scales of 1–10 m. Multivariate analysis indicated a total of eight major assemblages that characterize different sections and depths of the channel. 2. Because tannins in the low-salinity surface water layer block light, foliose algae reach 5 m depth in the marine western region but do not penetrate below 1 m in the east. By contrast, sessile invertebrates are most abundant below 5 m depth in the west and below 2 m in the east. Deeper assemblages are unlikely to be continuous with assemblages in deeper waters off the Tasmanian coast as they are highly constrained by depth within particular sections of the estuary. 3. While the species composition of the Bathurst Channel biota is most similar to that found elsewhere in Tasmania, the structural character of the biota in terms of major taxonomic groups is more closely allied to that found in fjords of south-western Chile and south-western New Zealand. These three regions all possess wilderness settings, high rainfall that is channelled through estuaries as a low-salinity surface layer, deep-water emergence of fauna, rapid change in biotic communities over short horizontal and vertical distances, and high levels of local endemism. They also include some of the most threatened aquatic ecosystems on earth due to increasing human activity from a near pristine base, and the potentially catastrophic impacts of climate change. Copyright r 2009 John Wiley & Sons, Ltd. Received 4 February 2009; Revised 2 September 2009; Accepted 13 September 2009 KEY WORDS: climate change; endemism; estuary; macroalgae; marine protected area; sessile invertebrates; threatened habitats INTRODUCTION indicated that the geomorphological resemblance with fjords is superficial (Baker and Ahmad, 1959). Regardless of origin, Bathurst Channel comprises a narrow 12 km long passage that physical processes operating within Bathurst Channel have connects the large coastal embayment of Port Davey with the many characteristics in common with fjords because of the equally large estuarine basin at Bathurst Harbour (Figure 1). shallow (o8 m depth) constricted sill at the western entrance It is located on the south-western coast of Tasmania within the and deeper waters within. The narrow central channel is South West National Park and Tasmanian Wilderness World generally about 300 m wide and 20–45 m deep mid- channel, Heritage Area, and forms part of the only large estuarine with shallows (o10 m depth) on the margins extending into system in southern Australia without significant human impact numerous adjacent bays. throughout the adjacent catchment (Edgar et al., in press). Recent studies in the region indicate that Bathurst Channel Because of steep-sided valleys and a convoluted shoreline, possesses a highly distinctive marine flora and fauna (Edgar, Bathurst Channel was long considered glacial in origin–the 1990; Resource Planning & Development Commission, 2003), only example of a fjord in Australia. Subsequent studies with high conservation value (Edgar et al., in press). Its biotic *Correspondence to: Neville S. Barrett, Tasmanian Aquaculture and Fisheries Institute, University of Tasmania, GPO Box 252-49, Hobart, Tasmania 7001. E-mail: [email protected] Copyright r 2009 John Wiley & Sons, Ltd. 398 N. S. BARRETT AND G. J. EDGAR Port Davey Bramble Cove Turnball Breaksea Island Island Munday Little Waterfall Woody Bay Beabey Island Island Joan Pt Pt Forrester Eve Pt Pt Bathurst Channel Platypus Bathurst Pt Harbour Figure 1. Location of sites surveyed by video in the Bathurst Channel region of south-western Tasmania in spring 2002. distinctiveness is due in part to the phenomenon of deep-water METHODS emergence, which involves uplifted vertical distribution (Edgar Field surveys and Barrett, in press), and also to a large component of endemic species, including a fish species not recorded elsewhere In total, 23 sites off rocky headlands distributed along (Edgar et al., in press). Partly in recognition of these Bathurst Channel from Port Davey to Bathurst Harbour biodiversity values, waters within the broader Port Davey were investigated in October and November 2002 (Figure 1). region were formally declared a multi-zoned marine protected Sites were chosen to describe variation in the sessile biota area (MPA) in January 2005. This MPA includes both Bathurst associated with the main channel from marine to upsteam Channel and Bathurst Harbour, which are together zoned as a ends, with patterns assessed using multiple transects separated fully protected sanctuary zone—the largest estuarine ‘no-take’ at scales of hundreds of metres nested along the estuary at conservation area in Australia and perhaps the largest scales of kilometres. At each site, single transect lines were set worldwide for a complete estuarine ecosystem. normal to the shore with the 0-m mark situated as close as Within Bathurst Channel, tannin-stained waters severely possible to the high tide mark. A diver swam the transect reel restrict light penetration, with consequent constriction of the offshore until the 20 m depth contour or the end of the 100 m depth range occupied by algal species. Below 5 m depth, the transect line was reached, whichever came first. Transects were benthic biota is dominated by filter feeding cnidarians, marked every 5 m so that positions along the transect line bryozoans, tube worms, ascidians and sponges, while could be interpreted from the subsequent video recordings. scavenging and predatory molluscs, crustaceans and After setting the transect line, the diver swam back to the echinoderms are relatively depauperate (Edgar et al., in press). shore along the line, recording the depth every 5 m to provide a Despite the scientific and conservation value of the depth profile of each transect for comparison with the video Bathurst Channel biota, no quantitative study has reported record. Concurrently, a second diver entered the water and on the distribution of species within the ecosystem, while swam down the line, using a Sony TRV 900 digital video relationships with biotas elsewhere have never been evaluated. camera in an underwater housing to record benthic The only other documented similar assemblages are found in assemblages present along the transect. Digital video was similar latitudes in fjords off south-western New Zealand and chosen as the format for this survey because output quality Chile, where the waters of coastal estuarine systems can be was immediately clear to the operator, and the video frame- tannin-stained and stratified, and taxa more typical of deeper grabbing capability was not substantially different to images water are present at depth o20 m, such as stony corals, black produced by digital still cameras available at the time. Higher corals, sea pens and brachiopods (Miller, 1997; Schiel and resolution film-based cameras were not used because variable Hickford, 2001; Fo¨rsterra and Haüssermann, 2003). underwater lighting in the region increased the possibility of The aim of the present study is to quantitatively describe poor film exposure during logistically-challenging surveys that the distribution of the dominant macroalgae and sessile could not be repeated. animals within Bathurst Channel, and to clarify broader In initial trials, digital images of benthic assemblages within biogeographic relationships. Our study is based on a a 0.5 m Â 0.5 m quadrat placed on the sea bed were recorded quantitative video survey of the region undertaken in spring using the still image capacity of the video camera. However, 2002 that also aimed to provide a baseline MPA data set for the distance between the camera and quadrat was found to be assessment of future human impacts. Human activity is too great for the use of photoquadrats in shallow tannin- increasing in Bathurst Channel, which is appropriately stained waters of Bathurst Channel. Despite optimal placing of regarded as environmentally sensitive due to the presence of video lights, species and substratum types could not be threatened and endemic species, and habitats with highly adequately resolved on images because of tannins in the restricted distributions and fragile structural complexity. water between the camera and the quadrat, and viscous mixing Potential risks to the ecosystem include nutrification of the of saline and brackish waters, which strongly attenuated and oligotrophic waters, disturbance to fragile biota on the sea bed distorted light. The video technique was consequently modified by scuba divers, invasion of exotic taxa, oil spills, climate by placing the camera closer to the sea bed. change, and mechanical damage due to propeller wash and The modified technique involved the diver moving the anchoring of vessels (Edgar et al., in press). vertically downward orientated video camera very slowly at a Copyright r 2009 John Wiley & Sons, Ltd. Aquatic Conserv: Mar. Freshw. Ecosyst. 20: 397–406 (2010) DISTRIBUTION OF BENTHIC COMMUNITIES IN A FJORD-LIKE ECOSYSTEM 399 height 0.5–0.7 m above the substratum, giving an image width With the exception of common species that were clearly of approximately 0.4 m. The camera was swum over a distance identifiable, reference images were taken of all species, life of about 10 m perpendicular to the transect at defined depths forms and substratum categories. All video collected during (0, 1, 2, 5, 10, 15, 20 m) below low water mark. During this study was transferred to MPEG format on DVD, which computer processing, still images were extracted from the should be durable and allow for reliable archiving.

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