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

Abstract

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 (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.

Planktonic protist diversity across contrasting Subtropical and Subantarctic waters of the southwest Pacific

“…The relative low diversity observed in the STF could be related to the increased phytoplankton biomass and productivity typically associated with the STF across the annual cycle (Murphy et al 2001; Pinkerton et al 2005) and the dominance of fewer ‘bloom-forming’ species in this highly productive zone (Chang and Gall 1998). The fact that species richness within STW was lowest during the more productive spring bloom conditions (TAN1212) is consistent with the view that more productive waters such as found in the STF, during the spring bloom, and locally on the Campbell Plateau (Gutiérrez-Rodríguez et al 2020) tend to decrease protistan diversity. However, the lower diversity associated with STF, relative to STW and SAW, was systematically observed across the different levels of nitrate and Chl a concentrations encompassed in this study (Figure S6) suggesting that other factors may contribute to this pattern.…”

“…In the S-STF, Chloropicon sieburthii made a substantial contribution in addition to the more dominant Chloroparvula pacifica (Figure S10). Whether this regional preference was linked to the bathymetric and hydrographic characteristics of the plateau (Forcén-Vázquez et al 2021; Neil et al 2004), the natural iron fertilization hypothesized for the region (Banse and English 1997; Gutiérrez-Rodríguez et al 2020) or a combination of these and other aspects cannot be concluded from our study. Moreover, an ASV belonging to this genus was also found to contribute substantially to protistan communities in coastal waters of the California Current Ecosystem (Gutierrez-Rodriguez et al 2019), highlighting the need of further studies to better understand the ecological drivers beyond coastal-oceanic trophic gradients responsible for the water mass preferences of such phytoplankton groups and species.…”

Planktonic protist diversity across contrasting Subtropical and Subantarctic waters of the southwest Pacific

“…In the S-STF, Chloropicon sieburthii made a substantial contribution in addition to the more dominant Chloroparvula pacifica (Figure S10). Whether this regional preference was linked to the bathymetric and hydrographic characteristics of the plateau (Forcén-Vázquez et al 2021;Neil et al 2004), the natural iron fertilization hypothesized for the region (Banse and English 1997;Gutiérrez-Rodríguez et al 2020) or a combination of these and other aspects cannot be concluded from our study. Moreover, an ASV belonging to this genus was also found to contribute substantially to protistan communities in coastal waters of the California Current Ecosystem (Gutierrez-Rodriguez et al 2019), highlighting the need of further studies to better understand the ecological drivers beyond coastal-oceanic trophic gradients responsible for the water mass preferences of such phytoplankton groups and species.…”

Salp blooms increase carbon export 5-fold in the Southern Ocean

“…Seawater collected with the Niskin bottles attached to the CTD rosette at 0200 h was used to fill a pair of 2.2-L polycarbonate bottles (100%, B and C) while a third bottle (A) was filled with 25% whole seawater diluted with 0.2-μm filtered seawater obtained immediately before by gravity filtration using an Acropak filter cartridge (Pall) directly from the same Niskin bottle. Nutrients (final concentrations in 2.2L bottles; nitrate 0.18 μM, ammonium 4.16 μM, phosphate 15.08, silicate 44.2 μM, and vitamins) were added to bottles A and B in order to ensure the assumption that the same phytoplankton intrinsic growth rate was occurring in WSW and FSW bottles despite dilution 39 . Bottles were then incubated in situ at the same six depths of collection using a drifting array.…”

“…After recovery, the entire content of the bottles were filtered onto 0.2-µm pore-size 25-mm polycarbonate filters and kept frozen until analysis. Once on land, filters were acidified with 200 µL 0.5 N HCl, Hi Safe 3 liquid scintillation cocktail was added and disintegrations per minute were then determined using a scintillation counter following procedures described elsewhere 39 .…”