Mesozooplankton taxonomic composition and copepod functional structure across contrasting hydrographic settings on the Amazon Continental Shelf

Abstract

Abstract We examined variation in mesozooplankton taxonomic composition and copepod functional structure across contrasting hydrographic settings on the Amazon Continental Shelf. Samples collected at 51 stations during three oceanographic surveys conducted in 2001 yielded 197 taxa, with copepods representing the richest and most abundant group. Hydrographic conditions were characterized using in situ environmental measurements, local depth, and, for Prospec XIX, reanalysis-derived sea surface salinity as complementary information. Survey-specific distance-based redundancy analyses (dbRDA) revealed a significant environment–assemblage relationship in North IV, where temperature, salinity, dissolved oxygen, and local depth jointly explained 41.4% of the variation in mesozooplankton composition (adjusted R2 = 0.284, p < 0.001). In contrast, global environmental models were not significant for Prospec XIX or Prospec XX. Survey-sector assemblages differed markedly and broadly corresponded to plume-influenced, transitional shelf-break, and more oceanic settings, with pronounced compositional turnover and distinct sets of indicator copepod taxa. Copepod functional diversity was highest in North IV, whereas functional richness was reduced in outer-shelf and oceanic sectors. Despite substantial taxonomic turnover, assemblages shared a comparatively restricted set of dominant trait modalities, particularly omnivory, mixed feeding, and ambush feeding. Taxonomic and functional diversity metrics showed significant positive covariation. Across the shelf–ocean continuum, mesozooplankton composition and copepod functional structure differed among surveys, as did their associations with measured environmental variables. Given the sampling design, these associations do not provide evidence of a direct causal response to plume influence. The 2001 dataset provides a historical reference for future assessments of pelagic biodiversity on the Amazon Continental Shelf.