<p>Calcifying organisms play essential roles in marine ecosystems and are valuable bioindicators of environmental conditions. This study investigates the variability in elemental composition within the calcitic skeletons of the barnacle <i>Amphibalanus improvisus</i> and bryozoan <i>Einhornia crustulenta</i>, inhabiting the low-salinity Baltic Sea. Seasonal analysis of 10 element/Ca ratios (Mg/Ca, Na/Ca, Sr/Ca, V/Ca, Mn/Ca, Co/Ca, Cu/Ca, Cd/Ca, Pb/Ca, U/Ca) revealed significant differences in elemental composition between the two species, indicating species-specific responses to the same environmental conditions. <i>A. improvisus</i> exhibited higher Na/Ca and Sr/Ca ratios, while other metals were present at higher levels in <i>E. crustulenta</i>. A dependence between Mg/Ca and temperature was found in <i>A. improvisus</i>, but not in <i>E. crustulenta</i>. Manganese variability in <i>A. improvisus</i> was linked to periods of high primary production and phytoplankton biomass, while no such trends were observed in <i>E. crustulenta</i>. The study also revealed differences in the incorporation of trace metals, including V/Ca, Co/Ca, Cu/Ca, Cd/Ca, Pb/Ca, and U/Ca, with species-specific accumulation patterns. For <i>A. improvisus</i>, trace element ratios showed consistent variation between periods and positive correlations with primary production, whereas no distinct patterns were observed for <i>E. crustulenta</i>. This study highlights the important role of biological factors in shaping the elemental composition of marine calcifiers while simultaneously underscoring their potential as bioindicators of environmental conditions, including temperature, salinity, and productivity. A comprehensive understanding of element/Ca variability in calcareous skeletons requires the simultaneous analysis of multiple chemical elements and their interrelationships.</p>