The ecological impacts of pollution on a river ecosystem: a community index and stable isotope approach
- Authors: Gininda, Simphiwe Linah
- Date: 2017
- Subjects: Water -- Pollution -- Bloukrans River , Water quality -- South Africa -- Makhanda , Stream ecology -- South Africa -- Makhanda , South African Scoring System , Sewage disposal in rivers, lakes, etc. -- South Africa -- Makhanda , Agricultural ecology -- South Africa -- Makhanda , Agricultural pollution -- South Africa -- Makhanda
- Language: English
- Type: text , Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/7993 , vital:21331
- Description: For decades, urbanized rivers have been modified to meet the needs of constantly expanding human populations in many countries around the world. The Bloukrans River in Grahamstown is one of the polluted and structurally modified urban rivers in South Africa, and there is no published information regarding its water quality and ecological status. Water quality is threatened by human activities including the disposal of treated and raw sewage, livestock farming, and agriculture. This study was conducted to determine the ecological status of the river by assessing its biological, chemical, and physical components in relation to man-mediated activities. Biological responses of macroinvertebrates were used to assess changes in water quality through space and time based on the South African Scoring System version 5 and the Average Score per Taxon water quality indices. The results showed poor water quality due to high nitrate and ammonium concentrations derived from sewage, and channel modifications by agricultural activities and dumping of solid waste. Channel width, water depth, dissolved oxygen, nitrate, and ammonium concentrations were the main drivers of macroinvertebrate distribution patterns and had the most influence on the variability in macroinvertebrates taxa richness, diversity and abundance. Diatoms were also used to assess water quality, specifically to indicate the trophic status of the river based on changes in the Trophic Diatom Index. The results suggested that the Bloukrans River was eutrophic during the course of this study. However, the trophic status varied with freshwater input, resulting in mesotrophic conditions during flooding and eutrophication in dry seasons. Changes in pH, phosphate concentration, water velocity (current speed), and temperature influenced the distribution of diatoms in the Bloukrans River. However, only pH was important at the community level and significantly influenced diatom abundances. Stable nitrogen isotope ratios (S15N) of autotrophs and primary and secondary consumers revealed noticeable differences between tissues of organisms exposed to treated sewage and those without any exposure. The S15N values in biota occurring above the sewage treatment discharge point were low, and those collected below the sewage point were higher. Although fertilizer derived nitrogen is generally depleted in 15N, agriculture-derived nitrogen could not be excluded as a possible source since animals at the sample site that was most affected by agricultural activities had the highest S15N values. This study provided valuable information on the ecological status of the Bloukrans River and identified the major activities associated with reduced biodiversity and water quality.
- Full Text:
Aquatic–terrestrial trophic linkages via riverine invertebrates in a South African catchment
- Authors: Moyo, Sydney
- Date: 2016
- Language: English
- Type: text , Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/54427 , vital:26564
- Description: Rivers play a vital role in human livelihoods and are likely to undergo substantial alteration due to climate and land use changes from an increasing human population. Mitigating the pressures facing rivers in the world requires scientists and environmental managers to understand the ecological mechanisms, and ultimately the strength, of connections between ecosystems. This understanding of connections between adjacent habitats will enable environmental managers to predict the consequences of perturbing these linkages in the future. In this thesis, aquatic-terrestrial linkages in rivers were investigated using ecologically meaningful variables including abundances, biomasses, stable isotopes and fatty acids. This study is part of a larger project entitled “Connectivity through allochthony: reciprocal links between adjacent aquatic and terrestrial ecosystems in South Africa”, in which a team of researchers assessed a variety of pathways connecting riverine and estuarine systems to land within a catchment in the Eastern Cape, South Africa. I conceptualised the flow of energy within a temperate southern hemisphere river (the Kowie River) within theoretical models of energy flow such as the River Continuum Concept (RCC; presents lotic systems as being longitudinally linked with food webs in shaded headwaters being principally driven by allochthonous energy, with the addition of autochthonous food as a minor carbon source in the lower reaches) and the Riverine Productivity Model (RPM; proposes consumers derive most of their energy from local production of phytoplankton, benthic algae and aquatic plants, as well as directly from riparian zones via terrestrial leaf litter). Using the RCC as a starting point, I collected macroinvertebrates (September 2012 to May 2013) along a longitudinal gradient and grouped them into functional feeding groups (FFGs). The results revealed that gatherers and filterers dominated in the Kowie River, and together represented 50 – 83% of the invertebrate assemblages. There was a general paucity of shredders (relative abundance was ≤ 10% across all sites and seasons). The changes in relative abundances of different FFGs did not follow predictions of the RCC along the longitudinal gradient, as there were no correlations of community structure with some physical attributes (stream width, canopy cover, distance of river) that changed along the river continuum. However, FFG abundances were related to water velocity, total dissolved solids and canopy cover. Broadly, the Kowie River data showed that changes in relative abundances of FFGs along the river continuum could not be explained by changes in physical attributes alone, and may be highly influenced by the availability of food and the chemistry of the stream. Analysis of stable carbon (δ13C) and nitrogen (δ15N) isotopes was used to estimate the contributions of algal and land-based production to consumers over space (six sites) and time (November 2012 to September 2013). Carbon contributions determined by the use of mixing models (Stable Isotope Analysis in R) revealed that consumers in the headwater assimilated mainly terrestrially-derived organic matter, with consumers in the middle and lower reaches assimilating autochthonous basal resources (macrophytes and algae). The findings from this river supported aspects of the RCC (at the headwaters; terrestrial organic matter made up 41% of consumer diets), but overall the data supported the predictions of the RPM (local production made the highest contributions of 50 – 86% to all FFGs across all seasons). The carbon isotopes of consumers and their food sources changed substantially every season, indicating that samples of food sources and consumers should be analysed many times throughout the year to capture that variability and to ensure that ephemeral components of the food web are not missed. To validate the findings from the isotope data, fatty acids were used as complementary tracers to determine the contributions of algal versus terrestrial organic matter to the consumers. Fatty acid tracers for terrestrial (Ʃω3/Ʃω6; 18:2ω6; 18:3ω3) vs aquatic (Ʃω3/Ʃω6; 20:5ω3) sources corroborated the findings from the isotope data set, as the mean ratio of Ʃω3/Ʃω6 in consumers was less than one at the headwaters (indicating allochthony), while middle and lower reaches were associated with Ʃω3/Ʃω6 > 1 (indicating autochthony). In addition to the tracer and FFG analyses for examining trophic connections between land and river, the bidirectional exchange of organisms between the riparian zone and the river was assessed using floating pyramidal traps (to measure emergence) and pan traps (for infalling invertebrates) placed at different sites in the river and the biomass in each trap was determined. The exchanges were variable over space and time, with emergence peaking in summer (169 to 1402 mg m-2 day-1) and declining in winter (3 to 28 mg m-2 day-1). Similarly, infalling invertebrates increased in summer (413 to 679 mg m-2 day-1) and declined in winter (11 to 220 mg m-2 day-1). Biomass measurements are indications of quantity that ignore nutritional quality, so I determined the bidirectional flow of invertebrates using absolute concentrations of physiologically important biochemical compounds (essential and polyunsaturated fatty acids). The fluxes of emergent and infalling arthropods peaked in summer (emergence = 0.3 to 18 mg m-2 day-1 and terrestrial infall = 0.3 to 3 mg m-2 day-1) and declined in winter (emergence = 0.01 to 0.51 mg m-2 day-1 and terrestrial infall = 0.01 to 0.03 mg m-2 day-1). However, during some seasons, no significant differences in polyunsaturated fatty acid flux in either direction were observed; this finding indicated the balance of reciprocal subsidisation via reciprocal flows of animals. Factors such as air temperature and algal productivity affected the reciprocal flows between adjacent habitats, with algal productivity being positively related to emergence while air temperature was positively correlated to infalling terrestrial invertebrates. This research enhances the growing body of literature on the function of riverine systems and offers some invaluable information on the flow of energy and the role played by invertebrates in translocating nutrients from terrestrial systems to aquatic systems and vice versa. This study unifies the concepts of the RCC and RPM and shows that these concepts are not limited only to large rivers, but are applicable to small southern temperate rivers too. However, some tenets of the theoretical models were challenged. For example, it challenges the proposition by the RCC that the fine particulate organic matter leaked from upstream breakdown of coarse particulate organic matter is predominantly allochthonous. Additionally, this study showed that in the headwaters, the RPM underestimated the role of autochthony. Overall, the results showed that the Kowie River and its riparian area are intrinsically connected. Once we understand the mechanisms controlling connections and subsidies across ecotones, we can then start to predict the consequences of disruptions to these connections by climate change and/or land use changes. To make predictions about future perturbations to rivers and riparian zones, studies like this, which considers the form and magnitude of subsidies, are needed to provide baseline information. Algal resources (e.g. epiphyton), macrophytes, riparian plants, terrestrial organisms and aquatic organisms all contributed to aquatic and terrestrial linkages in the Kowie River; therefore, it is important to conserve the different components of these ecosystems.
- Full Text:
Community structure and trophic ecology of shallow and deep rocky reefs in a well-established marine protected area
- Authors: Heyns, Elodie R
- Date: 2016
- Language: English
- Type: text , Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/54438 , vital:26565
- Description: The now formally adopted ecosystem approach to fisheries (EAF) considers not only commercially important species, but the entire ecosystem and the processes that support these species. A key component of EAF management is the implementation of no-take Marine Protected Areas (MPAs). Shallow water fish stocks are depleted and fishing effort is moving deeper and further offshore to keep up with demands. This situation calls for a detailed investigation of deep nearshore reefs to provide critical information relevant to policy uptake and management decisions regarding existing and new MPAs in terms of zonation and use. To address this need, the aim of this thesis was to investigate reefs that lie between 45 and 75 m and compare them in terms of community structure and function to the relatively well-studied shallow reefs that lie within SCUBA diving depth (<25 m). Ecological collections were made in the centre of a large and well-established MPA, Tsitsikamma National Park, to ensure that data represented non-anthropogenically impacted communities. Data were collected from two study sites; Rheeders Reef, (shallow reef) and Middlebank, a deep reef complex situated near the Storms River Mouth. The first step to address the aim of this study was to obtain baseline data on the distribution patterns of both the macrobenthic invertebrates and fish assemblages. Baseline data were obtained by underwater video methods and included the use of a remotely operated vehicle, baited remote underwater stereo-video systems (stereo-BRUVs) and traditional underwater camera equipment operated by SCUBA divers. To establish functional differences between the two study sites, fatty acid (FA) and stable isotope (SI) analyses were employed. These biomarker techniques provided insight into the importance of different sources of primary production, nutritional condition and species packing. From 360 photoquadrats examined for macrobenthic invertebrate distribution patterns, 161 invertebrates were identified that demonstrated a clear changeover of species along the depth gradient. Species richness was highest on the shallow reef and decreased with an increase in depth. To understand how the measured environmental variables impacted the macrobenthic assemblage data a LINKTREE analysis was performed. LINKTREEs produce hierarchical cluster analysis based on the macrobenthic assemblage data and provide a threshold of environmental variables that correspond to each cluster. The outcome of the LINKTREE analysis indicated that the changeover of species resulted in four distinct clusters, each cluster associated with a particular set of environmental variables that fell within a depth range. On the shallowest sites, the high energy environment resulting from wave action and surge prevented the settlement of suspended particles. The high energy environment of the shallow reef selected for low-growing encrusting species. High light intensities supported great abundances of benthic algae, and as light was lost with increasing depth, algal cover gradually diminished until it was completely absent on the deep reef. The reduced impact of surface wave action on the deep reef caused increased levels of settled suspended particles. The high levels of settled particles likely caused clogging of feeding parts of the encrusting species. Consequently, upright growth forms were more common in the lower energy environment of the deep reef. A total of 48 fish species were identified from 51 stereo-BRUVs samples. Fish assemblages differed significantly between the shallow and deep reefs. The shallowest sites were characterised by many small and juvenile fish species that fed at lower trophic levels. The deep reef supported the majority of the large predatory fish that fed at higher trophic levels. Many species demonstrated depth-related ontogenetic shifts in habitat use, and as such the deep reef hosted the majority of the sexually mature individuals. The fish assemblages also demonstrated a strong association with the macrobenthic clusters identified as habitat types by the LINKTREE analysis. The results from 201 FA and 191 SI samples provided information on specific feeding interactions, but more importantly shed some light on different processes that supported the shallow and deep reef communities. The shallow reef community was characterised by greater diversity of food sources, a pattern that could be explained by the presence of benthic algae and terrestrial inputs. Greater diversity of carbon sources at the bottom of the food web meant that a larger variety of species could be supported. Higher species richness increased the number of distinct taxa that performed similar functions, rendering the shallow reef more redundant and consequently more resilient to disturbance. In contrast, the deep reef demonstrated a food web supported mainly by pelagic production, which was more variable both over space and time. The deep reef was less redundant when compared to the shallow reef, as fewer species demonstrated similar trophic niches. These factors, in addition to the increased presence of sensitive calcareous macrobenthic species on the deep study site, rendered the deep reef more vulnerable to disturbance when compared to the shallow reef. Although the data presented here were from a single study area, the limitations typically associated with these inaccessible and challenging sampling environments made the dataset a significant contribution to the knowledge of reef ecosystems. The study addressed priority research questions for South Africa as identified during the National Biodiversity Assessment. The observable differences in structure, function and vulnerability point to the need for continued protection of our shallow reefs and offshore expansion of our MPA networks. Future research should determine if the patterns identified here are common throughout the Agulhas Ecoregion to provide managers with robust evidence for the extension our MPAs offshore.
- Full Text:
Predators of aerial insects and riparian cross-boundary trophic dynamics: web-building spiders, dragonflies and damselflies
- Authors: Chari, Lenin Dzibakwe
- Date: 2016
- Language: English
- Type: text , Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/55791 , vital:26734
- Description: This thesis characterises the cross-boundary trophic interactions of a relatively small model ecosystem, the Kowie River (Eastern Cape of South Africa), to explore their epistemic implications for systems ecology. Using web-building spiders and odonates (dragonflies and damselflies) as model organisms, I sought to investigate whether the diets of predators of aerial insects could be used to assess the strength of the trophic connectivity between freshwater and terrestrial systems in relation to variables such as stream width, distance from the river and aquatic insect emergence rates and abundances. Predator diet composition was determined by using a combination of diet analysis tools: direct observations of cross-subsidies, naturally-abundant stable (carbon and nitrogen) isotope analysis and fatty acid analysis. I also sought to reveal feeding niches and guilds among riparian aerial predators and investigate how the environment influenced predators’ access to aquatic prey subsidies. As emergent aquatic insect abundances decreased with an increase in distance from the river, and increased with stream width and seasonal changes from winter to summer, stable isotope and fatty acid analyses revealed distinct changes in web-building spider diet composition. Examination of the fatty acid eicosapentaenoic acid, a component commonly used as an indicator of consumer reliance on aquatic nutritional subsidies, showed that aquatic subsidies extended further inland at the wider sections of the river. Spiders and odonates at the wider sections of the Kowie River generally received more subsidies (56 – 70%) than those at the narrower sections (25 – 60%). When terrestrial insect biomass was distinctly low in winter, the benefit of aquatic subsidisation to spiders was relatively lower at the narrower sections of the Kowie River relative to the wide sections. As such, riparian areas adjacent to wide parts of the river were more likely to support larger populations of aerial predators than those at the narrow sections. Apart from the diet changes across time and space, there was evidence of inter-specific niche partitioning in both spiders and odonates, but no differences were observed between males and females of the same species. Results showed odonates of different sizes and hunting strategies had separate dietary niches, hence varied access to aquatic nutritional subsidies. The larger odonate taxa that frequently foraged mid-air had more varied diets and relied less on aquatic emergent insects than the smaller odonates that foraged from perches near the river. There was also evidence of niche partitioning amongst the spiders, as those that built horizontal webs captured more aquatic insects (40 – 78%) than the vertical orb-web builders (20 – 66%). This study showed that the nature and extent of trophic cross-boundary linkages in riparian areas largely depended on the availability of subsidies that varied seasonally and spatially. The width of the stream and seasonal variability emerged as important predictors of emergent insect abundances/biomasses that influenced predator feeding niches. The high mobility of odonates made their reliance on aquatic nutritional subsidies different from the less mobile spiders. The link between the width of the river and the extent of trophic connectivity has implications for riparian area management and definition of riparian buffer zones. However, the variation in diet niches amongst terrestrial consumers makes the results area-specific, and more studies are required that incorporate additional terrestrial predators in other fluvial systems so that we can make some generalizations on the dynamics of riparian trophic cross-boundary links.
- Full Text:
The trophic ecology of waterbirds in a small temperate estuary: a stable isotope and lipid tracer approach
- Authors: Hean, Jeffrey William
- Date: 2016
- Language: English
- Type: text , Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/54356 , vital:26557
- Description: Waterbirds are often overlooked as predators in aquatic ecosystems, despite the fact that waterbirds congregate in great numbers in and around waterways, notably estuaries. To fully appreciate the effect that aquatic feeding waterbird species may have on aquatic prey communities and the role that they play in estuarine food webs, stable isotopes and fatty acid profiles were employed to examine the seasonal diet of selected waterbirds in the Kowie Estuary, Eastern Cape Province, South Africa. Population counts were conducted every month for four seasons to examine the demography of waterbirds in the lower reaches of the estuary. The mean monthly energy consumption, along with dry matter intake of all waterbird species observed, were calculated and compared to similar estuaries in South Africa and elsewhere. Three duck species, one migrant sandpiper and one piscivore were selected for more detailed investigation at several temporal scales. This thesis has revealed that stable isotope analysis of waterbird tissues provides more informative data than fatty acid analysis for investigating waterbird diet and basal resource-tertiary consumer nutrient coupling. Stable δ15N and δ13C isotopes from several body tissues, in conjunction with SIAR models, were used to determine the seasonal diet of each waterbird species, while fatty acid profiles were investigated to examine the trophic transfer of fatty acids from basal resources to waterbird predators via the benthic fauna. Stable isotopes revealed that Cape Shoveller, Cape Teal and Yellow-Billed Duck shifted their diet over both long and short temporal scales, while the migratory Ruff and piscivorous Little Egret maintained a relatively consistent diet over time. Isopods, amphipods, copepods and Mysidacea were the main foods of all three duck species and the Ruff (>30%). Little Egret fed mainly on flathead mullet throughout the year. Fatty acid analysis revealed evidence for trophic transfer of specific fatty acids from basal resources to waterbirds in the Kowie Estuary but provided little information on seasonal diet of waterbirds. Waterbirds foraging in the Kowie Estuary appeared to shift their diet to coincide with resource abundance pulses, but also displayed seasonal dietary overlap. This study highlights the role that waterbirds play in aquatic food webs. The subject requires more attention so that we can better understand all the predatory drivers on aquatic communities.
- Full Text:
Spatio-temporal variation in the phytobenthos and phytoplankton community structure and composition of particulate matter along a river-estuary continuum assessed using microscopic and stable isotope analyses
- Authors: Dalu, Tatenda
- Date: 2015
- Language: English
- Type: text , Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/54416 , vital:26563
- Description: Phytoplankton and phytobenthos communities play an important role in lotic systems as primary producers providing essential biomolecules to higher trophic oganisms and are important indicators for environmental or ecological change. In this thesis, field studies (observational and experimental) along a river–estuary continuum were conducted to assess the spatio-temporal variation and development of phytobenthos and phytoplankton communities using a combination of stable isotope and community analyses in a temperate southern African system across four study periods: September (early spring) and November/December (late spring) 2012, and February (summer) and May/June (winter) 2013. Additionally, the sources and composition of the particulate organic matter were also analysed using stable isotope (δ15N and δ13C) analysis. The effects of substrate type and flood occurrence were assessed through experimental studies at an up- and downstream site of the river after a major flood event that occurred between October and November 2012. Common household tiles were used as artificial substrates to study the development/succession of phytobenthos communities after the flood disturbance. Distinct diatom communities were observed between upstream and downstream sites and at each site, community structure changed with time indicating succession. In addition to recording diatom characteristics on three natural substrates, namely; macrophytes, rocks and sediment, artificial substrates observations were also made on three different types of artificial substrates, namely; brick, brown clay and grey clay tiles. The natural (species richness 78) and artificial substrates (sp. richness 93) had different communities with the latter having greater species richness. Common phytobenthos taxa were not restricted to a single substrate but preference was generally high for the artificial substrates, especially brown tiles (mean sp. richness 47). Results of the redundancy analysis (RDA) analysis indicated that ammonium, conductivity, total dissolved solids, salinity, pH, oxygen reduction potential, phosphate and water depth were the major determinants of the phytobenthos composition at the two sites. The spatio–temporal variation of phytoplankton and phytobenthos communities and allochthonous organic matter along the river–estuary continuum was assessed at 8 sites using a combination of community and stable isotope analyses. A total of 178 species belonging to 78 genera were recorded with diatoms being predominant, accounting for 81.9 % of the total abundance. The total chl-a concentration along the river-estuary continuum increased from spring to a high in summer before decreasing to a low in winter. Periphyton communities were observed to be significantly different across sites (p < 0.05) in terms of species richness, abundances and isotopically The high periphytic δ15N values (range 7.9–15.2 ‰) recorded at the downstream sites compared to the pristine upstream sites (δ15N values range 4–7 ‰) suggest nutrient enrichment most likely derived from anthropogenic sources. Overall, our results reveal general patterns of periphyton communities and stable isotopes and provide improved information in the use of periphyton δ15N as an excellent indicator of anthropogenic nitrogen pollution. Ecologists are interested in the factors that control, and the variability in, the contributions of different sources to mixed organic materials traveling through lotic systems. We hypothesized that the source matter fuelling mixed organic pools in a river-estuary continuum varies over space and time, with the upper reaches of a system characterized by allochthonous-dominated material and autochthonous contributions becoming more important in the lower reaches. Samples of the mixed organic pools and allochthonous and autochthonous source materials were collected during the four study periods. The C:N ratios of suspended particulate matter (SPM) collected during summer and winter indicated that the lower reaches of the system had similar organic matter contributions from the freshwater and terrestrial sources. Stable isotope analysis in R revealed that the contributions of autochthonous organic matter were high in SPM along the entire continuum, and aquatic macrophytes were significant contributors to SPM specifically in the upper reaches. The terrestrial leaves made major contributions to the SPM in the middle regions of the system (i.e. downstream sites of the river, particularly in early and late spring). Bulk detritus had large allochthonous matter components in the lower reaches (estuary), and the contributions of aquatic macrophytes and benthic algae to bulk detritus were high (> 50 %) in the upper to middle reaches (river), but low (< 20 %) in the lower reaches (estuary). The current investigation represents the first attempt to assess the validity of the River Continuum Concept (RCC) in a southern African temperate river. The phytoplankton and phytobenthos communities, and chl-a concentration followed a trend similar to that proposed for the river continuum concept (RCC). The middle reaches based on the phytobenthos or phytoplankton communities and chl-a concentrations which were employed as proxies for primary production, were the most productive, while the upper reaches were the least primary productive. The evaluation of organic matter contributions to the SPM and detritus along the river–estuary continuum provided a baseline assessment of the nature and sources of potential food for consumers inhabiting different locations during different times of the year. Incorporating such spatio-temporal variations in SPM and detritus into food web studies will improve our understanding of the flow of carbon through aquatic systems.
- Full Text:
The diets of co-occuring anurans in a small South African river: assessments using stomach contents, stable isotope ratios and fatty acid profiles
- Authors: Sikutshwa, Likho
- Date: 2015
- Language: English
- Type: text , Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/54405 , vital:26562
- Description: During their life cycle amphibians change their habitat and feeding, and are important consumers in both terrestrial and aquatic habitats. The trophic ecology of anurans has been studied intensively using gut content analysis, but very little work has been done using biochemical techniques such as stable isotope and fatty acid analyses. These biochemical techniques produce data that allow for additional inferences about the trophic ecology of anurans, as they reveal what the organisms assimilated over time rather than what was recently ingested. The investigation of the feeding ecology and trophic interactions of anurans using gut content analyses together with stable isotope and fatty acid analyses will help us to better understand their ecological roles. The objective of this thesis was to assess the feeding ecology of four anurans (Amietia angolensis, Amietophyrnus rangeri, Strongylopus grayii and Xenopus laevis) in a temperate river using a combination of gut content, stable isotope and fatty acid analyses. Frogs, tadpoles and potential food sources were sampled in two sites (upstream and downstream) in the Kowie River, Eastern Cape Province, South Africa. Gut content analyses identified 147 prey items belonging to 12 prey orders in the stomachs of the sampled specimens. In both A. angolensis and S. grayii the most important prey category was Coleoptera, followed by Hemiptera, Diptera and Hymenoptera at both sites. Aquatic prey items (aquatic hemipterans, chirinomids and blackflies) were the most important food sources for X. laevis (Alimentary Index (IAi) = 6.4; 5.2; 4.2). In Am. rangeri, Hymenoptera was the most important prey category (IAi = 8.3). The trophic niche overlap between A. angolensis and S. grayii was biologically significant (> 0.6), and in the remaining species there was no significant trophic niche overlap. Amietia angolensis showed a larger trophic niche (Levin’s measure; B = 7.7 and Bst = 0.84 downstream, and upstream B = 7.6 (Bst = 0.82) compared to the other species. The gut content analyses showed that frogs feed on a variety of prey items that constitutes food sources from both aquatic and terrestrial habitats. Stable isotopes indicated that aquatic derived sources contributed significantly more towards the diets of X. laevis, A. angolensis tadpoles and S. grayii tadpoles compared with the other anurans, whereas aquatic and terrestrial derived food sources contributed equally to the diets of A. angolensis and S. grayii. Increased trophic positions in A. angolensis and S. grayii occurred throughout their development. The four different species had similar fatty acid profiles in the upstream region, and fairly similar δ13C values, suggesting that they probably consumed similar food. Fatty acid profiles of anurans in the downstream region showed distinct separations among the species. Tadpoles had high levels of diatom-associated fatty acids (20:5ω3; A. angolensis tadpole – 8.4 %, S. grayii tadpole – 9.4 % upstream and downstream; 9.1 and 6.1 % total fatty acids (TFA), respectively). All four species had substantial contributions from bacterial fatty acids, and large proportions of saturated fatty acids (30.6 - 50.0 %) including those with 14 and 18 carbons, indicating that bacterial and detritus food sources played an important role in their diets. The fatty acid profiles revealed high proportions of polyunsaturated fatty acids (PUFAs) and essential fatty acids (EFAs) in all species, indicating a good quality of food and that the quality of food consumed was similar among species. The results demonstrated the usefulness of a combination of traditional techniques (gut content techniques) and biochemical techniques (stable isotopes and fatty acid analysis) for assessing consumption and assimilation. The amphibian assemblages examined derived much of their energy from terrestrial and aquatic sources. This information will allow more precise and comprehensive assessments of trophic interactions in freshwater habitats, along with aiding in future amphibian conservation and management efforts.
- Full Text: