Connectivity of urban water bodies has been substantially changed due to hydrological, physicochemical, and biological alterations throughout upland, riparian, and aquatic environments. For example, drainage networks of impervious surface areas use storm drains to allow runoff to bypass infiltration or riparian zones, causing flashier hydrographs and increased contaminant loading. In addition to direct effects on water bodies, urbanization can also alter reciprocal connections from aquatic to terrestrial environments. For example, direct connections between wastewater and urban water bodies causes increased pharmaceutical loading in urban streams. While pharmaceuticals have myriad direct effects on ecosystem structure and function within stream ecosystems, they also impact the emergence of aquatic macroinvertebrates, reducing the connection between aquatic and terrestrial ecosystems. In addition, various strategies for rehabilitating urban ecosystem functioning (e.g., retention ponds, floodplain restorations) are often aimed at enhancing connectivity, either explicitly or implicitly. Using results from the Baltimore Ecosystem Study and other work focused on common urban contaminants, we will summarize the impacts of altered connectivity in urban aquatic ecosystems. In particular, we will focus on how numerous problems associated with urbanization are driven primarily by altered connectivity, and how these problems may be addressed by directly improving connectivity issues across the urban landscape.
Results/Conclusions
By altering the natural hydrology of the landscape, urbanization alters connectivity across multiple dimensions, having consequences for urban water quality and ecosystem functioning. In the longitudinal dimension, urban development increases connectivity by replacing headwater streams and wetlands with storm drain systems, sewers, and impervious surfaces, impacting material, energy, and nutrient transport downstream. For example, dissolved organic nitrogen and carbon concentrations increase by >100% moving from headwaters to the outlet of a major Baltimore stream network. In the lateral dimension, urbanization disconnects aquatic ecosystems from flood plains, with lowered water tables leading to drier riparian soils and altered floodplain biogeochemical cycling. In the vertical dimension, impervious surfaces short-circuit connections between surface water and groundwater, but the complex network of underground infrastructure can increase vertical connectivity via leaky pipes or inundation and infiltration processes. Finally, urbanization alters reciprocal connectivity (i.e., transfer of materials from aquatic to terrestrial environments), as urban chemical stressors can increase emergence of aquatic insects by up to 18%, subsequently affecting energy transfer into terrestrial environments. There are both intentional (i.e., stormwater management) and unintentional (i.e., urban chemical stressors) effects of urbanization on urban ecosystem connectivity, causing a range of impacts within aquatic environments and on downstream water quality.