Artificial-substratum periphyton and water quality in the lower La Trobe River, Victoria

Abstract

Between February 1975 and March 1977, periphyton from artificial substrata (glass microscope slides) and water samples for physicochemical analysis were obtained from eight sites on the lowland section of the La Trobe River, which flows through agricultural, urban and industrial areas. Total organic matter on the slides, estimated as weight loss on ignition, was usually highest in summer or autumn when river flows were low. However, chlorophyll a densities generally peaked in late winter and spring when nitrate concentrations were high, except at a site upstream of major urban and industrial areas, where a summer-autumn increase occurred. Thermal discharges from major power stations had no obvious effect on chlorophyll abundance, but did appear to substantially influence diatom assemblage composition from late summer to early winter, when river temperatures were highest. Downstream of the Morwell River confluence, diatom assemblages were influenced by a sharp increase in dissolved solids concentration and probably also by the grazing activities of snails (Ferrissia petterdi and Physastra gibbosa). The diatom flora at the most downstream site showed some evidence of recovery from thermal effects.