Mixtures of tetrakis hydroxymethyl phosphonium chloride and ammonium influence plankton community structure in blackwater pond mesocosms

Mixtures of chemicals from human sources are pervasive in aquatic systems, yet their effects on aquatic communities are relatively understudied. Emerging contaminants such as organophosphorus flame-retardants (e.g. tetrakis hydroxymethyl phosphonium chloride (THPC)), are toxic to planktonic species at environmentally relevant concentrations. Additionally, inorganic ammonium from agriculture and industry is likely to enter surface waters simultaneously with flame retardant chemicals. While effects of ammonium on plankton are known, its presence has the potential to influence the toxicity of other contaminants. To study mixture effects on plankton communities, floating mesocosms were dosed with THPC (0.08 or 0.8 mg L⁻¹) with or without ammonium (0.3 mg L⁻¹). Weekly measurements of zooplankton community composition and phytoplankton abundance (as chlorophyll a) were performed. Each factor alone had no effect on zooplankton abundance or species composition. However, the mixture decreased the proportion of calanoid copepods and increased rotifers during weeks 1–3. Chlorophyll a was unaffected by treatments, but concentrations were highly variable for the duration of the experiment. The interaction between THPC and ammonium suggests that the contaminants together reduce abundance of large zooplankton, releasing smaller zooplankton from predation and competitive pressure while potentially limiting food availability to planktivorous fish.