Abstract
A bryozoan-dominated benthic community on the Otago continental shelf (45° 50’ S, 170° 50’ E) has been the subject of a number of earlier studies. Although previous studies have helped define the nature of benthic communities associated with the bryozoan thickets, the sampling methods used (e.g., trawl and dredge sampling) have not allowed for a quantitative assessment of the community. A quantitative survey was, consequently, undertaken during May-September 2003 to further assess the diversity and fine-scale distribution of the epibenthic taxa of Otago' s bryozoan thickets. In consideration of the destructive sampling previously carried out within the area, an attempt was made to devise a minimally-invasive benthic sampling technique, whereby an underwater photographic camera system was used to obtain samples of the thicket epibenthos. Five cross-shelf transects (A to E) were sampled at 10 m depth increments, covering a water depth range of 60-110 m. More than 500 photographs of the seafloor were taken at 22 sampling stations, from which a range of information pertaining to local species diversity, abundance, distribution and intraspecific relationships was attained.
Forty-eight epibenthic taxa were identified from photographic samples, including framebuilding and tube-forming bryozoans, sponges, asteroids, ophiuroids, other echinoderms, ascidians, anemones, gastropod and bivalve molluscs, decapod crustaceans and fish. The bryozoans Cinctipora elegans, Hornera robusta, Adeonellopsis spp., poriferans, echinoderms and ascidians dominated the epibenthos. The highest abundance of epibenthic fauna occurred at water depths of 80-90 m on the narrowest region of the shelf between the promontory of Otago Peninsula and Saunders Canyon, while species richness and diversity was found to be greatest towards the northern and southern limits of the study area at 70- 80 m water depths. Throughout the study area the spatial distribution of epibenthic fauna was highly patchy even at the scale of individual photographs, affirming the description of this assemblage as a 'thicket'.
Environmental factors influencing the fine-scale distribution of epibenthos were statistically assessed, specifically factors associated with the bathymetric and latitudinal gradients of the Otago shelf. Water depth was the most significant factor influencing the variability in epibenthic faunal cover, although the latitudinal distribution of sampling transects was also associated with changes in abundance and distribution of the epibenthos. However, water depth and latitude, per se, do not directly influence the distribution of epibenthos, but rather related physical and biological factors. The influence of substratum type, hydrological parameters, sedimentation, concentration of nutritional resources and species interactions as well as factors associated with the methodology are discussed as possible determinants for the spatial distribution and abundance patterns of the bryozoan thickets.
The dominant bryozoan fauna of the Otago shelf has repeatedly been referred to as 'frame-building' and, consequently, the function of these species as ecosystem engineers and bioconstructors within the thicket community was examined. An initial assessment suggested that the frame-building bryozoans may control the availability of some resources to other organisms, including the provision of substratum complexity, access to food resources, protection for fragile and juvenile organisms and the maintenance of recruitment and reproduction, thus serving to enhance and sustain local benthic biological diversity.
The photographic sampling technique trialled during this study was highly effective, producing images that were suitable for a quantitative analysis of faunal coverage, while causing only minimal damage to the delicate taxa of the Otago bryozoan thickets. Thus, this sampling methodology resulted in a large set of photographs providing a comprehensive and permanent visual record of the benthic community.