Water-mass structuring of tintinnid communities within a mesoscale eddy in the Kuroshio Extension

Mesoscale eddies create persistent hydrographic compartments that can reorganize microzooplankton assemblages, yet the extent to which tintinnid communities are structured by eddy-associated water masses remains unclear. We investigated tintinnid community organization across a mesoscale eddy in the Kuroshio Extension using two intersecting transects and a hydrography-based delineation of four water masses (Kuroshio Water, Eddy Center Water, Eddy Periphery Water, and Deep Water). Community composition showed clear water-mass structuring in ordination space and was supported by permutation-based tests. Importantly, compositional differentiation among water masses remained evident when comparisons were constrained to overlapping depth intervals, indicating that water-mass identity captured community variability beyond depth-related gradients. Alpha-diversity patterns were consistent with these shifts, with the eddy core exhibiting higher richness and diversity but lower evenness relative to surrounding waters. Species-level diagnostics further supported water-mass specificity: indicator-species analyses identified distinct assemblage signatures for each water mass, while SIMPER highlighted a recurrent subset of taxa contributing disproportionately to inter-water-mass dissimilarity. Decomposition of beta diversity suggested contrasting modes of within-water-mass variability across the eddy, characterized by stronger turnover in Kuroshio Water and greater richness-difference components within eddy waters. Collectively, these results identify eddy-associated water masses as a robust organizing framework for tintinnid community structure in the Kuroshio Extension.