Experiment One
To examine top-down and bottom-up effects in the presence of snail
predators, we established experimental freshwater ponds in 75 800L
mesocosms filled with 500L of water (mean pH = 9.09) in the summer of
2011 at a facility approximately 20 miles southeast of Tampa, FL, USA.
Tap water in mesocosms was allowed to age for 48 hours before being
seeded with algae (periphyton and phytoplankton) and zooplankton
collected from local ponds. Algal and zooplankton communities were
allowed to establish over a four-week period and water was mixed among
tanks weekly to attempt to homogenize initial communities before
application of agrochemical treatments. Plastic containers with sediment
(1 L play sand and 1 L organic topsoil (The Scotts Company, Marysville,
OH, USA)) and five rooted shoots of H. verticillata were added to
each tank on 5 July 2011. Immediately before application of agrochemical
treatments on 11 July 2011 (Week 0), we added snails (21 Bi .glabrata [NMRI strain] and 12 Bu. truncatus[Egyptian strain], provided by NIAID Schistosomiasis Resource
Center) and snail predators (2 juvenile Procambarus allenicrayfish, 7 Belostoma flumineum water bugs, and 3Lethocerus spp. water bugs collected from local ponds) to each
tank.
Tanks were randomly assigned to one of fifteen treatments (5 tanks per
treatment) including twelve pesticides at their respective EEC
(estimated environmental concentration), fertilizer (440 µg/L
Phosphorus; 4400 µg/L Nitrogen, observed levels from the field; (Chase,
2003)), solvent control (0.0625 mL/L acetone), and water control in five
replicated spatial blocks. Water and solvent controls were used to
ensure that any observed effects of pesticides could not be attributed
to the presence of solvent. All pesticides were dissolved in acetone and
applied at their respective estimated peak environmental concentrations
(Tables S1-S2). EEC values were determined using the USEPA’s GENEEC
(v2.0, USEPA, Washington, D.C.) software, manufacturers’ label
application recommendations, and the physicochemical properties of each
pesticide (Table S1-S2).