Coastal hydrodynamics regulate population dynamics through the distribution and dispersal of the meroplankton of many benthic invertebrates. I examined the hydrodynamics at four different sites on the south-east coast of South Africa and coupled them with larval sampling done at high temporal and spatial resolution. Day and night sampling was done at all four sites and a continuous 24 hour study was done in one site, both forms of sampling were carried out in autumn and spring. Samples were taken at two stations, 900 metres offshore and 300 metres apart, within each site. Water properties measured were depth, temperature and current velocity and direction. Plankton samples were collected using a plankton pump at various depths, from the surface, bottom and either side of the thermocline when present. A wide range of taxa (mostly bryozoans, bivalves, barnacles and decapods) was examined. 2-way ANOVAs were used to test the effects of time and depth on each taxon. In addition, multiple regression analyses were performed on each taxon to investigate the effects of hydrodynamics on the distribution of larvae. Bryozoanlarvae proved to be positively phototactic whilst bivalve veligers, barnacle larvae and decapod zoeae performed diel vertical migration. Turbulence and temperature had an effect on the vertical distribution/migration of decapod zoeae. These results highlight the role of taxon-specific responses to flow and the potential differential effects on larval retention and ultimately connectivity of benthic populations.

Benthic filter feeders have a key functional role in the dynamics of coastal food web as an intermediate trophic level and bioengineers. A wide variety of factors, operating across multiple spatial scales (e.g. hydrographic regime, human activities), can affect the composition of the water column and thus the availability of food for benthic populations. Food availability in turn affects the growth, reproductive rates and survival of benthic organisms, and consequently, can influence the functioning of the entire ecosystem. This study aims to evaluate how various environmental factors may modify the diet of intertidal filter feeders living along the South African coast. Specifically, the effects of biogeography, upwelling, urbanization and freshwater input on the dietary regimes of five species of filter feeders (two mussel and three barnacle species) were investigated using fatty acid (FA) and stable isotope (SI) analyses.Strong interspecific differences were found among the five species considered. However, all species responded to factors operating at large (100s km) and meso (10s- 100s km) scales (i.e. biogeography and upwelling respectively). The barnacles exhibit habitat segregation and showed different FA and SI signatures from each other, while the two mussel species, an invasive and native species that co-occur in the same mussel beds, had partially overlapping diets. Differences in their diets were found only using FA analysis, while their SI signatures differed on only one occasion. This highlights the importance of using the appropriate tool, and ideally combined techniques, to investigate diets.FA and SI signatures of all species considered changed among the three biographical provinces (west, south and east coasts of South Africa) exhibiting similar patterns that reflect the two oceanographic regimes that characterize the coastline: the eutrophic Benguela Current on the west coast and the oligotrophic Agulhas Current on the other two coasts. Upwelling had a significant effect on FA and SI signatures, with stronger effects on the west coast than the south coast. The results indicate that benthic filter feeders at upwelling areas consumed a mix of coastal macroalgal detritus and phytoplankton, which was probably brought onshore during downwelling events. At smaller spatialscales and using repeated sampling, the influence of upwelling on the west coast was found to be pervasive, rather than discrete, so that it may be more appropriate to categorize upwelling by referring to upwelling centres and downstream areas. SI underlined a significant effect of urbanization on the diet of filter feeders with an enrichment in the δ¹⁵N being characteristic of anthropogenic effect. Although a large number of rivers characterize the South African coast, no distinct effect of freshwater input was found for either the SI or FA signatures of the filter feeders. This contrasts with earlier work on demersal species and suggests that freshwater input does not significantly affect food availability for intertidal filter feeders, and that other factors (e.g. hydrogeography) are more important in determining the diet of these populations. These results highlight that environmental and anthropogenic factors operating at different spatial and temporal scales have a profound effect on benthic ecosystems, and that they control the relationship between primary production and primary consumers in coastal areas. Above all, this work highlights the importance of understanding the spatial and temporal scales at which different factors affect feeding regimes, and their critical role in coastal food webs.

published_or_final_version / abstract / toc / Ecology and Biodiversity / Master / Master of Philosophy

Understanding the influence of disturbance, both natural and human-induced, is a persistent challenge in ecology. Recently, attempts to predict future environments have focused on the consequences of broad scale disturbances. In this thesis I focus on environmental and trophic disturbances as shapers of benthic assemblages. There is growing recognition of the need for greater scientific investment in understanding environmental disturbances to balance the continuing focus of research assessing trophic theories (e.g. herbivory). Historically, it is these theories that have provided a cornerstone to describe and manage subtidal rocky coasts worldwide. In this thesis, therefore, I first assess how our ecological perception of such disturbances (i.e. water pollution and harvesting grazers) may vary as a consequence of the choice of taxonomic classifications used to observe benthic patterns (Chapter 2). I then assess how mechanical disturbance (i.e. wave exposure) may affect the morphology of benthic habitat (Chapter 3) and how temperature disturbances (i.e. oceanographic, cold water pulsing) may affect the consumers of these habitats (Chapter 4). The critical first finding centred on the effectiveness of alternate scales and metrics of taxonomic classification to detect the effects of water pollution (i.e. nutrient enhancement) as the largest disturbing agent on the benthos, and that this effect may be exacerbated by loss of grazers. While observations of the benthos as morphological groups detected the effects of enhanced nutrients, species diversity (as a measure of phylogenetic relatedness) was the only one of the chosen measures sensitive enough to detect the interaction of both top-down and bottom-up stressors. This chapter highlights the importance of choice of classification (e.g. morphology v. species) and indices (e.g. Shannon index v. ABC curves and phylogenetic diversity) in their potential to predetermine our perception of ecological change and thereby predict future environments. Mechanical disturbance (i.e. wave exposure) has been widely studied as a mechanism that creates new space for colonisation by alternate species, but is less well studied as a force that can change the shape of communities by mediating their morphology. Macroalgal morphology varies in response to wave exposure such that individuals at high exposures are often smaller than individuals in more sheltered environments. Observations not only confirmed these patterns for a general assemblage on a wave exposed southern coast, but reciprocal transplants of assemblages between exposures also revealed that morphological differences were likely to be a product of flexibility in morphological response of algae to local environments (Chapter 3). In contrast to the often multi-directional responses of a complex suite of morphological characters (e.g. smoothness, stipe length, frond width), overall size has the potential to be used as a broad and predictive tool to identify hydrodynamic stressors across an entire exposure gradient or geographic range. Strong trophic interactions are often considered characteristic of aquatic systems and due to their perceived ubiquity on temperate rocky coasts, there has been an emphasis in the literature on the influence of herbivores in determining assemblages. Given the importance of the link between herbivores and assemblage structure, in my final chapter I investigated the potential for disturbance to act indirectly on benthos by affecting the survivorship of an herbivorous urchin (Chapter 4). I used observations of variable temperature regimes in a region of upwelling to design an experiment that tested whether this temperature variation could negatively affect the survivorship of settling sea urchins (Heliocidairs erythrogramma). When exposed to cold water, mortality increased by up to 70 %, within 12 h of settlement, representing a massive loss of benthic consumers within a very short time scale. This result was used to assess the potential of temperature to indirectly influence benthic habitats across several spatial scales, a process that may have been profoundly underestimated. In summary, this thesis provides insight into environmental and trophic disturbances as shapers of benthic assemblage patterns, both as natural and human-induced phenomena. I show that our perception of ecological response to the combination of such disturbances can be contingent on the organisational scales and metrics used. Subtle differences in initial choice of such observational units may not only have large affects on the kinds of benthic patterns and disturbances ecologists detect, but also those that they pursue. I recognise that while some physical disturbances can appear subtle (e.g. morphological variation) or strong (e.g. high mortality rate of herbivores), their relative impacts on the broader assemblage (e.g. understorey flora and fauna) will often be dependent on biogeography. Integrating local-scale biological interactions with regional-scale physical processes, therefore, appears to be a potentially progressive line of future enquiry. Indeed, consideration of responses from the physiological through to physical and biogeographical scales will not only strengthen our understanding of the effects of alternate disturbance regimes, but also our predictive power to anticipate future change. / http://proxy.library.adelaide.edu.au/login?url= http://library.adelaide.edu.au/cgi-bin/Pwebrecon.cgi?BBID=1339116 / Thesis (Ph.D.) - University of Adelaide, School of Earth and Environmental Sciences, 2008

The aim of this thesis has been to: 1) assess the state of pollution in the two study areas and relate them to recent changes found in the Kattegat - Belt Sea area; 2) examine the observed spatial and temporal variability in species composition, abundance and biomass in Arhus Bay and the Formes area and relate the variability to antropogenic and natural causes; 3) discuss and assess methods, particularly for the estimation of secondary production and the use of multivariate analyses as methods for examining changes in macrozoobenthic communities. Macrozoobenthos were sampled at 15 stations in Arhus Bay, Denmark from 1985 to 1991 while data from Formes (reference area) included 55 sampling stations from 1986 to 1990. Monthly sampling took place at one station in Arhus Bay in 1990 and 1991. The two study areas are both situated on the eastcoast of Jutland in the Kattegat - Belt Sea area at 13-17 m depth and both receive waste water from long sea outfalls. Although both areas are Subjected to salinity stratification for most of the year, the exposed position of the Formes area on the open Kattegat coast prevents it from suffering from severe oxygen deficiencies, unlike the Arhus Bay which is a sheltered, semi-enclosed sedimentation area where oxygen concentrations in the bottom water can be very low. At Formes the sediment is sandy while it is silty in Arhus Bay. The spatial and temporal variability in the benthos in Arhus Bay could to a great extent be explained by the variation in 7 important species: Abra alba, Corbula gibba, Mysella bidentata, Nepthys hombergii, N. ciliata, Ophiura albida and Echinocardium cordatum. The fluctuations in the number and biomass of A. alba had a pronounced effect on the total abundance and biomass in Arhus Bay. The severe winter of 1986/87 with low temperatures and oxygen depletion under the ice cover practically eliminated A. alba from the bay. A. alba quickly recolonized the area and was found in high numbers in 1988. Studies of growth of A. alba in 1990 and 1991 showed that by the end of 1990 the population had reached an average length of 10 mm while the average shell length was only 5 mm by the end of 1991. The difference between the two years could be attributed to the difference in sedimentation of phytoplankton from the water column. As in other parts of the Kattegat - Belt Sea area, Arhus Bay has experienced low oxygen concentrations in the bottom water in late summer early autumn throughout the 1980s. Only the oxygen depletion under the ice cover in early spring 1987 and the local oxygen deficiencies south of the outlet in 1989 and 1990 actually killed parts of the benthic fauna. Apart from 1981, the oxygen deficiencies have thus been less severe in Arhus Bay than in other parts of the southern Kattegat in the 1980s. The number of species, abundance and biomass decreased at Fornres from 1980 to 1985 while the discharge of BOD was fairly constant during the same period. From 1986 there was a slight decrease in the discharge of BOD but a considerable increase in the number of species, abundance and biomass. At least for the second half of the 1980s there was no straightforward relation between the organic enrichment from the outlet and species composition, abundance and biomass and suggests that other factors are also important influencing the fluctuations in the benthic fauna. Estimates of total secondary community production were found to be very dependent on the method used. The method described by Brey (1990) was found acceptable for estimating secondary production in Arhus Bay but care should be exercised when comparisons are made with other areas where different methods have been used to estimate production. Secondary production was estimated more accurately for some of the abundant species in Arhus Bay on the basis of monthly samplings by the method described by Crisp (1984). Among the multivariate analyses the Detrended Correspondence Analysis (DCA) and nonmetric Multidimensional Scaling (MDS) proved to be the most successful with the Arhus Bay and Fornes data sets. Two Way INdicator SPecies ANalysis (TWINSPAN) did not work well with the Fornes data because it imposed discontinuities on data sets with continous variation in distribution of species among samples. As community types existed to a certain degree in Arhus Bay TWINSPAN worked well with these data. DCA and MDS were found to be useful techniques for analysing large data sets because they can summarize the data matrices to a manageable form and find possible patterns in the data sets. The results of the analyses can then be used as starting point for more detailed investigations of single species/samples or groups of species/samples. By using different transformations of the raw data the role of dominant or rare species can be assessed. A major 'problem in the assessment of multivariate techniques is the lack of external standards to compare with. The results of multivariate analyses must therefore be assessed critically on the basis of a careful examination of the species list combined with the knowledge and experience of the investigator. The methods used all had their advantages and limitations but each of the different methods added some important information to the picture of the benthic community in Arhus Bay and Fornes. It was thus an considerable advantage to use several different methods to analyse the spatial and temporal variability in the benthic fauna in relation to antropogenic and natural causes.