Cecelia C. S. Hannides

Environmental drivers of mesozooplankton biomass variability in the North Pacific Subtropical Gyre

The environmental drivers of zooplankton variability are poorly explored for the central subtropical Pacific, where a direct bottom-up food-web connection is suggested by increasing trends in primary production and mesozooplankton biomass at station ALOHA (A Long-term Oligotrophic Habitat Assessment) over the past 20 years (1994–2013). Here we use generalized additive models (GAMs) to investigate how these trends relate to the major modes of North Pacific climate variability. A GAM based on monthly mean data explains 43% of the temporal variability in mesozooplankton biomass with significant influences from primary productivity (PP), sea surface temperature (SST), North Pacific Gyre Oscillation (NPGO), and El Nino. This result mainly reflects the seasonal plankton cycle at station ALOHA, in which increasing light and SST lead to enhanced nitrogen fixation, productivity, and zooplankton biomass during summertime. Based on annual mean data, GAMs for two variables suggest that PP and 3–4 year lagged NPGO individually account for ~40% of zooplankton variability. The full annual mean GAM explains 70% of variability of zooplankton biomass with significant influences from PP, 4 year lagged NPGO, and 4 year lagged Pacific Decadal Oscillation (PDO). The NPGO affects wind stress, sea surface height, and subtropical gyre circulation and has been linked to mideuphotic zone anomalies in salinity and PP at station ALOHA. Our study broadens the known impact of this climate mode on plankton dynamics in the North Pacific. While lagged transport effects are also evident for subtropical waters, our study highlights a strong coupling between zooplankton fluctuations and PP, which differs from the transport-dominated climate influences that have been found for North Pacific boundary currents.

Migrant solution to the anammox mystery.

Every night a massive migration takes place in ocean waters. Zooplankton and micronekton, a diverse group of organisms <1 mm to 10 cm in size, swim up from midwater depths (200–700 m) to the surface ocean to feed and return down to resting depths before daybreak. The importance of this diel vertical migration to marine biogeochemical cycles has been recognized for some time. Research has primarily focused on the migrant-mediated movement of elements including nitrogen (N) from the surface ocean to midwaters. Such an “active transport” involves feeding at night in the upper ocean and the consequent excretion of ammonium (1) and dissolved organic N (2, 3) at midwater depths during the day. However, within the midwater realm, the effect of such migratory behavior on elemental cycles is less well understood. This “twilight zone” hosts many of the reactions that drive global biogeochemical cycles, including the remineralization of particulate organic matter into nutrients such as nitrate and the production of dinitrogen (N2) gas. In PNAS, Bianchi et al. (4) significantly advance our knowledge of the role of animals in midwater biogeochemistry by linking the activity of migrant zooplankton and micronekton to an important N transformation pathway, anammox.