Environment Institute Leaders publications
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Browsing Environment Institute Leaders publications by Advisors "Gillanders, Bronwyn"
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Item Open Access Declining water quality as a driver of changes to subtidal communities.(2009) Gorman, Daniel; Connell, Sean Duncan; Gillanders, Bronwyn; School of Earth and Environmental Sciences : Ecology and Evolutionary BiologyThis body of work examines the influence of land use on nearshore water quality, and how this can drive changes to algal and invertebrate communities along Australia's southern coastline. The overall aim of the thesis was to investigate links between increasing coastal water-column nitrogen concentrations (derived from terrestrial inputs) and the expansion of turf-forming habitats that can alter the structure and function of subtidal ecosystems. I initially tested whether human activities in coastal catchments can increase subsidies of nitrogen to open rocky coasts. I identified landscape-scale variation in the supply of Dissolved Inorganic Nitrogen (DIN) to coastal waters adjacent to natural, agricultural and urban catchments. Compared to natural catchments, subsidies of DIN were 8 - 407 times greater in urban catchments, and 1 - 63 times greater in agricultural catchments. Subsidies of nitrogen from urban catchments were attributed to the release of sewage effluent, as delineated by δ¹ ⁵N isotopic values of transplanted algae. Having made this link, I then assessed whether catchment-scale variation in nitrogen subsidies may predict patterns of subtidal habitat structure, particularly as related to theories of regime shifts from forested landscapes to structurally depauperate turf-forming habitats. I validated this hypothesis, demonstrating that both relative covers and patch-sizes of turfed habitat were greater where the ratio of terrestrial nitrogen inputs to ambient coastal resources was large. An important realisation was that loss of forests may be more strongly related to the size of subsidy (i.e. the relative increases in water column nitrogen concentrations along urban coasts) rather than the size of coastal populations. Together, these data link coastal development with modified land-to-sea subsidies, and indirectly support the model that ecological effects may be proportional to the disparity between donor and recipient resources. Having demonstrated a link between nitrogen subsidies and subtidal habitat change, I then investigated factors likely to initiate and maintain such shifts. My results demonstrate that nutrient elevation can alter the natural phenology of turfs, sustaining dense covers throughout periods of natural senescence (winter). Perennial turf covers are able to accumulate large volumes of sediment; a synergy can impede the winter recruitment of canopy-forming species (kelps and fucoid algae). My observations of reduced forest recovery along urban coasts serve to highlight the complex interaction between elevated nutrients, persistent turf covers and increased sediment accumulation, which can reduce the resilience of coastal ecosystems to disturbance. In recognition that regime shifts are likely to have consequences for higher trophic levels, I compared the diet of invertebrate herbivores from healthy and degraded coastlines using stable isotope analysis (δ¹³C and δ¹⁵N). Dietary modelling showed that turfs contributed more to the diet of consumers along degraded coastlines where turfed landscapes have replaced extensive covers of macroalgal forest. Additionally, there were strong correlations between covers of turfed habitat, herbivore diet and relative densities. Changes to ambient food quality associated with regime shift may be an important aspect of nutrient-driven change along human-dominated coastlines. The final component of my thesis redressed some of the uncertainty about restoration initiatives for urban coasts by demonstrating that regime shifts are not necessarily permanent. I showed that turf removal can facilitate the recovery of degraded forests. Future restoration, therefore, is a possible outcome of polices that aim to decouple the link between nutrient inputs and recalcitrant turfed habitats that prevent forest recovery. Initiatives that reduce nutrient discharge to coastal waters (e.g., wastewater recycling) are likely to restore the resilience of nearshore marine ecosystems and promote their rehabilitation.Item Open Access The ecology of subtidal turfs in southern Australia.(2005) Russell, Bayden D.; Gillanders, Bronwyn; Connell, Sean Duncan; School of Earth and Environmental SciencesAssemblages of algae are altered by both bottom - up ( e.g. nutrient availability ) and top - down ( e.g. herbivory ) processes. As a result of the increasing human population in coastal areas, massive changes are forecast to benthic habitats in response to increasing coastal nutrient concentrations and a reduction in consumers. To identify the scales over which nutrients may have an effect, abundance of turf - forming algae growing as epiphytes on kelp ( Ecklonia radiata ) were related to water nutrient concentration across temperate Australia. In general, the percentage cover of epiphytes was greatest at sites with the greatest nutrient concentrations. By experimentally elevating mean nitrate concentration from the low 0.064 ± 0.01 µmol L [superscript - 1 ] to 0.121 ± 0.04 µmol L [superscript - 1 ], which was still only ~ 5 % of that measured on a more eutrophic coast, I was able to increase the percentage cover of epiphytes to match those seen on nutrient rich coasts, despite not matching the nutrient concentrations on those coasts. Hence, it appears that the effects of elevated nutrients will be disproportionately large on relatively oligotrophic coasts. Nutrient concentrations were also experimentally elevated to test whether the presence of an algal canopy or molluscan grazers were able to counter the effects of nutrient enrichment on algal assemblages. The loss of canopy - forming algae is likely to be a key precursor to nutrient driven changes of benthic habitats, because nutrients had no direct effect on algal assemblages in the presence of canopy - forming algae. In the absence of canopy - forming algae, space was quickly monopolised by turf - forming algae, but in the presence of elevated nutrients grazers were able to reduce the monopoly of turf - forming algae in favour of foliose algae. This switch in relative abundance of habitat may reflect greater consumption of nutrient rich turf - forming algae by grazers, possibly creating more space for other algae to colonise. Importantly, greater consumption of turf - forming algae in the presence of elevated nutrients may act as a mechanism to absorb the disproportionate effect of nutrients on oligotrophic coasts. In southern Australia, canopy - forming algae have a negative impact on the abundance of turf - forming algae. To assess the mechanisms by which an algal canopy may suppress turf - forming algae, abrasion by the canopy and water flow were experimentally reduced. Abrasion by the canopy reduced the percentage cover and biomass of turf - forming algae. In contrast to predictions, biomass and percentage cover of turf - forming algae were also reduced when water flow was reduced. Light intensity was substantially reduced when there was less water flow ( because of reduced movement in algal canopy ). However, the reduction in available light ( shading ) did not account for all of the observed reduction in biomass and percentage cover of turf - forming algae, suggesting that other factors are modified by water flow and may contribute to the loss of turf - forming algae. Habitat loss and fragmentation are well known to affect the diversity and abundance of fauna in habitat patches. I used experimental habitats to assess how fragmentation of turf habitats affects the diversity and abundance of two taxa of macroinvertebrates with different dispersal abilities. I established that increased isolation of habitats reduced the species richness and abundance of invertebrates with slow rates of dispersal, while the species richness and abundance of invertebrates with fast rates of dispersal were greatest in habitats that were far apart. In summary, this thesis provides an insight into some of the impacts associated with human populations in coastal areas, namely increased nutrient inputs, loss of grazers ( e.g. harvesting ), and loss of canopy algae and fragmentation of habitats. I show that increased nutrient concentrations in coastal waters can alter the relative abundance of algal species, and that some effects of elevated nutrients can be absorbed by the presence of grazers. I also show that elevated nutrients have no effect on algal assemblage in the presence of canopy - forming algae, and that canopies can suppress the colonisation of turf - forming algae. Finally, I show that the fragmentation of turf habitats affects taxa of invertebrates with different dispersal abilities in different ways. Whilst the contemporary ecology of much of the temperate Australian subtidal coast is considered to be relatively unaffected by human activity, this thesis shows that changes to top - down and bottom - up processes could have large consequences for habitats and their inhabitants.Item Open Access Regional and local patterns in kelp morphology and benthic assemblages(2005) Fowler-Walker, Meegan J.; Connell, Sean Duncan; Gillanders, Bronwyn; School of Earth and Environmental SciencesMost ecologists work at scales where complexity is greatest ( i.e. local ), and it is not surprising, therefore, that we tend to be captivated by the description and explanation of local variation whilst being pessimistic about the existence of broader patterns. Using a character ( kelp morphology ) known for its local and unaccounted variation, the morphology of the canopy - forming algae Ecklonia radiata ( Phaeophyta ) was quantified across > 5000 km of temperate Australian coastline, ( i ) between different configurations of algal stand ( i.e. monospecific vs mixed - species stands ) and ( ii ) across multiple spatial scales. A key result was that despite variation at local scales ( km ), differences between stands became increasingly clear at broad scales ( 1000 ' s km ), which supports the idea that large - scale patterns can emerge from apparent stochasticity at small scales. Within each stand, regional scale differences in morphological characters were evident ( i.e. Western Australia = South Australia ≠ Eastern Australia ). These characters correlated with geographic and environmental variables to indicate that the majority of morphological variation across temperate Australia was accounted for by longitude, wave exposure, water temperature and plant density. Morphological differences associated with environmental factors may reflect a plastic response to the local environment, or alternatively may reflect genetically fixed traits ( i.e. ecotypes ). An independent test of morphological variation associated with wave exposure environments, using a reciprocal transplant experiment, revealed that morphological plasticity was the mechanism enabling E. radiata to adopt different morphologies between exposure environments. The presence of kelp canopies has strong spatial relationships with organisms growing underneath them, and variation in the morphology of these canopies may facilitate distinct assemblages within the understorey habitat. Variation in the morphology of E. radiata was found to be associated with the structure of understorey assemblages, over broad spatial scales. This canopy - understorey association revealed two ' types ' of kelp forest ; one characteristic of Western and Southern Australia and the other of Eastern Australia. Patterns of canopy - benthos association have mostly been done on horizontal surfaces and experimental tests showed that such patterns on horizontal surfaces were not representative of vertical surfaces, which enables us to recognize the conditions for which we can reliably anticipate the structure of benthic organisms, thereby improving the predictive power of models that account for widespread patterns in subtidal heterogeneity. In conclusion, this thesis suggests that there are fundamental differences between the ecology of kelp forests at local scales ( i.e. between types of stand ) and at regional scales ( i.e. between the south and east coast of temperate Australia ), reflecting differences in kelp morphology that may be caused by environmental conditions ( e.g. exposure ) and may influence associated taxa ( e.g. understorey ). Consideration of such local - scale variation ( specificity ) when testing for the existence of broad - scale phenomena ( generality ) not only strengthens our understanding of the ecology of subtidal forests, but will also improve the predictive power of further research in this system.Item Open Access Stable isotopes of estuarine fish: experimental validations and ecological investigations.(2012) Bloomfield, Alexandra Louise; Gillanders, Bronwyn; Connell, Sean Duncan; School of Earth and Environmental SciencesStable isotopes of carbon and nitrogen are commonly used in ecological research to determine food webs and trace anthropogenic inputs. These applications rely on understanding isotope signature differences between an animal and its food. When an animal consumes a food item, or changes diet, it does not instantaneously reflect the isotope ratios of that food item. The isotopic signature of animal tissue gradually approaches equilibrium with the isotopic signature of its food, as molecules are turned over and new food items are assimilated into tissues. Stable isotope ratios also change between food consumed and animal tissues that are commonly sampled. The difference in stable isotope ratios between an animal's tissue and the food it consumes is called discrimination. The rate of change, or tissue turnover, and discrimination of stable isotopes varies among and within animals, and with environmental factors. I investigated the effects of temperature and diet on these isotope parameters for two fish species and applied results to improve determination of autotrophic sources within estuaries. I studied two common, omnivorous, estuarine fishes found in South Australia: black bream (Acanthopagrus butcheri) and yellow-eye mullet (Aldrichetta forsteri). Temperature and diet affected both tissue turnover rates and discrimination of carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope ratios in fish muscle. Fish reared at warmer temperatures generally had faster tissue turnover rates and smaller discrimination factors than fish reared at cooler temperatures. However, temperature interacted with diet quality to affect δ¹³C discrimination. Fish fed diets with low C:N ratios had larger δ¹³C discrimination at warmer temperatures than at cooler temperatures. This may be caused by fish catabolising more protein for energy and therefore being able to store more lipids at cooler temperatures than warmer temperatures. Fish fed diets with high C:N ratios were the opposite, with larger δ¹³C discrimination at cooler temperatures than at warmer temperatures. Compound-specific δ¹⁵N analyses were performed on amino acids from experimental black bream muscle tissues to see if the change in δ¹⁵N of amino acids could explain the bulk change in δ¹⁵N of whole muscle tissue. Some amino acid δ¹⁵N results mirrored those of bulk δ¹⁵N analyses suggesting that they may be non-essential amino acids, although there was large variation among individual fish. Wild fish commonly consume more than one dietary item, necessitating the use of mixing models to determine source contributions to diets. Omnivores consume animal and plant matter that can differ greatly in their elemental composition and this can affect the uptake of isotopic signatures from different food sources. I tested the importance of using elemental concentration in mixing models by combining two diets with different carbon and nitrogen concentrations and feeding them to yellow-eye mullet. I compared measured δ¹³C and δ¹⁵N of fish muscle with predicted values calculated with and without using elemental concentration. Using elemental concentration in mixing models improved estimates of predicted isotopic signatures. The experimentally derived discrimination factors for black bream and yellow-eye mullet were used to investigate the relative importance of autotrophic sources to their diets in four estuaries in South Australia. Isotope signatures of carbon and nitrogen can also be used to investigate ecological niches of animals, as isotope signatures reflect what an animal has eaten from different habitats and environments. I expected the isotopic niches of black bream and yellow-eye mullet to overlap, due to their shared environmental tolerances and feeding habits, as they are commonly found in the same estuaries. However, I found no overlap in isotopic niches between black bream and yellow-eye mullet. In some estuaries the autotrophic sources that black bream and yellow-eye mullet relied on were similar, however, in these estuaries fish appeared to be either feeding at different trophic levels or were likely not in competition with one another as they were caught in different areas within estuaries. The separate isotopic niches of black bream and yellow-eye mullet may be caused by habitat partitioning or interspecific competition within the estuaries studied. I used δ¹⁵N of black bream muscle to trace anthropogenic inputs of nutrients across a range of estuaries and related nutrient concentrations of estuarine waters to black bream abundance and recruitment. Black bream abundance and recruitment showed subsidy-stress responses to nutrient concentrations of ammonia, oxidised nitrogen and orthophosphorus, with peaks in abundance and recruitment occurring at low concentrations. A positive linear relationship was found between ammonia concentration of estuarine waters and δ¹⁵N of black bream. This suggests that anthropogenic ammonia was being taken up into the food web, or directly by black bream, and affecting black bream abundance and recruitment. In summary, I found environmental factors affected stable isotope signatures of fish muscle tissue. These results further show how important it is to quantify isotope parameters for individual species. Future research should focus on how to quantify influences on isotope signatures that cannot be determined in the field, such as ration intake, and how to account for these factors in field studies.