Flame retardant chemicals enter aquatic systems through municipal and industrial wastewater, and despite being detected in surface waters, most have not been thoroughly assessed for environmental risk. Halogenated flame retardants, known to be persistent and highly toxic to aquatic organisms are being phased out, and replaced with new or under-tested organophosphorus flame retardants. Organophosphorus flame retardants, such as tetrakis hydroxymethyl phosphonium chloride (THPC), often have similar applications to and are generally not as persistent as halogenated flame retardants. Single-species toxicity tests of organophosphorus flame retardants reveal a wide range of toxicity values within the range of environmentally relevant concentrations in surface waters. Additionally, other contaminants from industry and agriculture are likely to accompany flame retardant chemicals in surface waters. For example, THPC may be found in combination with ammonia, which is released directly with THPC in textile effluent and is a common component of agricultural fertilizers. Ammonia is a form of inorganic nitrogen that can stimulate primary production, but in excess can shift phytoplankton community composition to dominance by less edible or harmful species. While the effects of excess nutrients on planktonic communities are well studied, recent findings suggest the presence of nutrients can reduce or enhance the effects of other contaminants on aquatic organisms. Floating, in situ mesocosms were dosed with two levels of THPC (0.08 and 0.8 mg L-1) with or without ammonium (0.3 mg L-1) in a fully crossed design to study the effects of THPC and ammonium in combination on plankton communities. I hypothesized that THPC and ammonium affect plankton communities differently alone than in combination. Water samples were collected weekly over four weeks to assess zooplankton community composition and phytoplankton abundance (as chlorophyll a) in response to treatment additions. The combination of THPC and ammonium influenced community composition, but each factor alone had no effect. Specifically, low THPC+ammonium decreased the proportion of calanoid copepods with a corresponding increase in rotifers due to lower predation pressure compared to each factor alone one to three weeks post exposure. Total zooplankton abundance changed over time across all treatments, decreasing by more than 50% after one and two weeks of exposure before increasing to abundances approximately 27% higher than initial by week 4. Chlorophyll a was not affected by THPC and nutrient addition, but concentrations fluctuated between 5-15 µg L-1 in all treatments throughout the duration of the experiment. The interaction of THPC and ammonium suggests that these contaminants in combination can reduce abundance of large zooplankton such as copepods, thereby limiting a food source for planktivorous fish, and releasing smaller crustacean zooplankton and rotifers from predation and competitive pressure
