Evaluating the effects of desalination antiscalants on phytoplankton and bacterial communities in oligotrophic environments

Antiscalants are used in seawater desalination facilities and are typically discharged with the brine back into the coastal environment. We investigated the impact of two commonly-used antiscalants; polyphosphonates and polycarboxylates-based, on phytoplankton and bacterial communities in the oligotrophic Gulf of Aqaba (Northern Red Sea) using mesocosms simulating different antiscalants concentrations. Our results indicate that the addition of polyphosphonates or poly-carboxylates antiscalants significantly altered phytoplankton and bacterial growth, despite being ‘theoretically’ unavailable for microbial utilization. Specifically, the addition of polyphosphonates resulted in a significant eutrophication leading to increase phytoplankton and heterotrophic bacterial biomass. Contrary, the addition polycarboxylates antiscalants triggered a moderate increase in heterotrophic bacterial biomass and a slight phytoplankton loss, summing to a zero change in bacterioplankton's organic biomass. Altogether, the enhanced autotrophic/heterotrophic biomass attributed to the discharge of polyphosphonates-based antiscalants could impair the pretreatment procedure and increase fouling propensity on the reverse-osmosis membranes. Therefore, it is recommended to minimize/avoid the use of polyphosphonates antiscalants in P-limited oligotrophic seas if the brine is discharged into oligotrophic environments.