Macroalgal blooms are increasing world-wide and have negative effects on benthic invertebrates and sediments. These include loss of species diversity and development of hostile sediment environments. This thesis considers ecological effects of Ulva lactuca L., and its mechanical removal on benthic invertebrates and sediments in Avon-Heathcote Estuary, New Zealand. Benthic communities comprised 34 species from 12 groups recorded from seven sites during seasonal general surveys. Dominant groups at each site were Gastropoda and Bivalvia. The most abundant species were Austrovenus stutchburyi, Micrelenchus tenebrosus and Amphibola crenata. Community composition varied significantly between sites, and there were significant site-specific differences in abundances of most species between winter and summer. U. lactuca had the greatest seasonal variation. Several species correlated with U. lactuca biomass, and the strength of correlation for different species varied between sites. There were seasonal changes in sediment physico-chemical variables between sites with greatest change in the silt/clay fraction. The sediment variables silt/clay fraction, dissolved oxygen and temperature correlated with seasonal changes of patterns in benthic community assemblages. A similar study was carried out by Bressington in 2003. In both studies, Bivalvia and Gastropoda were the most abundant groups, with Gastropoda having a higher, and Bivalvia a lower, proportion in the present study compared with 2003. Summer communities were significantly different between the two studies. Compared with 2003 there were higher percentages of sediment pore water and volatile solids present in 2005. Experimental removal of U. lactuca was conducted by mechanical broom at two sites: an open, exposed central sand flat, and McCormacks Bay, a shallow, sheltered mud flat. Removing U. lactuca had several immediate effects. These included a significant decrease in abundance of mobile epifauna (Micrelenchus tenebrosus and Zeacumantus subcarinatus) and an increase in abundance of infauna, including Arthritica bifurca and Austrovenus stutchburyi. There was no effect of U. lactuca removal on Austrovenus stutchburyi condition and 46 days following removal, invertebrate abundances approached pre-removal levels at each site. U. lactuca removal also caused short-term increases in dissolved oxygen and temperature of pore water. The greatest visual impact of removing U. lactuca was to sediments in McCormacks Bay from trampling. It was concluded that the variables having the greatest effect on seasonal species distribution and abundance at each site were temperature and sediment grain size. Differences between the present study and the study in 2003 were due to differences in sampling procedure mainly due to the two different quadrat sizes. Greater accuracy in representing long-term changes in ecosystems would be achieved by using standard sampling protocols. Removal of U. lactuca by mechanical broom was effective and had low impact on benthic invertebrates and physico-chemical variables, but it should be used only in sandy habitats because of severe disturbance to soft-sediment environments. Options for management and control of U. lactuca in Avon-Heathcote Estuary are discussed.

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.

Macroalgal blooms are increasing world-wide and have negative effects on benthic invertebrates and sediments. These include loss of species diversity and development of hostile sediment environments. This thesis considers ecological effects of Ulva lactuca L., and its mechanical removal on benthic invertebrates and sediments in Avon-Heathcote Estuary, New Zealand. Benthic communities comprised 34 species from 12 groups recorded from seven sites during seasonal general surveys. Dominant groups at each site were Gastropoda and Bivalvia. The most abundant species were Austrovenus stutchburyi, Micrelenchus tenebrosus and Amphibola crenata. Community composition varied significantly between sites, and there were significant site-specific differences in abundances of most species between winter and summer. U. lactuca had the greatest seasonal variation. Several species correlated with U. lactuca biomass, and the strength of correlation for different species varied between sites. There were seasonal changes in sediment physico-chemical variables between sites with greatest change in the silt/clay fraction. The sediment variables silt/clay fraction, dissolved oxygen and temperature correlated with seasonal changes of patterns in benthic community assemblages. A similar study was carried out by Bressington in 2003. In both studies, Bivalvia and Gastropoda were the most abundant groups, with Gastropoda having a higher, and Bivalvia a lower, proportion in the present study compared with 2003. Summer communities were significantly different between the two studies. Compared with 2003 there were higher percentages of sediment pore water and volatile solids present in 2005. Experimental removal of U. lactuca was conducted by mechanical broom at two sites: an open, exposed central sand flat, and McCormacks Bay, a shallow, sheltered mud flat. Removing U. lactuca had several immediate effects. These included a significant decrease in abundance of mobile epifauna (Micrelenchus tenebrosus and Zeacumantus subcarinatus) and an increase in abundance of infauna, including Arthritica bifurca and Austrovenus stutchburyi. There was no effect of U. lactuca removal on Austrovenus stutchburyi condition and 46 days following removal, invertebrate abundances approached pre-removal levels at each site. U. lactuca removal also caused short-term increases in dissolved oxygen and temperature of pore water. The greatest visual impact of removing U. lactuca was to sediments in McCormacks Bay from trampling. It was concluded that the variables having the greatest effect on seasonal species distribution and abundance at each site were temperature and sediment grain size. Differences between the present study and the study in 2003 were due to differences in sampling procedure mainly due to the two different quadrat sizes. Greater accuracy in representing long-term changes in ecosystems would be achieved by using standard sampling protocols. Removal of U. lactuca by mechanical broom was effective and had low impact on benthic invertebrates and physico-chemical variables, but it should be used only in sandy habitats because of severe disturbance to soft-sediment environments. Options for management and control of U. lactuca in Avon-Heathcote Estuary are discussed.

The Avon-Heathcote Estuary is of significant value to Christchurch due to its high productivity, biotic diversity, proximity to the city, and its cultural, recreational and aesthetic qualities. Nonetheless, it has been subjected to decades of degradation from sewage wastewater discharges and encroaching urban development. The result was a eutrophied estuary, high in nitrogen, affected by large blooms of nuisance macroalgae and covered by degraded sediments. In March 2010, treated wastewater was diverted from the estuary to a site 3 km offshore. This quickly reduced water nitrogen by 90% within the estuary and, within months, there was reduced production of macroalgae. However, a series of earthquakes beginning in September 2010 brought massive changes: tilting of the estuary, changes in channels and water flow, and a huge influx of liquefied sediments that covered up to 65% of the estuary floor. Water nitrogen increased due to damage to sewage infrastructure and the diversion pipeline being turned off. Together, these drastically altered the estuarine ecosystem. My study involves three laboratory and five in situ experiments that investigate the base of the food chain and responses of benthic microalgae to earthquake-driven sediment and nutrient changes. It was predicted that the new sediments would be coarser and less contaminated with organic matter and nutrients than the old sediments, would have decreased microalgal biomass, and would prevent invertebrate grazing and bioturbation activities. It was believed that microalgal biomass would become similar across new and old sediments types as the unstable new sediments were resuspended and distributed over the old sediments. Contact cores of the sediment were taken at three sites, across a eutrophication gradient, monthly from September 2011 to March 2012. Extracted chlorophyll a pigments showed that microalgal biomass was generally lower on new liquefied sediments compared to old sediments, although there was considerable site to site variation, with the highly eutrophic sites being the most affected by the emergence of the new sediments. Grazer experiments showed that invertebrates had both positive and negative site-specific effects on microalgal biomass depending on their identity. At one site, new sediments facilitated grazing by Amphibola crenata, whereas at another site, new sediments did not alter the direct and indirect effects of invertebrates (Nicon aestuariensis, Macropthalmus hirtipes, and A. crenata) on microalgae. From nutrient addition experiments it was clear that benthic microalgae were able to use nutrients from within both old and new sediments equally. This implied that microalgae were reducing legacy nutrients in both sediments, and that they are an important buffer against eutrophication. Therefore, in tandem with the wastewater diversion, they could underpin much of the recovery of the estuary. Overall, the new sediments were less favourable for benthic microalgal growth and recolonisation, but were less contaminated than old sediments at highly eutrophic sites. Because the new sediments were less contaminated than the old sediments, they could help return the estuary to a noneutrophic state. However, if the new sediments, which are less favourable for microalgal growth, disperse over the old sediments at highly eutrophic sites, they could become contaminated and interfere with estuarine recovery. Therefore, recovery of microalgal communities and the estuary was expected to be generally long, but variable and site-specific, with the least eutrophic sites recovering quickly, and the most eutrophic sites taking years to return to a pre-earthquake and non-eutrophied state. changes in channels and water flow, and a huge influx of liquefied sediments that covered up to 65% of the estuary floor. Water nitrogen increased due to damage to sewage infrastructure and the diversion pipeline being turned off. Together, these drastically altered the estuarine ecosystem. My study involves three laboratory and five in situ experiments that investigate the base of the food chain and responses of benthic microalgae to earthquake-driven sedimen tand nutrient changes. It was predicted that the new sediments would be coarser and less contaminated with organic matter and nutrients than the old sediments, would have decreased microalgal biomass, and would prevent invertebrate grazing and bioturbation activities. It was believed that microalgal biomass would become similar across new and old sediments types as the unstable new sediments were resuspended and distributed over the old sediments. Contact cores of the sediment were taken at three sites, across a eutrophication gradient, monthly from September 2011 to March 2012. Extracted chlorophyll a pigments showed that microalgal biomass was generally lower on new liquefied sediments compared to old sediments, although there was considerable site to site variation, with the highly eutrophic sites being the most affected by the emergence of the new sediments. Grazer experiments showed that invertebrates had both positive and negative site-specific effects on microalgal biomass depending on their identity. At one site, new sediments facilitated grazing by Amphibola crenata, whereas at another site, new sediments did not alter the direct and indirect effects of invertebrates (Nicon aestuariensis, Macropthalmus hirtipes, and A. crenata) on microalgae. From nutrient addition experiments it was clear that benthic microalgae were able to use nutrients from within both old and new sediments equally. This implied that microalgae were reducing legacy nutrients in both sediments, and that they are