Assessing the structure of the circalittoral epibenthic community in the Slovenian Sea using video sledge

One of the main factors shaping a marine benthic community is sediment composition of the sea floor. In this study, we observed an epibenthic community in an offshore area of the Slovenian Sea represented mainly by sandy bottom with gradually increasing pelite fraction towards the coastline. The observations were carried out along linear transects using a video sledge (SKIKAM) in the spring of 2022 and 2023. The video sledge is comprised of three video cameras, two laser pointers and lights fixed to a metal construction, with a pair of skiis attached on the bottom. At each location, the videos of the sea bottom were recorded together with additional data on GPS position, speed and duration of trawling. Each video transect was divided into sub-sections with the duration of approximately 3 minutes, which were then analysed separately. The video analysis included taxonomic identification of organisms, their size and abundance together with a visual observation of changes in community structure, the amount and distribution of organic detritus, comprised of mollusc shells and dead organism skeletons, along with changes in the seafloor topography including presence of bumps and recessions on the bottom. The overall number of taxa observed was 171, of which 125 were determined to the species level. The highest species diversity belonged to tunicates (37 taxa, 22%) followed by sponges (28 taxa, 16%) and cnidarians (18 taxa, 11%). The highest abundance presented the echinoderms (3000/100m2, 73%) followed by sponges (500/100m2, 12%) and cnidarians (300/100m2, 6%). The dominant species was the brittle star Ophiothrix quinquemaculata (2800/100m2, 70%). Furthermore, we observed several changes in the visual appearance of the community throughout the sampling area. Firstly, we related the variation in community structure to five classes, defined by the pelite fraction in the sediment (up to 20%, up to 30%, up to 40%, up to 50% and up to 60% pelite) obtained from previous studies in the area. We applied a non-parametric test of significant difference between more groups based on the Bray-Curtis distance measure, which showed a significant difference in community composition and species abundance between the pelite classes. After performing a SIMPER analysis, we obtained the overall average dissimilarity of 59.12 between all pelite classes and the highest dissimilarity of 61.77 between the classes of up to 30% and up to 40% pelite fraction in the sediment. In all pelite classes, O. quinquemaculata conrtibutes to the greatest difference (65%) between classes, followed by C. pedunculatus (6%) and Paguroidea (3%). Secondly, we observed visible changes in species distribution, organism abundance, changes in the seafloor topography and the amount and distribution of coarse detritus material on the bottom. We recognised similar community characteristics in transects, sampled in the areas with similar pelite fraction in the sediment. We distinguished between the community type found on muddy-sand (from up to 20% to up to 40% pelite), sandy-mud (from up to 50% to up to 60% pelite) and transitional area (from up to 30% to up to 50% pelite) which included the characteristics of both groups. We applied two non-parametrical tests based on the Bray-Curtis distance measure, which presented a significant difference between the three clusters, along with the result of the SIMPER analysis, which showed the overall average dissimilarity of 60.28, with the highest dissimilarity 67.34 between muddy-sand and sandy-mud clusters. Again, O. quinquemaculata contributed to the greatest difference (64%) between all clusters. The proportion of the variance explained by the environmental factors pelite and community type was 11% (CCA), along with a significant effect of factors pelite and community type on the species distribution and abundance (NMDS). Finally, in some areas we observed damages on the seabed that can be associated with fishing activity.