CEINT 2010-2011 Wetland Mesocosm Experiment:
Our largest endeavour has been the ongoing monitoring of treatment effects in a replicated experiment using 24 slantboard wetland mesocosms. These mesocosms were constructed to allow us to examine the impact of silver nanoparticles in wetland ecosystems in a fully replicated experiment. Planted in March of 2010, these 4’x12’ mesocosms consist of an upland, aquatic, and periodically flooded area. The slantboard design allows examination of the fate, transport, and impacts of nanomaterials in a wide range of conditions in a single experimental unit. The 24 mesocosms were planted with 6 wetland plant species, and 4 macrophytes. The open design also allowed recruitment of many different taxa of intvertebrates and algae.
The mesocosms were dosed on August 17th, 2010, and the experiment is ongoing. There are six treatments: three silver with final concentrations in the water of 2.5mg Ag/L (gum arabic coated AgNPs, GA-AgNPs; polyvinylpyrrolidone coated AgNPs, PVP-AgNPs; and AgNO3), and three control with no added silver (control, GA coating control, and PVP coating control). To account for the NO3- added in the AgNO3 treatment, an equivalent amount of KNO3 was added to all mesocosms in the replicated experiment. There were 3 replicates of all treatments with the exception of the control treatment, which had 4 replicates. Of the 24 mesocosms established, 19 were included in the fully replicated silver experiment. The other five were used in an unreplicated study of nanomaterial fate and transport by the Wiesner lab.
After dosing with silver, the aquatic macrophytes senesced in silver treated mesocosms. Likely from the senescing macrophytes, there was a pulse of dissolved organic C (DOC), and small increases in the dissolved nutrients NH4+, NO3-, and phosphate. These impacts were strongest in the GA-AgNP and AgNO3 treatments, with less pronounced increases in the PVP-AgNP treatment. Associated with this increase in DOC and loss of macrophyte biomass, there was also a large decrease in dissolved O2 in the GA-AgNP and AgNO3 treatments from increased respiration and decreased photosynthesis.
The most intriguing ecosystem functional response was a 2-3 order of magnitude increase in methane concentration in the watercolumn in silver amended treatments (day 0 being the start of the experiment). This was not simply a result from increased DOC and decreased dissolved O2; both GA-AgNPs and AgNO3 had stronger impacts on DOC and O2 than PVP-AgNPs, yet all three treatments had similar impacts on methane production. Similar results have been observed by our group in laboratory experiments, and further work is planned to see what exactly causes this increase in methane concentration.