Stable isotope ratios indicate differential omnivory among syntopic rocky shore suspension-feeders
- Authors: Richoux, Nicole B , Vermeulen, Ilke , Froneman, P William
- Date: 2014
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/68261 , vital:29224 , https://doi.org/10.1007/s00227-013-2358-6
- Description: Publisher version , We utilised stable isotope ratios to assess differences in diet among three indigenous and syntopic rocky shore suspension-feeders (mussels Perna perna, barnacles Tetraclita serrata, and polychaetes Gunnarea gaimardi). We also determined the spatial and temporal variability in the suspension-feeder diets by collecting specimens on two occasions from two regions adjacent to hydrologically distinct river mouths (i.e. one with larger annual freshwater throughput than the other). The results showed that the isotopic niches (used as proxies for trophic niches) of the three species did not overlap and that the barnacles occupied a trophic position (3.4) well above those of the mussels (2) and polychaetes (2.6). We ascribed the interspecific differences primarily to the disparate feeding mechanisms used by the species. Large-scale regional (~50 km) differences in suspension-feeder diets were apparent, but not small-scale (up to a few km north and south of each estuary mouth). The regional differences in diet resulted from the increased availability of estuarine-origin suspended particulate matter (SPM) and zooplankton in the region adjacent to the river with relatively larger freshwater output, although overall incorporation of zooplankton versus mixed SPM into consumer diets was relatively consistent between regions and through time. Temporal shifts in suspension-feeder diets were apparent from stable carbon isotope ratios in the consumers. Our results showcase the measurable effects of regional-scale processes that can alter the food sources for dominant primary consumers in the rocky intertidal, thus potentially affecting entire food webs through bottom-up processes. The clear evidence for trophic niche partitioning offers valuable insights into how potentially strong competitors can coexist.
- Full Text: false
- Date Issued: 2014
Surface distribution of microphytoplankton of the south-west Indian Ocean along a repeat transect between Cape Town and the Prince Edward Islands
- Authors: Froneman, P William , Pakhomov, E A , Meaton, V
- Date: 1998
- Language: English
- Type: text , Article
- Identifier: vital:6933 , http://hdl.handle.net/10962/d1011953
- Description: Surface chlorophyll-a (chl-a) concentrations, microphytoplankton (>20 μm) species composition and distribution along a repeat transect between Cape Town and the Prince Edward Islands were investigated in early austral autumn (April/May) 1996. Samples were collected at approximately 30 nautical mile intervals for the analysis of size-fractionated chl-a and the identification and enumeration of microphytoplankton species. Peaks in total chl-a (>1 μg 1 [superscript -1]) were recorded at the Subtropical Convergence (STC), at the Sub-Antarctic Front (SAF) and in the waters surrounding the Prince Edward Islands. In addition, a minor peak in chl-a concentration was recorded in the continental shelf waters. At stations where elevated chl-a concentrations were recorded, microphytoplankton generally formed a substantial contribution (-10%) to total chlorophyll. Outside these regions, total chlorophyll concentrations were lower (<0.9 μg 1 [superscript -1]) and almost entirely dominated by nano- and picophytoplankton, which contributed >95% of the total. Microphytoplankton species composition along both transects were dominated by chain-forming species of the genera Chaetoceros (mainly C. neglectum, C. peruvianus and C. constrictus), Nitzschia spp. and Pseudoeunotia doliolus. Cluster and ordination analysis based on species composition identified five distinct microphytoplankton assemblages, which were closely associated with the different water masses in the region between Cape Town and the Prince Edward Islands. The microphytoplankton species composition and biogeographic zones identified during this investigation are in general agreement with similar studies conducted in the south-west Indian Ocean during the austral summer, which suggests that there are little seasonal trends in both the microphytoplankton species composition and biogeographic zonation.
- Full Text:
- Date Issued: 1998
Temporal and spatial variability in the abundance, biomass and distribution of Palaemon peringueyi (Decapoda: Crustacea) are influenced by biological but not physico-chemical factors in the permanently open Kariega Estuary, South Africa
- Authors: Nyalungu, Nonhlanhla P , Hodgson, Alan N , Froneman, P William , Dopolo, M , Masubelele, M
- Date: 2014
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/68106 , vital:29200 , https://doi.org/10.2989/16085914.2014.960792
- Description: Publisher version , Biological and physico-chemical factors affecting abundance and biomass of shrimp Palaemon peringueyi were investigated in 2010–2011 in the lower, middle and upper reaches of the freshwater-deprived permanently open Kariega Estuary. Shrimp abundance and biomass ranged from 0 to 88 ind. m−2 and 0 to 4.4 g wwt m−2, respectively. Shrimps were most abundant in the lower reach, and highest abundances and biomasses were recorded in the lower reach in June when salinities were low. The shrimps were almost always absent in the upper reach. Juveniles were found mostly in the lower and middle reaches, whereas adults were distributed in all three reaches. Regression analyses showed no statistically significant relationship for either abundance or biomass of the shrimps with temperature, salinity and dissolved oxygen (p > 0.05). The relationship between shrimp abundance and biomass and vegetation cover was positive and statistically significant (R2 = 0.109 and 0.185, respectively; p < 0.05). Vegetation cover plays an important role in determining the presence, distribution and abundance or biomass of P. peringueyi in this estuary.
- Full Text: false
- Date Issued: 2014
Temporal variability in the effects of grazing by the territorial limpet Patella longicosta on the productivity of the crustose alga Ralfsia verrucosa
- Authors: Kaehler, Sven , Froneman, P William
- Date: 1999
- Language: English
- Type: Article
- Identifier: vital:6935 , http://hdl.handle.net/10962/d1011956
- Description: This study reports the effects of grazing by the territorial limpet Patella longicosta on the productivity of the encrusting alga Ralfsia verrucosa. Grazing significantly increased algal growth during the first month of the study (August). Simulated grazing by brushing also enhanced productivity. However, the productivity of grazed and ungrazed algae did not differ significantly during the ensuing three months. As the growth of ungrazed plants remained the same throughout the study, the elevated productivity of grazed plants during August was attributed to increased limpet activity. The variation in grazer-induced algal growth was possibly associated with increased feeding when the limpets embark on energetically costly periods of reproduction.
- Full Text: false
- Date Issued: 1999
The diet of the calanoid copepod, Pseudodiaptomus hessei, in a permanently open southern African estuary inferred from fatty acid analyses
- Authors: Noyon, Margaux , Froneman, P William
- Date: 2014
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/70901 , vital:29758 , https://doi.org/10.1093/plankt/fbu037
- Description: The fatty acid (FA) composition of Pseudodiaptomus hessei, one of the most abundant copepod species in southern African estuaries and lakes, reveals potential food selectivity and the importance of flagellate prey in their diets. Differences in FA profiles between males and females were recorded; however, none of these patterns matched the reproductive activities of the copepod. Our findings stress the importance of improving our knowledge of FA biosynthesis pathways to employ fatty acids as trophic markers in feeding studies of zooplankton in temperate environments.
- Full Text:
- Date Issued: 2014
The effects of increased freshwater inflow on metal enrichment in selected Eastern Cape estuaries, South Africa
- Authors: Orr, Kyla K , Burgess, Jo E , Froneman, P William
- Date: 2008
- Language: English
- Type: Article
- Identifier: vital:6967 , http://hdl.handle.net/10962/d1012032
- Description: The concentrations of select metals (Cd, Co, Cu, Fe, Pb, Ni and Zn) within the water column and sediment of the permanently open Kariega Estuary and temporary open / closed Riet and East Kleinemonde Estuaries were investigated during a dry and a wet season. Enrichment factors (EFs), using Fe as a reference element, and baseline linear regression models for metals vs Fe were used to assess the extent of metal enrichment in the sediments. The results of the study indicate that Cd, Co Ni and Pb were enriched above baseline concentrations (1.0 < EF < 4.1) in the sediments of all three estuaries. Co, Pb and Ni enrichment in the Kariega Estuary sediments was significantly higher during the dry season, and the mean concentrations of Pb and Cd in the water column were 19-fold and 66-fold higher in the dry season. The elevated concentration of metals during the dry season could be related to accumulation of diffuse pollution from human activities within the catchment area. Conversely, inflow of freshwater into the estuary had the net effect of reducing the concentration and enrichment of these metals within the Kariega Estuary due to scouring and outflow of estuarine water and sediment into the marine environment. The temporal variations in metal concentrations and enrichment factors were less pronounced in the temporary open / closed estuaries than the Kariega Estuary. The observed trend can probably be related to the low anthropogenic impact within the catchment areas of these systems, and the relatively smaller size of the catchments. Significant spatial variations existed in metal enrichment in the sediment of both the East Kleinemonde and Riet Estuaries, with the highest degrees of enrichment occurring in the sediments from the marine environment and lower reaches.
- Full Text:
- Date Issued: 2008
The importance of phytoplankton size in mediating trophic interactions within the plankton of a southern African estuary
- Authors: Froneman, P William
- Date: 2006
- Language: English
- Type: text , Article
- Identifier: vital:6928 , http://hdl.handle.net/10962/d1011944
- Description: The influence of the phytoplankton size composition in mediating the trophic interactions between the bacteria, phytoplankton, microheterotrophs (<200 μm) and mesozooplankton (>200 μm) was investigated on three occasions in a warm temperate, temporarily open/closed estuary situated along the southern African coastline. Results of the investigation indicated that the microheterotrophs represented the most important consumers of bacteria and chlorophyll (chl)-a <5.0 μm. The low impact of the mesozooplankton on the bacteria and chl-a <5.0 μm during the study appeared to be related to the inability of the larger zooplankton to feed efficiently on small particles. During those periods when total chl-a concentration was dominated by picophytoplankton (<2.0 μm) and microphytoplankton (>20 μm), mesozooplankton were unable to feed efficiently on the chl-a due to feeding constraints. In response to the unfavorable size structure of the phytoplankton assemblages, mesozooplankton appeared to consume the microheterotrophs. The negative impact of the mesozooplankton on the microheterotrophs resulted in a decrease in the impact of these organisms on the bacteria and the chl-a <5.0 μm. This result is consistent with the predator-prey cascades. On the other hand, when the total chl-a was dominated by nanophytoplankton (2–20 μm), mesozooplankton were able to feed directly on the phytoplankton. Results of the study indicate that size structure of the phytoplankton assemblages within estuaries plays an important role in mediating the trophic interactions between the various components of the plankton food web.
- Full Text:
- Date Issued: 2006
The natural history and conservation of Indian Ocean Humpback Dolphins (Sousa plumbea) in South African waters
- Authors: Plön, Stephanie , Cockcroft, Victor G , Froneman, P William
- Date: 2015
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/68207 , vital:29219 , https://doi.org/10.1016/bs.amb.2015.08.005
- Description: Publisher version , Although most knowledge on the biology of Sousa plumbea has primarily come from South African waters, a number of research gaps remain on the natural history and status of the species in the region. Research on two populations in South African waters for which some historical data exist may aid in highlighting long-term changes in the biology and natural history of this little known coastal delphinid. Recent studies on the age, growth and reproduction of animals incidentally caught in shark nets in Richards Bay, KwaZulu-Natal, yielded a lower maximum age estimate of 24 (previously 46) growth-layer-groups (GLGs), sexual maturity of 7.5 and 8 GLGs in males and females (previously 12–13 and 10 GLGs, respectively), an ovulation rate of 0.2 and a 5-year calving interval (previously 0.3 and 3-year calving interval) than previously reported. These differences may be due to a difference in the interpretation of GLGs between observers or a predominance of young males being caught in the shark nets. Stomach content analysis revealed a change in the relative proportions of the main prey items over the past 25 years, but no difference in species richness or diversity was found between the sexes. No change in trophic level was recorded between 1972 and 2009. Field studies in Algoa Bay, Eastern Cape, conducted 16 years apart indicated a decline in the mean group size (from 7 to 3 animals), a decline in the maximum group size (from 24 to 13 animals), an increase in solitary individuals (15.4–36%), and a change in behaviour from predominantly foraging (64–18%) to mainly travelling (24–49%). The observed changes are suggestive of a change in food availability, resulting in a range shift or a potential decline in numbers. These studies indicate the importance of long-term studies to monitor population changes and their possible causes. A number of threats, such as shark nets, pollution (noise and chemical), and coastal development and disturbance, to the humpback dolphin populations in South Africa have been identified. Urgent action is required to ensure continued existence of the species in South African waters.
- Full Text: false
- Date Issued: 2015
The Southern Ocean Group at Rhodes University: seventeen years of biological oceanography in the Southern Ocean reviewed
- Authors: McQuaid, Christopher D , Froneman, P William
- Date: 2004
- Language: English
- Type: Article
- Identifier: vital:6834 , http://hdl.handle.net/10962/d1010423
- Description: This paper reviews the main findings of the Southern Ocean Group at Rhodes University over the last 17 years. A primary contribution has been the development of conceptual models of the physical-biological driving mechanisms that support enormous seasonal populations of land-based top predators at the Prince Edward Islands. Collectively, these models are referred to as the life-support system of the islands. Near-shore subcomponents of the ecosystem, including inshore feeding predators, are largely supported by autochthonous primary production of kelps and localized diatom blooms. These energy sources feed indirectly into top predator populations via the benthic communities. A crucial link is formed by the bottom-dwelling shrimp, Nauticaris marionis, which feeds largely on benthic species and detritus and is eaten by a number of diving seabirds. The frontal systems that lie north and south of the islands are important feeding grounds for offshore feeding birds. A decadal-scale southward shift in the position of the Sub-antarctic Front towards the islands is reflected in increases in populations of these species. , Rhodes Centenary issue
- Full Text:
- Date Issued: 2004
Trophic interactions in an austral temperate ephemeral pond inferred using stable isotope analysis
- Authors: Dalu, Tatenda , Weyl, Olaf L F , Froneman, P William , Wasserman, Ryan J
- Date: 2016
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/68022 , vital:29185 , https://doi.org/10.1007/s10750-015-2533-5
- Description: Publisher version , Ephemeral ponds are vulnerable aquatic habitats which are difficult to protect given their dynamic nature and sensitivity to degradation during dry periods. Little information is available on these habitats in austral regions, with almost no information on food-web structure and complexity. The study aimed to assess trophic interactions among dominant organisms in an ephemeral pond food web, and investigate the importance of autochthonous and allochthonous carbon, using 13C and 15N isotopes. Results of the investigation suggest that the food web comprised four trophic levels, with the top predators being Notonectids (Notonecta sp.) and diving beetles (Cybister tripunctatus (Olivier)). Intermediary trophic levels comprised zooplankton (daphniids and copepodids), macroinvertebrates (e.g. micronectids and molluscs) and tadpoles. Generalist feeders dominated the higher trophic levels (>3) with specialists comprising the lower trophic levels (≤3). The consumers preferred autochthonous fine particulate organic matter, epiphyton and submerged macrophyte organic matter sources over allochthonous sources. Autochthonous organic matter was transferred to the food web via zooplankton and select macroinvertebrates including Micronecta sp. and Physa sp. The food-web structure within the pond appeared to reflect the secondary stage of trophic structural complexity in the evolution of ephemeral ponds over the course of their hydro-period.
- Full Text: false
- Date Issued: 2016
Using multivariate analysis and stable isotopes to assess the effects of substrate type on phytobenthos communities
- Authors: Dalu, Tatenda , Richoux, Nicole B , Froneman, P William
- Date: 2014
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/68123 , vital:29201 , https://doi.org/10.5268/IW-4.4.719
- Description: Publisher version , For more than a century, artificial substrates have been employed in phytobenthos studies. In the present study, we compared the phytobenthos community structure in a field experiment over 3 seasons (summer, autumn, and winter) on 3 types of artificial substrates (brick, brown clay tiles, and grey clay tiles) and 3 natural substrates (macrophytes, rocks, and sediment) in a small, temperate system. A combination of multivariate analyses (cluster, multi-response permutation procedure, indicator species [IndVal], and canonical correspondence analysis [CCA]) and stable isotope analysis was used. We identified 96 total phytobenthos taxa. Artificial substrates resulted in different substrate communities, as shown by stable isotope analysis, cluster analysis, and a multi-response permutation procedure, with only those communities growing on grey tiles being similar to natural substrate communities. Overall, artificial substrates exhibited slightly higher species richness compared to natural substrates over the 3 seasons, although there were no significant differences (p> 0.05). Phytobenthos grown on brown tiles, rocks, and bricks showed seasonal variability of the carbon isotope δ13C values using one-way ANOVA (p< 0.05). Phytobenthos community structure did not show great seasonal variation; however, CCA identified water flow, conductivity, ammonium, phosphate, and water depth as important in structuring phytobenthos communities on different substrates. IndVal analysis showed that common phytobenthos taxa were not restricted to a single substrate, but preference was generally high for natural substrate, especially rocks, compared to artificial substrates. Substrate microhabitat type seems to influence the communities within the study areas.
- Full Text: false
- Date Issued: 2014