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Long-term monitoring projects are becoming more than ever crucial in assessing the effects of climate change on marine communities, especially in Antarctica, where these changes are expected to be particularly dramatic. Detailed studies of the Antarctic benthos are in fact particularly important for a better understanding of benthos dynamics and potential climate-driven shifts. Here, due to the extreme fragility of benthic communities, non-destructive techniques represent the best solution in long-term monitoring programs. In this paper we report new results from 2017, 2018, 2019 photogrammetric campaigns within the Italian National Antarctic Research Program (PNRA). A new protocol of data acquisition and multi-temporal processing that provides co-registered 3D point clouds between the three years without control points nor direct georeferencing methods is presented. This is achieved by adding a level of image understanding leveraging semantic segmentation with convolutional neural network (CNN) of the benthic features. Slow growing (estimated less than a mm per year) organisms, such as Corallinales (Rhodophyta algae), represent a natural stable pattern, leveraged to automatically orient in the same reference system the photogrammetric surveys of different epochs. This approach is also proved to be effective in improving the orientation of adjacent strips acquired within the same campaign. Within the paper an in depth analysis of the achieved results shows the effectiveness of the implemented procedure. [ABSTRACT FROM AUTHOR]

The need for sound baseline information about community structure and composition against which changes can be detected and quantified is a well-recognised priority in Antarctica. Here, the collection of such data is challenging, especially at sea, where long-term monitoring is usually logistically feasible only in the proximity of permanent research stations. In recent years, underwater photogrammetry has emerged as a non-destructive and low-cost method for high-resolution topographic reconstruction. We decided to apply this technique to videos, recorded during standard SCUBA surveys of Antarctic benthos in Tethys Bay (Ross Sea, Antarctica) in 2006 and 2015 and originally not meant for photogrammetry. Our aim was to assess the validity and utility of the photogrammetric method to describe benthic communities from the perspective of long-term monitoring. For this purpose, two of the transects surveyed in 2015 were revisited in 2017. Videos were processed with photogrammetric procedures to obtain 3D models of the seafloor and inhabiting organisms. Overall, a total of six 20 m-long transects, corresponding to a total area of ~ 200 m2 of seafloor were analysed. Accuracy of the resulting models, expressed in terms of Length Measurement Error (LME), was 1.9 mm on average. The 2017 transects showed marked differences in some species, such as a 25–49% increase in the number of sea urchins Sterechinus neumayeri (Meissner, 1900) and the complete disappearance of some sponges Mycale (Oxymycale) acerata Kirkpatrick, 1907. Our analyses confirm the efficacy of photogrammetry for monitoring programmes, including their value for the re-analysis of legacy video footage. [ABSTRACT FROM AUTHOR]