Small-scale spatial patterns of Austrovenus stutchburyi in natural and translocated beds

Abstract

Ecosystems are patterned and patchy as a result of spatial and temporal variation in the abundance and distribution of essential resources, as well as due to geological and ecological processes. Large-scale patterns are usually formed by abiotic stressors while small-scale patterns are a result of biotic factors such as inter- and intra-specific interactions. Many suspension feeding bivalves have been reported to form both small and large-scale spatial patterns in soft-sediment environments, in a response to boost resilience from environmental stressors and increase facilitation. However, small-scale spatial patterns of infaunal suspension feeding bivalves are currently not as well studiedfor, despite playing important roles in ecosystem functioning e.g. eutrophication control, resuspend buried nutrients and modify habitats through increased bed roughness. This thesis set out to recognize small-scale spatial patterns of Austrovenus stutchburyi and whether patterns differed with density or environmental properties in Tauranga Harbour. A field survey was used to distinguish small-scale spatial patterns of Austrovenus stutchburyi in nine established Austrovenus stutchburyi beds. At each site a 4 m² plot was established in a ‘low’, ‘medium’ and ‘high’ density section of Austrovenus stutchburyi aggregates, with core samples taken at 0.5 m intervals within the plots. Austrovenus stutchburyi were then transplanted at low, medium and high densities in 4 m² plots into an area of low ambient densities to observe any spatial pattern formation and environmental effects with density. Austrovenus stutchburyi did not exhibit spatial patterns no different from random in natural beds and was not dependent on density or environmental variables. This was indicated by most of the plots accepting the null hypothesis of Moran’s I which indicates a random distribution. Following the same sampling design as the field survey a translocation experiment was performed to distinguish any small-scale spatial patterns of Austrovenus stutchburyi in a low ambient density environment. Transplanted Austrovenus stutchburyi beds exhibited clustering in high density treatments, indicated by a positive Moran’s I value in all three high density plots (0.335 – 0.420). Both low and medium plots exhibited random patterns, accepting the null hypothesis of Moran’s I. Transplanted Austrovenus stutchburyi beds significantly improved chlorophyll a content with increasing density treatments as chlorophyll a increased by almost a factor of two between ambient and high plots from 11.3 µg g⁻¹ dw to 20.4 µg g⁻¹ dw. However, only a small increase in organic matter content was observed and no modifications were seen towards sediment grain size properties. The results from this study can enrich the knowledge of the currently understudied small-scale patterns of infaunal suspension feeding bivalves including acting as a baseline for future hypotheses and sampling schemes. There is also potential for the high-density translocated plots to act as nursery grounds for reseeded juvenile Austrovenus stutchburyi due to the modified environment becoming nutrient-rich further improving conservation and restoration of estuarine habitats.