Ocean acidification driven by the uptake of anthropogenic
Ocean acidification and its repercussions on marine ecosystems constitute an
important consequence of the ongoing rise in atmospheric carbon dioxide
(
The combination of these chemical reactions is most often referred to as
ocean acidification (OA). As it proceeds, the dissolved inorganic carbon (DIC)
concentration (the sum of [
Marine calcifying organisms, many of which are important primary producers
(e.g., coccolithophores), extract the constituents of their calcitic or
aragonitic tests (shells) from seawater. In most cases, their ability to do
so is directly dependent on the saturation state of the surrounding water.
Supersaturated seawater (
The Canadian Arctic Archipelago (CAA) and its adjacent deep basins, the
Canada Basin (CB) and Baffin Bay (BB, Fig. 1), are part of the region
projected to undergo the largest reduction in ice cover and, consequently,
the largest decrease in surface pH (
Map of the study area with dominant surface circulation flow paths (following McLaughlin et al., 2004, and Proshutinsky et al., 2009). CAA stands for Canadian Arctic Archipelago. Created using Ocean Data View (Schlizter, 2016).
In this study, we use a large observational dataset for this part of the Arctic to (1) describe the recent state of the carbonate chemistry and its spatial variability in the Canadian Arctic Archipelago and adjacent basins, (2) investigate the interannual variability in carbonate system parameters and identify detectable temporal trends using time series spanning from 2003 to 2016, and (3) estimate the contribution of the temporal change in biological activity to the observed variability of surface DIC.
The Canada Basin (CB), Canadian Arctic Archipelago and Baffin Bay
accommodate the flow of surface waters from the North Pacific to the North
Atlantic (Stigebrandt, 1984), as well as circulation of Atlantic waters at
greater depths. The water mass structure of the southern Canada Basin is
representative of these broad circulation patterns and can be summarized as
follows (Carmack et al., 1989; MacDonald et al., 1989; Lansard et al.,
2012): a relatively cold and fresh surface layer that contains significant
fractions of meteoric water (river discharge and precipitation) and sea ice
melt in the summer and becomes homogeneous in winter; an intermediate layer
(
The CAA is a series of islands on the Canadian continental shelf, through
which complex circulation patterns unfold in narrow and relatively shallow
channels (
The oceanographic regime of Baffin Bay is distinct from that of the CAA and
CB, as it receives multiple inputs from both the Arctic and Atlantic oceans.
Cold and relatively fresh Arctic- and Pacific-derived waters enter this
2300 m deep semi-enclosed basin through the Nares Strait and the Jones
and Lancaster sounds (Muench, 1971; Jones et al., 1998, 2003). Warmer and
more saline Atlantic Ocean waters are transported from the Labrador Sea by
the West Greenland Current (WGC) into Baffin Bay through the eastern side of
Davis Strait and circulate cyclonically, i.e., in an anti-clockwise direction,
before joining the southward Baffin Island Current (BIC), which exits Baffin
Bay through the western Davis Strait (Bourke et al., 1989; Münchow et al.,
2015). Atlantic Ocean waters are modified as they mix with Arctic inflows in
northern Baffin Bay, near the North Water Polynya (Melling et al., 2001).
The resulting water mass structure is described by Tang et al. (2004) as follows: (1) a cold (
Geographical location of the oceanographic stations covered by the dataset, colour-coded according to the year of sampling, with the approximate boundaries of the main areas mentioned in the text.
The dataset used in this study comprises data from 420 stations visited
during various research cruises carried out aboard the Canadian Coast Guard
Ship (CCGS)
Research cruises carried out by the CCGS
Seawater was sampled separately for each measured parameter from Niskin
bottles mounted on a Rosette system equipped with a Seabird SBE 911plus
conductivity–temperature–depth (CTD) sensor, which recorded in situ practical
salinity (
In situ pH
In order to assess the robustness of the computed DIC values, we calculated
DIC from
Questionable
In order to quantify the error associated with the calculated carbonate
system parameters reported in this study, we used the CO2SYS program
modified by Orr et al. (2018), which applies error propagation to
instrumental and constant-related uncertainties. For simplicity, we report
the mean uncertainty for each parameter (see Table 2), as the variance is
minimal within our dataset. We found the additional uncertainty associated
with the unavailability of nutrient concentrations (P and Si) as input
parameters in CO2SYS to be negligible (up to 0.0006 pH units, 1.5
Mean uncertainties of parameters computed in CO2SYS, their standard deviations and their relative weight with respect to the mean value of each parameter, for different carbonate parameter input pairs.
To characterize the recent state of seawater carbonate chemistry in the
study area, we use data obtained in the late summers (August–September) of
2014, 2015 and 2016. Uncertainties for each parameter can be found in the
Methods (Sect. 3.3). The mean 2014–2016 surface (
Surface water (
Surface water (
Mean surface water (
The saturation state of surface waters (
Surface water (
Surface water (
In the Queen Maud Gulf, surface
Surface waters throughout the study area are supersaturated with respect to
calcite, with
Depth profiles of pH
Depth profiles of pH
The most prominent feature in profiles of carbonate system parameters in the
Canada Basin is the Upper Halocline Layer (UHL), a layer of water
originating from the Pacific Ocean with a relatively low pH due to its
high metabolic
The Amundsen Gulf and the western portion of the Parry Channel (Fig. 7b, f,
j) exhibit a similar water mass structure and carbonate system chemistry as
the Canada Basin, as the dominant circulation pattern pushes water eastward
from the CB to the CAA. Undersaturation with respect to aragonite does not
occur at the surface in these areas, owing to higher salinities. Although
the amplitudes of the
The shallow bathymetry of the central CAA restricts the eastern flow
originating from the CB to the layer of Pacific water (UHL) and the
overlying surface water.
East of Barrow Strait, where a sill restricts the eastward flow to the upper
125 m of the water column (Bidleman et al., 2007), the water mass regime
changes. With the exception of one profile that captures the Pacific outflow
through the western portion of Lancaster Sound at station 301 (Fig. 7d, h,
l, blue line), this change is clearly visible in profiles of carbonate
system parameters in Lancaster Sound and Baffin Bay, where the upper 500 m
of the water column is supersaturated with respect to aragonite at each of
the visited stations. Although surface waters in Northern Baffin Bay are a
mixture of multiple inputs from the CAA and the Arctic Ocean through Nares
Strait, the warm and saline Atlantic water inflow from the Labrador Sea
dominates the water mass structure in the region and accounts for the high
alkalinity of these waters relative to the CB and CAA (Münchow et al.,
2015). In Baffin Bay, waters become undersaturated with respect to aragonite
and calcite at depths of
Number of comparable data points in the top 100 m of the water column, obtained from measurements made at the same station and within set depth intervals (of 3.5 m at the surface, increasing progressively to 15 m to a depth of 100 m), between each year of the dataset.
Of the 420 stations that make up our dataset, 24 were visited in at
least 2 different years and match our comparability criteria for
time series. These criteria are (1) the stations were sampled within 31 calendar days of each other (this criterion is not ideal since seasonality
is highly variable and driven by complex sea ice processes, including ice
break-up) and (2) the stations are located within a 5 km radius. The mean
time difference and distance between comparable stations are 12 calendar
days and 1.81 km, respectively. Eight were visited three times, the
remainder were visited twice. With the exception of one site in northern
Baffin Bay, all recurrently sampled stations are located within or on the
outskirts of the CAA. A total of 16 of the 24 time series span 3 years or less.
Within all comparable stations, we identified the measurements made at
similar depths in the top 100 m of the water column (Fig. 8). The 2014–2015
and 2014–2016 intervals have the largest number of comparable data points
(
Location of the stations visited over an interval of 5 years or more. Stations are designated by area-related acronyms (LS is Lancaster Sound, CAA is Canadian Arctic Archipelago, AM is Amundsen Gulf) that are not the station identifiers used during the expeditions.
Depth profiles of
To quantify near-surface change, we averaged data from the top 25 m of the
water column. Across this depth interval, between 2007 and 2016, the
temperature-normalized
Depth profiles of
Mean
Four stations located on the transect extending from Cape Bathurst to Banks
Island complete the seven time series (Fig. 11).
Three of these time series (AM1, AM2, AM4) span 2003–2009 or 2004–2009.
Thus, given their spatial and temporal proximity, we expect a certain
consistency in the trends they exhibit. Surprisingly, salinity increased at
each of the stations over the study period, by 1.46, 0.70 and 0.13,
respectively. Despite the consistent salinity trend, stations AM2 and AM4
show opposite trends in carbonate system parameters, the former exhibiting a
positive change in
Below 25 m depth, variations in atmospheric conditions and biological
activity become decreasingly influential on carbonate system parameters,
relative to changes in water mass properties resulting from mixing. The
largest change in our time series occurs at Station LS1, where
As previously stated, these time series are snapshots in time and cannot be
assumed to represent the continuous evolution of the carbonate chemistry in
the Canadian Arctic. Nonetheless, even with a small sample size, we can
confidently state that the temporal evolution of carbonate system parameters
in the region does not display a systematic trend on sub-decadal timescales.
Moreover, most of the significant changes that our time series exhibit are
associated with variations in the physical oceanography of the region
(water mass distribution and circulation) or surface processes (melting of
sea ice). Given the well-documented rapid melting of the sea ice cover in
the region (e.g., Tivy et al., 2011), we did not expect to observe increases
in summer surface salinity over time intervals of 5 to 9 years. Our time
series, therefore, offer proof of the strong interannual variability of this
highly dynamic system. Discerning the ocean acidification signal amid the
various physical and biological sources of change would require continuous
time series over a longer period of time. We estimated this period to be 23 to
35 years for pH, 25 to 37 years for
We define
Figure 12 shows
It is unlikely that the consistent directions of change we observe for the
months of August and October represent actual trends, given the small data
pool and inconsistent reference states used to make those observations.
Nevertheless, we can use this analysis to constrain the contribution of
fluctuations in biological activity to the interannual variability of the
DIC. In the top 25 m of the water column, the maximum amplitude of
The extremely weak (
Field observations of carbonate system parameters made between 2014 and 2016
in the Canadian Arctic reveal that surface waters of the region serve as a
net
Time series of carbonate system parameters, although relatively short
(
The time of emergence (ToE) of a process affecting a natural system is the time required for the measurable effects of this process to emerge from the natural variability of the system. The concept is predominantly applied in global climate change modelling studies, for which the results are either “years of emergence” based on a pre-industrial steady state (e.g., Friedrich et al., 2012) or time intervals over which observations must be made in order to distinguish an anthropogenic signal from its natural variability. Few of these studies have used observations (e.g., Sutton et al., 2016), and, to our knowledge, none of them have focused specifically on the Arctic.
We define the time of emergence according to the following equation:
Although the assumption of relative equilibration with atmospheric
The results of this analysis are presented in Table A2. At the surface,
pH
Values of natural variability (
Calculated times of emergence of carbonate system parameters at various depths, rounded to the closest year. The
Our results show a slight increase of ToE values from the surface to the
90–110 m depth interval (except for
Number of data points included in the annual means used in the time of emergence calculations.
Despite their similarity, our calculated times of emergence are consistently
longer than those reported in modelling studies (Keller et al., 2014;
Rodgers et al., 2015). This is consistent with the fact that coastal waters,
that comprise a large portion of our dataset, exhibit a much higher
variability in pH (and other carbonate system parameters) than open oceans
(Duarte et al., 2013). Furthermore, direct observations are likely to
integrate variability on temporal and spatial scales that are too small to
be resolved by models. It is also important to note that distinct
measurement techniques and their associated uncertainties create an
analytical bias between different parameters, a bias that is not present in
the same form in modelling studies. The relative uncertainties of in situ pH
The validity of these conclusions depends on a methodology that differs considerably from its modelling equivalent, even if the results from both approaches are consistent with each other. In addition to the instrumental bias mentioned previously, our observations are subject to a sampling bias, since we only use data gathered in the summer months. Consequently, the natural variability used in our ToE calculations does not encompass the entire annual cycle. Nevertheless, because we define the natural variability of the system in terms of interannual rather than seasonal changes, the former should not change, assuming the amplitude of the seasonal cycle is constant through time. The other form of sampling bias possibly affecting our results is spatial, as cruise tracks and durations varied every summer.
In order to estimate the relative fractions of sea ice melt and meteoric
water (mostly river water) in the Queen Maud Gulf, we use
Using the intercept of the trend line equation (
The raw data collected as a part of the ArcticNet program, on which most of the observations presented in this paper are based, can be accessed through the Polar Data Catalogue (Mucci, 2017). Complementary datasets, some of which are part of larger databases, are also available on various online repositories (François et al., 2012; Chierici et al., 2013; Giesbrecht et al., 2014; Papakyriakou et al., 2017).
AM and ABL conceived the project. AM and HT acquired much of the data prior to 2016. ABL carried out the data analysis and wrote the first draft of the paper, whereas AM and HT provided editorial and scientific recommendations. HT provided results of alkalinity and dissolved inorganic carbon analyses and scientific recommendations.
The authors declare that they have no conflict of interest.
We would like to thank the captains and crew of the CCGS
This research has been supported by the Network of Centers of Excellence-Tri-Council (grant no. 3.8 ArcticNet/Manitoba), NSERC Discovery grants (grant no. RGPIN/39679-2013), and the NSERC-MEOPAR (Marine Environmental Observation, Prediction and Response Network) (project no. 13: Canadian Ocean Acidification Research Partnership).
This paper was edited by Jean-Pierre Gattuso and reviewed by Leif Anderson and one anonymous referee.