Southeast Tasmania Temperate Reef Survey Post-Survey Report

This report provides a description of the CERF Marine Biodiversity Hub survey of southeast Tasmanian temperate reefs, aboard RV Challenger, as part of the Hub’s Surrogates Program. The survey was undertaken as a collaboration between the Tasmania Aquaculture and Fisheries Institute (TAFI, University of Tasmania) and Geoscience Australia (GA), and was completed in two stages during 2008 and 2009. The purpose of field surveys in the Surrogates Program is to collect highresolution, accurately co-located physical and biological data to enable the robust testing of a range of physical parameters as surrogates of patterns of benthic biodiversity at relatively fine spatial scales. The objective is to test these relationships in strategically selected areas that are representative of much more extensive benthic environments, and where the bio-physical data collected complement existing data for these areas. This report describes the methods employed in the mapping and video characterisation of shallow-shelf temperate reef habitats across seven survey sites in southeast Tasmania: Freycinet Peninsula; Maria Island; Tasman Peninsula (Fortescue area); Port Arthur; Huon Estuary/D’Entrecasteaux Channel; The Friars; and Tinderbox (D’Entrecasteaux Channel). Preliminary results are provided of the analysis of multibeam sonar and underwater video data. Examples of the types of biota encountered in the towed video and stills images, and initial interpretations of the benthic communities are also provided. In addition, initial results are presented from the deployment of an Autonomous Underwater Vehicle (AUV) to collect high resolution photographs of reefs and associated biota. For the seven sites surveyed, the geomorphology of the nearshore and shelf is characterised by a mix of hard and soft seabed features that provide a range of potential habitat types. The most extensive reefs are formed where outcrops of bedrock extend offshore from headlands and nearshore islands, with small areas of reef occurring as isolated patch reefs on the shelf. Away from the reefs, the seabed is typically sediment covered and flat. The morphology of the mapped reefs ranges from high relief reefs, which rise several tens of metres above the surrounding seafloor (e.g. The Hippolyte Rocks), to low relief reefs that are only a few metres high and often partially covered in sediment (e.g. Freycinet Peninsula, outer shelf). In detail, reef morphology displays strong geological control, as shown by the highly fractured dolerite reef surface at The Friars and the stepped morphology of some inshore sandstone reefs along the Tasman Peninsula. Reefs in the more sheltered areas of Port Arthur and the Huon River / D’Entrecasteaux Channel are less extensive than along the open coast, occurring as narrow zones of hard-ground along shorelines and surrounding small islands. Otherwise, the mapping in these areas mostly reveals relict landforms such as incised river channels, drowned terraces and other soft sediment features (e.g. pockmarks). In this survey, large linear areas of seafloor were video-taped and the video footage used to characterise the benthic habitats evident in five of the seven survey areas. A range of benthic habitats was identified including high-relief bedrock reef at The Hippolyte Rocks, low-lying and partially sediment-covered bedrock in the Freycinet Marine Protected Area (MPA), transitional patch reef adjacent to sandy seabed at the margin of bedrock exposures, and extensive sand flats. A variety of sand wave and rippled habitats was often recorded in and around the reefs themselves. Biological habitats were also diverse with several distributional patterns recorded. The most dominant pattern was a strong depth zonation, with a kelp forest zone (dominated by Ecklonia radiata) in water depths < 45 m, which quickly transitioned into a sponge-dominated deep reef zone (reef depths > 45 m). Beyond the reefs, the shelf sediments were often carpeted with screw shells and where present in high densities provided hard substrata for a range of suspension-feeding invertebrates. Although this depth pattern was very consistent between locations, some differences were observed. For example, differences in the level of exposure to wave energy between locations appeared to influence the density and structure of these zones. Kelp morphologies were thinner and longer in more exposed sites (e.g. The Friars), while sponges here were less dense and smaller in size. Screw shells, which occurred extensively on shelf sediments and within the sheltered inlets and channels of the Huon Estuary and Port Arthur, varied in their density between locations. The densest shell beds were recorded north of the Nuggets, while sparse screw shells were recorded in areas of higher wave energy (e.g. around The Friars) and where the substratum was muddy (e.g. Huon Estuary). In future work, the morphological characteristics of reefs in the study area will be quantified by a range of metrics, including slope, relief, rugosity and surface curvature. These parameters will be used to test for co-variance with spatial and bathymetric patterns in reef biological assemblages, as defined by the video characterisations. This analysis for co-variance will also consider variations in reef biological communities that may be a function of differences in wave energy regime between the study sites. The aim of this work is to improve our understanding of the degree of influence of physical characteristics on the spatial distribution of biological communities that exist on temperate reefs. Significantly, this assessment will incorporate a statistical measure of the degree to which derived physical parameters can be used as surrogates to map and model patterns of marine biodiversity. In turn, these outputs can be used to better inform the management of similar shallow marine systems elsewhere in Australia