Estuaries are productive ecological transition zones between freshwater and marine environments that provide important commercial, recreational, aesthetic and cultural resources. The Avon-Heathcote estuary in Christchurch, New Zealand, is no exception, and its close proximity to different kāinga and Christchurch city has provided cultural, recreational and aesthetic values for centuries, especially Mahinga kai for Tangata Whenua. Tuangi (The New Zealand cockle, Austrovenus stutchuryi) is an important source of Mahinga kai to the iwi Ngai Tahu, but also an important ecological ecosystem engineer that provides internal habitat to parasites and, through its shell production, external habitat (hard substratum) for epibiota species. Several parasites, in particular the metacercariae echinostome parasite Curtuteria australis, depend on Austrovenus as its intermediate host, and these parasites can be considered allogenic engineers because they turn living material (here the host) from one physical state into a second. This is particularly evident in intertidal sedimentary estuaries where parasites, including Curtuteria, can alter the behaviour and fitness of the ecosystem-engineering hosts and thereby alter entire community structures. Similarly, several epibiota species depend on the shell of Austrovenus as a substratum on which to live. This hard substratum is particularly important for epibiota in estuaries that are devoid of rocky reefs, including autogenic ecosystem engineers like common large macroalgae (e.g. Ulva spp.). However, the Avon-Heathcote estuary, like many estuaries around the world, has become nutrient-enriched following sewage wastewater discharges, input from rivers and encroaching urban development, facilitating enhanced growth of algae attached to shells. Following recruitment and rapid growth on the shells, large algal fronds can break off and accumulate into thick mats that may cause anoxia and detrimental effects on many estuarine organisms.

The main objective of this thesis is to quantify key linkages between three types of ecosystem engineers; the cockle Austrovenus, its internal parasites and its external epibiota community, including large macroalgae that can detach from the shell and develop into free-living mats. To address these objectives, spatial-temporal field surveys and laboratory and field experiments investigated (i) when and where Austrovenus provide internal habitat to parasites and external habitat to epibiota, and (ii) if and how parasites and epibiota affect survival and positioning of Austrovenus in or on the sediment. It was hypothesized that parasites and epibiota species would be abundant in and on Austrovenus and that their densities would vary across seasons and environmental gradients. It was also expected that parasites would reduce the ability of Austrovenus to bury themselves, so that surface-lying cockles would have higher parasite densities and be more susceptible to predation. Finally, it was hypothesized that a cover of macroalgae would decrease the susceptibility of Austrovenus to predation, but have negative effects on associated epibiota species, and that herbivorous epibiota species, through grazing, could control the abundance of epibiotic Ulva recruits.

Seasonal collections of Austrovenus showed that parasite densities varied in different environments within the estuary (mean ranged from 3-129 for buried hosts and 7-187 for surface-selected hosts). However, host parasite loads did not vary between seasons. Parasite infestation was, found to be slightly higher in hosts exposed above the sediment compared to those buried in sediment. However, the test factor host position accounted for < 1% of the total data variability and therefore host position is of relative low ecological importance. Spatial variability in host parasite loads was significantly correlated to host sizes (its width, Rho = 0.72), individual epibiota species (Anthopleura and Elminius, Rho = –0.11 and Rho = 0.1, respectively), percentage coarse ssediment (Rho = 0.55), and less so to salinity (Rho = 0.42) and elevation level (Rho = 0.33), although the latter two variables were not statistically significant. A laboratory experiment did not confirm the expected hypothesis that hosts with high parasites loads had impaired burrowing ability. A 6-week field experiment, where the burrowing ability of the host was manipulated to increase its visibility, showed that hosts with reduced burrowing abilities did not have higher mortality than hosts with normal burring ability. Epibiota species were also highly variable in the estuary. A spatial survey from 15 sites found four encrusting and 11 solitary epibiota species with highly variable densities across sites and seasons. Factors that accounted for epibiota richness and density included host size and seasonality (particularly for macroalgal species), whereas environmental gradients and co-occurrence patterns with different epibiota species explained additional variability for only a few species. Foliose and tubular forms of Ulva spp. were the most abundant epibiota species throughout estuary (on average 2.3 and 1.7 per host, respectively) and were therefore studied in more detail. A 6-week field experiment showed that drift macroalgal mats had little effect on densities of either Austrovenus or epibiota species. Similarly, another field experiment showed that predators had no impact on Austrovenus abundances, irrespective of its size, if Austrovenus was allowed to bury or not, and if it was unconcealed or concealed under macroalgal mats. Finally, a laboratory experiment showed that small meso-grazers, under natural background densities, could not reduce densities or sizes of Ulva recruits on shells or barnacles (attached to Austrovenus shells). This study has shown that a single species of estuarine shell-forming ecosystem engineer provides ubiquitous internal and external habitat for other species throughout an estuary. The study has helped clarify how ecosystem engineers can directly control species abundances (here of parasites and epibiota) but also function as nursery grounds for other important ecosystem engineers (here bloom-forming drift algae). Furthermore, and in contrast to past research, this study did not find strong relationships between parasites and Austrovenus or its epibiota, suggesting that past generalisations about parasite effects may not be applicable within and between all estuaries. Finally, the study documented that drift macroalgae and consumers, in natural background densities, had very little impact on Austrovenus and its epibiota. Previous studies have shown that hosts with high parasite loads are commonly found on the sediment surface. These studies have suggested that this impaired burial ability makes the host more vulnerable to predation (by the parasites final host). However, at the same time, surface-lying host are also more exposed to fouling by epibiota species, which could reduce predation (by the final host) because epibiota may conceal it. However, this thesis found little support for either of these opposing ecological processes; parasite loads did not decrease burial ability, and host exposed the surface were not predated more, irrespective of being concealed or not Clearly, future studies should aim to identify thresholds in space, time, and densities where parasites, macroalgae and consumers have stronger impacts on Austrovenus and each other than shown here.