Abstract

Availability of dissolved iron and light are both regulating factors for primary productivity in high (macro)nutrient, low chlorophyll regions of the Southern Ocean. Here, using on-board iron/light incubation experiments conducted in 2015 in the Atlantic sector of the Southern Ocean, we show that irradiance limited significant phytoplankton growth (in chlorophyll-a and particulate organic carbon) north of the Polar Front (46 °S 08 °E), while iron addition resulted in growth stimulation even at low light levels in the Antarctic zone (65 °S 0 °E). The phytoplankton community in the Polar Frontal Zone showed a greater functional diversity than the one in the Antarctic Zone. The community structure changed over the course of the incubations in response to increased iron and light. The observed increase in chlorophyll-a under high light in the Polar Frontal Zone was driven predominantly by an increase in pico- (0.2-2 μm) and large (>5 μm) nanophytoplankton. Pigment fingerprinting indicated an increase in the contribution of diatoms and Phaeocystis over the course of the incubation. In contrast, in the Antarctic Zone, the increase in chlorophyll-a after iron enrichment was predominantly due to an increase in the contribution of diatoms and large nanophytoplankton. The photochemical efficiency (Fv/Fm) was low at both sites at the beginning of the incubations, but increased upon iron fertilization in both water masses, indicating stress relief. However, the acclimation strategies fundamentally differed between the two communities. The ratio of photoprotective versus light-harvesting pigments increased under high light in the Polar Frontal Zone independent of iron enrichment, whereas this ratio declined upon iron enrichment in the Antarctic Zone even under high light. At the same time, the functional cross section of photosystem II (σPSII) decreased upon iron enrichment in the Antarctic Zone, but not in the Polar Frontal Zone. Our experiments support the need to take biogeographical differences between Southern Ocean water masses into account when interpreting ecosystem dynamics.

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Phytoplankton composition at the end of each treatment for (a) incubation experiment S54-46 in the PFZ and (b) incubation experiment S54-65 in the AAZ. Initial = Initial community with no treatment; L1 = low light; L2 = high light; +Fe = iron enrichment.

Abstract

Active fluorescence measurements can provide rapid, non-intrusive estimates of phytoplankton primary production at high spatial and temporal resolution, but there is uncertainty in converting from electrons to ecologically relevant rates of CO2 assimilation. In this study, we examine the light-dependent rates of photosynthetic electron transport and 13C-uptake in the Atlantic sector of the Southern Ocean to derive a conversion factor for both winter (July 2015–August 2015) and summer (December 2015–February 2016). The results revealed significant seasonal differences in the light-saturated chlorophyll specific rate of 13C-uptake, (PBmax), with mean summer values 2.3 times higher than mean winter values, and the light limited chlorophyll specific efficiency, (αB), with mean values 2.7 times higher in summer than in winter. Similar patterns were observed in the light-saturated photosynthetic electron transport rates (ETRRCIImax, 1.5 times higher in summer) and light limited photosynthetic electron transport efficiency (αRCII, 1.3 times higher in summer). The conversion factor between carbon and electrons (Φe:C (mol e mol C-1)) was derived utilizing in situ measurements of the chlorophyll-normalized number of reaction centers (nRCII), resulting in a mean summer Φe:C which was 3 times lower than the mean winter Φe:C. Empirical relationships were established between Φe:C, light and NPQ, however they were not consistent across locations or seasons. The seasonal decoupling of Φe:C is the result of differences in Ek-dependent and Ek-independent variability, which require new modelling approaches and improvements to bio-optical techniques to account for these inter-seasonal differences in both taxonomy and environmental mean conditions.

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(a) Correlation of non photochemical quenching, NPQNSV, and Φe:C/nRCII (mol e- mol C-1 mol Chl a-1 mol RCII-1); (b) correlation of NPQNSV and Φe:C (mol e- mol C-1). Values of NPQNSV and Φe:C were derived from light response curves of 13C-uptake and FLC measurements. Linear regressions calculated for summer (excluding points on far left from ICE station—see Supporting Information Fig. S12): all p<0.001.

(a) Correlation of non photochemical quenching, NPQNSV, and Φe:C/nRCII (mol e mol C-1 mol Chl a-1 mol RCII-1); (b) correlation of NPQNSV and Φe:C (mol e mol C-1). Values of NPQNSV and Φe:C were derived from light response curves of 13C-uptake and FLC measurements. Linear regressions calculated for summer (excluding points on far left from ICE station—see Supporting Information Fig. S12): all p<0.001.

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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Fv/Fm (a, c, e) and chlorophyll a (Chl a) responses (mg m3) (b, d, f), from the control and Fe addition treatments of experiments initiated in the sub-Antarctic zone over early summer (a, b), midsummer (c, d) and late summer (e, f). Displayed here are averages with standard deviations (n = 3-5 for all time points, except the end time point where n = 6-12; see Table S1 for exact sample numbers). Please note the different scales in panels (a) and (b).

Fv/Fm (a, c, e) and chlorophyll a (Chl a) responses (mg m3) (b, d, f), from the control and Fe addition treatments of experiments initiated in the sub-Antarctic zone over early summer (a, b), midsummer (c, d) and late summer (e, f). Displayed here are averages with standard deviations (n = 3-5 for all time points, except the end time point where n = 6-12; see Table S1 for exact sample numbers). Please note the different scales in panels (a) and (b).

Abstract

In the Southern Ocean, increasing evidence from recent studies is highlighting the need for high-resolution sampling at fine spatial (meso- to sub-mesoscale) and temporal scales (intra-seasonal) in order to understand longer-term variability of phytoplankton and the controlling physical and biogeochemical processes. Here, high-resolution glider data (3 hourly, 2 km horizontal resolution) and satellite ocean colour data (2-4 km) from the Sub-Antarctic zone (SAZ) were used to 1) quantify the dominant scales of variability of the glider time series, 2) determine the minimum sampling frequency required to adequately characterise the glider time series and 3) discriminate how much of the variability measured with a glider is the result of temporal variations versus spatial patchiness. Results highlight the important role of signals shorter than 10 days in characterising surface chlorophyll (chl-a) variability , particularly in spring (97%) and to a lesser degree in summer (27%). These small scales of variability were also evident in the physical indices of SST, wind stress and mixed layer depth. Further analysis revealed that sampling at high frequencies (

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Box-and-whisker plots for the mean and standard deviation for subsampling at periodic frequencies (blue boxes) and the corresponding random extraction (red boxes) for the summer surface chl-a time series.

Box-and-whisker plots for the mean and standard deviation for subsampling at periodic frequencies (blue boxes) and the corresponding random extraction (red boxes) for the summer surface chl-a time series.

Abstract

Autonomous platforms will begin to address the space-time gaps required to improve estimates of phytoplankton distribution, which will aid in the quantification of baseline conditions necessary to detect long-term trends that can be attributed to factors such as climate change. However, there is a need for high quality controlled and verified datasets. In vivo fluorescence provides a proxy for chlorophyll pigment concentration, but it is sensitive to physiological downregulation under incident irradiance (fluorescence quenching). Quenching can undermine the validity of these datasets by underestimating daytime fluorescence derived chlorophyll across regional and temporal scales. Existing methods from the literature have corrected for quenching, however, these methods require certain assumptions to be made that do not hold true across all regions and seasons. The method presented here overcomes some of these assumptions to produce corrected surface fluorescence during the day that closely matched profiles from the previous (or following) night, decreasing the difference to less than 10%. This method corrects daytime quenched fluorescence using a mean nighttime profile of the fluorescence to backscattering ratio multiplied by daytime profiles of backscattering from the surface to the depth of quenching (determined as the depth at which the day fluorescence profile diverges from the mean night profile). This method was applied to a 7-month glider time series in the sub-Antarctic Southern Ocean together with four other methods from the literature for comparison. In addition, the method was applied to a glider time series from the North Atlantic to demonstrate its applicability to other ocean regions.

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Time series of surface midnight fluorescence (black line) and midday uncorrected fluorescence (grey line) from 28 July 2015 to 8 February 2016, together with midday corrected fluorescence using 5 different methods (Xing et al. 2012; Biermann et al. 2012; Swart et al. 2015; Hemsley et al. 2015; Thomalla et al. this study).

Time series of surface midnight fluorescence (black line) and midday uncorrected fluorescence (grey line) from 28 July 2015 to 8 February 2016, together with midday corrected fluorescence using 5 different methods (Xing et al. 2012; Biermann et al. 2012; Swart et al. 2015; Hemsley et al. 2015; Thomalla et al. this study).

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 24h incubations were performed within the austral summer of 2015/16 to determine the photosynthetic parameters αB, PBmax and Ek. 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 αB, 72.55 ± 27.97 (μg (μg Chl a)−1 h−1) for PBmax and 50.84 ± 11.89 (μM photons m−2 s−1) for Ek, 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 αB, 62.44 ± 36.96 (μg (μg Chl a)−1 h−1) for PBmax and 55.81 ± 19.60 (μM photons m−2 s−1) for Ek. 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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Depth-integrated primary production for each transect, including the mean for each treatment, the mean absolute differences between and the mean percentage difference between the treatments.

Depth-integrated primary production for each transect, including the mean for each treatment, the mean absolute differences between and the mean percentage difference between the treatments.

Abstract

Eleven incubation experiments were conducted in the South Atlantic sector of the Southern Ocean to investigate the relationship between new production (ρNO), regenerated production (ρNH+), and total carbon production (ρC) as a function of varying light. The results show substantial variability in the photosynthesis–irradiance (P vs E) parameters, with phytoplankton communities at stations that were considered iron (Fe)-limited showing low maximum photosynthetic capacity (Pmax) and low quantum efficiency of photosynthesis (αB) for ρNO3, but high Pmax and αB for ρNH4, with consequently low export efficiency. Results at stations likely relieved of Fe stress (associated with shallow bathymetry and the marginal ice zone) showed the highest rates of Pmax and αB for ρNO3 and ρC. To establish the key factors influencing the variability of the photosynthetic parameters, a principal components analysis was performed on P vs E parameters, using surface temperature, chlorophyll-a concentration, ambient nutrients, and an index for community size structure. Strong covariance between ambient nitrate (NO3) and αB for ρNO3 suggests that Fe and possibly light co-limitation affects the ability of phytoplankton in the region to access the surplus NOreservoir. However, the observed relationships between community structure and the P vs E parameters suggest superior performance by smaller-sized cells, in terms of resource acquisition and Fe limitation, as the probable driver of smaller-celled phytoplankton communities that have reduced photosynthetic efficiency and which require higher light intensities to saturate uptake. A noticeable absence in covariances between chlorophyll-a and αB, between Pmax and αB, and between temperature and αB may have important implications for primary-production models, although the absence of some expected relationships may be a consequence of the small dataset and low range of variability. However, significant relationships were observed between ambient NO3 and αB for ρNO3, and between the light-saturation parameter Ek for ρNO3 and the phytoplankton community’s size structure, which imply that Fe and light co-limitation drives access to the surplus NO3 reservoir and that larger-celled communities are more efficient at fixing NO3 in low light conditions. Although the mean Pmax results for ρC were consistent with estimates of global  production from satellite chlorophyll measurements, the range of variability was large. These results highlight the need for more-advanced primary-production models that take into account a diverse range of environmental and seasonal drivers of photosynthetic responses.

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Spatial distribution map of  Maximum photosynthetic capacity (PB max) for carbon uptake

Spatial distribution map of Maximum photosynthetic capacity (PB max) for carbon uptake

Thomalla S.J., Gilbert Ogunkoya, Vichi M., Swart S.
Abstract

One approach to deriving phytoplankton carbon biomass estimates (Cphyto) at appropriate scales is through optical products. This study uses a high-resolution glider data set in the Sub-Antarctic Zone (SAZ) of the Southern Ocean to compare four different methods of deriving Cphyto from particulate backscattering and fluorescence-derived chlorophyll (chl-a). A comparison of the methods showed that at low (<0.5 mg m−3) chlorophyll concentrations (e.g., early spring and at depth), all four methods produced similar estimates of Cphyto, whereas when chlorophyll concentrations were elevated one method derived higher concentrations of Cphyto than the others. The use of methods derived from particulate backscattering rather than fluorescence can account for cellular adjustments in chl-a:Cphytothat are not driven by biomass alone. A comparison of the glider chl-a:Cphyto ratios from the different optical methods with ratios from laboratory cultures and cruise data found that some optical methods of deriving Cphyto performed better in the SAZ than others and that regionally derived methods may be unsuitable for application to the Southern Ocean. A comparison of the glider chl-a:Cphyto ratios with output from a complex biogeochemical model shows that although a ratio of 0.02 mg chl-a mg C−1 is an acceptable mean for SAZ phytoplankton (in spring-summer), the model misrepresents the seasonal cycle (with decreasing ratios from spring to summer and low sub-seasonal variability). As such, it is recommended that models expand their allowance for variable chl-a:Cphyto ratios that not only account for phytoplankton acclimation to low light conditions in spring but also to higher optimal chl-a:Cphyto ratios with increasing growth rates in summer.

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Time-evolution of chl-a:Cphyto ratios at the surface (10 m) derived from the 30%POC method (solid lighter green top line), the B05 method (red line), the M13 method (blue line), and the S09 method (pink line). In addition, Chl-a:Cphyto ratios were calculated using the chl-a:POC ratio from the cruise data (which implies that all POC is phytoplankton specific) and is presented as 100%POC (darker green bottom dashed line). Included for comparision are (1) the chl-a:Cphyto ratios derived from the original equation from Sathyendranath et al. (2009) presented as S09original (purple line), (2) the satellite range of ratios from Behrenfeld et al. (2005) (black dotted lines) and (3) the ratios derived from the PELAGOS025 model (McKiver et al., 2015, extracted from the model for the same geographical co-ordinates as the glider transect in time but for a year 2011 simulation, solid black line). The inset shows a detail of the daily signal for the B05 method.

Time-evolution of chl-a:Cphyto ratios at the surface (10 m) derived from the 30%POC method (solid lighter green top line), the B05 method (red line), the M13 method (blue line), and the S09 method (pink line). In addition, Chl-a:Cphyto ratios were calculated using the chl-a:POC ratio from the cruise data (which implies that all POC is phytoplankton specific) and is presented as 100%POC (darker green bottom dashed line). Included for comparision are (1) the chl-a:Cphyto ratios derived from the original equation from Sathyendranath et al. (2009) presented as S09original (purple line), (2) the satellite range of ratios from Behrenfeld et al. (2005) (black dotted lines) and (3) the ratios derived from the PELAGOS025 model (McKiver et al., 2015, extracted from the model for the same geographical co-ordinates as the glider transect in time but for a year 2011 simulation, solid black line). The inset shows a detail of the daily signal for the B05 method.

Abstract

The Southern Ocean forms a key component of the global carbon budget, taking up about 1.0 Pg C yr−1 of anthropogenic CO2 emitted annually (∼10.7 ± 0.5 Pg C yr−1 for 2012). However, despite its importance, it still remains undersampled with respect to surface ocean carbon flux variability, resulting in weak constraints for ocean carbon and carbon – climate models. As a result, atmospheric inversion and coupled physics-biogeochemical ocean models still play a central role in constraining the air-sea CO2fluxes in the Southern Ocean. A recent synthesis study (Lenton et al., 2013a), however, showed that although ocean biogeochemical models (OBGMs) agree on the mean annual flux of CO2 in the Southern Ocean, they disagree on both amplitude and phasing of the seasonal cycle and compare poorly to observations. In this study, we develop and present a methodological framework to diagnose the controls on the seasonal variability of sea-air CO2 fluxes in model outputs relative to observations. We test this framework by comparing the NEMO-PISCES ocean model ORCA2-LIM2-PISCES to the Takahashi 2009 (T09) CO2 dataset. Here we demonstrate that the seasonal cycle anomaly for CO2fluxes in ORCA2LP is linked to an underestimation of winter convective CO2 entrainment as well as the impact of biological CO2 uptake during the spring-summer season, relative to T09 observations. This resulted in sea surface temperature (SST) becoming the dominant driver of seasonal scale of the partial pressure of CO2 (pCO2) variability and hence of the differences in the seasonality of CO2 sea-air flux between the model and observations.

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Global ocean summer and winter air-sea CO2 flux climatologies contrasting Takahashi, 2009 (T09) observations for reference year 2000 (a–b), and NEMO-PISCES (1993–2006) (c–d), units mmol C m−2 day. It shows seasonal climatological biases between the model and observations in the Southern Ocean.

Global ocean summer and winter air-sea CO2 flux climatologies contrasting Takahashi, 2009 (T09) observations for reference year 2000 (a–b), and NEMO-PISCES (1993–2006) (c–d), units mmol C m−2 day. It shows seasonal climatological biases between the model and observations in the Southern Ocean.

Abstract

In the Sub-Antarctic Ocean elevated phytoplankton biomass persists through summer at a time when productivity is expected to be low due to iron limitation. Biological iron recycling has been shown to support summer biomass. In addition, we investigate an iron supply mechanism previously unaccounted for in iron budget studies. Using a 1-D biogeochemical model, we show how storm-driven mixing provides relief from phytoplankton iron limitation through the entrainment of iron beneath the productive layer. This effect is significant when a mixing transition layer of strong diffusivities (kz > 10−4 m2 s−1) is present beneath the surface-mixing layer. Such subsurface mixing has been shown to arise from interactions between turbulent ocean dynamics and storm-driven inertial motions. The addition of intraseasonal mixing yielded increases of up to 60% in summer primary production. These results stress the need to acquire observations of subsurface mixing and to develop the appropriate parameterizations of such phenomena for ocean-biogeochemical models.

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Comparisons of (a and b) primary production, (c and d) DFe, and (e and f) integrated PP, surface PP*64, MLD, and surface DFe between the 'SXLD surface mixed-layer deepening' and the 'subsurface mixing run'.

Comparisons of (a and b) primary production, (c and d) DFe, and (e and f) integrated PP, surface PP*64, MLD, and surface DFe between the ‘SXLD surface mixed-layer deepening’ and the ‘subsurface mixing run’.

Ryan-Keogh T J, Liza M. DeLizo, Walker O. Smith Jr., Peter N. Sedwick, Dennis J. McGillicuddy Jr., C. Mark Moore, Thomas S. Bibby
Abstract

The bioavailability of iron influences the distribution, biomass and productivity of phytoplankton in the Ross Sea, one of the most productive regions in the Southern Ocean. We mapped the spatial and temporal extent and severity of iron-limitation of the native phytoplankton assemblage using long- (>24 h) and short-term (24 h) iron-addition experiments along with physiological and molecular characterisations during a cruise to the Ross Sea in December–February 2012. Phytoplankton increased their photosynthetic efficiency in response to iron addition, suggesting proximal iron limitation throughout most of the Ross Sea during summer. Molecular and physiological data further indicate that as nitrate is removed from the surface ocean the phytoplankton community transitions to one displaying an iron-efficient photosynthetic strategy characterised by an increase in the size of photosystem II (PSII) photochemical cross section (σPSII) and a decrease in the chlorophyll-normalised PSII abundance. These results suggest that phytoplankton with the ability to reduce their photosynthetic iron requirements are selected as the growing season progresses, which may drive the well-documented progression from Phaeocystis antarctica- assemblages to diatom-dominated phytoplankton. Such a shift in the assemblage-level photosynthetic strategy potentially mediates further drawdown of nitrate following the development of iron deficient conditions in the Ross Sea.

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Spatial distribution of iron stress response with concurrent variable fluorescence changes in long-term iron addition experiments.

Spatial distribution of iron stress response with concurrent variable fluorescence changes in long-term iron addition experiments.

Meredith, M., Swart S., Monteiro P.M.S., et al.
Abstract

The Southern Ocean exerts a disproportionately strong influence on global climate, so determining its changing state is of key importance in understanding the planetary-scale system. This is a consequence of the connectedness of the Southern Ocean, which links the other major ocean basins and is a site of strong lateral fluxes of climatically important tracers. It is also a consequence of processes occurring within the Southern Ocean, including the vigorous overturning circulation that leads to the formation of new water masses, and to the strong exchange of carbon, heat, and other climatically relevant properties at the ocean surface. However, determining the state of the Southern Ocean in a given year is even more problematic than for other ocean basins, due to the paucity of observations. Nonetheless, using the limited data available, some key aspects of the state of the Southern Ocean in 2014 can be ascertained.

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BAMS Sate of the Climate 2014 cover

BAMS Sate of the Climate 2014 cover

Thomalla S.J., Dr Marie-Fanny Racault, Swart S., Monteiro P.M.S.
Abstract

In the Southern Ocean, there is increasing evidence that seasonal to subseasonal temporal scales, and meso- to submesoscales play an important role in understanding the sensitivity of ocean primary productivity to climate change. This drives the need for a high-resolution approach to resolving biogeochemical processes. In this study, 5.5 months of continuous, high-resolution (3 h, 2 km horizontal resolution) glider data from spring to summer in the Atlantic Subantarctic Zone is used to investigate: (i) the mechanisms that drive bloom initiation and high growth rates in the region and (ii) the seasonal evolution of water column production and respiration. Bloom initiation dates were analysed in the context of upper ocean boundary layer physics highlighting sensitivities of different bloom detection methods to different environmental processes. Model results show that in early spring (September to mid-November) increased rates of net community production (NCP) are strongly affected by meso- to submesoscale features. In late spring/early summer (late-November to mid-December) seasonal shoaling of the mixed layer drives a more spatially homogenous bloom with maximum rates of NCP and chlorophyll biomass. A comparison of biomass accumulation rates with a study in the North Atlantic highlights the sensitivity of phytoplankton growth to fine-scale dynamics and emphasizes the need to sample the ocean at high resolution to accurately resolve phytoplankton phenology and improve our ability to estimate the sensitivity of the biological carbon pump to climate change.

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Time series of (a) modelled MLD and water column integrated NPP (mg C m-2 d-1), (b) modelled respiration (mg C m-2 d-1) (Sverdrup 1953), with standard mean error (shaded area), (c) same as for (c) but for NCP (mg C m-2 d-1), and (d) f-ratio approximation of the export efficiency (PP/mean NCP) (solid line).

Time series of (a) modelled MLD and water column integrated NPP (mg C m-2 d-1), (b) modelled respiration (mg C m-2 d-1) (Sverdrup 1953), with standard mean error (shaded area), (c) same as for (c) but for NCP (mg C m-2 d-1), and (d) f-ratio approximation of the export efficiency (PP/mean NCP) (solid line).

Thomalla S.J., Dr Marie-Fanny Racault, Swart S., Monteiro P.M.S.
Abstract

In the Southern Ocean, there is increasing evidence that seasonal to subseasonal temporal scales, and meso- to submesoscales play an important role in understanding the sensitivity of ocean primary productivity to climate change. This drives the need for a high-resolution approach to resolving biogeochemical processes. In this study, 5.5 months of continuous, high-resolution (3 h, 2 km horizontal resolution) glider data from spring to summer in the Atlantic Subantarctic Zone is used to investigate: (i) the mechanisms that drive bloom initiation and high growth rates in the region and (ii) the seasonal evolution of water column production and respiration. Bloom initiation dates were analysed in the context of upper ocean boundary layer physics highlighting sensitivities of different bloom detection methods to different environmental processes. Model results show that in early spring (September to mid-November) increased rates of net community production (NCP) are strongly affected by meso- to submesoscale features. In late spring/early summer (late-November to mid-December) seasonal shoaling of the mixed layer drives a more spatially homogenous bloom with maximum rates of NCP and chlorophyll biomass. A comparison of biomass accumulation rates with a study in the North Atlantic highlights the sensitivity of phytoplankton growth to fine-scale dynamics and emphasizes the need to sample the ocean at high resolution to accurately resolve phytoplankton phenology and improve our ability to estimate the sensitivity of the biological carbon pump to climate change.

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Abstract

In the Southern Ocean there is increasing evidence that seasonal to sub-seasonal temporal scales, meso- and submesoscales play an important role in understanding the sensitivity of ocean primary productivity to climate change. In this study, high-resolution glider data (3 hourly, 2km horizontal resolution), from ~6 months of sampling (spring through summer) in the Sub-Antarctic Zone, is used to assess 1) the different forcing mechanisms driving variability in upper ocean physics and 2) how these may characterize the seasonal cycle of phytoplankton production. Results highlight the important role meso- to submesoscale features have in driving vertical stratification and early phytoplankton bloom initiations in spring by increasing light exposure. In summer, the combined role of solar heat flux, mesoscale features and subseasonal storms on the extent of the mixed layer is proposed to regulate both light and iron to the upper ocean at appropriate time scales for phytoplankton growth, thereby sustaining the bloom for an extended period through to late summer. This study highlights the need for climate models to resolve both meso- to submesoscale and subseasonal processes in order to accurately reflect the phenology of the phytoplankton community and understand the sensitivity of ocean primary productivity to climate change.

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Glider sections of (a) temperature (°C), (b) stratification and (c) chlorophyll-a concentration (mg m-3) during the 'spring bloom initiation phase' of SOSCEx. The MLD is depicted using a white curve.

Glider sections of (a) temperature (°C), (b) stratification and (c) chlorophyll-a concentration (mg m-3) during the ‘spring bloom initiation phase’ of SOSCEx. The MLD is depicted using a white curve.

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