The Weddell Sea represents a point of origin in the Southern Ocean where
globally important water masses form. Biological activities in Weddell Sea
surface waters thus affect large-scale ocean biogeochemistry. During
January–February 2019, we measured net primary production (NPP), nitrogen
(nitrate, ammonium, urea) uptake, and nitrification in the western Weddell
Sea at the Antarctic Peninsula (AP) and Larsen C Ice Shelf (LCIS), in the
southwestern Weddell Gyre (WG), and at Fimbul Ice Shelf (FIS) in the
south-eastern Weddell Sea. The highest average rates of NPP and greatest
nutrient drawdown occurred at LCIS. Here, the phytoplankton community was
dominated by colonial
The Southern Ocean is an important driver of Earth's climate as it
transports large quantities of heat and dissolved gases and supplies
65 %–85 % of the global ocean's nutrients
(Keffer
and Holloway, 1988; Sarmiento et al., 2004; Frölicher et al., 2015;
Keller et al., 2016; Fripiat et al., 2021). Despite the Southern Ocean's
central role in atmospheric
The Weddell Sea is separated from the Antarctic Circumpolar Current (ACC) and open Southern Ocean by the Weddell Sea fronts (Orsi et al., 1995). The general large-scale circulation takes the form of the cyclonic, wind-driven, and topographically steered Weddell Gyre (WG) (Fahrbach et al., 1994, 1995; Orsi et al., 1995). The production of bottom water is thought to occur at two sites in the Weddell Sea: at Filchner–Ronne Ice Shelf (FRIS) and Larsen C Ice Shelf (LCIS) (Gordon et al., 1993; Schröder et al., 2002; Schodlok et al., 2002). Here, Modified Warm Deep Water (MWDW) intrudes onto the continental shelf and mixes with Antarctic Surface Water (ASW), which alters its physical and chemical properties, ultimately resulting in the formation of dense bottom waters. Upon reaching the Antarctic Peninsula (AP), the transformed bottom waters either spill out over the shelf and re-enter the ACC or are entrained into the eastward-flowing limb of the WG (Orsi et al., 1993; Locarnini et al., 1993).
The surface waters of the open Weddell Sea are warm and
saline, while those over the continental shelf are relatively cool and fresh
(Nicholls et al., 2004). These different
waters are separated by the Antarctic Slope Front
(ASF; Jacobs, 1986, 1991), a fast-flowing
jet situated between the 500 and 1000 m isobath that separates the Open
Ocean Zone (OOZ) from the Coastal and Continental Shelf Zone (CCSZ;
Jacobs, 1986, 1991; Muench and
Gordon, 1995). The Antarctic CCSZ has been observed to host high rates of
productivity in the summer (e.g.
Smith and Nelson, 1990; Arrigo et
al., 2008) as melting sea ice supplies dissolved iron and increases water-column stratification, yielding favourable conditions for phytoplankton
growth (Lannuzel et al., 2008). Inputs of
dissolved iron from continental shelf sediments and coastal runoff further
elevate the ambient iron concentrations, such that the CCSZ seldom
experiences iron depletion
(Klunder
et al., 2014; Dinniman et al., 2020). As a result, the large phytoplankton
blooms of the CCSZ can at times almost completely deplete the surface
nitrate concentrations
(Jennings
et al., 1984; Hoppema et al., 2000; Henley et al., 2017), supporting high rates
of carbon export that fuel the benthic community on the underlying
continental shelf
(Isla
et al., 2006, 2011; Pineda-Metz et al., 2019) and/or eventually lead to
long-term storage of atmospheric
On an annual basis, phytoplankton growth in the euphotic zone that is
fuelled by nitrate supplied from below (i.e. “new production”) must be
balanced by the export of sinking organic matter into the ocean interior
(i.e. “export production”), thus driving
Since phytoplankton in the CCSZ of the Weddell Sea consume much of the
nitrate supplied to the surface
(Jennings et al., 1984;
Hoppema et al., 2000), they should, by mass balance, drive the export of a
significant amount of atmospheric
Observations suggest that Weddell Sea phytoplankton blooms are initially
dominated by smaller species (e.g.
The Weddell Sea is particularly understudied near LCIS where thick-sea-ice
conditions persist all year round. To our knowledge, the only biogeochemical
study conducted in the vicinity of LCIS was undertaken in the austral summer
of 1992/93. Using measurements of nutrient depletion, Hoppema et al. (2000)
estimated primary production in the vicinity of LCIS to be
47.5–95
Sampling was conducted in January–February 2019 during the Weddell Sea
Expedition on board the R/V
Maps of the Weddell Sea, Larsen C Ice Shelf (LCIS; insert
Seawater was collected from discrete depths using a rosette of twenty-four
12 L Niskin bottles. At each station, seawater samples for nutrient analysis
were collected throughout the water column (typically at 15 discrete
depths), while samples for phytoplankton taxonomy and rate experiments were
taken from three to six depths (see below) that were selected based on profiles of
temperature, chlorophyll
Simulated in situ experiments were conducted to determine the rates of net primary
production (NPP), N uptake (as nitrate (
Seawater samples for the
The net decrease in euphotic zone nutrient concentrations following nutrient
recharge in winter (i.e. the extent of nutrient depletion due to
consumption by phytoplankton), between the start of the growing season until
the time of our sampling, can be estimated for each station as
Seasonal melting of sea ice in the Weddell Sea introduces low-salinity,
low-nutrient waters that dilute the biogeochemistry of the mixed layer
(Eicken, 1993), potentially leading to an
overestimation of phytoplankton-driven nutrient depletion. We correct for
the depletion in the surface [
Depth profiles of
Incubation filters were oven-dried for 24 h at 40
The specific rates of carbon fixation (
The
To determine the relative carbon export potential at each station, we calculated
the
At all stations, microphytoplankton samples were collected between the
surface and 30 m using a HYDRO-BIOS conical plankton net (
Onshore, each preserved net sample was homogenized, and one drop
(40
An aliquot of 5 mL from each preserved sample was cleaned by removing
carbonate particles and organic matter using 10 % hydrochloric acid and
37 % hydrogen peroxide, respectively. After thorough rinsing with
distilled water, permanent slides were prepared by pipetting the cleaned
material onto acid-washed coverslips, air-drying them overnight, and
mounting the cover slips onto glass slides using Naphrax® mountant (refractive index is 1.7). The permanent slides were examined
using a Zeiss Axioscope A1 LM equipped with differential interference
contrast at
The average size (
Flow cytometry samples were analysed using a BD LSR II SORP flow cytometer
with blue–red–green laser configuration. The size class to which each cell
belonged was defined based on its forward scatter area (FSC-A) relative to
the FSC-A of 2.8 and 20
Throughout the study region, relatively cool and fresh (
Variability in the density of ASW was observed among the stations (Fig. 2a). The surface density profiles at the AP, WG, and early-summer FIS stations were very similar, while the late-summer density profile at FIS revealed lower-density waters in the upper 100 m. At LCIS, the surface density profiles were highly variable, and no consistent pattern was observed, although the most northern stations (L9 and L10, Fig. 1) were characterized by the lowest densities. Stations L1 and L3, situated closest to the ice shelf, were characterized by the highest densities, contiguous with the underlying WW layer.
The MLD appeared most strongly controlled by salinity at all stations and
was always shallower than the depth of the euphotic zone (
The concentrations of the regenerated N forms (i.e.
Depth profiles (0–500 m) of
The concentrations of
The euphotic zone concentrations of
Depth profiles (0–150 m) of
Variations in the depletion ratios of
The highest concentrations of POC and PON were observed in the surface at
all stations (Fig. 5a and b), decreasing towards
Bar plots of
Euphotic zone-averaged nutrient concentrations, nutrient
depletions, and nutrient depletion ratios at each station occupied in the
Weddell Sea in January–February 2019. Values shown are averages
Euphotic-zone-integrated and averaged rates at each station
occupied in the Weddell Sea in January–February 2019. Values shown are
averages
At all stations, NPP was generally highest at the surface (Fig. 6a) and
decreased towards
Daily rates of
As per NPP, the rates of
At all stations, rates of
Rates of
Rates of
Depth profiles of
At the stations where urea uptake was measured (LCIS stations and WG1, 11
out of 19 stations; Fig. 6, Table 2),
Equation (7) may overestimate urea uptake at some of the stations,
particularly where low urea concentrations were measured. Theoretically,
Euphotic-zone-integrated
The euphotic-zone-integrated
The flow cytometry data show that the phytoplankton community was
numerically dominated by picoplankton at all stations, with
The
From the samples collected using the phytoplankton net (i.e. single cells
or colonies
For the regions of the Weddell Sea that we sampled in summer 2019, the euphotic-zone-integrated rates of NPP and N uptake were generally lower at the OOZ stations than at the CCSZ stations, with the highest depth-specific uptake rates observed in surface waters at LCIS (Fig. 6a–d, Table 2). The few studies that have previously measured summertime rates of NPP and N uptake in the Weddell Sea report similar results, with rates in the marginal ice zone (MIZ) and CCSZ that were up to 5 times higher than in the OOZ (El-Sayed and Taguchi, 1981; Smith and Nelson, 1990; Park et al., 1999). The summertime CCSZ of the Weddell Sea can thus be broadly characterized as a highly productive region with elevated biomass accumulation driven by increased water-column stratification and iron-replete conditions, both the result of sea-ice melt (Semeneh et al., 1998; Lannuzel et al., 2008; Klunder et al., 2011). That said, we observed considerable variability in the biogeochemical rates measured in each region of the Weddell Sea, particularly at LCIS; we examine the possible drivers of and controls on the inter- and intra-regional differences below.
Euphotic zone-averaged rates of
Throughout the sampling region, the average euphotic zone rates of
The lowest regenerated N concentrations occurred at the stations with the
lowest rates of NPP and
At LCIS, the stations closest to the ice shelf were characterized by low
SSTs and low rates of NPP and N uptake (stations L1 and L3; Figs. 1 and S4
and S5a in the Supplement, Table 2). The low SSTs can be attributed either to the formation
of sea ice or to the upwelling of WW along the ice shelf. Sea-ice formation,
in addition to decreasing SST, also increases the salinity of ASW due to
brine rejection (Gill, 1973). While
the salinity of ASW at the low-SST stations was indeed elevated, the oxygen
concentrations were relatively low (
Relatively cold, saline surface waters have previously been observed at the ice edge off Larsen A and B ice shelves and shown to hinder NPP (Cape et al., 2014). In that case, the dense surface waters were surmised to result either from offshore wind stress at the inshore region that induced localized mixing or from the advection of surface waters offshore by coastal upwelling. Both mechanisms would decrease water-column stability and, by extension, productivity. Cape et al. (2014) observed an increase in NPP with distance from the coast at Larsen A and B, a trend that we did not observe, likely because of the proximity of our LCIS stations to the ice shelf (within 75 km for all stations). Instead, our rates of NPP and N uptake were positively coupled with SST at the ice edge (Figs. S4 and S5). We propose that surface SST at LCIS can be used as an indicator of water mass age, with cooler SSTs indicating newly upwelled WW and warmer SSTs designating older surface waters that have had time to absorb heat from the atmosphere. The higher rates of NPP and N uptake in the warmer surface waters occur because phytoplankton experience favourable growing conditions for an extended period, resulting in biomass accumulation. By contrast, persistent localized upwelling along LCIS inhibits productivity in the adjacent surface waters, with implications for the spatial distribution of biomass and the potential for organic carbon export.
High iron concentrations have previously been measured in surface waters in
the CCSZ and northern Weddell Sea (as high as 7 nM;
Lannuzel et al., 2008; De Jong et al., 2012). Iron is supplied to the mixed layer in these
regions via sea-ice melt, ice shelf melt, continental runoff, vertical and
lateral advection, and resuspension of continental shelf sediments
(Lannuzel
et al., 2008; De Jong et al., 2012; Klunder et al., 2014). In contrast, the
central WG is iron-limited as iron is supplied to surface waters mainly by
wind-induced vertical mixing (Hoppema
et al., 2015). During our sampling, sea-ice concentrations were high at the
WG stations, which would have dampened the effect of wind stress on surface
waters and thus hindered vertical mixing. At FIS in early summer, iron
should have been replete as phytoplankton would not have had sufficient time
to exhaust the surface reservoir. Here, the sea-ice concentrations were
elevated, and the mixed layers were deep such that light, rather than iron,
likely limited phytoplankton growth. Indeed, light-limited diatoms have been
observed to consume
Accounting for regenerated N uptake greatly alters the
Scatterplots of
We can also use the
At LCIS, a coastal sensible heat polynya persisted throughout the sampling
period. The opening of such polynyas along the eastern AP is linked to the
occurrence of warm, föhn winds that originate over the continent and
blow over the AP, influencing the coastal north-western Weddell Sea
(Cape et al., 2014). Föhn winds drive the
offshore movement of sea ice, which initiates the opening of polynyas that
persist because the winds are warm, thus hindering the formation of new
sea ice (Cape et al., 2014). The development of
coastal sensible heat polynyas results in relatively deep mixed layers and a
weakly stratified water column. The polynya at LCIS opened in late November,
approximately 2 months prior to our sampling. At this time (i.e. the
beginning of the growing season), motile
At the other (non-LCIS) sampling sites, diatoms dominated the phytoplankton
community. We hypothesize that at the beginning of the growing season,
melting sea ice alleviated light and, to a lesser extent, iron limitation,
providing favourable conditions for diatom growth. At the same time, the
generally lower iron concentrations characteristic of open Weddell Sea
surface waters may have selected against
Previous
Estimates of the
The low rates of euphotic zone nitrification are consistent with the
previous (limited) data available for the summertime OOZ and CCSZ of the
Southern Ocean. For example, Mdutyana et al. (2020) measured euphotic zone
rates of
Although the highest
Throughout the Weddell Sea,
Previous studies conducted in the MIZ and CCSZ of the Weddell Sea have shown
that
LCIS is a region of deep-water formation, such that the biogeochemical
properties of ASW influence those of MWDW and the bottom waters. Our data
indicate significant net depletion of nutrients from ASW over the summer
growing season. These nutrients would have been converted to organic matter
that was consumed by zooplankton; exported from the euphotic zone
to be decomposed by heterotrophic bacteria, in the water column or on the
shelf; or consumed by the benthic community. The subsurface remineralization
of organic matter acts to increase the
As SSTs rise and sea ice melts, a shift from
We investigated the summertime productivity of understudied regions of the
Weddell Sea, including LCIS, along with the potential importance of
different phytoplankton groups for biomass production, nutrient consumption,
and carbon export potential. Our data show that mixed-layer nutrient
depletion ratios are determined by the dominant phytoplankton group. The
lowest
Although the waters adjacent to LCIS were characterized by the highest
The data discussed in this paper are available
in the Zenodo database and can be found at
The supplement related to this article is available online at:
RFF led the study and writing of the manuscript. SEF contributed substantially to writing the manuscript and designed the experiments with RF and TB. RF and JMB carried out the experiments. JMB, TGB, SEF, KAMS, and SS assisted with sampling and data generation and contributed to writing the manuscript.
The contact author has declared that neither they nor their co-authors have any competing interests.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
This article is part of the special issue “The Weddell Sea and the ocean off Dronning Maud Land: unique oceanographic conditions shape circumpolar and global processes – a multi-disciplinary study (OS/BG/TC inter-journal SI)”. It is not associated with a conference.
We thank Captain Knowledge Bengu, Captain Freddie Ligthelm, and the
exceptional crew of the R/V
This research was supported by the Flotilla Foundation through a grant to Sarah E. Fawcett and Thomas G. Bornman; the South African National Antarctic Programme through grants 105539, 117035, and 129232 to Sarah E. Fawcett; the South African National Research Foundation through postgraduate fellowships to Raquel F. Flynn, Jessica M. Burger, Shantelle Smith and Kurt A. M. Spence; and the African Academy of Sciences and Royal Society through a FLAIR Fellowship to Sarah E. Fawcett. Additionally it was supported by UCT through a Science Faculty Fellowship to Raquel F. Flynn; Vice-Chancellor Doctoral Research scholarships and Postgraduate Merit awards to Raquel F. Flynn, Jessica M. Burger, and Shantelle Smith; and a Vice-Chancellor Future Leaders 2030 award to Sarah E. Fawcett. The authors were also financed by the South African Department of Science and Innovation's Biogeochemistry Research Infrastructure Platform (BIOGRIP) and Shallow Marine and Coastal Research Infrastructure (SMCRI).
This paper was edited by Christian Haas and reviewed by Sebastien Moreau and two anonymous referees.