The influence of a seasonal coastal current on the distribution of gelatinous zooplankton, in particular the siphonophore Muggiaea atlantica

Coastal currents can shape planktonic communities over regional scales. The Irish coastal current, a seasonal jet around southwest Ireland, is well studied in the context of phytoplankton, but its influence on gelatinous zooplankton remains poorly understood. A 7-day research survey assessed the gelatinous zooplankton community in relation to the current using in situ hydrographic measurements and multinet sampling (300 µm mesh, 0.25 m² opening). We identified 23 gelatinous zooplankton taxa, of which Aglantha digitale (2.56 ± 5.14 ind. m⁻³), Pleurobrachia pileus (1.00 ± 1.03 ind. m⁻³) and Nanomia sp. (0.65 ± 1.28 ind. m⁻³) were most abundant. Community composition differed significantly between inshore/coastal current and offshore waters, driven by A. digitale, P. pileus and Muggiaea atlantica. M. atlantica, a transient summer visitor, was significantly more abundant inshore (1.36 ± 1.85 ind. m⁻³) and in the current (0.56 ± 0.55 ind. m⁻³) than offshore (0.08 ± 0.16 ind. m⁻³). Our findings suggest the current may facilitate M. atlantica transport to Ireland, seeding inshore populations along the current trajectory. This highlights the potential role of smaller hydrodynamic features in structuring zooplankton communities and species connectivity in the Celtic Sea.