Ocean alkalinity enhancement (OAE) via marine enhanced rock weathering (mERW) of olivine is a proposed strategy for atmospheric carbon dioxide removal (CDR). However, the potential co-release of trace metals to marine ecosystems, most notably nickel (Ni) and chromium (Cr), raises environmental concerns.</p> <p>Here, we investigated total alkalinity (A<sub>T</sub>) release, trace metal dynamics and metal bioaccumulation during mERW using two complementary approaches. First, a 28-day microcosm experiment showed that bioturbation by the lugworm <em>Arenicola marina</em> in pure olivine sand enhanced A<sub>T</sub> release nearly fivefold and more than doubled the release of dissolved Ni (DNi). Second, in a long-term (> 1 year) mesocosm experiment, we added fine-grained olivine at sediment loading levels of 6% and 10%, and coarse-grained olivine at 10%, to natural intertidal sediments inhabited by <em>A. marina</em>. We quantified sediment–water fluxes and porewater concentrations of A<sub>T</sub>, DNi, and DCr, and assessed metal accumulation in lugworm tissues. Olivine amendment increased sediment–water A<sub>T</sub> fluxes approximately two- to threefold, without significant effects of grain size or loading level. Fine-grained olivine treatments resulted in the highest DNi fluxes and the greatest Ni bioaccumulation in lugworms. Lugworm Ni concentrations increased up to tenfold relative to controls. In contrast, olivine amendment did not increase DCr fluxes and Cr bioaccumulation was minor compared to Ni.</p> <p>These results show that bioturbation can enhance olivine weathering and that olivine amendment can increase A<sub>T</sub> release from natural coastal sediments. However, finer grain size disproportionately increased Ni release and bioaccumulation without a detectable additional A<sub>T</sub> benefit. This decoupling suggests that A<sub>T</sub> generated by olivine weathering may have been partly offset by suppression of natural sedimentary A<sub>T</sub> generation. Under the conditions tested here, coarse olivine therefore provided a more favourable balance between net A<sub>T</sub> release and ecotoxicological risks from Ni exposure during mERW.