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Underwater image-based sampling procedures, using SCUBA, are compared using imagery collected from a temperate hard-substratum community. The effectiveness of a low-budget, high-resolution image mosaicing technique is assessed by comparing the relative efficiency of data collection, extraction and analysis among sampling procedures. In addition, a manipulative experiment tested whether the sampling procedures could detect the physical removal of 10% of the reef community. Overall, four factors were explored within the data: data collection media (stills and video), cover and community composition estimation techniques (visual cover estimation, frequency of occurrence and point extraction), change detection (pre- and post-impact) and depth (8, 14, 18 and 22 m). Stills imagery sampled the reef community at a higher image resolution than the video imagery, which enabled identification of more species and less-conspicuous benthic categories. Using the visual cover estimation technique, the stills imagery also had the greatest benefit in terms of efficiency and species identification. However, the experimental impact was detected using only the point extraction technique. The recommendations are that: (1) the image mosaicing technique is applied to fixed-station monitoring; (2) the point extraction technique be used for efficient and cost-effective monitoring at coarse taxonomic resolutions; and (3) survey depths remain constant over the duration of hard-substratum community monitoring.

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ABSTRACT1. Photoquadrats enable efficient and cost-effective quantitative estimation of epibenthic communities. Despite their utility, however, there has been limited use of photoquadrats for such purposes in temperate north-west Europe, where there is also a growing need for standardized approaches to marine monitoring.2. A rapid photoquadrat-based methodology was trialled by scuba divers on a heterogeneous boulder-slope habitat in a Special Area of Conservation (SAC) in Northern Irish waters. To investigate the monitoring potential of the method, the benthic community was randomly sampled in the summers of 2009 and 2010. The community was represented at species level (community composition) and by functional group (community structure), and the data-extraction resolution was varied using different numbers of point intercepts per image (25, 50, and 100) to assess the balance of precision and efficiency with regard to detection of community change.3. The method was efficient in situ and yielded sufficient sample images for estimation of local benthic community diversity (number of species). The community varied significantly, with six distinct sub-communities identified within the survey area. High spatial variability obscured detection of temporal changes in the overall community composition and structure. However, spatial variability was substantially reduced by testing only the dominant sub-community, in which significant changes were detected between 2009 and 2010.4. The ability of the photoquadrat to detect individual taxa was related to data resolution: the more point-intercepts sampled, the more taxa were discovered, but the data-extraction effort was greater. After considering the ability of the photoquadrat method to quantify number of species and to detect change in community structure, as well as its precision and efficiency, the inspection of 50 point intercepts per sample image was found to be optimal. These findings demonstrate the benefits of photoquadrat-based methods and highlight their potential as a standard approach to marine monitoring.

ABSTRACT1. In marine temperate waters, the diverse epibenthic communities of hard substrata require robust, efficient, high-resolution methods to better understand community dynamics and biological processes, such as growth, recruitment and mortality that drive changes within communities.2. The epibenthic community of a harbour wall was monitored from fixed stations over three seasons in 2009 to evaluate the utility of a high-resolution photomosaic method. The community was described by visually estimating cover and abundance of taxa within the photomosaics. In addition, areas occupied by the solitary cup coral Caryophyllia smithii, the encrusting sponge Hymeniacidon perleve, and the red macroalga Plocamium cartilagineum were digitized and their seasonal growth, recruitment and mortality estimated over the study period.3. Significant changes occurred to the community in every season. The fewest number of taxa and the lowest total taxa cover was observed in the winter. In the spring, there was significant growth of red macroalgae, brown macroalgae, and ascidian taxa. In the summer there was significant growth of green macroalgae and turf taxa, while red macroalgae experienced photoinhibition, and ascidians significantly declined in abundance.4. The photomosaics enabled the extraction of data at a high taxonomic resolution, reduced the effects of water-column turbidity, and through digitization, enabled estimation of the seasonal growth, recruitment, and mortality of C. smithii, H. perleve and P. cartilagineum. However, the spring and summer canopies of macroalgae and turf obscured the detection of many understory taxa, and thus, limited the accuracy of understorey assessments. Provided the limitations of photomosaics are recognized, the method demonstrated in this study has many potential benefits of advantage for fixed-station monitoring studies in temperate waters.