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The relationship between periphyton, flow and nutrient status in south-western cape foothill rivers and the implications for management
Expanded Title:One of the key aspects of river management is to allow for the Ecological Reserve or water needed to sustain aquatic ecosystems. Defining the Ecological Reserve in rivers is heavily dependent on the ability to understand and quantify the relationship between any one ecosystem component and the flow regime. In South Africa, fish, riparian vegetation, and macro-invertebrates are the main biotic components currently considered to set Ecological Reserves and in monitoring the effects of altered flow on ecosystem integrity. Despite its importance to the biotic integrity of river ecosystems, the periphyton is not currently included in the suite of biotic components used for Reserve determinations, in South Africa. This report constitutes the first detailed assessment of periphyton community structure and biomass in South African rivers. It explains how periphyton responds to flow regimes and nutrient enrichment, and reveals the importance of periphyton dynamics in water resources management. It contributes to the body of knowledge that should be used for the determination of the Ecological Reserve and can help guide future monitoring of aquatic ecosystem health. The study examined natural seasonal changes in periphyton biomass and community composition in the foothills of south-western Cape rivers and the effects of enrichment and flow alteration. Flood frequency, the timing of the spring disturbance flood event and dry season low flow conditions were identified as the key components of the flow regime for the maintenance of healthy periphyton communities. A reduction in the frequency of wet season disturbance floods or a loss of the spring flood may result in proliferations of periphyton, which would naturally have been maintained as a thin mat in early stages of accrual. Algal proliferations are often not palatable to invertebrates that rely on algae as a primary source of food. This may result in the loss of sensitive taxa, which would ultimately affect the ecological integrity of the system. Elevated dry season base flows can increase the mucilage content of the periphyton mat, which would lead to habitat degradation for invertebrates and fish and a reduction in food quality for invertebrate grazers. High mucilage would also increase the resistance of the mat to flood disturbance, which has implications for the size of the flood necessary to reset the periphyton community back to its pioneer state typical of the wet season. The factors or combination of factors, (i.e. hydrodynamics, temperature, light, grazing, habitat and water quality) which explain differences in periphyton communities under natural and altered conditions were studied further to identify their relative importance. The differences in periphyton communities in biotopes typical of foothills in south-western Cape rivers were studied in different nutrient conditions and seasons. Periphyton communities with different pre-flood growth forms were compared in terms of their response to individual flood events of different magnitude. Recovery following a flood was also monitored. This study indicates that periphyton could contribute substantially to the suite of biotic indicators used to predict ecosystem response to flow alteration in South Africa. Periphyton indicators based on a combination of algal division (i.e. diatoms, green algae and cyanobacteria) and growth form, as well as Chl - a biomass and the Autotrophic Index, are proposed for predicting and monitoring the biotic response to flow alteration. Chl a biomass was identified as a reliable indicator of trophic status in rivers during this study and it its recommended that measures of Chl a biomass be incorporated into routine river health monitoring initiatives in South Africa.
Date Published:01/11/2012
Document Type:Research Report
Document Subjects:Water Resource Management/IWRM - Catchment Management, Ecosystem - Invertebrates
Document Format:Report
Document File Type:pdf
Research Report Type:Standard
WRC Report No:1676/1/12
ISBN No:978-1-4312-0303-1
Authors:Ewart-Smith J; King JM
Project No:K5/1676
Originator:WRC
Organizations:Fresh Water Research Unit, University of Cape Town; Water Matters cc
Document Size:7 009 KB
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