The Effect of Organic Pollution on Organisms in the Wainuiomata River

Abstract

This thesis is the account of a chemical and biological survey of the Wainuiomata River, made between October 1970 and May 1972. The aims were: 1. To assess the degree of organic pollution caused by effluent from a sewage treatment plant. 2- To investigate the effects of organic pollution on the main invertebrate fauna of the river. 3. To investigate and evaluate different methods of data presentation and analysis, in particular cluster analysis. Flow in the river and thus dilution of the sewage effluent are greatly influenced by discharge from a water supply dam. Discharge ceases during summer and autumn, producing wide differences in the condition of the river between summer and winter. Water and quantitative benthic samples were collected each month from eleven stations situated along a 19 km stretch of the river between Morton Dam and the catchpole stream. Water samples were analysed for total alkalinity, turbidity, cations and nutrient anions, by the Cawthron Institute, Nelson. Concentrations of all ions measured increased markedly immediately below the sewage outfall. Cations and total alkalinity stabilised rapidly at concentrations only slightly higher than concentrations at upstream stations, but elevated nutrient concentrations persisted throughout the survey area below the outfall, partly due to farmland run-off. Enrichment of the water stimulated the growth of sewage fungus immediately below the outfall, and abundant filamentous green algae within the next three kilometers. Extensive beds of Elodea canadensis developed in the quieter reaches during periods of low flow, but were removed by high discharge during winter. Deoxygenation to 50% of saturation occurred in summer at weedy stations below the outfall. Changes in concentration of different forms of nitrogen are discussed with respect to seasonal effects and uptake of nutrients below the sewage outfall. Sodium concentrations have been used to estimate dilution of influents and to demonstrate uptake of nutrients. Factors contributing to the growth of E. canadensis below the sewage outfall are discussed. The distributions of the twenty more common invertebrate taxa are presented individually in densograms and discussed with reference to the literature. The animal communities present at each station are described and their interactions with their physical and floral environments are discussed. Communities at adjacent stations are compared, giving particular emphasis to the effects of changes in water chemistry which occur between stations. The effects of a major flood which occurred half way through the survey are discussed. A diverse and abundant fauna dominated by Potamopyrgus antipodarum, Chironomidae and Deleatidium, and including a large variety of Trichoptera, was present at upstream stations. Many fewer species were collected below the sewage outfall, but the insects Hydora picea, Oxyethira albiceps and Chironomidae were usually found. P. antipodarum and Physa showed great increases in summer and autumn coincident with the increase in weed growth. Further downstream, total numbers decreased but numbers and diversity of Trichoptera and Ephemeroptera increased in response to a reduction in weed growth which accompanied the reduction in nutrient concentrations. Pycnocentrodes aureola and H. picea dominated the fauna at these stations. Two methods of relating changes in animal communities with changes in chemistry are investigated. A subjective graphical method is compared, with the so called objective method of cluster analysis, and some important limitations of cluster analysis are discussed with reference to its application in the assessment of mild organic pollution. A prediction/ observation method for assessing the usefulness of individual species as indicator organisms is proposed, and the value of indicator organisms is discussed and evaluated.