A PHOTOGRAPHIC SURVEY OF TEMPORAL VARIABILITY IN THE UNIQUE BIOTA OF REEFS IN BATHURST CHANNEL, PORT DAVEY, 2002-2023

The benthic reef communities of Bathurst Channel represent a unique component of Tasmania’s marine ecology and diversity, requiring ongoing protection of the catchment and the estuary itself to protect the fragile invertebrate communities present. Containing a number of fragile deep-water invertebrate species growing at accessibly shallow depths, the reef habitats are both susceptible to impacts and are of scientific importance. This study was initiated in 2002 following recognition of the continuing need for a practical, quantitative monitoring program to provide information on species composition and species distribution throughout the channel, and to detect any adverse changes occurring over time. The current survey, conducted in March 2023, follows earlier surveys in 2002 and 2010, collecting digital image data from depth intervals ranging from the intertidal zone to 20 metres depth at 13 monitoring sites extending throughout Bathurst Channel, to document change through time. The high-resolution imagery collected in 2023 was used to build on a descriptive catalogue of the biota observed, first developed in 2010, for use in future monitoring and species referencing. Species and substrate percentage cover in the photos was analysed using the national online image access, annotation and data storage tool Squidle+, enabling data files to be stored, accessed publicly and reanalysed.

Our analysis describes the changes in species composition throughout Bathurst Channel through time over two decades, and thus provides some insight into the relevant environmental and biological factors structuring variation in distribution of algal and invertebrate species throughout the estuary. At the spatial level, our results were consistent with previous descriptions of the community types within the Channel, showing that this system is inherently stable over these time frames. Patterns in assemblage distribution reflected strong species’ responses to gradients in tannin levels and salinity throughout the estuary, distance from the ocean and the strength of currents/mixing. Percentage abundances of a number of key species and species groupings (e.g. lace bryozoans) between 2023 and previous survey data were also comparable, albeit with a few differences resulting from an improvement in image quality between the earlier 2002 survey and subsequent 2010 and 2023 surveys. However, there were also several notable changes. The most notable one was an apparent >90% decline in sea whip abundance at both Munday Island and Forrester Point at 5 m depth relative to 2002 levels. This is presumed to be a consequence of drought conditions leading up to both the 2010 and 2023 surveys, reducing the tannin concentration in surface waters to a point where algal growth became possible at 5 m depth in the mid-estuary, smothering components of the invertebrate fauna. This decline was matched by increases in the cover of some macroalgal species that are presumably responding positively to increased light levels. Ongoing monitoring will enable recovery rates to be determined, improving our understanding of the natural variability of these fragile assemblages in a changing climate. The decline in sea whip numbers does indicate that there is a delicate balance between invertebrates and algae in the lower photic zone within the channel, one that corresponds with the depth occupied by many fragile invertebrate species, and that this balance could be significantly altered under climate change scenarios that relate to reduced rainfall. Due to the new image analysis protocols and pipeline established during this study utilising the national Squidle+ platform, and the development of high-resolution digital photography since 2002, further surveys and analysis of data will be easily comparable using the same sites, depths, and the methods of analysis as established here. This will allow for the successful provision of reliable monitoring data to feed into ongoing management and conservation of the unique ecosystems found in Bathurst Channel.