Community structure and trophic relations in marine tufa stromatolite pools of the Eastern Cape
- Authors: Rishworth, Gavin Midgley
- Date: 2017
- Subjects: Bioturbation -- South Africa -- Eastern Cape
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10948/7116 , vital:21236
- Description: Microbialites were the dominant life-form of most shallow oceans during the Precambrian. These structures are formed by the deposition of calcium carbonate by cyanobacteria as well as the binding and trapping of sediment by these and other microalgae. In modern environments they are scarce due to several factors, including grazing pressures by metazoans, altered calcium carbonate saturation states of seawater and competition with macroalgae. The recent discovery of an extensive network of actively accreting layered microbialites (stromatolites) along the South African coastline is potentially informative from this perspective. These stromatolites form within the peritidal zone, at the interface of groundwater seepage and periodic marine incursion, forming pools trapped by the accreting fabric. The aim of this thesis was to characterise the ecosystem dynamics of a representative selection of the South African locations. During a comprehensive monthly assessment over an annual cycle, as well as for additional seasonal collections, physico-chemical measurements were monitored together with biological components such as benthic and pelagic microalgae as well as the invertebrate fauna inhabiting the stromatolite pools. These components were then assessed in terms of the potential physical and biological drivers which might explain patterns of variability. Finally, to link all of the ecosystem components, a food-web analysis was conducted, to determine the trophic linkages and, importantly, the reliance by the various consumer organisms on the stromatolite material as a food resource. Results show that the stromatolite pools are driven by a regular interplay between freshwater and marine salinity states, this being determined by tidal amplitude and ocean storm cycles. Furthermore, marine incursion represents the primary source of phosphorus for the stromatolite pools, while available nitrogen is consistently provided by the freshwater inlet stream at each site. This results in an optimum zone of primary biomass within the main stromatolite pool supported by nutrient conditions, while the shifts in salinity state occurring over a weekly tidal schedule likely exclude organisms and macrophytes that are not halotolerant. This is reflected in the benthic microalgae that form the stromatolite accretions in that they are primarily driven by salinity conditions, in addition to seasonal patterns. Interestingly, the variable nutrient conditions, both between sites and temporally, did not contribute as an important driver of the benthic microalgae but did significantly relate to the pelagic microalgae (phytoplankton). This, together with the higher biomass of benthic microalgae compared to its pelagic counterpart, suggests that the stromatolite pools are a benthic-driven system. The short duration of water retention within the stromatolite pools as a result of the constant freshwater inflow, likely also precludes nutrient build-up and favours the benthic, sessile ecosystem component, especially the stromatolite-forming microalgae. In terms of the metazoan infauna, the South African stromatolite pools support a persistent assemblage. This might be surprising given the apparently destructive influence of grazing and burrowing animals on microbial mats in terms of restricting the formation of layered accretions. However, metazoans that burrow within the stromatolite fabric were observed to coexist with clear, layered accretions. This supports the observations in some other modern microbialite habitats to suggest that metazoan disruption is clearly not the only or primary factor responsible for modern microbialite scarcity. When assessing the possible drivers of the metazoan community occupying the stromatolite matrix, both salinity patterns and resource conditions in terms of nutrient supply and macroalgal cover were consistently best related to infaunal abundance and presence/absence. This further demonstrates the role of salinity conditions in terms of providing a habitat that is restrictive to most metazoan organisms, while also suggesting that the metazoans are responding to macroalgal rather than the stromatolite microalgal conditions. To further develop this observation, the results from the stable isotope work clearly reflect a dominance of pool macroalgae in the diets of invertebrate consumers, with little to no stromatolite material consumed. This suggests that there is limited apparent destructive grazing influence by the metazoans on the stromatolite matrix, in addition to the burrowing bioturbation mentioned previously. Furthermore, the metazoan grazers may be indirectly benefitting the stromatolites by restricting macroalgal biomass, which might otherwise outcompete its microalgal counterpart. This study provides a valuable understanding of benthic-driven peritidal stromatolite ecosystems, and also, from a geological perspective of past stromatolite habitats, suggests some of the mechanisms as to why metazoans may be able to coexist with layered microbialites. Given the threats to similar habitats globally, especially in terms of water resources, management measures necessary to ensure stromatolite persistence in modern environments such as these are proposed. The possible ecological role of peritidal stromatolite habitats within the broader environment, as well as recommendations for future work, is also contextualised.
- Full Text:
- Date Issued: 2017
- Authors: Rishworth, Gavin Midgley
- Date: 2017
- Subjects: Bioturbation -- South Africa -- Eastern Cape
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10948/7116 , vital:21236
- Description: Microbialites were the dominant life-form of most shallow oceans during the Precambrian. These structures are formed by the deposition of calcium carbonate by cyanobacteria as well as the binding and trapping of sediment by these and other microalgae. In modern environments they are scarce due to several factors, including grazing pressures by metazoans, altered calcium carbonate saturation states of seawater and competition with macroalgae. The recent discovery of an extensive network of actively accreting layered microbialites (stromatolites) along the South African coastline is potentially informative from this perspective. These stromatolites form within the peritidal zone, at the interface of groundwater seepage and periodic marine incursion, forming pools trapped by the accreting fabric. The aim of this thesis was to characterise the ecosystem dynamics of a representative selection of the South African locations. During a comprehensive monthly assessment over an annual cycle, as well as for additional seasonal collections, physico-chemical measurements were monitored together with biological components such as benthic and pelagic microalgae as well as the invertebrate fauna inhabiting the stromatolite pools. These components were then assessed in terms of the potential physical and biological drivers which might explain patterns of variability. Finally, to link all of the ecosystem components, a food-web analysis was conducted, to determine the trophic linkages and, importantly, the reliance by the various consumer organisms on the stromatolite material as a food resource. Results show that the stromatolite pools are driven by a regular interplay between freshwater and marine salinity states, this being determined by tidal amplitude and ocean storm cycles. Furthermore, marine incursion represents the primary source of phosphorus for the stromatolite pools, while available nitrogen is consistently provided by the freshwater inlet stream at each site. This results in an optimum zone of primary biomass within the main stromatolite pool supported by nutrient conditions, while the shifts in salinity state occurring over a weekly tidal schedule likely exclude organisms and macrophytes that are not halotolerant. This is reflected in the benthic microalgae that form the stromatolite accretions in that they are primarily driven by salinity conditions, in addition to seasonal patterns. Interestingly, the variable nutrient conditions, both between sites and temporally, did not contribute as an important driver of the benthic microalgae but did significantly relate to the pelagic microalgae (phytoplankton). This, together with the higher biomass of benthic microalgae compared to its pelagic counterpart, suggests that the stromatolite pools are a benthic-driven system. The short duration of water retention within the stromatolite pools as a result of the constant freshwater inflow, likely also precludes nutrient build-up and favours the benthic, sessile ecosystem component, especially the stromatolite-forming microalgae. In terms of the metazoan infauna, the South African stromatolite pools support a persistent assemblage. This might be surprising given the apparently destructive influence of grazing and burrowing animals on microbial mats in terms of restricting the formation of layered accretions. However, metazoans that burrow within the stromatolite fabric were observed to coexist with clear, layered accretions. This supports the observations in some other modern microbialite habitats to suggest that metazoan disruption is clearly not the only or primary factor responsible for modern microbialite scarcity. When assessing the possible drivers of the metazoan community occupying the stromatolite matrix, both salinity patterns and resource conditions in terms of nutrient supply and macroalgal cover were consistently best related to infaunal abundance and presence/absence. This further demonstrates the role of salinity conditions in terms of providing a habitat that is restrictive to most metazoan organisms, while also suggesting that the metazoans are responding to macroalgal rather than the stromatolite microalgal conditions. To further develop this observation, the results from the stable isotope work clearly reflect a dominance of pool macroalgae in the diets of invertebrate consumers, with little to no stromatolite material consumed. This suggests that there is limited apparent destructive grazing influence by the metazoans on the stromatolite matrix, in addition to the burrowing bioturbation mentioned previously. Furthermore, the metazoan grazers may be indirectly benefitting the stromatolites by restricting macroalgal biomass, which might otherwise outcompete its microalgal counterpart. This study provides a valuable understanding of benthic-driven peritidal stromatolite ecosystems, and also, from a geological perspective of past stromatolite habitats, suggests some of the mechanisms as to why metazoans may be able to coexist with layered microbialites. Given the threats to similar habitats globally, especially in terms of water resources, management measures necessary to ensure stromatolite persistence in modern environments such as these are proposed. The possible ecological role of peritidal stromatolite habitats within the broader environment, as well as recommendations for future work, is also contextualised.
- Full Text:
- Date Issued: 2017
Time-activity budgets of cape gannets (Morus Capensis) at Bird Island, Algoa Bay
- Authors: Rishworth, Gavin Midgley
- Date: 2014
- Subjects: Birds of prey -- Behavior -- South Africa -- Algoa Bay , Gannets -- South Africa -- Algoa Bay , Birds -- Breeding -- South Africa -- Algoa Bay
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:10728 , http://hdl.handle.net/10948/d1021046
- Description: For several decades now, seabirds have been considered to be useful indicators of the state of their prey resources because of how they reflect lower-level trophic variability through proxies such as diet or behaviour. However, collection of appropriate data is often challenging in the marine environment because of logistical or financial constraints. In this study, time-activity budgets were studied in the Cape gannet (Morus capensis), a seabird which has been advocated as a potential bioindicator for local epipelagic prey. VHF transmitters attached to PVC leg-rings were fitted to 50 adult breeding pairs during the 2011/2012 and 2012/2013 breeding seasons at Bird Island, Algoa Bay, to determine whether an automated method of continuously collecting time-activity budget data can replace conventional laborious direct observations. To validate that the foraging trip duration data generated from the automated method was a reflection of foraging effort, Time-Depth Recorders (TDRs) were simultaneously equipped to birds with VHF transmitters for three weeks. In order to assess the influence of factors other than prey availability on parent time-activity budgets, sex, chick age and body condition were measured. Additionally, chick growth and survival were recorded in order to investigate the gender-specific effects of parental time-activity budget variability on these parameters. Attachment of VHF transmitters to leg-rings of adult Cape gannets had no observable negative effects on the adults or their chicks in terms of adult body condition, nest attendance and foraging trip durations, or chick growth and survival. Furthermore, the frequency distribution of behaviour bout (foraging trip and nest attendance) durations was no different between automated and directly observed data. However, the automated method did record shorter behaviour bouts, largely attributed to the increased likelihood of direct observations missing birds returning briefly to their nests during older chick provisioning. Additionally, foraging trip duration was highly correlated to foraging effort in terms of time spent resting on the sea surface, flying and diving. The automated method therefore appears to be a good reflection of direct nest attendance observations and foraging effort. Cape gannet time-activity budgets were related to chick age and parental sex. Especially as chicks neared fledging, females spent significantly longer periods of time foraging than males, with males consequently provisioning their chicks more often. Furthermore, adults departing their nests earlier spent more time away from the nest foraging as more same-day daylight hours were available. Chick growth was a function of parent foraging trip duration and associated prey delivery rates. Chick survival was most strongly affected by the amount of time which chicks were left unattended by both parents and consequently exposed to predation by kelp gulls (Larus dominicanus) or to adverse weather conditions. Additionally, for females (but not males) there appeared to be a trade-off between foraging trip duration and chick survival. The Cape gannet appears to demonstrate a strategy whereby the costs of reproduction to the female are shifted towards male-dominated chick provisioning as the chick nears fledging. Drivers of time-activity budget variability such as chick age and parental sex therefore need to be considered if using data on foraging trip duration as a proxy of foraging effort and prey availability.
- Full Text:
- Date Issued: 2014
- Authors: Rishworth, Gavin Midgley
- Date: 2014
- Subjects: Birds of prey -- Behavior -- South Africa -- Algoa Bay , Gannets -- South Africa -- Algoa Bay , Birds -- Breeding -- South Africa -- Algoa Bay
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:10728 , http://hdl.handle.net/10948/d1021046
- Description: For several decades now, seabirds have been considered to be useful indicators of the state of their prey resources because of how they reflect lower-level trophic variability through proxies such as diet or behaviour. However, collection of appropriate data is often challenging in the marine environment because of logistical or financial constraints. In this study, time-activity budgets were studied in the Cape gannet (Morus capensis), a seabird which has been advocated as a potential bioindicator for local epipelagic prey. VHF transmitters attached to PVC leg-rings were fitted to 50 adult breeding pairs during the 2011/2012 and 2012/2013 breeding seasons at Bird Island, Algoa Bay, to determine whether an automated method of continuously collecting time-activity budget data can replace conventional laborious direct observations. To validate that the foraging trip duration data generated from the automated method was a reflection of foraging effort, Time-Depth Recorders (TDRs) were simultaneously equipped to birds with VHF transmitters for three weeks. In order to assess the influence of factors other than prey availability on parent time-activity budgets, sex, chick age and body condition were measured. Additionally, chick growth and survival were recorded in order to investigate the gender-specific effects of parental time-activity budget variability on these parameters. Attachment of VHF transmitters to leg-rings of adult Cape gannets had no observable negative effects on the adults or their chicks in terms of adult body condition, nest attendance and foraging trip durations, or chick growth and survival. Furthermore, the frequency distribution of behaviour bout (foraging trip and nest attendance) durations was no different between automated and directly observed data. However, the automated method did record shorter behaviour bouts, largely attributed to the increased likelihood of direct observations missing birds returning briefly to their nests during older chick provisioning. Additionally, foraging trip duration was highly correlated to foraging effort in terms of time spent resting on the sea surface, flying and diving. The automated method therefore appears to be a good reflection of direct nest attendance observations and foraging effort. Cape gannet time-activity budgets were related to chick age and parental sex. Especially as chicks neared fledging, females spent significantly longer periods of time foraging than males, with males consequently provisioning their chicks more often. Furthermore, adults departing their nests earlier spent more time away from the nest foraging as more same-day daylight hours were available. Chick growth was a function of parent foraging trip duration and associated prey delivery rates. Chick survival was most strongly affected by the amount of time which chicks were left unattended by both parents and consequently exposed to predation by kelp gulls (Larus dominicanus) or to adverse weather conditions. Additionally, for females (but not males) there appeared to be a trade-off between foraging trip duration and chick survival. The Cape gannet appears to demonstrate a strategy whereby the costs of reproduction to the female are shifted towards male-dominated chick provisioning as the chick nears fledging. Drivers of time-activity budget variability such as chick age and parental sex therefore need to be considered if using data on foraging trip duration as a proxy of foraging effort and prey availability.
- Full Text:
- Date Issued: 2014
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