Optimal sampling and spatiotemporal change in epibenthos at a sub-Antarctic Marine Protected Area
Thesis (MSc)--Stellenbosch University, 2017.
ENGLISH ABSTRACT: There have been notable regional and temporal changes in many biological systems around the Prince Edward Islands (PEI) in the Southern Ocean, but little work has focused on epibenthic assemblages. The aim of this study was to investigate spatial and temporal changes in the epibenthos, as they may be important indicators of change. During the austral summer of 2013 and 2015, a benthic towed camera system was employed to survey benthic habitats around Marion and Prince Edward Islands. This photographic sampling survey was conducted to align with a previous epibenthic photographic survey completed between 1984 and 1989. As marine photographic surveys have a propensity to underrepresent the species richness present, investigations into the optimal sampling intensity required per substrate type were conducted initially in order to ensure rigorous representative sampling. Historical photographic data of epibenthic assemblages in the upstream and downstream regions of the islands were then compared to data from the new photographic surveys to assess changes in regional species composition over time. Due to the various challenges associated with using underwater photography as a sampling method, identifying an optimal photographic sampling intensity for the epibenthos at the PEIs was imperative to this study. To achieve this, species-area relationships were used in conjunction with substrate type to determine the number of samples needed for given thresholds of species representation to be reached. The asymptotic richness estimator Chao 2 was used to compare the species richness detected in an area to a statistical estimation of the possible total species present. The point where species accumulation curves started to decelerate was identified as the point where a 20% increase in samples results in <5% increase in the number of new species observed. Points of deceleration were subsequently compared among substrate types. Substrate type was not found to be an accurate predictor of epifaunal species richness at the PEIs. There was no difference in the species area curves among substrate types and all curves accelerated at the same rate irrespective of substrate type. An average of 20 samples was required to reach a point at which at least 80% of the total asymptotic richness had been sampled. A strategy using this sampling intensity can, therefore, be applied across substrate types at the PEIs. With the required sampling intensity determined, historical and resampling surveys in the upstream and downstream regions of the PEIs were compared. The Shannon-Wiener index indicated that diversity differed over time but did not differ between regions. In contrast, however, there were regional differences in Pielou’s evenness index but no difference over time. There were significant differences in species composition between regions over time. The temporal differences in assemblages, however, were due to dispersion effects. The temporal change in benthic assemblages may mirror shifts in the prevailing primary productivity patterns at the islands. Climate-driven changes are thought to have caused variations in mesoscale oceanographic regimes and subsequent primary productivity patterns, which influence the benthos upon sinking. The photographic dataset compiled in this study provide the second permanent record of the epibenthic environment at the PEIs and will aid future monitoring studies in this region, especially in the face of imminent climate changes. The results of this study will advance the long-term understanding of the epibenthos at the PEIs.
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