Tannin-based coagulants and P-adsorbents have no acute effects on a widespread tropical copepod at applicable mitigation doses

Abstract Eutrophication mitigation techniques, like “Floc and Sink” and “Floc and Lock” uses phosphorus (P)-adsorbents and coagulants to control algal blooms. However, their potential impact on non-target zooplankton is a concern. This study assessed the acute lethality of two P-adsorbents [beige limestone—BL; lanthanum-modified bentonite—LMB) and two tannin-based coagulants (Tanfloc® (TAN) and Mimosa tenuiflora tannin—Jurema-Preta (JUR)] on Notodiaptomus iheringi (Wright, 1935), a dominant neotropical copepod. Our tests revealed a dose-dependent toxicity for all materials. P-adsorbents, BL and LMB, showed similar lethality, likely due to shared physical interaction mechanisms. However, Tanfloc® (LC50: 223.55 mg.L−1) was significantly more lethal than Jurema (LC50: 705.41.mg.L−1), a difference attributed to their varying content of condensed tannins. Crucially, at applicable mitigation dosages (50 mg.L−1 for coagulants; 100 mg.L−1 for P-adsorbents), these products, alone or in combination, caused no significant lethality. These findings suggest that tannin-based coagulants and clay P-adsorbents at relevant environmental concentrations are generally safe for N. iheringi. The study highlights the importance of including copepods in ecotoxicological assessments and supports tannin-based products as promising, safer alternatives to conventional aluminum-based coagulants for sustainable eutrophication management.