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Urban runoff is a major cause of stream degradation. For appropriate management action, there is a need to establish whether or not the runoff is degrading or potentially will degrade the receiving water. This work explored the use of local periphyton communities for assessing the ecological impacts of runoff of two urban catchments. Periphyton communities were colonized on glass substrate for 2 weeks in riverine waters receiving the urban runoff and in non-receiving waters at an upstream reference site. The receiving communities were evaluated against the reference community for photosynthetic efficiency and tolerance to copper, identified to be a significant runoff stressor. Photosynthesis efficiency was measured as a PSII quantum yield, and community tolerance was assessed using a laboratory ecotoxicological test. The hypothesis tested is that the runoff degrades by causing communities in receiving waters to become more sensitive to runoff stressors. The bioassessment indicated a much higher copper threshold than provided for in the generic water quality guidelines. The significance in the use of local community is that it allows for ecological risk analysis of exposure to runoff stressors, providing site-specific information relevant to management.

Abstract-Protection of ecological health of aquatic ecosystems is a primary focus of water quality programs globally. Of special concern is the degradation impact from urban stormwater. Urban storm runoff is highly variable in both physical and chemical stressors, and can impact by changing the structure and physiology of ecological communities. We propose the use of photosystem II (PSII) activity of local periphyton communities for detecting impacts on the ecological health of aquatic ecosystems. This study evaluated the sensitivity of PSII response of periphyton communities to environmental stress in the field and stress (copper toxicity) simulated in the laboratory. The field study was done on natural periphyton communities at two sites in the Swan River estuary, Western Australia receiving urban stormwater discharge. Natural community at a non-receiving upstream site was used as the control. The laboratory study involved a chronic and an acute exposure to copper using periphyton community collected at the control upstream site. Both the laboratory and field studies indicated that PSII response was a sensitive indicator to the different exposures than the response in biomass, chlorophyll a content, or in-vivo chlorophyll fluorescence. Response in PSII also correlated strongly with the structural changes of the communities in total individuals (N) and species number (S) (Pearson's coefficient 0.9 at sig < 0.01). Communities pre-exposed to chronic copper, exhibited enhanced PSII activity that correlated positively with the increased community tolerance to copper toxicity. The PSII activity of communities receiving urban stormwater was indicative of the stormwater quality in that lower activity corresponded to poorer water quality. The results demonstrated that community PSII response to be sufficiently sensitive to environmental stress, including urban stormwater that is episodic and highly variable.Index Terms-PSII quantum yield, periphyton community, ecological impact, variable exposure, urban stormwater.