The stable isotopic composition of particulate organic carbon
(
The marine environment is undergoing rapid changes as atmospheric carbon
dioxide increases, with the greatest change occurring in the upper ocean
(Gruber et al., 1999; Sabine and Tanhua, 2010). Anthropogenic carbon inputs
and the increase of greenhouse gases in the atmosphere are causing ocean
warming (Cheng et al., 2019), changes to upper ocean stratification (Bopp et
al., 2001; Capotondi et al., 2012), and altered distributions of nutrients
and carbon (Khatiwala et al., 2013; Quay et al., 2003; Gruber et al., 2019).
Marine phytoplankton are diverse, and are already responding to ocean
warming, including changes to productivity (Behrenfeld et al., 2006; Arrigo
and van Dijken, 2015), the length of growing season (Henson et al., 2018)
and phytoplankton cell size (Finkel et al., 2010). Alterations to
phytoplankton diversity and/or productivity will likely have knock-on
effects on marine food web dynamics. Investigating such changes in
remote marine environments requires tracers that can pinpoint shifts in
dietary sources. The
During photosynthesis, marine phytoplankton take up aqueous
The
Phytoplankton growth rate, cell size and cell geometry are also important
controls on
The carbon fixation pathway can vary amongst phytoplankton species through
the assimilation of bicarbonate via active transport as opposed to diffusive
All of the processes that alter the uptake fractionation of [
In this study we investigate the mechanisms for isotopic fractionation in
Samples were collected onboard the RRS
Measurements of total
[
Samples for the measurement of the stable isotopes of carbon in dissolved
inorganic carbon (
Particulate samples were collected onto ashed, pre-weighed GF/F microfibre
filters (0.7
Phytoplankton pigments were analysed by high-performance liquid chromatography (HPLC) analysis. Between 500 and 2000 mL of seawater was filtered through 25 mm GF/F filters. The filters were placed in 2 mL cryovials and flash frozen
in liquid nitrogen. Next, filters were transferred to a
The SSTC is characterized by the convergence of contrasting biogeochemical regimes. In the colder Subantarctic Surface Waters (SASW), located south of the SSTC, concentrations of macronutrients are elevated and primary production is primarily limited by iron availability (Browning et al., 2014). The subtropical waters to the north of the SSTC are associated with the South Atlantic subtropical gyre and are principally macronutrient limited, or possibly macronutrient–iron co-limited (Browning et al., 2014, 2017). The three subtropical water masses, the Agulhas Current (AC), the South Atlantic Central Water (SACW) and the Brazil Current (BC), can be readily identified by warmer temperatures and higher salinities; the influence of the Malvinas Current (MC) separates the core of the SACW and BC (Fig. 1).
Map and longitudinal transects across the south subtropical
convergence.
Higher [
Satellite images of surface chlorophyll concentrations across this region
indicate elevated standing stocks of phytoplankton in comparison with the
South Atlantic gyre and subantarctic waters further south (Browning et al.,
2014). Chlorophyll concentrations peak between austral spring and summer,
and the south subtropical convergence (SSTC) moves south as a result of the
expansion of the Agulhas and Brazil currents. Depth profiles showed that the
subantarctic waters have elevated and uniform chlorophyll concentrations
(0.2–0.9 mg m
If
Modelled
Using this model we tested how
Correlations between [
To investigate the spatial variability across the SSTC, [
Distribution of
To test whether cell size (and thus the cellular surface area to volume ratio)
plays an important role in determining
The fractional contribution of phytoplankton size classes to total
chlorophyll as estimated from phytoplankton pigments. The size classes are
defined as pico (
An estimate of the approximate average community cell size was calculated by
defining a specific cell size for each of the three defined size classes
(picophytoplankton were defined as 1
Estimated average cell radii were generally smaller at the core of the
subtropical water masses compared with the SASW (Fig. 5; depth range
The estimated average phytoplankton community cell radius. The
average radius (white contour lines) was calculated using the proportions of
pico-, nano- and microplankton in Fig. 4. We estimate the average radius
using assumed cellular radii of 0.5, 2.5 and 25
When
The samples with a larger estimated community cell size on both the east and
western margins were not included in this correlation analysis as they show
a significant offset from this relationship (Fig. 6b). These samples have
a larger estimated cell size compared with measured
South of the SSTC, the phytoplankton community is dominated by haptophytes
(Browning et al., 2014). A lower species diversity south of the front may
explain the closer alignment between [
The biological fractionation of carbon isotopes during uptake by
phytoplankton can be estimated using
Empirical estimates of
Variation in modelled and measured
We predict the variability in
If the flow of [
The influence of cell size on the expression of
The changing [
Global distributions of
In the low-latitude ocean, previous studies have shown that this trend
becomes decoupled: [
The results from our field study demonstrate that the phytoplankton
assemblage has a key role in determining
Thus, regions where frequent physical changes stimulate variable and diverse
phytoplankton assemblages may be more likely to have a decoupled
relationship between
Ambient [
Observational studies show rapid warming in the world's oceans in response to climate change, and that most of the ocean heat uptake is stored in the upper 75 m (Cheng et al., 2019). Predicted ocean warming trends are variable in different regions, with a greater rate of increase predicted in the polar regions (IPCC, 2013). Warming at the ocean surface promotes thermal stratification, which, in the Southern Ocean may decrease light limitation, whereas in the subtropics it is likely to promote further nutrient limitation (Sarmiento et al., 2004). Thus, climate change will promote varying responses from phytoplankton communities and their physiology across the global ocean (Rousseaux and Gregg, 2015).
In the oligotrophic gyres, ocean–atmosphere global climate models project increased stratification and decreases in net primary productivity with the onset of climate change (Boyd and Doney, 2002; Capotondi et al., 2012; Le Quere et al., 2003). Nutrients are already the limiting factor for net primary productivity and small cells are readily adapted to these oligotrophic environments, where recycled nutrients such as ammonium are the main nutrients available to phytoplankton (Fawcett et al., 2011). Many studies observe a shift to phytoplankton communities dominated by picoplankton as the water column becomes stratified and increasingly nutrient depleted (Atkinson et al., 2003; Bouman et al., 2003; Latasa and Bidigare, 1998; Lindell and Post, 1995; Irwin and Oliver, 2009). These observations suggest that the average community cell size may decrease further with ongoing climate change.
There may be large-scale shifts in community structure, including the
physiological dependencies of phytoplankton on light and nutrients and their
ecological diversity (Bouman et al., 2005; Behrenfeld et al., 2005; Siegel
et al., 2005). A decrease in cell size may lead to a faster pace of
metabolism (Brown et al., 2004). However, recent work suggests that
At higher latitudes, models predict increases in net primary production with
improved light availability in the mixed layer and an extended growing
season (Bopp et al., 2001; Sarmiento et al., 2004). Warming and reduction in
sea ice is likely to initiate an earlier onset of bloom with a predicted
5–10 d shift per decade (Henson et al., 2018). Therefore, in the
subantarctic ocean we may expect decreased light limitation, higher growth
rates and decreases in community cell size. The results from this study
demonstrate the different physiology of phytoplankton across the SSTC and
the expression of carbon uptake on
We use the results from this study to predict how the isotopic fractionation
during carbon uptake may alter with increased [
Projected changes in
In the subantarctic waters, although predicted
The sensitivity of
Seawater warming, which is expected to accompany future increases in
[
Stable isotope analysis of organic matter has emerged as the primary means
of examining marine food web structure and variability (Middelburg, 2014).
Carbon isotope signatures in particulate organic carbon vary substantially
from the relative influence of terrestrial and marine carbon, carbon uptake
pathways and the influence of carbon concentrating mechanisms (Jasper and
Gagosian, 1990; Ganeshram et al., 1999). In contrast to nitrogen isotopes,
there is negligible fractionation of carbon isotopes through trophic levels,
which allows for the accurate estimation of dietary sources of carbon (Minagawa
and Wada, 1984). Therefore, the factors that contribute to variability at
the base of the food web need to be well understood in order to accurately
comprehend marine food web dynamics (Peterson and Fry, 1987). These findings
could also have implications for the distribution of
This study highlights the importance of cell size as a primary determinant
of the extent of isotopic fractionation in particulate organic carbon during
uptake and the subsequent signature imparted at the base of the food web.
Our findings support previous work predicting increases in
We use our results from the field study to understand how increased
All carbon and isotope data have been submitted to the British Oceanographic Data Centre as part of the UK GEOTRACES programme. The data in this study are also available on request from the corresponding author.
The supplement related to this article is available online at:
RET analysed the
The authors declare that they have no conflict of interest.
We thank the crew and scientists of the RRS
This research has been supported by the NERC (grant no. NE/H008497/1NERC).
This paper was edited by Christoph Heinze and reviewed by two anonymous referees.