Laboratory- and field- scale bioassays for predicting responses of wild rice (Zizania palustris L.) to exposures of site-specific sediments and waters

Abstract

Four bioassays were used to expose wild rice (WR) to site-specific waters and sediments within the boundaries of site-specific conditions. Mesocosm- and microcosm- scale bioassays were developed to measure responses of WR to sediment exposures. In mesocosms, WR height (HT), dry weight biomass (DWB), and seed production (SP) were statistically lower for Cleaver and Unnamed Lake sediment-grown plants compared to Rat River Bay sediment-grown plants. An accelerated-growth microcosm-scale bioassay accurately represented the mesocosm-scale bioassay, while decreasing overall time, sediment, water, and space. WR developed to near reproductive maturity. Significant differences were not identified between mesocosm:microcosm ratios for WR DWB or SP, which appeared to be primarily influenced by sediment ammonia-nitrogen concentrations. Rafts were deployed in two select aquatic systems: three in the Seine River (non-industry-influenced); and two rafts in each of three legacy-mine influenced pits to determine the life stage (aerial, floating leaf, submerged) more sensitive to water depth and water depth increases. Based on data obtained during this study, floating leaf plants were determined more sensitive to depth increases; aerial stage was least sensitive to depth increases. WR developed to aerial stage in 20 and 40 cm water depth treatments in all legacy-mine influenced pits indicating no adverse responses to pit waters. Two flow-through WR paddies were constructed adjacent to separate legacy-mine influenced pits with elevated sulphate concentrations. Seeded WR in each paddy developed according to typical phenology during each of multiple successive growing seasons. In the Pit A paddy, no statistical decreases in HT or DWB were observed between growing seasons; average SP statistically increased in 2019; and seed DWB remained statistically similar between growing seasons. Over two successive growing seasons, WR stem density remained statistically similar in the Pit C paddy. Despite conditions potentially conducive to iron sulphide root coating formation, this was not identified via SEM-EDX characterization. No adverse WR responses observed throughout this study were determined resultant of Pit A or Pit C water exposures. Representativeness of natural WR areas is paramount to bioassay data defensibility. Bioassays described herein were designed to represent field conditions to the extent possible given the scale of the bioassay.