Abstract Metal speciation in solution is uncontrolled during algal growth in the traditional algal bottle assay. A resin-buffered nutrient solution was developed to overcome this problem and this was applied to test the effect of chloride (Cl−) on cadmium (Cd) uptake. Standard nutrient solution was enriched with 40mM of either NaNO3 or NaCl, and was prepared to contain equal Cd2+ but varying dissolved Cd due to the presence of CdCln2−n complexes. Both solutions were subsequently used in an algal assay in 100mL beakers that contained only the solution (designated “−R”) or contained the solution together with a cation exchange sulfonate resin (2gL−1, designated “+R”) as a deposit on the bottom of the beaker. Pseudokirchneriella subcapitata was grown for 72h (1.4×105–1.4×106cellsmL−1) in stagnant solution and shaken three times a day. Growth was unaffected by the presence of the resin (p>0.05). The Cd concentrations in solution of the −R devices decreased with 50–58% of initial values due to Cd uptake. No such changes were found in the +R devices or in abiotic controls. Cd uptake was unaffected by either NaNO3 or NaCl treatment in the +R device, confirming that Cd2+ is the preferred Cd species in line with the general concept of metal bioavailability. In contrast, Cd uptake in the −R devices was two-fold larger in the NaCl treatment than in the NaNO3 treatment (p<0.001), suggesting that CdCln2−n complexes are bioavailable in this traditional set-up. However this bioavailability is partially, but not completely, an apparent one, because of the considerable depletion of solution 109Cd in this set-up. Resin-buffered solutions are advocated in the algal bottle assay to control trace metal supply and to better identify the role of metal complexes on bioavailability.