• Energy Research

This article presents a historical overview of estuarine microphytobenthos (MPB) research in South Africa published over the period 1950 to the present, highlighting major milestones, challenges an...

Within a given ecosystem, species persistence is driven by responses to the effects of biotic and abiotic stressors. Ongoing climatic shifts and increased pollution pressure have created the need to assess potential effects and interactions of physical and biotic factors on coastal ecosystem processes to project ecosystem resilience and persistence. In coastal marine environments, primary production dynamics are driven by the interaction between bottom-up abiotic effects and biotic effects induced by top-down trophic control. Given the many environmental and climatic changes observed throughout coastal regions, we assessed the effects of interactions among temperature, nutrients and grazing in a laboratory-based microcosm experiment. We did this by comparing chlorophyll-a (chl-a) concentrations at two temperatures in combination with four nutrient regimes. To test for subsequent cascading effects on higher trophic levels, we also measured grazing and growth rates of the calanoid copepod Pseudodiaptomus hessei. We observed different phytoplankton and zooplankton responses to temperature (17 °C, 24 °C) and nutrients (nitrogen only (N), phosphates only (P), nitrogen and phosphates combined (NP), no nutrient additions (C)). Contributions of predictors to model fit in the boosted regression trees model were phosphates (42.7%), copepods (23.8%), nitrates (17.5%) and temperature (15.9%), suggesting phosphates were an important driver for the high chl-a concentrations observed. There was an increase in total phytoplankton biomass across both temperatures, while nutrient addition affected the phytoplankton size structure prior to grazing irrespective of temperature. Phytoplankton biomass was highest in the NP treatment followed by the N treatment. However, the phytoplankton size structure differed between temperatures, with microphytoplankton being dominant at 24 °C, while nanophytoplankton dominated at 17 °C. The P and C treatments exhibited the lowest phytoplankton biomass. Copepod abundances and growth rates were higher at 17 °C than at 24 °C. This study highlights that bottom-up positive effects in one trophic level do not always positively cascade into another trophic level. It was, however, evident that temperature was a limiting factor for plankton abundance, productivity and size structure only when nutrients were limiting, with top-down pressure exhibiting minimal effects on the phytoplankton.

Ephemeral wetlands in arid regions are often degraded or destroyed through poor land-use practice long before they are ever studied or prioritized for conservation. Climate change will likely also have implications for these ecosystems given forecast changes in rainfall patterns in many arid environments. Here, we present a conceptual diagram showing typical and modified ephemeral wetlands in agricultural landscapes and how modification impacts on species diversity and composition.

Abstract Land use intensification and climate change are two prominent drivers of variation in biological communities. However, we know very little about how these two potential environmental stressors interact. Here we use a stable isotope approach to quantify how animal communities respond to urban and agriculture land use, and to latitudinal variation in climate (rainfall and temperature), in 29 streams across South Africa. Community structure was shaped by both land use and climatic factors. The taxonomic diversity of invertebrates was best explained by an independent negative effect of urbanization, while abundance declined in summer. However, we could not use our variables to predict fish diversity (suggesting that other factors may be more important). Both trophic functional diversity (quantifed using isotopic richness) and food chain length declined with increasing temperature. Functional redundancy (quantifed using isotopic uniqueness) in the invertebrate community was high in wet areas, and a synergistic interaction with urbanization caused the lowest values in dry urban regions. There was an additive effect of agriculture and rainfall on abundance‐weighted vertebrate functional diversity (quantified using isotopic dispersion), with the former causing a decline in dispersion, with this partially compensated for by high rainfall. In most cases, we found that a single dominant driver (either climate or land use) explained variation between streams. We only found two incidences of combined effects improving the model, one of which was amplified (i.e. the drivers combined to cause an effect larger than the sum of their independent effects), indicating that management should first focus on mitigating the dominant stressor in stream ecosystems for successful restoration efforts. Overall, our study indicates subtle food web responses to multiple drivers of change, only identified by using functional isotope metrics—these are a useful tool for a whole‐systems biology understanding of global change. A free Plain Language Summary can be found within the Supporting Information of this article.

Increased sedimentation and siltation associated with anthropogenic environmental change may alter microbial biofilms and the carbohydrates they produce, with potential bottom-up effects in these ecosystems. The present study aimed to examine to what extent carbohydrate (associated with biofilm exopolymer) concentration and benthic algal biomass vary among different sediment types (size-structure categories) using a microcosm experiment conducted over a period of 28 days. Substrate treatment and time had a significant effect on the total chlorophyll-a concentrations, whilst a significant interaction was present in the case of total sediment carbohydrates. Total sediment carbohydrates did not relate significantly to chlorophyll-a concentrations overall, nor for any substrate treatments owing to a non-significant 'chlorophyll-a × substrate' interaction term. The diatom community characteristics across sediment sizes were unique for each treatment in our study, with unique dominant diatom taxa compositions within each sediment size class. The finest sediment particle-size (<63 μm) may be the least stable, most likely due to lower binding. We anticipate that the current study findings will lead to a better understanding of how different sediment types due to sedimentation and siltation will impact on primary productivity and the composition of diatom communities in aquatic systems.

AbstractResting egg production is considered the most common form of dormancy in aquatic invertebrates. Given that many taxa at least partially terminate resting egg state using environmental cues, knowledge on environmental drivers of hatching success is important, particularly within the context of climate change and environmental degradation. Fairy shrimp (anostracans) are temporary wetland specialists that are reliant on resting egg production for population persistence. Temporary wetlands are common in many arid regions projected to experience increases in temperature, and in areas often compromised by human‐mediated activities. In this study, we assessed the combined effects of light and temperature on the hatching success of Streptocephalus cafer (Anostraca) dormant eggs from temporary wetlands in an arid environment. Both temperature and light altered hatching success, with emergent effects evident. Light caused a significant threefold increase in hatching success overall, while temperature effects were non‐linear, with hatching optimised at 27°C, and especially under light conditions. These results are discussed within the context of shifting climates and disturbances to temporary wetland ecosystems.

As part of a larger investigation, the effect of apex predation on estuarine bacterial community structure, through trophic cascading, was investigated using experimental in situ mesocosms. Through either the removal (filtration) or addition of specific size classes of planktonic groups, four different trophic scenarios were established using estuarine water and its associated plankton. One such treatment represented a "natural" scenario in which stable apex predatory pressure was qualified. Water samples were collected over time from each of the treatments for bacterial community evaluation. These samples were assessed through pyrosequencing of the variable regions 4 and 5 of the bacterial 16S rRNA gene and analysed at the species operational taxonomic unit (OTU) level using a community procedure. The blue-green group dominated the samples, followed by Proteobacteria and Bacteroidetes. Samples were the most similar among treatments at the commencement of the experiment. While the bacterial communities sampled within each treatment changed over time, the deviation from initial appeared to be linked to the treatment trophic scenarios. The least temporal deviation-from-initial in bacterial community was found within the stable apex predatory pressure treatment. These findings are consistent with trophic cascade theory, whereby predators mediate interactions at multiple lower trophic levels with consequent repercussions for diversity.

In many coastal regions, mean coastal atmospheric and water temperatures are projected to shift as climate change ensues. Interaction strengths between organisms are likely to change along with environmental changes, given interspecific heterogeneity in responses to physico-chemical variables. Biological interaction outcomes have the potential to alter food web production and trophic level biomass distribution. This is particularly pertinent for key species that are either abundant or play disproportionately large roles in ecosystem processes. Using a functional response approach, we quantified the effects of shifting temperatures on interactions between key mysid species-sympatric in their distribution across a biogeographic transition zone along the east coast of South Africa. The Rhopalophthalmus terranatalis functional response type toward Mesopodopsis wooldridgei prey was independent of temperature, with all treatments producing Type II functional responses. Temperature effects on predator-prey dynamics were, however, evident as interaction strength was greatest at 21 °C, as measured by maximum feeding rates. Unlike maximum feeding rate, attack rates increased linearly with increasing temperature across the experimental treatments. Our findings suggest that interaction strength between the mysid shrimp species is likely to vary spatially along the current length of their sympatric distribution and temporally in certain regions where temperatures are projected to change. Such experimental interaction investigations are becoming increasingly important given our relatively poor understanding of the consequences of environmental change for effects on interactions among species and their wider ecosystem implications.