Morphology, distribution and assemblage impacts of three non-indigenous foraminiferal species on the Swedish west coast: Implications for shipping-related biopollution

Global shipping facilitates introductions of marine non-indigenous species (NIS), a form of biological pollution. Yet, microeukaryotes such as benthic foraminifera (Rhizaria) remain underrepresented in bioinvasion surveys, though they can significantly impact ecosystem functioning. Here we present new occurrence data on three foraminiferal NIS (Ammonia confertitesta, Trochammina hadai, and Nonionella sp. T1) from 12 Swedish west coast sites, including a fjord, commercial harbors, and small marinas. The new data are combined with a regional synthesis of all published records to date for these NIS in Scandinavia and northern Germany. We compile and map regional NIS occurrences, provide operational morphological criteria for NIS identification, and assess their spatial distribution, relative abundances, and impact on assemblage structure and diversity. Nonionella sp. T1 dominates foraminiferal communities in the Scandinavian fjords (e.g., up to 70-75% of assemblages in Gullmar Fjord) and is associated with pronounced declines in evenness and diversity. In contrast, A. confertitesta and T. hadai are strongly associated with harbors, cumulatively reaching 50-70% of living assemblages in industrial harbors while remaining less abundant in small boat harbors. At low to moderate abundance (≤∼20%), these two species increase total foraminiferal abundances and can enhance foraminiferal diversity, whereas their abundance >∼20% are linked to reduced evenness and diversity, indicating biotic homogenization. These patterns, together with NIS tolerance of organic enrichment, hypoxia and metal-contaminated sediments, suggest that foraminiferal NIS are tightly coupled to pollution gradients in harbors and fjords and hold promise as bioindicators of shipping pressure, pollution, and bioinvasions.