Hidden diversity in gelatinous zooplankton assemblages from selected Pacific Ocean ecosystems

Gelatinous and soft-bodied zooplankton (GZ) are critical components of almost all marine food webs. They act as intermediate predators and prey, contribute to nutrient cycling, and help shape microbial communities. Despite these characteristics, GZ were traditionally seen as ‘trophic dead-ends’ with limited ecosystem value. This view has changed in recent decades. As a result, improved representation of GZ in food web, fisheries, and biogeochemical models is now possible. Despite GZ assemblages being comparably well-studied in the Pacific Ocean, many questions beyond the taxonomic class or species levels remain unanswered. This dissertation aims to provide new insights into inter- and intraspecific differences in GZ biology, demography, stoichiometry, bioenergetics, and distribution in the subarctic Northeast Pacific and the Chatham Rise in the Southwest Pacific. This dissertation demonstrates that GZ organic content, elemental composition, and energy content vary among species, between life cycle stages, and with organism size. New species-resolved energy content data support previous in situ observations and reject the notion of GZ organisms as ‘trophic dead-ends’. GZ are also shown to utilise a stoichiometrically wide prey spectrum, challenging the linear carbon-to-phosphorus food web dogma. Moreover, in and ex situ observations of a widespread pelagic tunicate species (Salpa thompsoni), revealed size-specific growth rates and size- and stage-dependent differences in day and night vertical distributions. Corresponding demographic data suggest a life span shorter than previously estimated. Ontogenetic morphological changes were demonstrated to be highly comparable between several macromorphologically similar salp species. The salps’ abundance was subsequently shown to be sensitive to season, year, bottom depth, and some ocean-atmospheric indices. The causation and size- and stage-specific diel vertical migration (DVM) patterns of two cold-water salp species were studied using demography and size-frequency distribution data. The resulting analyses provide support for several DVM-related hypotheses. Overall, this dissertation contributes significantly to our understanding of GZ biology and ecology by highlighting key inter- and intraspecific characteristic differences. These new insights form a comprehensive baseline for the realistic parameterisation and inclusion of GZ in food web, fisheries, and biogeochemical models.Science, Faculty ofEarth, Ocean and Atmospheric Sciences, Department ofGraduat