Inactivation by adding different phosphorus (P) adsorbents is one of the most frequently used methods for combating inland water eutrophication. The aim of this work was to assess the toxic effects of novel P adsorbents (magnetic particles, MPs) on the phytoplankton community. An outdoor microcosm experiment, containing lake water and surface sediment from a hypertrophic Mediterranean lake, was carried out following a factorial design (n = 5) with three different treatments: control (C), where no MPs were added; Treatment-Water (T-W) and Treatment-Sediment (T-S). In T-W and T-S treatments, MPs were added on the surface water layer and on the sediment, respectively, to obtain a final concentration of 1.4 g MP L−1. This concentration was based on both the sedimentary mobile P concentration of the study site and the maximum P adsorption capacity of the MPs, obtained from the literature. After 24 h of contact time, the MPs were removed using a magnetic rake. Physicochemical measurements and biological samples were taken after 24 h of exposure to the MPs and at different time points after such exposure (day 2, 7, 21, 35 and 70). Changes in phytoplankton community such as abundance (biovolume and Chla), species composition and taxonomic groups were assessed, as well as changes in the Shannon-Wiener diversity index. Additionally, the eutrophic metric Algae Group Index (AGI), one of the metrics proposed in the Water Framework Directive, was also calculated. Our results indicate that there is no strong evidence to infer that MPs caused an effect on the phytoplankton community, since no significant differences (GLM test; p > 0.05) were found between controls and treatments in any of the studied variables (phytoplankton taxonomic groups, AGI, Chla concentration, biovolume, diversity and community responses). Accordingly, MPs did not cause any toxic effects on the phytoplankton community of the lake, encouraging the use of MPs in a future whole-lake restoration strategy. However, if the final goal of the restoration plan is to combat nuisance cyanobacteria blooms, higher initial MPs doses or repeated MPs applications are required to achieve a reduction in P concentrations below biological thresholds in order to prevent algal blooms.
