Decoupling Between Phytoplankton Growth and Microzooplankton Grazing Enhances Productivity in Subantarctic Waters on Campbell Plateau, Southeast of New Zealand

Abstract

AbstractThe Subantarctic zone is one of the largest High‐Nutrient Low‐Chlorophyll zones of the Southern Ocean. Despite widespread iron limitation, phytoplankton accumulation (chlorophyll a (chla) > 0.3 mg m−3) often occurs near islands and bathymetric features such as on the Campbell Plateau, southeast of New Zealand. To investigate the processes responsible for localized increases in chla commonly observed by satellites, we characterized phytoplankton biomass structure, production, and microzooplankton grazing on Campbell Plateau and surrounding waters in austral autumn (March 2017). Chla on the plateau tended to be higher, more variable (0.52 ± 0.38 mg chla m−3, mean ± standard deviation), and characterized by larger phytoplankton forms (22 ± 27%chla > 20 μm) than surrounding waters (0.29 ± 0.12 mg chla m−3, 5 ± 2%). The increased contribution of diatoms, together with higher photosystem II maximum photochemical efficiency (Fv/Fm = 0.45 ± 0.05) and lower effective absorption cross‐section (σPSII = 774 ± 90 Å RCII−1) on the plateau, suggests an alleviation of iron stress relative to surrounding waters (Fv/Fm = 0.37 ± 0.04, σPSII = 974 ± 89 Å RCII−1). Phytoplankton growth (μ0 = 0.42 ± 0.20 day−1) and production rates (6.1 ± 3.2 mg C m−3 day−1) were also higher compared to surrounding waters (0.27 ± 0.04 day−1, 3.5 ± 1.9 mg C m−3 day−1). While microzooplankton grazing (g = 0.28 ± 0.18 day−1) balanced phytoplankton growth off the plateau (g:μ0 = 1.13 ± 0.18), the imbalance observed on Campbell Plateau (g = 0.25 ± 0.25 day−1) allowed a substantial proportion of primary production to escape microzooplankton grazing control (g:μ0 = 0.48 ± 0.31). Overall, the degree of coupling tended to decrease with the depth of the mixed layer (R2 > 0.6, p < 0.001). We hypothesize that the entrainment of deeper water into the mixed layer regulates the onset and fate of the autumn bloom by altering nutrient supply and microzooplankton grazing pressure.