The Effect of Urbanization and Dam Removal on Stream Metabolism

The main goal of this dissertation is to explore the influence of urbanization and dam removal on stream function represented by stream metabolism (i.e., gross primary production (GPP), ecosystem respiration (ER) and net ecosystem production (NEP)). Moreover, the research examines dam removal as a means of restoring stream metabolism. In a study of seven sites around the Chicago region during summer and fall of 2009-2013, results showed that stream GPP and ER decreased during storms, but on average ER show significantly greater resistance than GPP. After floods, both ER and GPP recovered to pre-flood levels within approximately 1-10 days, with no significant difference between GPP and ER resilience to floods. Results from this study demonstrate that GPP in urban streams is more susceptible to disturbance than the ER. Low production as a result of continued flood events can result in low oxygen levels in water and therefore can affect organisms that prefer specific DO levels. This dissertation demonstrates that ecosystem metabolism changes significantly following dam removal. More specifically, dam removal increased GPP and ER levels at the upstream and downstream dam removal sites, immediately after removal of the dam. Away from the studied dam, a reference site showed only a small seasonal decline in GPP and ER across the same months. Metabolism in the upstream and downstream sections of the restored rivers shows similar GPP and ER rates and similar rates to a reference site following dam removal. Moreover, dam removal moves the streams towards autotrophy by increasing GPP more than ER. Overall, I concluded that dam removal can restore aquatic ecosystems by enhancing metabolism rates even in non-production seasons (i.e., winter). The study suggests that dam removal should be considered as a preferable means of restoration for areas with low GPP and ER. Moreover, the dam removal timing can also play an important role on the behavior of metabolism response to dam removal (e.g., if the dam was removed in spring where algae bloom, GPP can increase to much higher levels). A future research should investigate multiple dam removal case studies to address the influence of seasonality on dam removal. Finally, in a study of 50 watersheds across a gradient in urban intensity (18 sites located on 13 watersheds in the Midwestern US during summer/fall periods.), urbanization was quantified using the urban land use gradient index (ULUG) which is derived from infrastructure, land cover, and population variables, in addition to the flashiness index which is derived from normalized variation of daily flow regime. I showed that both GPP and ER decreased sharply (in absolute value) above 23.8 ULUG. Moreover, results concluded that urbanization is a major controlling factor on stream metabolism as it increases heterotrophy by reducing GPP more than ER. This study suggests that metabolism restoration projects should be targeted to high urban intensities. A future study can include much larger number of sites with different site characteristics (i.e., organic matter availability, canopy cover)