Diurnal variation in Te Waihora water quality

Abstract

During the summer of 2010/2011 research was carried out at Te Waihora/Lake Ellesmere to measure the extent of diurnal variation in important water quality parameters. Diurnal variation is the variation which occurs over a 24 hour period and in aquatic environments large diurnal variation can have significant adverse effects on the aquatic ecology. While water quality testing currently takes place on a regular basis at Te Waihora/Lake Ellesmere, spot testing is likely to miss important changes that take place over a 12 or 24 hour timeframe. The purpose of this study was to document the extremes of diurnal variation and to investigate the extent to which localised weather conditions influenced results. The study ran for a duration of 10 weeks with sampling taking place once a week. In total, seven 12 hour and three 24 hour sampling excursions occurred. On each occasion water temperature, pH, dissolved oxygen and conductivity were measured on an hourly basis. During 24 hour monitoring nutrient and turbidity measurements were taken on a by-hourly basis. It was expected that the diurnal variation in Te Waihora/Lake Ellesmere would be large as the lake is shallow in nature, has little riparian vegetation and shade, and is eutrophic. Dissolved oxygen and pH were expected to peak at the warmest part of the day and return to normal overnight. The results showed diurnal variation to be significant within the lake, and pH and dissolved oxygen peaked much later in the day than expected. Weather conditions did influence results with the largest variations occurring on hot, calm days and the smallest variations on windy, overcast days.