UC San Diego

Plastic is found across the globe and ubiquitous as a pollutant in aquatic environments. The primary source of plastic for the environment is the mismanagement of waste. Through runoff, plastic eventually enters freshwater environments and becomes a hazard to aquatic life. Although plastic’s effects on organisms is well documented, it is unknown how these effects propagate through food webs and affect ecosystem functions. Biodegradable forms of plastic have been developed that may be less persistent and therefore have smaller environmental footprints. My thesis investigates the effects of one non-biodegradable thermoplastic polyurethane (TPU) and two biodegradable TPUs on freshwater ecosystems using experimental pounds. In a gradient design, microplastics were added to 27 experimental ponds with zooplankton and algae communities. The concentrations of both plankton types were monitored over 97 days. We show that algal blooms and eutrophic conditions occurred consistently in ponds treated with medium to high concentrations of the petroleum TPU, and less often in those treated with the biodegradable plastics. The density and biomass of zooplankton were significantly lower in tanks above a threshold concentration of non-biodegradable TPU within 36 days of exposure (p<0.05). This threshold concentration increased over time from 0.023 grams per liter on day 36 to 0.072 grams per liter on day 97 (p<0.05). The two bioplastics had much weaker effects on zooplankton densities than the petroleum based TPU. My thesis suggests that plastic pollution could contribute significantly to degrading of water quality by promoting algal blooms and eutrophic conditions through reducing zooplankton grazing. Biodegradable plastics offer an alternative to petroleum-based plastics and have less impact on plankton communities. The spread of plastic pollution in recent decades has coincided with expanding zones of eutrophication, deoxygenation and algal blooms. My thesis is the first experimental evidence from the field to suggest a causal relationship between such water quality trends.