Defining the ecologically relevant mixed‐layer depth for Antarctica's coastal seas

Carvalho, Filipa; Kohut, Josh; Oliver, Matthew J.; Schofield, Oscar

doi:10.1002/2016gl071205

Cited by 83 publications

(84 citation statements)

References 30 publications

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“…The bloom region is more distinctive when we consider the depth in the water column where the subsurface N 2 maximum occurs, which has been shown to be an ecologically relevant depth corresponding to phytoplankton vertical distribution (Carvalho et al, ). This is plotted for SOSE in Figure c and for individual Argo profiles in Figure d.…”

Section: Resultsmentioning

confidence: 99%

“…More recently, Sverdrup's critical depth hypothesis has been questioned for several reasons including its failure to distinguish between the hydrographically defined mixed layer and the actively turbulent layer (Behrenfeld, ; Carranza et al, ; Franks, ; Taylor & Ferrari, ). Still, despite the limitations of the critical depth hypothesis, mixed‐layer depth (MLD) has been shown to be closely linked to primary productivity in the Southern Ocean (Carvalho et al, ; Fragoso & Smith, ; Mitchell et al, ; Mitchell & Holm‐Hansen, ).…”

Section: Introductionmentioning

confidence: 99%

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Physical Drivers of Phytoplankton Bloom Initiation in the Southern Ocean's Scotia Sea

et al. 2019

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The Scotia Sea is the site of one of the largest spring phytoplankton blooms in the Southern Ocean. Past studies suggest that shelf‐iron inputs are responsible for the high productivity in this region, but the physical mechanisms that initiate and sustain the bloom are not well understood. Analysis of profiling float data from 2002 to 2017 shows that the Scotia Sea has an unusually shallow mixed‐layer depth during the transition from winter to spring, allowing the region to support a bloom earlier in the season than elsewhere in the Antarctic Circumpolar Current. We compare these results to the mixed‐layer depth in the 1/6° data‐assimilating Southern Ocean State Estimate and then use the model output to assess the physical balances governing mixed‐layer variability in the region. Results indicate the importance of lateral advection of Weddell Sea surface waters in setting the stratification. A Lagrangian particle release experiment run backward in time suggests that Weddell outflow constitutes 10% of the waters in the upper 200 m of the water column in the bloom region. This dense Weddell water subducts below the surface waters in the Scotia Sea, establishing a sharp subsurface density contrast that cannot be overcome by wintertime convection. Profiling float trajectories are consistent with the formation of Taylor columns over the region's complex bathymetry, which may also contribute to the unique stratification. Furthermore, biogeochemical measurements from 2016 and 2017 bloom events suggest that vertical exchange associated with this Taylor column enhances productivity by delivering nutrients to the euphotic zone.

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Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Physical Drivers of Phytoplankton Bloom Initiation in the Southern Ocean's Scotia Sea

et al. 2019

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“…NCP was calculated from seasonal changes in DIC as the depth‐integrated (surface to the MLD) difference between the winter value (DIC winter ) and the salinity‐normalized measured value. MLD was determined from the maximum of Brunt‐Väisälä buoyancy frequency (Carvalho, Kohut, Oliver, & Schofield, ). NCP and ice melt decrease DIC while organic matter remineralization increases DIC (Bates et al, ).…”

Section: Methodsmentioning

confidence: 99%

“…Surface ML light is expected to change in the future as upper ocean circulation, stratification, and sea ice cover continue to undergo rapid and dramatic changes (Martinson et al, ; Meredith et al, ; Moffat & Meredith, ; Schofield et al, ; Stammerjohn et al, ). In the long‐term future, the combination of these physical changes will likely lead to a deepening of the ML as sea ice is lost and winds increase, resulting in more variable and overall lower light in the ML (Carvalho et al, ; Meredith et al, ; Schofield et al, ; Turner et al, ; Vernet et al, ). Although dFe inputs are likely to increase with warming, a deeper ML would dilute available dFe (Annett et al, ).…”

Section: Introductionmentioning

confidence: 99%

Light Is the Primary Driver of Early Season Phytoplankton Production Along the Western Antarctic Peninsula

et al. 2019

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Light and dissolved iron (dFe) availability control net primary production (NPP) in much of the Southern Ocean, but the primary controller during spring in the western Antarctic Peninsula has never been assessed. Underwater light and dFe availability are sensitive to climate‐induced changes in upper ocean circulation, stratification, and sea ice cover, which can affect NPP and phytoplankton community composition, both of which can alter carbon drawdown and food web structure. We estimated in situ NPP, net community production, and heterotrophic respiration and contextualized our field measurements with satellite‐based historical NPP estimates. Average light exposure mainly controlled NPP, while low dFe was associated with greatest NPP, indicating that spring phytoplankton growth is light‐limited and not dFe‐limited. Using experiments that simulated varying mixed layer depths by exposing phytoplankton to a short period of in situ surface light (up to 150× the mean light in the mixed layer, comparable to the difference in light experienced by phytoplankton mixed from 50 m to the surface), we assessed the effect of phytoplankton photoacclimation on NPP and relative success of individual taxa. At moderate light exposure (<30×), phytoplankton experienced little photodamage or changes in NPP, and Phaeocystis antarctica grew more than diatoms. Conversely, phytoplankton exposed to high light (>60×) experienced significant photodamage, declines in NPP, and declines in P. antarctica, with no consistent changes in diatoms. These results support the idea that P. antarctica is better adapted to variable light than diatoms and suggest that deeper mixed layers with variable light will favor P. antarctica.

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“…Mixed layer depth (MLD) was defined using the depth at which the Brunt-Väisälä frequency (BVF) was highest, as recommended recently for the WAP (Carvalho et al, 2017). Ocean Data View v4.7.8 was used to calculate the BVF (Schlitzer, 2016).…”

Section: Water Column Stability and Stable Oxygen Isotopesmentioning

confidence: 99%

Assessing Drivers of Coastal Primary Production in Northern Marguerite Bay, Antarctica

Rozema

Kulk

Veldhuis³

et al. 2017

Front. Mar. Sci.

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The coastal ocean of the climatically-sensitive west Antarctic Peninsula is experiencing changes in the physical and (photo)chemical properties that strongly affect the phytoplankton. Consequently, a shift from diatoms, pivotal in the Antarctic food web, to more mobile and smaller flagellates has been observed. We seek to identify the main drivers behind primary production (PP) without any assumptions beforehand to obtain the best possible model of PP. We employed a combination of field measurements and modeling to discern and quantify the influences of variability in physical, (photo)chemical, and biological parameters on PP in northern Marguerite Bay. Field data of high-temporal resolution (November 2013-March 2014) collected at a long-term monitoring site here were combined with estimates of PP derived from photosynthesis-irradiance incubations and modeled using mechanistic and statistical models. Daily PP varied greatly and averaged 1,764 mg C m −2 d −1 with a maximum of 6,908 mg C m −2 d −1 after the melting of sea ice and the likely release of diatoms concentrated therein. A non-assumptive random forest model (RF) with all possibly relevant parameters (M RFmax ) showed that variability in PP was best explained by light availability and chlorophyll a followed by physical (temperature, mixed layer depth, and salinity) and chemical (phosphate, total nitrogen, and silicate) water column properties. The predictive power from the relative abundances of diatoms, cryptophytes, and haptophytes (as determined by pigment fingerprinting) to PP was minimal. However, the variability in PP due to changes in species composition was most likely underestimated due to the contrasting strategies of these phytoplankton groups as we observed significant negative relations between PP and the relative abundance of flagellates groups. Our reduced model (M RFmin ) showed how light availability, chlorophyll a, and total nitrogen concentrations can be used to obtain the best estimate of PP (R 2 = 0.93). The resulting estimates from our models suggest summer PP to have been between 214.4 and 176.1 g C m −2 . Through the employment of a modeling technique without any assumptions apart from a representative sampling strategy, we showed and estimated how PP in this climatically sensitive and changing region can best be predicted and described.

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Defining the ecologically relevant mixed‐layer depth for Antarctica's coastal seas

Cited by 83 publications

References 30 publications

Physical Drivers of Phytoplankton Bloom Initiation in the Southern Ocean's Scotia Sea

Physical Drivers of Phytoplankton Bloom Initiation in the Southern Ocean's Scotia Sea

Light Is the Primary Driver of Early Season Phytoplankton Production Along the Western Antarctic Peninsula

Assessing Drivers of Coastal Primary Production in Northern Marguerite Bay, Antarctica

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