This study assessed spatial and seasonal variation in cadmium, lead, copper, zinc and mercury in the sea urchin Paracentrotus lividus and the mussel Mytilus galloprovincialis at three harbour stations on the Algerian west coast. Monthly sampling from January to December 2025 at one fixed station in each of Oran, Arzew and Mostaganem yielded 216 composite samples, accompanied by biometric and environmental measurements. Metals were quantified by atomic absorption spectrometry following microwave digestion. Spatial comparisons used station–month means with month as a blocking factor, supported by mixed models, robust tests and adjustment for body size. Inference was restricted to the three sampled stations because independent spatial replication within ports was absent. Both species exhibited the same concentration ranking, Oran > Arzew > Mostaganem, for all five metals, with significant pairwise differences after Tukey adjustment. Station and season effects were significant, without a significant interaction; concentrations peaked in summer at approximately 1.35 times winter levels. The station effect persisted after adjustment for body size. Mussels accumulated more cadmium, lead, zinc and mercury, whereas copper concentrations were higher in sea urchins. Allometric exponents were 2.774 for sea urchins and 2.932 for mussels. After adjustment for species and season and correction for multiple testing, metal concentrations remained associated with turbidity and suspended particulate matter at Oran and with temperature and dissolved oxygen at Mostaganem; no association persisted at Arzew. These relationships were interpreted as associations rather than evidence of contaminant pathways. The metal pollution index reproduced the spatial ranking, decreasing from 2.77 at Oran to 1.65 at Mostaganem. The findings document persistent differences among the sampled stations and complementary accumulation patterns in the two species, supporting their combined use in regional biomonitoring. Generalisation to entire ports requires additional independent stations.