First Record of the Non‐Indigenous Copepod Acartia ( Acanthacartia ) tonsa Dana, 1849 and Its Spatio‐Temporal Co‐Occurrence With the Native Acartia ( Acartiura ) clausi Giesbrecht, 1889 Along an Anthropogenic Gradient on the Algerian Coast (South‐Western Mediterranean Sea)

Non‐indigenous copepods are increasingly reported in the Mediterranean Sea, where anthropogenically disturbed coastal environments may facilitate their establishment and their co‐occurrence with native congeners. This study provides the first record of the non‐indigenous calanoid Acartia tonsa along the Algerian coast and examines its spatio‐temporal distribution relative to the native A. clausi . Zooplankton was sampled during four seasonal campaigns covering a complete annual cycle at four coastal stations of Bordj El Kiffan (eastern Algiers Bay, 36°45′ N, 3°22′ E), arranged along an anthropogenic pressure gradient from a brine‐discharge outlet to an offshore reference station. We hypothesised that (1) disturbed conditions generated by brine discharge and coastal enrichment favour the establishment of A. tonsa , and (2) as an early‐stage invader, A. tonsa currently co‐occurs with A. clausi with a high degree of niche overlap rather than excluding it. A. tonsa was recorded at all stations and seasons (4.80–5280 adults m −3 ), with a marked summer maximum, and consistently reached higher abundances than A. clausi (5.60–371.20 adults m −3 ). Both species peaked in summer and were positively associated with temperature, and niche‐overlap indices remained high throughout the annual cycle, indicating spatial and temporal co‐occurrence rather than exclusion. The relative abundance of the two congeners decreased from the enriched nearshore stations towards the offshore reference station, linking the dominance of A. tonsa to the local anthropogenic gradient.