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An analysis of changes in mesophotic reef communities in the South east Australian Marine Park (AMP) network using benthic imagery collected by an autonomous underwater vehicle (AUV).
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Citations
... We used the last year of surveys at each site as the basis for estimation and then simulation of future change. The same Bayesian priors for spatial and temporal correlation outlined in Perkins et al. (2021a) were used. ...
Image-based surveys of the marine benthos are being increasingly adopted as a monitoring tool for habitats and biota, particularly in mesophotic depths (~30-150 m) which are technically difficult to survey. Many modern tools for these surveys, such as remotely operated vehicles and autonomous underwater vehicles, can capture thousands of images in a single deployment. Turning this into quantitative data typically involves human annotation which is often time-consuming and costly. Percent cover of organisms is one of the most common metrics for monitoring changes in abundance, which may be attained through visual estimation, digitization, or point-count approaches. However, alternative metrics of abundance such as density (direct counts) or presence-absence, as well as metrics that quantify condition (e.g., bleaching) and size-structure can also provide quantitative information for tracking change. Understanding the statistical power of different approaches is critical to designing effective image-based monitoring programs. Given the differing statistical power and time taken using different approaches, program managers need to decide where to allocate resources. Here we use benthic imagery from two long-term monitoring sites in southeastern Australia to annotate three example morphospecies (morphologically distinct organisms) using three approaches: point-count (percent cover), full count and presence-absence within imagery. Also, we compare the performance of the point count and full count approaches for monitoring bleaching in one of our morphospecies. We use spatio-temporal models to quantify trends in the empirical data and simulations to quantify the power of these approaches to detect different levels of temporal change using different sampling efforts (either 100 or 200 images). Additionally, we examine the additional insights that size-structure information can provide for two morphospecies. We find that the full count approach provides a higher statistical power to detect change than the other approaches for our example mor-phospecies, including tracking bleaching status. Size-structure information can provide additional insights such as the occurrence of recruitment or mortality events or growth of individuals. We recommend that monitoring programs using benthic imagery should consider the choice of annotation approach as this is likely to impact observed temporal patterns, particularly when the focus is specific indicator species rather than total biodiversity.
... Our findings are important from a conservation perspective, as these deeper reefs are known to be biodiverse and are likely to contain species that are not yet described by science. Present evidence points to our target cup sponge morphospecies being a potential indicator for ongoing monitoring of the impacts of warming events across MEs in the south-east marine park network as it is reliably identified, widespread and relatively abundant [12] . Clearly, further work is required to establish threshold temperatures and exposure durations for bleaching to occur, how long bleaching persists for, how these factors differ across the range of this cup sponge morphospecies, and to better couple the timing of surveys with MHW events. ...
Climate change driven extreme events such as marine heatwaves (MHWs) can have dramatic impacts on ecosystems, with thermal stress often resulting in localised die-offs and visible signs of impacts such as bleaching of organisms. Such impacts are reported widely in shallower ecosystems but are less studied on deeper mesophotic ecosystems (MEs) where collecting data is more expensive. However, these deeper reefs are often biodiverse and play important ecological roles, and so understanding climate change impacts at these depths is important. Here we use benthic imagery collected as part of a large-scale monitoring program to explore bleaching in a cup sponge ‘morphospecies’ (i.e. morphologically distinct organisms readily identified in imagery) in MEs across eastern Tasmania, a region experiencing rapid ocean warming. We find an increased incidence of bleaching in surveys following MHWs, but currently no evidence for mass mortality following bleaching. Our results suggest that this cup sponge morphospecies may be useful for tracking climate change impacts on MEs in the region. Future efforts should be directed towards a better understanding of the physiological limits of this morphospecies across its range and timing surveys to more closely follow MHW events. Sponges form an important and dominant component of temperate MEs and monitoring the impacts of climate change on sponges across these ecosystems should therefore be an ongoing priority.
