The seasonal cycle of mixed layer dynamics and phytoplankton biomass in the Sub-Antarctic Zone: A high-resolution glider experiment

Swart, Sebastiaan; Thomalla, Sandy J.; Monteiro, Pedro M. S.

doi:10.1016/j.jmarsys.2014.06.002

Cited by 110 publications

(167 citation statements)

References 52 publications

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“…Reasons for this growth limitation include the high iron requirements of the photosynthetic apparatus (Raven, 1990;Raven et al, 1999;Shi et al, 2007;Strzepek and Harrison, 2004) particularly under low-light conditions and a lack of iron sources (Duce and Tindale, 1991;Tagliabue et al, 2014). Phytoplankton blooms in the SAZ are characterized by high inter-annual and intra-seasonal variability with an extended duration that sustains high chlorophyll concentrations late into summer (Carranza and Gille, 2015;Racault et al, 2012;Swart et al, 2015;Thomalla et al, 2011Thomalla et al, , 2015. The longevity of these late summer blooms is unusual as iron limitation at this time of year is expected to be limiting growth (Boyd, 2002).…”

Section: Discussionmentioning

confidence: 99%

“…Instead, Tagliabue et al (2014) propose that biologically recycled iron within the mixed layer is the dominant mechanism for sustaining summertime blooms. However, there is now compelling evidence to suggest that storm events may also play a critical role in extending the duration of summertime production through intra-seasonal entrainment of dissolved iron from a subsurface reservoir (Carranza and Gille, 2015;Fauchereau et al, 2011;Swart et al, 2015;Thomalla et al, 2011). This mechanism was tested using a 1-D biogeochemical model by Nicholson et al (2016) whose results suggest that intra-seasonal mixed-layer perturbations may offer relief from iron limitation in summer, particularly if there is sufficient subsurface vertical mixing beneath the surface mixed layer.…”

Section: Discussionmentioning

confidence: 99%

“…Tagliabue et al (2014) postulated that due to weak diapycnal inputs of iron, there must be a heavy reliance on Fe recycling within the mixed layer to meet the iron demand. An alternative hypothesis is that summer storms can sustain mixed-layer biomass through the entrainment of limiting nutrients, particularly in the SAZ (Carranza and Gille, 2015;Nicholson et al, 2016;Swart et al, 2015). As a storm passes through the SAZ, it deepens the mixed layer accessing the subsurface iron reservoir; the subsequent re-shoaling of this buoyant water fuels surface water phytoplankton growth in a high-light and replenished-nutrient environment.…”

Section: T J Ryan-keogh Et Al: Seasonal Development Of Iron Limitamentioning

confidence: 99%

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Seasonal development of iron limitation in the sub-Antarctic zone

Ryan‐Keogh

Thomalla

Mtshali³

et al. 2018

Biogeosciences

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Abstract. The seasonal and sub-seasonal dynamics of iron availability within the sub-Antarctic zone (SAZ; ∼ 40-45 • S) play an important role in the distribution, biomass and productivity of the phytoplankton community. The variability in iron availability is due to an interplay between winter entrainment, diapycnal diffusion, storm-driven entrainment, atmospheric deposition, iron scavenging and iron recycling processes. Biological observations utilizing grow-out iron addition incubation experiments were performed at different stages of the seasonal cycle within the SAZ to determine whether iron availability at the time of sampling was sufficient to meet biological demands at different times of the growing season. Here we demonstrate that at the beginning of the growing season, there is sufficient iron to meet the demands of the phytoplankton community, but that as the growing season develops the mean iron concentrations in the mixed layer decrease and are insufficient to meet biological demand. Phytoplankton increase their photosynthetic efficiency and net growth rates following iron addition from midsummer to late summer, with no differences determined during early summer, suggestive of seasonal iron depletion and an insufficient resupply of iron to meet biological demand. The result of this is residual macronutrients at the end of the growing season and the prevalence of the high-nutrient low-chlorophyll (HNLC) condition. We conclude that despite the prolonged growing season characteristic of the SAZ, which can extend into late summer/early autumn, results nonetheless suggest that iron supply mechanisms are insufficient to maintain potential maximal growth and productivity throughout the season.

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Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Section: T J Ryan-keogh Et Al: Seasonal Development Of Iron Limitamentioning

confidence: 99%

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Seasonal development of iron limitation in the sub-Antarctic zone

Ryan‐Keogh

Thomalla

Mtshali³

et al. 2018

Biogeosciences

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“…This offers a higher degree of granularity than recent studies that have analyzed bulk seasonal differences in flow characteristics, which are then used to infer submesoscale activity. Both numerical models (Mensa et al 2013;Sasaki et al 2014) and observations (Callies et al 2015;Swart et al 2015;Buckingham et al 2016) provide strong evidence that there are seasonal modulations in submesoscale activity. In general, these reflect a steepening of the kinetic energy spectra during summer, typically with slopes around k…”

Section: Introductionmentioning

confidence: 93%

Open-Ocean Submesoscale Motions: A Full Seasonal Cycle of Mixed Layer Instabilities from Gliders

Thompson

Lazar

Buckingham

et al. 2016

Journal of Physical Oceanography

175

227

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The importance of submesoscale instabilities, particularly mixed layer baroclinic instability and symmetric instability, on upper-ocean mixing and energetics is well documented in regions of strong, persistent fronts such as the Kuroshio and the Gulf Stream. Less attention has been devoted to studying submesoscale flows in the open ocean, far from long-term, mean geostrophic fronts, characteristic of a large proportion of the global ocean. This study presents a year-long, submesoscale-resolving time series of near-surface buoyancy gradients, potential vorticity, and instability characteristics, collected by ocean gliders, that provides insight into open-ocean submesoscale dynamics over a full annual cycle. The gliders continuously sampled a 225 km 2 region in the subtropical northeast Atlantic, measuring temperature, salinity, and pressure along 292 short (;20 km) hydrographic sections. Glider observations show a seasonal cycle in near-surface stratification. Throughout the fall (September-November), the mixed layer deepens, predominantly through gravitational instability, indicating that surface cooling dominates submesoscale restratification processes. During winter (December-March), mixed layer depths are more variable, and estimates of the balanced Richardson number, which measures the relative importance of lateral and vertical buoyancy gradients, depict conditions favorable to symmetric instability. The importance of mixed layer instabilities on the restratification of the mixed layer, as compared with surface heating and cooling, shows that submesoscale processes can reverse the sign of an equivalent heat flux up to 25% of the time during winter. These results demonstrate that the openocean mixed layer hosts various forced and unforced instabilities, which become more prevalent during winter, and emphasize that accurate parameterizations of submesoscale processes are needed throughout the ocean.

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“…Weak diapycnal inputs and a heavy reliance on iron recycling was suggested by Tagliabue et al (2014) to match approximate phytoplankton utilization within the pelagic zones. An alternative theory that postulates the importance of summer storms may also be pivotal in understanding the seasonal dynamics of phytoplankton primary productivity (Nicholson et al, 2016;Swart et al, 2015;Thomalla et al, 2015), with respect to the sustained bloom observed in the sub-Antarctic Zone (SAZ). Here, summer storms are said to periodically deepen the mixed layer to below the ferricline followed by rapid shoaling during quiescent periods that balances the supply of light and iron in the upper oceans favouring phytoplankton growth that culminates in a sustained summer bloom .…”

Section: Introductionmentioning

confidence: 99%

Modelled estimates of spatial variability of iron stress in the Atlantic sector of the Southern Ocean

Ryan‐Keogh

Thomalla²,

Mtshali³

et al. 2017

Biogeosciences

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Abstract. The Atlantic sector of the Southern Ocean is characterized by markedly different frontal zones with specific seasonal and sub-seasonal dynamics. Demonstrated here is the effect of iron on the potential maximum productivity rates of the phytoplankton community. A series of iron addition productivity versus irradiance (PE) experiments utilizing a unique experimental design that allowed for 24 h incubations were performed within the austral summer of 2015/16 to determine the photosynthetic parameters α B , P B max and E k . Mean values for each photosynthetic parameter under iron-replete conditions were 1.46 ± 0.55 (µg (µg Chl a) −1 h −1 (µM photons m −2 s −1 ) −1 ) for α B , 72.55 ± 27.97 (µg (µg Chl a) −1 h −1 ) for P B max and 50.84 ± 11.89 (µM photons m −2 s −1 ) for E k , whereas mean values under the control conditions were 1.25 ± 0.92 (µg (µg Chl a) −1 h −1 (µM photons m −2 s −1 ) −1 ) for α B , 62.44 ± 36.96 (µg (µg Chl a) −1 h −1 ) for P B max and 55.81 ± 19.60 (µM photons m −2 s −1 ) for E k . There were no clear spatial patterns in either the absolute values or the absolute differences between the treatments at the experimental locations. When these parameters are integrated into a standard depth-integrated primary production model across a latitudinal transect, the effect of iron addition shows higher levels of primary production south of 50 • S, with very little difference observed in the subantarctic and polar frontal zone. These results emphasize the need for better parameterization of photosynthetic parameters in biogeochemical models around sensitivities in their response to iron supply. Future biogeochemical models will need to consider the combined and individual effects of iron and light to better resolve the natural background in primary production and predict its response under a changing climate.

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The seasonal cycle of mixed layer dynamics and phytoplankton biomass in the Sub-Antarctic Zone: A high-resolution glider experiment

Cited by 110 publications

References 52 publications

Seasonal development of iron limitation in the sub-Antarctic zone

Seasonal development of iron limitation in the sub-Antarctic zone

Open-Ocean Submesoscale Motions: A Full Seasonal Cycle of Mixed Layer Instabilities from Gliders

Modelled estimates of spatial variability of iron stress in the Atlantic sector of the Southern Ocean

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