Effects of urbanisation and a wastewater treatment plant on microplastic densities along a subtropical river system

Global freshwaters are increasingly threatened by pollutants emanating from human activities around watersheds. Microplastic pollution is an increasing problem for rivers worldwide, potentially threatening ecological integrity, ecosystem services and human health. We present quantifications and characterisations of sediment microplastic pollution in a subtropical river system in southern Africa, and relate distributions to wastewater treatment works, abiotic variables and urban environments. We additionally apply several diversity indices to decipher how microplastic types differ across the river system seasonally. Over two thousand microplastic particles were found across five sites and three seasons in the river system, comprising microbeads of various colours and microfibres. Microplastic concentrations were highest and most diverse in the hot–wet (mean range 76.0 ± 10.0–285.5 ± 44.5 microplastic kg−1) season as compared to the cool–dry (16.5 ± 4.5–27.0 ± 5.0 microplastic kg−1) and hot–dry (13.0 ± 4.0–29.0 ± 10.0 microplastic kg−1) seasons, and were mostly dominated by microfibres. However, no clear patterns were found in relation to wastewater treatment operations spatially, or in relation to abiotic variables in the river system. This study therefore finds a diverse range of microplastic types widely distributed in the river system that differ across seasons. Our results provide important, novel insights into plastic pollution in an understudied area of the Global South, and point to extensive pollution from sources outside of wastewater treatment works

Nutrient release dynamics associated with native and invasive leaf litter decomposition: a Mesocosm Experiment

Leaf litter contributes to the functioning of aquatic ecosystems through allochthonous inputs of carbon, nitrogen, and other elements. Here, we examine leaf litter nutrient inputs and decomposition associated with four plant species using a mesocosm approach. Native sycamore fig Ficus sycomorus L., and silver cluster–leaf Terminalia sericea Burch. ex DC. decomposition dynamics were compared to invasive tickberry Lantana camara L. and guava Psidium guajava L., whereby phosphate, nitrate, nitrite, silicate, and ammonium releases were quantified over time. Leaf inputs significantly reduced pH, with reductions most marked by invasive L. camara. Conductivity was heightened by all leaf input treatments, except native T. sericea

Behavioural Responses and Mortality of Mozambique Tilapia Oreochromis mossambicus to Three Commonly Used Macadamia Plantation Pesticides

The use of pesticides in agricultural systems may have deleterious effects on surrounding environments. Aquatic systems are no exception and are increasingly polluted through the leaching of pesticides from agricultural activities. However, the pesticide pollution effects on key aquatic species have not been studied in many regions. In southern Africa, increasing pesticide use associated with macadamia tree Macadamia integrifolia farming presents a growing risk to surrounding aquatic ecosystems. This study assessed behavioural responses of an important and widely-distributed freshwater fish, Mozambique tilapia Oreochromis mossambicus, following exposure to three commonly used macadamia pesticides (i.e., Karate Zeon 10 CS, Mulan 20 SP, Pyrinex 250 CS) at different concentrations (0.7–200 µL, 0.3–1000 mg, and 0.7–8750 µL, respectively) over 24 h. Behavioural responses, i.e., swimming erratically, surfacing, vertical positioning, loss of equilibrium, being motionless and mortality were observed after pesticides exposure. Lethal dose 50 (LD50) values of Karate Zeon 10 CS, Mulan 20 SP and Pyrinex 250 CS were 2.1 µL (per water litre dilution—WLD), 5.2 mg (WLD) and 21.5 µL (WLD), respectively. These concentrations are therefore expressed as a maximal threshold usage in the environment around macadamia farms and a minimum distance of the plantations to water systems should be considered. Further studies should examine effects on other fish species and aquatic invertebrates to inform on pesticide pollution threats and mitigation plans for the region

Assessing the effects of MACADAMIA ORCHARD pesticide inputs on recipient aquatic ecosystems

PhDENVDepartment of Geography and Environmental SciencesAquatic environments, while not the target of many pesticide applications, often receive chemicals through catchment runoff dynamics. In this regard, the use of pesticides in agricultural systems may have deleterious effects on aquatic ecosystems within the same catchment area. Here using a series of in–situ and ex–situ experiments, the study assessed (a) pesticide concentrations of acetamiprid and chlorpyrifos across MACADAMIA ORCHARD and communal area reservoirs using liquid chromatography tandem mass spectrometry (LC–MS/MS), (b) the behavioural responses of Mozambique tilapia, Oreochromis mossambicus, following exposure to three commonly used macadamia pesticides (i.e., Karate Zeon 10 CS, Mulan 20 SP, Pyrinex 250 CS at different concentrations, (c) macadamia Macadamia integrifolia leaf litter decomposition and mosquito colonisation following pesticide exposure, and (d) macroinvertebrate colonisation associated with introduced stone substrates in cages within the MACADAMIA ORCHARD and control reservoirs to explore whether macadamia pesticides affected on macroinvertebrate colonisation dynamics. Firstly, analysis of acetamiprid and chlorpyrifos associated with sediments in MACADAMIA ORCHARD reservoirs revealed mean pesticide concentrations of 14.48 μg L–1 and 5.67 μg L–1, respectively, whereas, in communal area reservoirs outside of agricultural catchments, both pesticides were not detected. Acetamiprid was not detected across reservoir water, whereas the mean pesticide concentration of chlorpyrifos of 6.51 μg L–1 (MACADAMIA ORCHARD) and 0.13 μg L–1 (communal area) were detected. Secondly, O. mossambicus demonstrated different onset behavioural responses, i.e., swimming erratically, surfacing, vertical positioning, loss of equilibrium, being motionless and mortality with high mortality at increased mortality. Thirdly, macadamia leaf litter tended to decompose faster when exposed to pesticide treatments, whereas chlorophyll–a were reduced. Furthermore, pesticide treatments seemed to promote mosquito (i.e., Culex spp.) and pupal abundances. Finally, a total of 644 macroinvertebrate individuals were recorded across macadamia and communal reservoirs, mostly dominated by Chironominae (55.1 %), Ostracoda (14.4 %), Trithemis sp. (5.2 %), Anax sp. (5.2 %) and Radix natalensis (5.0 %). The results indicate that MACADAMIA ORCHARD reservoirs are highly contaminated by acetamiprid and chlorpyrifos and that these contaminants have implications for the ecological functioning of aquatic ecosystems. Reduction of species diversity adjusted community structure and altered energy flow and nutrient recycling were all demonstrated. The results of the study collectively show how common pesticides used in the macadamia plantation may be exert pressure on adjacent freshwater ecosystems. Future studies should examine effects on trophic interactions, pesticide accumulation in faunal groups and allochthonous trophic transfer dynamics to terrestrial environments. Such information would be useful for managers tasked with mitigation plans for the region.National Research Foundation (NRF

Assessing the effects of invasive and native leaf litter decomposition dynamics in agricultural water impoundments

MENVSCDepartment of Ecology and Resources ManagementLeaf litter contributes to the functioning of aquatic ecosystems through allochthonous inputs of carbon, nitrogen and other elements. In many freshwater ecosystems, leaf litter inputs are among the most important cross-ecosystem nutrient contributions. However, native plant communities are under threat from invasive plant species, with largely unexplored consequences for recipient aquatic ecosystems. Broadly, ecological impacts of invasive alien species can be unpredictable and simultaneously span multiple habitat types and taxonomic groups. Invasive alien plants can have particularly severe ecological impacts, and plant inputs into aquatic environments can alter abiotic and biotic aquatic dynamics. Lakes and reservoir ecosystems are regarded as heterotrophic detritus-based habitats which are dependent upon allochthonous organic matter for the majority of energy inputs. Allochthonous detritus is extremely important for the trophic dynamics of the microbial organisms, macroinvertebrates and benthic plants in lakes and reservoirs. In the present study, leaf litter nutrient inputs, decomposition and colonisation associated with four plant species was examined using a combination of mesocosm and field experimental approaches. Native sycamore fig Ficus sycomorus L., and silver cluster–leaf Terminalia sericea Burch. ex DC. decomposition dynamics were compared to invasive tickberry Lantana camara L and guava Psidium guajava L., whereby phosphate, nitrate, nitrite, silicate and ammonium releases were quantified over time. Leaf inputs significantly reduced pH, with reductions most marked by invasive L. camara. Conductivity was heightened by all leaf input treatments, excepting native T. sericea. Leaf inputs significantly affected all nutrient levels monitored in the water over time, except for silicate. In particular, leaf litter from invasive L. camara drove significantly increased nutrient concentrations compared to other native plant species, whilst effects of invasive P. guajava were less statistically clear. The end weights of the leaf litter demonstrated decomposition differences among the species types, following a decreasing order of P. guajava > T. sericea > F. sycomorus > L. camara, further suggesting high organic inputs from invasive L. camara. Furthermore, ex-situ larval mosquito colonisation of with the above-mentioned native and invasive species leaves were assessed. Larval mosquito abundances differed significantly accordingly to leaf treatment, whilst no mosquitoes colonised leaf-free controls. Leaf litter from the invasive L. camara, invasive P. guajava and native F. sycomorus drove significant increases in mosquito abundances relative to native T. sericea. In situ macroinvertebrate colonisation, and quantify decomposition rates, of four species of native and invasive terrestrial plants was also assessed. Leaf treatments had a significant, group-specific effect on abundances and composition among focal macroinvertebrates. Invasive leaves reduced Physidae and Oligochaeta abundances, yet Ostracoda were significantly more abundant in the presence of invasive P. guajava. Chironomidae relative abundances increased under invasive L. camara treatments, whilst differences among leaf treatment effects on Coenogrionidae abundances were not statistically clear. In turn, macroinvertebrate diversity did not differ significantly among plant treatment groups, but the contributing taxa varied. The decomposition rate of the leaf litter demonstrated differences among the species types, following a decreasing order of L. camara > F. sycomorus > T. sericea > P. guajava. The study results highlight that differential leaf litter decomposition rates of invasive and native plant types play a significant role in nutrient release, thereby potentially supporting increased aquatic ecosystem productivity. The study highlights that shifting terrestrial plant communities following invasion may alter aquatic nutrient availability and how insect communities may utilise such resources. In addition, the study highlights that even semi-aquatic organisms such as mosquitos are affected by differing leaf-litter inputs and this may have broader societal implications through vectoring of mosquitoborne disease. While the study showed that invasive leaf litter generally decomposes faster than native litter, the overall findings suggest that plant species-specific, rather than invasive versus native, considerations were important for colonization and nutrient release dynamics. As such, future studies should continue to assess characteristics of other dominant native and invasive plant species within the context of leaf litter allochthonous inputs into recipient aquatic ecosystems. Larger datasets will hopefully prove useful in developing a predictive framework for how riparian plant community shifts will impact on aquatic ecosystem functioning.NR

Aquatic macroinvertebrate community colonisation and succession in macadamia orchard and communal area reservoirs: a case study of Luvuvhu River valley, South Africa

The demand for macadamia nut production worldwide has led to increased use of pesticides and chemicals for pest and disease control. Reservoirs in these macadamia farming sectors are at . The current study investigated macroinvertebrate colonisation and succession associated with two macadamia orchards and two communal area reservoirs. The potential effects of stressors from these land uses was tested for and compared using stone substrates over a 6-week period. Stone substrates from both reservoir types were abundantly colonised over time and the total macroinvertebrate taxa and abundance showed an increasing trend across the sampled weeks, with macadamia orchards having the highest number of macroinvertebrate taxa. Strong ecological succession was observed across reservoirs, with the initial succession of early colonisers – i.e., Chironominae (, Ostracoda (and Anax sp. (predator) within communal area reservoirs, and Chironominae, Ostracoda Radix natalensis (scrapers) in macadamia orchard reservoirs – followed by predatory colonisers such as Gyrinidae larvae, Trithemis sp. (macadamia orchard reservoirs), and Ranatra sp. (communal area reservoirs). Macroinvertebrate community structure differed significantly across sites and weeks, with no similarity being observed for communities across the different reservoirs. Redundancy analyses further highlighted 11 sediment chemistry variables (i.e., pH, resistivity, P, K, Na, Ca, Mg, Cu, B, Fe and S) which were significantly related to macroinvertebrate community structure. Thus, sediment variables were found to be better predictors of macroinvertebrate community structure in macadamia orchard reservoirs than communal area reservoirs. Consequently, we concluded that differences in colonisation ability among taxa and environmental stressors were important factors driving succession. These results add to the understanding of the macroinvertebrate colonisation processes and environmental stressors within agricultural landscapes, which can aid in the development of conservation management of freshwater ecosystems

Macroinvertebrate colonisation associated with native and invasive leaf litter decomposition

Lake and reservoir ecosystems are regarded as heterotrophic detritus-based habitats which are dependent on both autochthonous and allochthonous organic matter for the majority of energy inputs. In particular, allochthonous detritus is in particular important for the trophic dynamics of microbial organisms, macroinvertebrates and benthic plants in freshwaters. Here, we assess macroinvertebrate colonisation, and quantify decomposition rates, of leaf litter from species of native and invasive plants in a small agricultural reservoir. Native fig Ficus sycomorus and silver cluster–leaf Terminalia sericea were compared to invasive tickberry Lantana camara and guava Psidium guajava, whereby macroinvertebrate colonisation was assessed over time. Leaf treatments had a significant, group-specific effect on abundances and composition among focal macroinvertebrates. Invasive leaves reduced Physidae and Oligochaeta abundances, yet Ostracoda were significantly more abundant in the presence of invasive P. guajava. Chironomidae relative abundances increased under invasive L. camara treatments, whilst differences among leaf treatment effects on Coenogrionidae abundances were not statistically clear. In turn, macroinvertebrate diversity did not differ significantly among plant treatment groups. The decomposition rate of the leaf litter demonstrated differences among the species, following a decreasing order of L. camara > F. sycomorus > T. sericea > P. guajava. The study results highlight that leaf litter species identity among invasive and native plants plays an important role in the colonisation of macroinvertebrates in small reservoirs, thereby differentially supporting aquatic environments and food webs. However, differences were not uniform across invader-native groupings. Nonetheless, certain invasive leaf litter decomposes faster than native litter, with possible implications for broader nutrient dynamics and subsequent community composition