Seasonal and spatial changes in carbon and nitrogen fluxes estimated using 234Th:238U disequilibria in the North Pacific tropical and subtropical gyre

Abstract

Particle dynamics are an essential component of global ocean biogeochemistry as they transport essential nutrients, carbon, and other reactive elements and compounds from the surface ocean to depth in the water column. The North Pacific Ocean is characterized by spatial and temporal variations in particle export mediated by a diverse food web and variations in environmental conditions, such as oxygenation. Here we explored temporal variability in the downward flux of particulate carbon (PC) and nitrogen (PN) using 238U234Th disequilibria, sediment traps and in situ pumps in winter, spring and summer at the time-series site Station ALOHA, a region characterized by a well-defined summer export pulse that influences the composition and structure of the biological community within the mesopelagic zone. We further explored spatial variability in PC and PN fluxes along a latitudinal gradient (17.5°-5°N x 150°W) that transitions from a low to high productivity region influenced by equatorial upwelling, with several stations further characterized by a shallow (~ 130 m) oxygen minimum zone. Winter PC and PN fluxes at 150 m at Station ALOHA were low, while summer and spring fluxes were significantly higher, coinciding with a seasonal export pulse associated with diazotrophs. PC and PN fluxes along the 155°W transect were also low at 150 m and similar to those measured at Station ALOHA in winter. At Station ALOHA zooplankton impart a greater influence over both small and large PC (and PN) fluxes in February relative to September or May, when heterotrophic bacteria play a proportionally larger role in particle remineralization and cycling. Along the transect stations, PC fluxes were too low to discern any clear trends with latitude, likely due to El Niño conditions at the time of sampling. Where vertical water column profiles of PC and PN fluxes were available, PC (and PN) fluxes were found to peak in the subsurface at 8°N (50 m) and 5°N (75 m) with zooplankton grazing and microbial remineralization following patterns similar to those found in February at Station ALOHA. Combined, these results support the hypothesis that small and large particles contribute to mesopelagic carbon demand depending on season, with smaller particles having greater contributions to mesopelagic food webs when surface derived particle export is low.