Simulating the multiple effects of a diatom bloom on metal bioaccumulation in clams

Coastal phytoplankton blooms influence fisheries production and ecosystem function, yet the potential impact of a bloom on metal trophic transfer remains unclear. We hypothesize that diatom blooms alter metal bioaccumulation in predators by modifying metal speciation and uptake pathways. To test this, we used a continuous-flow culture system that better simulates natural water conditions to grow stable-isotope-labeled (⁶⁵Cu, ⁶²Ni, ¹¹³Cd, and ²⁰⁶Pb) diatoms (Thalassiosira weissflogii) and examined how their presence affects metal bioaccumulation in clams (Ruditapes philippinarum). Our results show that the presence of diatoms significantly reduced availability of dissolved Cu to bivalves, but had little effect on Cd, Ni, and Pb. For diatom-associated metals, the assimilation efficiencies of Cd, Ni, and Cu were similar regardless of whether they were bound to diatom frustules or intracellular components, but frustule-bound Pb was significantly less assimilated (17 %) than the whole-cell Pb (32 %). At the steady-state diatom concentration, diatom-bound Cu, Cd, and Pb contributed 79 % to 85 % of their bioaccumulation in the bivalves, while diatom-bound Ni contributed 34 %. The presence of diatoms enhanced overall bioaccumulation of Cd, Ni, and Cu by factors of 4.8, 1.9, and 1.7, respectively, while having negligible effect on Pb (0.98 ± 0.32-fold). These findings highlight the necessity of integrating algal bloom dynamics into metal risk assessments, especially given increasing bloom occurrences due to climate change.