Tatenda Dalu

Sinks and sources: Assessing microplastic abundance in river sediment and deposit feeders in an Austral temperate urban river system

Microplastics are important novel pollutants in freshwaters but their behaviour in river sediments is poorly understood due to the large amounts of coloured dissolved organic matter that impede sample processing. The present study aimed to 1.) estimate the microplastic pollution dynamics in an urban river system experiencing temporal differences in river flow, and 2.) investigate the potential use of chironomids as indicators of microplastic pollution levels in degraded freshwater environments. Microplastic levels were estimated from sediment and Chironomus spp. larvae collected from various sites along the Bloukrans River system, in the Eastern Cape South Africa during the summer and winter season. River flow, water depth, channel width, substrate embeddedness and sediment organic matter were simultaneously collected from each site. The winter season was characterised by elevated microplastic abundances, likely as a result of lower energy and increased sediment deposition associated with reduced river flow. In addition, results showed that particle distribution may be governed by various other external factors, such as substrate type and sediment organic matter. The study further highlighted that deposit feeders associated with the benthic river habitats, namely Chironomus spp. ingest microplastics and that the seasonal differences in sediment microplastic dynamics were reflected in chironomid microplastic abundance. There was a positive, though weakly significant relationship between deposit feeders and sediment suggesting that deposit feeders such as Chironomus spp. larvae could serve as an important indicator of microplastic loads within freshwater ecosystems.

Phytoplankton community diversity along a river-estuary continuum

In this paper we examine the aspects of phytoplankton communities along a riverine-estuarine continuum in the Kowie system, Eastern Cape (South Africa). This study aims to relate the phytoplankton population community structure to physicochemical parameters and estimate the different chlorophyll-a concentrations for the Kowie system. Eight sampling sites along a 70 km reach of the Kowie system were sampled over a one-year period. One hundred and seventy-eight species belonging to 78 genera were recorded within the riverine-estuarine continuum. Diatoms were predominant, accounting for 81.9% of the total abundance. The estuary had 98 species (55 genera) recorded whereas the river had 141 species (67 genera). Using a two-way analysis of variance (ANOVA), species richness was found to differ significantly amongst seasons. The chl-a concentration along the estuarine-riverine continuum increased from spring to a high in summer (river mean = 7.9 mg m−3 and estuary mean = 3.3 mg m−3) before decreasing to a low in winter. Kruskal–Wallis ANOVA showed significant differences among chl-a concentration with seasons and sites. Redundancy analysis identified five factors – salinity, water depth, aerial cover, nitrates and ammonia – that were significant (p < 0.05) in affecting phytoplankton variation. The relatively small study indicates the need for further monitoring to gain a better understanding of Kowie system phytoplankton.

Temporal variation of the plankton communities in a small tropical reservoir (Malilangwe, Zimbabwe)

ABSTRACT Seasonal variations of plankton, expressed in terms of species composition and cell density in relation to selected physicochemical water parameters, were investigated in Malilangwe reservoir, Zimbabwe, between February and October 2011. In total, 98 phytoplankton species (2 Dinophyta, 4 Euglenophyta, 14 Bacillariophyta, 50 Chlorophyta, 13 Desmids and 15 Cyanophyta), 66 zooplankton species (13 Cladocerans, 7 Calanoids/Cyclopoids, 46 Rotifers) and two others (Chaoborus sp. and Hydracarina) were recorded. Chlorophyta, Dinophyta and Cyanophyceae comprised the bulk of the phytoplankton, while Rotifera and Cladocera dominated the zooplankton community. A typical seasonal succession of plankton species occurred from February to October. Algal blooms were observed during May to July, dominated by Anabeana sp., Nostoc sp., Anabeana circinalis, Zygenema sp., Anabeana sporiodes, Ceratium hirudinella and Perinidium sp. Redundancy Analysis (RDA) showed that water level, conductivity, pH, dissolved oxygen, temperature, reactive phosphorus and macrophyte cover accounted for most of the distribution of the plankton species.

Detecting the impact of heavy metal contaminated sediment on benthic macroinvertebrate communities in tropical streams.

The effects of heavy metal pollution on benthic macroinvertebrate communities in tropical streams draining ultramafic systems have not been explored, despite a pressing need for ecological risk assessment to protect and manage aquatic ecosystems in these areas. The objective of this study was to examine benthic macroinvertebrate community composition in relation to metal concentrations in stream sediments and other physico-chemical variables in the Manyame River system, which drains part of the Great Dyke of Zimbabwe. Benthic macroinvertebrate sampling and community composition analysis, sediment collection, processing and metal analysis and assessment of other variables in the water column were done once at 55 sampling sites: 12 urban, 30 communal (i.e. sparsely populated rural areas, with livelihoods centred around subsistence agriculture) and 11 Great Dyke sites. Canonical correspondence analysis and partial canonical correspondence analysis (pCCA) were used to determine the importance of sediment heavy metal concentrations in explaining benthic macroinvertebrate community composition in comparison with other factors. Water quality ranged from very poor for urban locations due to sewage pollution, to good in communal locations. Significantly high concentrations of metals (ANOVA, p<0.05) and high magnesium/calcium (Mg/Ca) ratio were recorded in sediments for the Great Dyke site locations. The Mg/Ca ratio, Ca2+, Cr3+ and K+ were found to be important metals structuring benthic macroinvertebrate communities in the study streams, with metals explaining a larger percentage (58.0%) of the total variation explained compared to other variables (35.9%). However, taxa richness, diversity, evenness, percentage of Ephemeroptera, Plecoptera Trichoptera related metrics were higher at some Great Dyke sites than at communal sites. Thus, measures of taxa richness, diversity and %EPT may provide misleading information when assessing heavy metals in moderately polluted environments as in this study. Assessment of tropical streams draining ultramafic systems that are heavy metal-contaminated should also include benthic invertebrate community structure analysis, as it is possible that common endpoints, such as %EPT, may not identify impacts to aquatic communities.

Nature and source of suspended particulate matter and detritus along an austral temperate river–estuary continuum, assessed using stable isotope analysis

Ecologists are interested in the factors that control, and the variability in, the contributions of different sources to mixed organic materials travelling through lotic systems. We hypothesized that the source matter fuelling mixed organic pools in a river–estuary continuum varies over space and time. Samples of the mixed organic pools were collected along a small temperate river (Kowie River) in southern Africa during early and late spring, summer and winter. 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. Stable isotope analysis in R revealed that aquatic macrophytes were significant contributors to SPM in the upper reaches. Bulk detritus had large allochthonous matter components in the lower reaches, and contributions of aquatic macrophytes and benthic algae were high (>50%) in the upper to middle reaches. The evaluation of organic matter contributions to 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 seasons. Incorporating SPM and detritus spatio-temporal variations in food web studies will improve our understanding of carbon flow in aquatic systems.

Using functional responses to quantify interaction effects among predators

Summary Predator diversity alterations have been observed in most ecosystems as a result of the loss and/ or addition of species. This has implications for predator–prey dynamics as non-trophic interactions among predators, so-called multiple predator effects (MPE), are known to influence predation success. In addition, there is often a density-dependent relationship between prey availability and prey consumption (functional response). While MPE investigations are common in the literature, functional responses have rarely been incorporated into this field of predation ecology. Here, we outline an experimental procedure that incorporates functional responses into multiple predator effect studies. Using three fish species with different functional traits as model predators (bluegill Lepomis macrochirus, southern mouthbrooder Pseudocrenilabrus philander and banded tilapia Tilapia sparrmanii), we assess intra- and interspecific predator interaction outcomes on predator–prey dynamics. This was done by contrasting observed functional responses of heterospecific and conspecific combinations of predators with expected responses based on those of individual predators. Multipredator combinations produced variable results. Bluegill were the only species in which observed conspecific multipredator functional responses matched those of expected based on individual performance (prey risk neutral effects). In contrast, prey risk reduction was observed for both mouthbrooder and tilapia conspecific multipredator trials. Heterospecific combinations revealed strong prey risk reduction effects for mouthbrooder–tilapia and bluegill–tilapia trials, while mouthbrooder–bluegill multipredator functional responses combined additively. These results are discussed within the context of behavioural traits of the species and the development of a trait-based predictive framework. Using a functional response approach allowed for the assessment of multiple predator effects across a range of prey densities. We propose that the incorporation of within-guild predator combinations into classic functional response investigations will enhance predictive capacity development in competition and predation ecology. A lay summary is available for this article.

Impact of irrigation based sugarcane cultivation on the Chiredzi and Runde Rivers quality, Zimbabwe

Agriculture is vital in sustaining human livelihoods. However, agriculture as it is currently practiced, is contributing to the degradation of freshwater ecosystems globally. We investigated impacts of irrigation return flows from sugarcane farming on water quality and health status of the Chiredzi and Runde Rivers, a biodiversity hotspot region in south-eastern Lowveld of Zimbabwe. The water quality at inlets from the crop field into the wetland system; wetland outlets into the river systems; and river sites upstream and downstream of wetland outlets were monitored during the dry and wet seasons. The wetland system formed naturally from excessive drainage from the cane fields but its purifying capacity was unknown to date. An assessment of the water physical-chemical variables (at all sites) and macroinvertebrate communities (at river sites only) was carried out. Results showed that the wetland was deficient in its purifying capacity as it was already saturated by salts and nutrients from high irrigation return flow loads. A significant seasonal variation was observed for conductivity, reactive phosphorus (RP), pH and total phosphorus (TP) concentrations among the inlets to the wetland whereas among the river sites significant seasonal differences were observed for ammonium, dissolved oxygen, nitrate, RP, pH, TP and turbidity concentrations during the dry season. From the macroinvertebrate community data the impact of the irrigation return flows on the river system was apparent, as the good water quality sites were characterised by a high diversity of pollution sensitive macroinvertebrate taxa, while the irrigation impacted sites were characterised and dominated by pollution tolerant taxa. High ion concentration (conductivity and salinity) and pH were found to be important in structuring macroinvertebrate communities as determined using multivariate analysis in the river system. In conclusion, the river water quality was significantly impacted by irrigation return flows and this greatly reduced the ecological health as highlighted by low macroinvertebrate diversity and increases in physical-chemical variables.

Trophic interactions in an austral temperate ephemeral pond inferred using stable isotope analysis

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.

A new species of Lovenula Schmeil (Copepoda, Calanoida, Diaptomidae) from the Eastern Cape Province of South Africa

Recent collections from the Eastern Cape Province of South Africa yielded an undescribed species of the freshwater diaptomid copepod genus Lovenula. It is probably the largest paradiaptomine known and closely resembles L. falcifera (Loven, 1845). This species, L. raynerae n. sp., differs in several characters, including the shape of the female genital operculum, but particularly in details of the male and female leg 5: on the female the size of the endopod, a distinctive protuberance on the first exopodal segment; on male leg 5, the features of the seta adjacent to the claw of the right leg, a subdistal endopodal spine of the same ramus and details of the claw, with a middle gap on the inner margin. As with L. falcifera, the new species was collected from an ephemeral habitat. A record of L. falcifera from Ethiopia probably represents an undescribed species. A key to the species of the genus is provided.

Aquatic macrophytes in a tropical African reservoir : diversity, communities and the impact of reservoir-level fluctuations

ABSTRACT Variation in species composition, distribution and abundance of macrophytes in the Malilangwe reservoir, a tropical African reservoir, was investigated at four sites in three seasons; March (hot-wet season), June (cool-dry season) and September (hot-dry season) over a nine-month period. Thirteen macrophyte species, representing nine families, were recorded during the study period. Submerged macrophytes had a patchy distribution and the mean transect cover per species was below 3% with the exception of Ceratophyllum demersum and Potamogeton pusillus. Canonical Correspondence Analysis (CCA) identified four distinct macrophyte groups closely associated with the three seasons; hot-wet, cool-dry and hot-dry and environmental factors; pH, dissolved oxygen, phosphorus and water level. Change of seasons and subsequent fluctuations in water levels resulted in changes in macrophyte community structure from the dominant emergent macrophytes (Cyperus involucratus, Panicum repens, Ludwigia stolonifera, Phragmites mauritianus and Schoenoplectus corymbosus) in the hot-wet season to submerged macrophytes (Najas horrida, Potamogeton crispus, Potamogeton pusillus, Potamogeton tricarinatus and Persicaria decipiens) in the hot-dry season. Although changes in water level variations seemed to influence macrophyte structure and composition changes, further research is needed to evaluate to what extent water level fluctuations interact with other seasonal factors acting independently.