Assessment of phytobenthos in the lower Phongolo River catchment in relation to changing environmental conditions
Floodplain ecosystems are important since they provide numerous services and resources, including food, wood and water to humans. The lower Phongolo River floodplain is one such ecosystem and is the most unique and diverse floodplain in South Africa due to its biodiversity and economic value. The floodplain is located in northern KwaZulu-Natal and stretches from the Pongolapoort Dam to the confluence between the Phongolo and Usuthu rivers in the Ndumo Game Reserve (NGR). The Pongolapoort Dam was constructed with the aim to supply nearby towns with water as well as provide irrigation water for sugarcane and cotton plantations. Controlled flood releases were implemented from the dam to ensure that the ecosystem infrastructure is maintained, but there have been no flood releases since December 2014 due to an ongoing drought in the area. The NGR is the only protected section in the floodplain area and is a Ramsar wetland of international importance due to its high biodiversity and unique wetlands. The lower Phongolo River floodplain is at risk due to increasing human population pressure (extensive fishing, irrigation schemes, water abstraction and agriculture) and spraying of DDT in the floodplain area for mosquito vector control. Only a few studies have focussed on the phytobenthos of South Africa’s floodplain ecosystems, with no published work on the diatom community of the lower Phongolo River floodplain. There are various advantages for including diatoms in ecological and ecotoxicology studies as they have a relatively short life span, are sensitive to any changes within their environment, are species rich, can colonise almost all substrata, are found in nearly all aquatic habitats, are primary producers in all aquatic ecosystems and can be preserved for many centuries in sediment due to their siliceous cell wall. The main aim of this study was to increase our knowledge on the spatial and temporal diatom community structures of the lower Phongolo River floodplain. Ecological and ecotoxicological studies were carried out to assess the influence that flow variation, physico-chemical water quality, changing environmental conditions and insecticides have on the structuring and vitality of the diatom communities. The results showed that the diatom community had observable differences between river sites during the presence and absence of a flood release event, with flow changes affecting the diatom community the most. During the absence of a controlled flood release, the species had to endure less variation in flow and physico-chemical conditions as there was a more stable environment. Temporal variation was noted across six surveys in the Usuthu River due to more natural flow patterns compared to the Phongolo River. A continuous base flow was maintained in the Phongolo River from the Pongolapoort Dam, which resulted in extremely low and consistent flows for the river. Due to the natural flows, fluctuations in the physico-chemical water variables were recorded for the Usuthu River with less fluctuations in the physico-chemical water variables for the Phongolo River. Flooding of the Usuthu River into Shokwe Pan resulted in the pan being nutrient enriched. The main drivers structuring the diatom community in Shokwe Pan and the ephemeral pans were nutrients. Nutrients enter these pans as runoff from the surrounding catchment area. Conductivity was the main driver in the structuring of the diatom community of Nyamithi Pan and adjacent Paradise Pan. This was expected as Nyamithi Pan is highly saline (conductivity values between 3500 and 11000 μS/cm) as it is situated on top of marine cretaceous deposits with natural saline groundwater seeping into the pan. Increased conductivity for Paradise Pan is due to the influence of Nyamithi Pan on Paradise Pan. When Nyamithi Pan overflows it fills Paradise Pan, which lies within the catchment area of Nyamithi Pan. Flooding of the Usuthu River during February 2017 had an influence on the nutrient concentrations of Shokwe Pan, as well as influencing the physico-chemical water variables and diatom community structure of the lower reaches of the Phongolo River and Nyamithi Pan. The influence of an Usuthu River flood on the lower reaches of the Phongolo River and Nyamithi Pan are indicated by similar driving forces (temperature, electrical conductivity and sulphate) shaping the diatom community structure of these sites. As the Phongolo River experienced extremely low flows (flow rate of 4–8.50 m3s-1) (due to an ongoing drought) during the study period, it had no influence on the floodplain pans. The δ15N and δ13C signatures remained consistant in the Usuthu River during summer and late summer rainfall seasons, with shifts noted in these stable isotope signatures in the Phongolo River between these seasons. For the Phongolo River, the δ15N increased and δ13C decreased between the two seasons. An increase in runoff from intensive agricultural activities within the floodplain area could cause an increase in the δ15N in the Phongolo River. The δ13C values decreased due to lower food availability and increased feeding (from competition) on periphyton during the late summer rainfall period. The δ13C values remained consistant for both floodplain pans (Nyamithi and Shokwe Pans) across seasons with a decrease in the δ15N values between summer and late summer rainfall seasons. Decreasing rainfall caused a decrease in influx of water into wetland ecosystems, which resulted in a decrease in nitrogen concentrations. Wetland ecosystems are seen as more stable environments than riverine ecosystems when considering changes in the nitrogen concentrations. This is due to the nitrogen spiralling effect where nitrogen is transported downstream within river ecosystems but remains within the same wetland ecosystem. A sediment core from Nyamithi Pan was dated as 1083 years old and indicated changing environmental conditions. Dominant diatom species identified within the core include H. coffeaeformis, C. meneghiniana, D. elliptica, Fragilaria sp., Navicula sp., Nitzschia sp. and N. palea. The relative abundances of these species decreased and fluctuated less in the more recent (i.e. past 300 years) section of the sediment core. An increase in the relative abundance of D. elliptica and decreased relative abundances of H. coffeaeformis, C. meneghiniana, Fragilaria sp., Navicula sp., Nitzschia sp. and N. palea indicates a freshening (decreasing salinity) and decreasing nutrient concentrations of Nyamithi Pan. Less extreme fluctuations in the species’ relative abundances are due to annual flooding events in the floodplain that results in fluctuations between desiccation and inundation of the floodplain pan. This is supported by the co-occurrence of salinity tolerant and in-tolerant species and lower relative abundance of the dominant species. Exposures to DDT, Deltamethrin and a mixture negatively influenced the diatom vitality during laboratory and microcosm exposures irrespective of the exposure concentration. The vitality and functioning of the diatom cells were influenced by these insecticides through changes that occurred to their chloroplast. These insecticides had a phototoxic effect on the diatom community and caused the chloroplasts of these organisms to either distort, dissolve or to have reduced the chlorophyll-α concentrations. For both exposure studies the percentage dead cells were higher in the exposed samples compared to the controls. The insecticides had a negative effect on diatom metrics (life-forms, ecological guilds and size classes), which resulted in a significant decrease in some diatom metrics after exposure to the selected insecticides. Results from both exposure studies indicated that diatoms are effective bio-indicators of pesticide exposure and could be used in ecotoxicology studies. Overall the results show the importance of including diatom analysis in floodplain ecosystem studies. It is important to study the floodplain as a whole and not only focus on either the river or floodplain pans. Changes in the environmental variables (biological, chemical and physical) will influence the diatom community structure and vitality. Long term analysis of the diatom community within the lower Phongolo River floodplain is essential, especially to study the change and influence on the diatom community when the Phongolo River floods the floodplain area (post-drought).