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Decoupling Between Phytoplankton Growth and Microzooplankton Grazing Enhances Productivity in Subantarctic Waters on Campbell Plateau, Southeast of New Zealand
Author(s) -
GutiérrezRodríguez A.,
Safi K.,
Fernández D.,
ForcénVázquez A.,
Gourvil P.,
Hoffmann L.,
Pinkerton M.,
Sutton P.,
Nodder S. D.
Publication year - 2020
Publication title -
journal of geophysical research: oceans
Language(s) - English
Resource type - Journals
eISSN - 2169-9291
pISSN - 2169-9275
DOI - 10.1029/2019jc015550
Subject(s) - phytoplankton , plateau (mathematics) , oceanography , grazing , chlorophyll a , productivity , environmental science , biomass (ecology) , nutrient , environmental chemistry , biology , chemistry , ecology , botany , geology , mathematical analysis , mathematics , macroeconomics , economics
The Subantarctic zone is one of the largest High‐Nutrient Low‐Chlorophyll zones of the Southern Ocean. Despite widespread iron limitation, phytoplankton accumulation (chlorophyll a (chl a ) > 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 chl a commonly observed by satellites, we characterized phytoplankton biomass structure, production, and microzooplankton grazing on Campbell Plateau and surrounding waters in austral autumn (March 2017). Chl a on the plateau tended to be higher, more variable (0.52 ± 0.38 mg chl a m −3 , mean ± standard deviation), and characterized by larger phytoplankton forms (22 ± 27%chl a > 20 μm) than surrounding waters (0.29 ± 0.12 mg chl a 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 (σ PSII = 774 ± 90 Å RCII −1 ) on the plateau, suggests an alleviation of iron stress relative to surrounding waters (Fv/Fm = 0.37 ± 0.04, σ PSII = 974 ± 89 Å RCII −1 ). Phytoplankton growth (μ 0 = 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:μ 0 = 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:μ 0 = 0.48 ± 0.31). Overall, the degree of coupling tended to decrease with the depth of the mixed layer ( R 2 > 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.

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