Extreme rainfall events across the US are projected to get 20% more severe and 200% more frequent in the next decades due to warming. This is concerning for the health of aquatic systems as runoff is a prevalent source of aquatic pollution. In farmlands, but also in urban areas, heavy rains wash from soil the excess nutrients from fertilizers. These nutrient-rich waters often find their way into water bodies fueling Harmful Algal Blooms (HAB) in rivers, freshwater lakes, lagoons and coastal areas. One common solution to mitigate the excess runoff alongside reducing flooding has been the construction of retention ponds/wetlands to catch stormwater. The role of these artificial reservoirs in protecting water quality and infrastructure is undoubtable, but also creates runoff-fed pond/wetland ecosystems that expose organisms to high levels of pollutants selecting for organisms with an increased tolerance to chemicals. Here we capitalize on these man-made environments to explore the ecological consequences of urban and suburban runoff in the diversity of water and sediments’ microbial communities, with special emphasis on those often linked to HABs, such as cyanobacteria and eukaryotic algae.
We used a combination of microbial community characterization using high-throughput sequencing of the 16S ribosomal gene, strain isolation methods and lab-based testing under common garden conditions to investigate the changes in community membership and diversity of the microbial communities in retention ponds heavily impacted by urban run-off in the Lake Ontario watershed (Rochester, NY). We contrasted the response of the organisms isolated from these environments with those collected in retention ponds, relatively unimpacted by anthropogenic actions. Common garden experiments indicated that members of the green algal community are extremely susceptible to the presence of NaCl road salt, MgCl2 road salt (marketed as environmentally friendly), or Miracle Gro Lawn Food. The common herbicide Roundup Weed & Grass Killer was able, even at extremely low concentrations, to inhibit algal growth completely. This has important consequences for blooms in freshwater environments as cyanobacteria, the major cause of HABs in lakes, are often resistant to herbicides. Combined stressors such as warmer temperatures, presence of nutrients and toxics and a decrease of other members of the phytoplankton can create ideal conditions for cyanobacteria HABs.