Urban runoff impacts on benthic communities of a receiving stream

Area of Science:

• Environmental Science
• Ecotoxicology
• Freshwater Ecology

Background:

• Urban and industrial rainfall runoff mobilizes chemical pollutants, forming complex mixtures that can harm aquatic ecosystems.
• These pollutant mixtures may have synergistic or additive effects, exceeding the impact of individual contaminants on aquatic organisms and ecosystem functions.

Purpose of the Study:

• To investigate the ecological impacts of urban runoff on river biofilms and macroinvertebrate communities in an urban stream.
• To differentiate the effects of urban runoff pollutants from hydrological disturbances caused by rainfall events.

Main Methods:

• Studied five runoff episodes in an urban stream (Girona, Spain) using a control (upstream) versus impact (downstream) design.
• Assessed effects on river biofilm structure and function (chlorophyll-a, phosphatase activity, bacterial density) and macroinvertebrate community composition.
• Monitored concentrations of dissolved organic carbon (DOC), nutrients, and specific pollutants (1,3-diphenylguanidine, zinc, copper).

Main Results:

• Urban runoff significantly increased nutrient, DOC, and pollutant concentrations, leading to higher chlorophyll-a, phosphatase activity, and bacterial densities in biofilms.
• Hydrological disturbance from small-scale floods detached biofilms, complicating the assessment of pollutant-specific impacts.
• Macroinvertebrate community composition shifts were primarily driven by temporal dynamics rather than direct pollutant effects.

Conclusions:

• Pollutant impacts from urban runoff were often masked by environmental factors like hydrological disturbance.
• Biofilm communities showed clearer responses to pollutants during moderate rainfall events that allowed pollutant-driven effects to emerge.
• Recurrent urban runoff poses increasing ecological risks to urban streams, especially with climate change and urbanization driving extreme weather events.