Assemblage organization over time prevents invasion success in phytoplankton

Abstract Invasion plays a central role in long‐term species coexistence, sometimes rejuvenating diversity and, at other times, collapsing it. Community resistance to invasions depends partly on species richness and structure, both of which change with community organization over time. Here, we examine the resistance of phytoplankton assemblages to invasion across varied initial species richness and structure, as well as assemblage organization over time. We hypothesize that invasion success will be higher in less organized assemblages and lower in more organized assemblages. To test this, we first experimentally constructed phytoplankton assemblages by mixing natural assemblages from regional lakes and manipulated richness along a dilutional gradient as part of a large‐scale mesocosm experiment. Phytoplankton assemblages from these mesocosms, varying by dilution rate, were selected and mixed to create unorganized assemblages of high and low richness. We considered these constructed phytoplankton assemblages unorganized because their structures were not the result of ecological interactions. For an invader, we used the green alga Golenkinia radiata , a taxon historically observed in the region, but absent from the natural assemblages at the time of sampling. We conducted four sequential invasion experiments, each lasting 7 days, initiated with increasingly organized assemblages from the mesocosms. We found that, while still unorganized early in succession, assemblages were vulnerable to invasion. However, as the assemblages organized with time, they became resistant to invasion. Assemblage richness, which ranged from 23 to 29 in the high‐richness mesocosms and from 15 to 24 in the low‐richness mesocosms, had only a marginal effect on invasion success. Instead, declining resources, notably phosphorus, reduced diversity, and the emerging dominance of strong competitors near the niche of the G. radiata invader explained the decreased success of invasions as the assemblages organized with time. In the more organized assemblages, G. radiata encountered formidable competitors in established populations of the diatom Nitzschia acicularis and the chrysophyte Synura sp., both of which had higher affinities for phosphorus than G. radiata . Our study highlights that assemblage organization with time plays a fundamental role in phytoplankton species coexistence, including stronger resistance to invasion as assemblages mature.