Responses of aquatic communities to physical and chemical parameters in agriculturally impacted coastal river systems

Abstract

The assessment of ecological integrity of river systems is a multidisciplinary research field necessary for effective river management. The objectives of the current study were: (i) to characterize the spatial and temporal distribution and community assemblages of macroinvertebrates and algae down a longitudinal gradient and; (ii) to determine the environmental variables that affect assemblage distribution. The study occurs in agriculturally influenced coastal rivers in the southern Cape, South Africa, over a period of two years (wet and dry seasons). At upstream sites low electrolytes (electrical conductivity: 12-30 mS m) and nutrients (total nitrogen: <1 mg L) were recorded. Moving down the longitudinal gradient agricultural influences impacted water quality and aquatic communities. Increased electrical conductivity (> 100 mS m) and nutrients were observed during the wet season. The downstream site showed a positive correlation between nitrogen, phosphates and silica and the river flow regime. Upstream sites were dominated by diatoms and macroinvertebrates indicative of low pH values, low electrolytes and oligotrophic conditions. The benthic substrate of the downstream site was dominated by benthic filamentous algae during the dry and wet seasons, which were associated with increased water alkalinities (34-48 mg L:dry; 24-63 mg L:wet), water hardness (CaCO3) (117- 133 mg L: dry; 24-67 mg L: wet) and nutrients (total phosphorus: 0.44 mg L: dry; 0.26 mg L:wet) under an altered flow regime. Mesotrophic conditions were a usual occurrence during both dry and wet seasons. The altered flow regime at this site played a role in algal and macroinvertebrate assemblages that occurred.