Toxicity of an Urban Creek: Effects of developmental exposure to water from Ellerbe Creek Watershed on zebrafish (Danio rerio) swimming behavior

Urban changes in land use, such as increasing impervious surface cover and the building of stormwater pipes, result in anthropogenically manipulated water drainage into local watersheds. Along with changes in drainage patterns, urbanization introduces new chemicals into the watershed or changes existing chemical concentrations. Therefore, contaminant sources in urban watersheds, such as stormwater runoff and municipal wastewater discharge, lead to complex mixtures of chemicals entering aquatic ecosystems. Urbanization is projected to increase across the world, resulting in more changes in hydrology and more chemicals entering urban watersheds. While it is known that urban infrastructure and pollution changes the chemical and physical properties of an urban watershed, there is little known about how these changes impact the developmental health of aquatic organisms that call the watershed home. It is important to understand developmental toxicity because changes in development can impact the adult fitness of organisms, ultimately impacting the population and potentially the entire ecosystem.

In Durham, North Carolina Ellerbe Creek Watershed is a highly developed watershed with 22% impervious surface by area. Ellerbe Creek (EC) cuts through Downtown Durham and has various urban pollution sources such as industrial, residential, and recreational development. Due to the exceedance of water quality standards, multiple segments of EC are considered impaired under the Clean Water Act.

EC is home to many species of fish. Understanding the influence of chemical contamination on fish populations is important for estimating the broader ecosystem health. To examine the effects that EC chemical constituents may have on fish development, we conducted a series of behavioral toxicity studies following treatment with EC water using zebrafish (Danio rerio). Specifically, we raised zebrafish in urban watershed samples collected during four seasons across sixteen points along EC. Sites were selected based on their location along the main branch or tributaries of EC. Main branch sites were selected based on their spatial relationship (upstream versus downstream) to a wastewater treatment plant. Tributary sites differed in their local land cover, resulting in highly varying chemical profiles at each site. Sampling seasons roughly corresponded with seasonal variance in water quality parameters. At five days post fertilization, we assessed the swimming behavior of the zebrafish. This allowed us to determine if developing in differing urban water samples had an adverse effect on zebrafish behavior, which can ultimately have an impact on survivability and competitiveness of fish larvae.

When analyzing our data, we wanted to answer four leading questions. The first was whether any effect on swimming behavior was seen between fish raised in EC samples compared to fish raised in control water. We then wanted to know if the wastewater treatment site’s effluent impacted the swimming behavior of zebrafish since we knew that the majority of water in sites downstream of the wastewater treatment plant was comprised of wastewater effluent. Similarly, we asked if fish raised in water samples from different tributaries showed differing swimming behavior. Tributaries act as a snapshot of the water that is feeding into them and the water chemistry of one does not change the water chemistry of another – allowing us to better understand the conditions that may result in urban water toxicity. Finally, we aimed to evaluate how the collection season of the water samples may influence swimming behavior.

Results show spatial and temporal heterogeneity in the impacts of water samples on zebrafish larval behavior. Fish raised in EC water samples had varying responses in their swimming behavior. Among fish raised in select EC water samples, there was a change in behavior compared to fish raised in control water. This change appeared to be both spatial, varying based on collection site, and temporal, varying based on collection month. All changes in fish swimming behavior were hypoactive compared to the controls. We did not see a difference in swimming behavior between fish raised in water samples from upstream of the wastewater effluent site compared to fish raised in water samples from downstream of the wastewater effluent site. There was, however, a difference in swimming behavior among fish raised in different tributary water samples. Significant site-specific heterogeneity among tributaries appeared to be driven by one to two collection sites and varied by collection month.

This study aimed to better understand how water from EC impacts the health and development of organisms living in the creek. The effects seen on swimming behavior of zebrafish raised in EC water samples suggest that the urban watershed has an adverse effect on the development of the zebrafish. Different sites and seasons had different chemical and physical properties that may have resulted in these changes and future research focused on identifying the drivers of this behavioral change is imperative for understanding and ameliorating urban watershed ecosystem health. Broadly, the impacts of urban development on watershed chemical constituents and their toxicity and on ultimate ecosystem-level consequences are an important consideration when planning future development. To this end, whole organismal toxicity assays can serve to improve ecological risk assessments.