Road Salt Runoff: The Relative Contribution of Direct and Indirect Effects in Pond Food Webs

Freshwater salinization is an emerging global concern, and in northern latitudes can be largely attributed to road deicer applications during winter and spring. In urbanizing areas, stormwater ponds now comprise a significant portion of standing water as these ponds are designed to sequester excess run-off and pollutants, such as road salt deicers. While these man-made ponds are not intended to serve as freshwater ecosystems, it is becoming clear that they do provide habitat for many organisms, but we know little of their ecosystem structure, function and interactions with pollutants. The purpose of my research was to investigate the effects of road salt contamination on pond food webs through manipulating trophic structure and salt in experimental pond communities as well as through field surveys of urban stormwater ponds. In the spring and summer of 2008 and 2009 I used 40, 600-L experimental pond mesocosms to explore the relationships between chloride concentration, tadpole ( Hyla versicolor) presence and algae/zooplankton inoculum source. In 2008 I used inoculum from a pristine habitat while in 2009 I collected inoculum from urban stormwater ponds. My 2008 mesocosm study results suggest that under elevated chloride conditions, tadpoles had more algal resources (measured as chlorophyll a concentration) available as a result of declines in zooplankton competitors, inducing a faster rate and size at metamorphosis. Road salts and tadpole grazers also interacted to impair microbial respiration and leaf litter decomposition. My 2009 mesocosm study results suggest that tadpoles and certain zooplankton species residing in urban stormwater ponds may be acclimating to road salt deicer inputs. From April through June 2009, I surveyed 8 stormwater ponds in the Red Run Watershed, Baltimore County, Maryland, USA to describe patterns in aquatic food webs associated with a gradient in road salt inputs. I observed a decline in zooplankton density along an increasing conductance gradient that is consistent with experimental results of road deicer effects, suggesting algal resources are possibly freed from grazing pressure as zooplankton consumers are negatively impacted by road salt exposure. My results highlight the potential for both direct and indirect effects of road deicers on freshwater communities.