A study on periphyton as indicator of water-quality in regulated rivers

In the central part of South Africa, where the average rainfall is 400–600 mm/yr and evaporation far exceeds precipitation, it is important to monitor the limited freshwater resources that are available and to keep the aquatic environment in an acceptable state. Excessive nutrients (N and P) lead to algal blooms and deterioration of other aquatic biota as the water quality declines. Biological monitoring methods and programmes have been instrumental in the management and monitoring of the health of aquatic ecosystems. Various biomonitoring indices have been developed, using fish, benthic macroinvertebrates, phytoplankton and periphyton (including Bacillariophyta), as site- or non-site-specific indicators of water quality. Periphyton forms the foundation of many food webs. It is adaptable to the availability of a habitat and is directly affected by changes in water quality. In unregulated rivers, “normal” flow patterns and disturbance regimes shape the benthic community composition, while in regulated rivers the “unpredictability” of flow (as an example) adds extra stress to the ecosystem. The overall objective of this study was to determine the position of periphyton (as a group) as a biomonitoring tool and which of its components would be best suited as indicators of water quality. This study was carried out over two periods of 24 months each at two sites on the Modder River and one on the Renoster Spruit. The sites were selected because SASS5 (standard benthic macroinvertebrate index of water quality) could be carried out on “stones in current”, as this is the preferred habitat for sampling periphyton. The physical and chemical factors sampled were temperature (°C), turbidity (NTU), flow (m/s), dissolved oxygen (mg/ℓ and % saturation), electrical conductivity (μS/cm) and total dissolved salts (mg/ℓ), pH, redox potential (mV), nutrients (including dissolved inorganic phosphorus and nitrogen) and chlorophyll-a. The biotas sampled were periphyton, phytoplankton and benthic macroinvertebrates. Statistical analyses were carried out on all sampled data. Correlations and patterns between the periphyton values and the physical, chemical and biological conditions were investigated. The effect of seasonality on the periphyton and the influence of hydrological phases (dry and wet periods) on the periphyton were examined. Results indicate that the composition of the periphyton is shaped by seasons. An increase of Bacillariophyta was found during winter, and Chlorophyta and Cyanophyta during summer. The increase of flow during wet periods had a negative effect on the biovolume of periphyton, as fewer filamentous and colonial algae were present during the wet period. The cell and chlorophyll-a concentration also decreased because of dislodgement during high flow. Even though the nutrients had an influence on all the periphytic algal components, the best correlations were found with the periphytic chlorophyll-a concentration. The periphyton composition and concentration were compared to the biomonitoring indices used on the sampled rivers and sites, namely FRAI (fish) and SASS5 (benthic macroinvertebrates), as well as phytoplankton. To conclude, it was found that periphyton could be used as a biomonitoring indicator in the monitoring and management of water quality. However, as the standard biomonitoring indices operate on different spatial scales and measures, the best results would be obtained if all, or a combination of indices, were used.