Seasonal phenology of appendicularian tunicates in the North Water, northern Baffin Bay

Appendicularian tunicates are marine, gelatinous zooplankton that are important grazers of protists and phytoplankton in Arctic polynyas. However, little is known at the seasonal scale about their life cycle and population phenology in the Arctic. The temporal and spatial pattern of abundance and biomass, body size, and maturity stage were determined over two cruises representing an entire productive season of the North Water (NOW), to determine when and where populations spawn, and whether population growth tracks the expansion of open water and the development of diatom blooms. Oikopleura vanhoeffeni began to spawn in April and May, in association with an early diatom bloom caused by the opening of the Jones Sound and Inglefield Bay polynyas in the central NOW. Fritillaria borealis began population growth in June, during the transition from the early diatom bloom to an extended bloom of Chaetoceros socialis (‘C-bloom’). Subsequent spawning and population growth of both species followed the progression of open water and the developing C-bloom, to the north and then south during June–September. Maximum abundance and biomass of both species occurred during September, supported by the C-bloom prey community. Temperature had a strong, positive relationship with the abundance and biomass of both species, and moderate-to-high concentrations of diatoms seemed necessary for the initiation of spawning in O. vanhoeffeni . Appendicularians matured quickly in the early-opening polynyas in the NOW despite very low water temperatures and achieved high population growth rates in response to diatoms produced over an extended growing season (April–September).