Spring/Summer 2011

24 Pages·2011·2.4 MB·English

Science
Ocean Carbon and Biogeochemistry
Studying marine biogeochemical
News
cycles and associated ecosystems in
the face of environmental change
Volume 4, Number 2 • Spring/Summer 2011
Preliminary Carbon Budgets for the Laurentian Great Lakes
By Galen McKinley, Noel Urban, Val Bennington, Darren Pilcher, Cory McDonald
Summary: the magnitude and controls on fluxes deep reservoir that provides the ongo-
of carbon in lakes on a global basis. ing capacity of the oceans to sequester
We offer three approaches to esti-
The Laurentian Great Lakes, an anthropogenic CO emissions. In con-
mating carbon budgets and lake-air 2
enormous freshwater resource, are a trast, each of the Great Lakes mixes
CO fluxes for each of the five Great
2
major component of the U.S. coast- top to bottom at least once each year.
Lakes, two based on literature review
line and have some similarities with Hence, the entire volume of the lakes
and one based on simple models for
the coastal oceans. The Great Lakes equilibrates annually with atmospheric
each lake. Results for the net lake-to-
contain nearly 20% of the surface CO . Seasonal storage of CO in the
air CO flux range over two orders of 2 2
2
fresh water of the earth. The length bottom waters of each lake occurs
magnitude from one-tenth to several
of the U.S. coastline on the Great (Atilla et al. 2011), and the timing of
10s of Tg C yr-1. To improve these pre-
Lakes is equal to 49% of the total seasonal mixing shows interannual
dictions, whole-lake biogeochemistry
ocean coastline of the lower 48 states. variability as well as long-term trends
and its spatio-temporal variability re-
One major distinction between the (e.g., Austin and Colman 2007; Trum-
quire substantial additional research,
Great Lakes and the global oceans is pickas et al. 2009). Future warming of
with particular focus needed on net
the circulation regime. The oceans the lakes will reduce the solubility of
primary productivity (NPP), respira-
have a large reservoir of deep water CO in the lakes and drive a degassing
tion, and surface-lake pCO . 2
2
that is isolated from the atmosphere tendency that will oppose the tenden-
1. Introduction and and that only slowly mixes with the cy for increased CO2 uptake driven by
Background oceanic surface water. The timescale increasing atmospheric pCO2.
for mixing of surface and deep ocean There are differences among the
There is a growing awareness that
waters is about 1000 years. It is this Great Lakes that are likely to affect
lakes are hotspots for carbon cycling
in the landscape (e.g., Buffam et al.
Table 1. Summary of Great Lake characteristics. Relative magnitude with respect to
2011; e.g., Christensen et al. 2007;
Lake Erie in parentheses.
Karlsson et al. 2010), and that, world-
wide, they may represent both sites Hydrologic
of significant CO release to the atmo- Surface Area Residence Mean depth Max. depth
2
sphere as well as carbon sequestration Lake (m2) Time (yr) (m) (m)
in sediments (Cole et al. 2007; Tranvik Superior 8.21x1010 174 150 406
et al. 2009). Compared to the IPCC’s (3.2) (67) (7.7) (6.3)
estimate of annual carbon storage on
Michigan 5.78x1010 104 85 282
the continents (2.2 Pg C yr-1) (IPCC
(2.2) (40) (4.5) (4.4)
2007), the burial of organic carbon in
Huron 5.96x1010 21 59 229
lake sediments (0.6 Pg C yr-1) (Tranvik
(2.3) (8.2) (3.1) (3.6)
et al. 2009) and the emission of CO
2
Erie 2.57x1010 26 19 64
from world lakes and rivers (1.2 Pg C
(1.0) (1.0) (1.0) (1.0)
yr-1) (Battin et al. 2009; Tranvik et al.
2009) are of comparable magnitude. Ontario 1.90x1010 7.3 86 244
Clearly, there is a need to understand (0.7) (2.8) (4.5) (3.8)
OCB NEWS • Spring/Summer 2011 1
Science
carbon cycling. These differences in- Figure 1. Alternative
(cid:36)
clude size, location within the chain of (cid:38)(cid:50) depictions of carbon
(cid:21)
lakes, trophic state, and geologic and (cid:51)(cid:75)(cid:82)(cid:87)(cid:82)(cid:86)(cid:92)(cid:92)(cid:81)(cid:87)(cid:75)(cid:72)(cid:86)(cid:76)(cid:86) cycling and the carbon
(cid:36)(cid:79)(cid:79)(cid:82)(cid:70)(cid:75)(cid:87)(cid:75)(cid:82)(cid:81)(cid:82)(cid:88)(cid:86)
geographic setting. Depending on mass balance. (a) This
(cid:50)(cid:85)(cid:74)(cid:17)(cid:3)(cid:38)
the dimension being considered, the (cid:50)(cid:88)(cid:87)(cid:73)(cid:79)(cid:82)(cid:90) framework categorizes
lakes differ among themselves only (cid:54)(cid:54)(cid:87)(cid:87)(cid:85)(cid:85)(cid:72)(cid:72)(cid:68)(cid:68)(cid:80)(cid:80)(cid:9)(cid:9) carbon according to
by factors of 3 to 8 (Table 1). Lake (cid:42)(cid:85)(cid:82)(cid:88)(cid:81)(cid:71)(cid:90)(cid:68)(cid:87)(cid:72)(cid:85) its source and depicts
(cid:39)(cid:44)(cid:38) the pathways taken by
Superior, the largest, has 3.2 times the
each category. (b) This
surface area of Lake Erie, the small-
(cid:37)(cid:88)(cid:85)(cid:76)(cid:68)(cid:79)
framework categorizes
est, and Lake Superior is also nearly
(cid:37) carbon according to
8 times deeper on average. The large
easily measurable
size of Lake Superior means that even
classes, and depicts
small process rates (expressed per transformations among
unit area) can represent large carbon (cid:39)(cid:50)(cid:38) those pathways.
(cid:39)(cid:39)(cid:44)(cid:44)(cid:38)(cid:38)
fluxes (Tg C yr-1). The much shal-
lower depth of Lake Erie implies that
the average summer temperature will (cid:51)(cid:50)(cid:38)
be much higher, that a larger fraction
of primary production will be buried
in the sediments, and that primary
spheric CO than lakes with low NPP rus (Dolan and McGunagle 2005) and
production will occur over a larger 2
(Del Giorgio et al. 1997). Recent mea- hence on lake NPP.
fraction of the water column; all of
surements confirm that the greater 2. Literature Review:
these factors will favour the produc-
algal abundance in the lower lakes in Carbon budgets for each
tion, and potentially also the burial,
of autochthonous organic carbon. summer is associated with lower pCO2 of the Great Lakes
in surface waters (Karim et al. 2011).
This may make the lake more likely The carbon budget for a lake or
There are a host of other features of
to be a net sink for atmospheric CO . coastal ocean may be depicted in
2 the geographic and geologic setting
The small size of Lake Erie also allows several ways. One approach is to
of the individual lakes that impact
the lake to cool sufficiently such that focus on processes, and to separate
carbon cycling. Much of the Lake
complete ice coverage occurs in most allochthonous and autochthonous
Superior basin lies in the Canadian
years; ice cover blocks the release of organic carbon in the lake (Figure 1a).
Shield; the slow weathering of the
CO throughout winter. The lakes The cycle of autochthonous carbon
2 volcanic rocks and shallow overlying
lower in the chain have a much larger consists of only three processes that
soils result in low carbonate and phos-
flow of water and carbon through must be balanced: photosynthesis,
phorus inputs into this lake relative
them than do the upper lakes. Conse- respiration, and burial (outflow may
to the other Great Lakes that in turn
quently, the lower lakes will have lower be important in the lower lakes).
contribute to low NPP and no annual
water retention times, and less time Similarly, allochthonous organic
calcite precipitation (whiting events)
for respiration of allochthonous DOC. carbon inputs are respired within the
in Lake Superior. Over the past 20
Small errors in the estimation of lake, passed through the lake to the
years, the intensity of whiting events
carbon concentrations in the inflows outflow, or converted to particulate
has declined due to the removal of cal-
from upstream lakes as well as in the matter and buried. A lake will have a
cium and carbonate from the water by
lake outflows can have major impacts net efflux of CO only if respiration
2
invasive Dreissenid mussels (Barbiero
on the carbon budgets; in other words, of allochthonous organic carbon
et al. 2006). Geologic and climatic
the carbon budgets are more sensitive plus degassing of inorganic carbon
setting also impact the loadings of
to the accuracy of carbon concentra- inputs is greater than the burial of
allochthonous DOC to the lakes; load-
tion measurements. Historically, the autochthonous organic carbon. Lakes
ings are higher to Lakes Superior and
Great Lakes have shown a wide range such as Superior with large inputs of
Huron than to the other lakes (Shih
of trophic state, with variability across allochthonous DOC, long residence
et al. 2010). The climate and geologic
the lakes and through time. In gener- times that allow for complete respi-
setting influenced the development of
al, lakes with higher algal productivity ration of that DOC, low NPP and
agriculture, now the dominant control
(net primary production or NPP) are low burial rates of organic matter
on anthropogenic inputs of phospho-
more likely to be net sinks for atmo- are likely to be net sources of CO to
2
OCB NEWS • Spring/Summer 2011 2
Science
(cid:20)(cid:19)(cid:17)(cid:19) (cid:20)(cid:19)(cid:17)(cid:19)
(cid:26)(cid:17)(cid:24) (cid:54)(cid:88)(cid:83)(cid:72)(cid:85)(cid:76)(cid:82)(cid:85) (cid:26)(cid:17)(cid:24) (cid:40)(cid:85)(cid:76)(cid:72)
(cid:24)(cid:17)(cid:19) (cid:24)(cid:17)(cid:19)
(cid:16)(cid:20)(cid:92)(cid:85)(cid:12) (cid:21)(cid:17)(cid:24) (cid:16)(cid:20)(cid:92)(cid:85)(cid:12) (cid:21)(cid:17)(cid:24)
(cid:38)(cid:3) (cid:38)(cid:3)
(cid:74)(cid:3) (cid:19)(cid:17)(cid:19) (cid:74)(cid:3) (cid:19)(cid:17)(cid:19)
(cid:55) (cid:55)
(cid:88)(cid:91)(cid:3)(cid:11) (cid:16)(cid:21)(cid:17)(cid:24) (cid:88)(cid:91)(cid:3)(cid:11) (cid:16)(cid:21)(cid:17)(cid:24)
(cid:41)(cid:79) (cid:41)(cid:79)
(cid:16)(cid:24)(cid:17)(cid:19) (cid:16)(cid:24)(cid:17)(cid:19)
(cid:16)(cid:26)(cid:17)(cid:24) (cid:16)(cid:26)(cid:17)(cid:24)
(cid:16)(cid:20)(cid:19)(cid:17)(cid:19) (cid:16)(cid:20)(cid:19)(cid:17)(cid:19)
(cid:20)(cid:19)(cid:17)(cid:19) (cid:20)(cid:19)(cid:17)(cid:19)
(cid:48)(cid:76)(cid:70)(cid:75)(cid:76)(cid:74)(cid:68)(cid:81) (cid:50)(cid:81)(cid:87)(cid:68)(cid:85)(cid:76)(cid:82)
(cid:26)(cid:17)(cid:24) (cid:26)(cid:17)(cid:24)
(cid:24)(cid:17)(cid:19) (cid:24)(cid:17)(cid:19)
(cid:16)(cid:20)(cid:38)(cid:3)(cid:92)(cid:85)(cid:12) (cid:21)(cid:17)(cid:24) (cid:16)(cid:20)(cid:38)(cid:3)(cid:92)(cid:85)(cid:12) (cid:21)(cid:17)(cid:24)
(cid:74)(cid:3) (cid:19)(cid:17)(cid:19) (cid:74)(cid:3) (cid:19)(cid:17)(cid:19)
(cid:55) (cid:55)
(cid:88)(cid:91)(cid:3)(cid:11) (cid:16)(cid:21)(cid:17)(cid:24) (cid:88)(cid:91)(cid:3)(cid:11) (cid:16)(cid:21)(cid:17)(cid:24)
(cid:41)(cid:79) (cid:41)(cid:79)
(cid:16)(cid:24)(cid:17)(cid:19) (cid:16)(cid:24)(cid:17)(cid:19)
(cid:16)(cid:26)(cid:17)(cid:24) (cid:16)(cid:26)(cid:17)(cid:24)
(cid:16)(cid:20)(cid:19)(cid:17)(cid:19) (cid:16)(cid:20)(cid:19)(cid:17)(cid:19)
(cid:20)(cid:19)(cid:17)(cid:19)
Figure 2. Summary of carbon mass balances for each of the
(cid:26)(cid:17)(cid:24) (cid:43)(cid:88)(cid:85)(cid:82)(cid:81)
Laurentian Great Lakes. Error bars indicate standard deviations
(cid:24)(cid:17)(cid:19) among multiple estimates; variability may reflect spatial or tempo-
(cid:16)(cid:20)(cid:92)(cid:85)(cid:12) (cid:21)(cid:17)(cid:24) ral variability as well as methodological imprecision or errors (for
(cid:38)(cid:3)
(cid:74)(cid:3) (cid:19)(cid:17)(cid:19) data sources, see Urban et al. in prep). Negative is a carbon loss
(cid:55)
(cid:88)(cid:91)(cid:3)(cid:11) (cid:16)(cid:21)(cid:17)(cid:24) from the lake.
(cid:41)(cid:79)
(cid:16)(cid:24)(cid:17)(cid:19)
(cid:16)(cid:26)(cid:17)(cid:24)
(cid:16)(cid:20)(cid:19)(cid:17)(cid:19)
the atmosphere. Lakes such as Erie Meyers and Ishiwatari 1993). Budgets outputs to the lake. The air-lake flux
that have higher NPP, higher burial for inorganic carbon have not been of CO is dependent on the changes in
2
efficiencies on account of shallower compiled for any of the lakes except magnitude of dissolved CO relative to
2
water depths, and short water resi- Superior and Ontario. While these atmospheric CO . While the pools of
2
dence times (hence low efficiencies for caveats suggest that the uncertainty carbon depicted in this framework are
oxidation of allochthonous organic about the magnitude of the net CO measurable, some individual fluxes
2
carbon) are more likely to be net sinks flux is large, they reinforce the conclu- are not. Because very few measure-
for atmospheric CO . sion that lakes Superior, Michigan ments of gas exchange or of dissolved
2
Although this approach gives crude and Huron are likely sources of CO pCO have been reported for the Great
2 2
estimates at best of the magnitude to the atmosphere. Correction for the Lakes, the tabulation of the mass
of fluxes, it does suggest that lakes errors mentioned above would likely balance (Figure 2) is a hybrid of the
Superior, Michigan and Huron are reduce the magnitude of the CO sink frameworks shown in Figure 1 and
2
sources for atmospheric CO , and estimated for lakes Erie and Ontario. retains terms for photosynthesis and
2
that lakes Erie and Ontario are sinks An alternative framework for the respiration.
(Table 2). The estimate of respira- carbon budget is shown in Figure 1b. While we emphasize the prelimi-
tion of allochthonous organic carbon This framework separates the measur- nary nature of the carbon budgets
could be improved by accounting able pools of carbon and considers the tabulated from the literature (Urban
for seasonal temperature effects on transformations among these pools. et al., in prep) in Figure 2, several
respiration. The values in Table 2 In this framework, photosynthesis important features of the carbon bal-
assume that all organic carbon in and respiration appear as internal ance for the Great Lakes are revealed.
the sediments is autochthonous (cf. processes rather than as inputs and First, the magnitudes of the fluxes
OCB NEWS • Spring/Summer 2011 3
Science
Table 2. Estimate of net CO fluxes (positive for out of lake) for each lake based on three prep) indicate that the dominant
2
component process rates (Tg C yr-1).a terms in the annual budget are NPP
and respiration, with other terms
Respiration Degassing of Burial of
being small. The U.S. EPA makes
Lake of Alloch. OC DIC inputs Autoch. OC Net CO2 flux
twice-annual surveys of the open lake
Superior 0.63 0.25 0.08 ± 0.17 +0.80
(Figure 3), collecting pH and alka-
Michigan 0.53 ? 0.28 ± 0.18 > +0.25 linity data from which pCO can be
2
estimated, albeit with significant un-
Huron 0.27 ? 0.20 ± 0.09 > +0.07
certainty in a freshwater system. These
Erie 0.02 ? 0.49 ± 0.40 > -0.47
data indicate that Lake Superior tends
Ontario 0.04 ? 0.57 ± 0.36 > -0.53 to be slightly supersaturated with
carbon dioxide during the spring and
a. Data sources summarized in Urban et al. (in prep.).
near equilibrium during the summer
(Tg C yr-1) generally decrease from 5 Great Lakes results in a flux to the (Atilla et al., 2011), which is consis-
Superior to Michigan to Huron to atmosphere of 2.3 Tg C yr-1. However, tent with limited influence from the
Erie and Ontario simply as a result until watershed inputs of DIC are watershed. Furthermore, analysis of
of the lake sizes. Second, the inflows better constrained, the net gas fluxes an eddy-resolving, coupled physical-
from upstream lakes constitute major calculated from these mass balances biogeochemical-carbon model of Lake
components of the carbon budgets should not be viewed as realistic. The Superior (MITgcm.Superior, Figure
for lakes Huron, Erie and Ontario; value of Figure 2 is in illustrating the 3, Bennington 2010, McDonald et al.
small errors in estimation of DIC relative magnitudes of fluxes for each 2011, Bennington et al. in prep) indi-
concentrations (e.g., lack of winter lake individually as well as among the cates that the annual cycles of NPP
samples) could lead to significant er- different lakes. Our analysis suggests and vertical mixing are the funda-
rors in carbon budgets for these lakes. that catchment inputs, photosynthe- mental controls on the seasonal cycle
For each of the lakes except Ontario, sis, and respiration would be the of surface lake pCO2, and thus of the
photosynthesis and respiration are areas most in need of improvement, seasonal cycle of air-lake CO2 flux.
the largest carbon cycling processes; both in terms of additional measure- Since we do not have spatially
these two processes are poorly charac- ments and further compilation of explicit coupled physical-biogeochem-
terized on a whole-lake basis. Again, existing data. ical-carbon models for the lower
these processes are included in the
mass balance only because of a lack Mean April 1997 pCO
2
of adequate measurements of pCO
2
(or direct flux measurements of CO ) 450
2 250
to enable estimation of lake-wide
gas exchange. Finally, inputs of DIC 200
400
from the catchments of all of the
150
lakes except Superior and Ontario are
350
poorly constrained. Here, these inputs 100
have been estimated by balancing the
50
inorganic carbon budget under the 300
assumption of no degassing of DIC
100 200 300 400 500 600
inflows; that assumption is almost
km
certainly incorrect. Because of this
assumption, estimates of net gas ex- Figure 3: April 1997 pCO2 (matm) from
3. Mechanistic Models MITgcm.Superior. DOC, DIC and ALK in-
change (calculated by difference from
Lake Superior has the best-known puts from nine major rivers are represented
all of the fluxes) are underestimated
lake-wide carbon budget due to several (Bennington 2010). Dots are EPA bi-annual
(i.e. in Figure 2 these fluxes should be
recent projects using data and models sampling locations.
more negative, indicating a smaller ef-
flux) for all lakes except Superior. in an attempt to balance the budget. Great Lakes (Michigan, Huron, Erie,
The sum of the gas exchange Consistent with Figure 2, whole-lake Ontario) available to us, we proceed
estimates shown in Figure 2 for the budgeting efforts (Urban et al. 2005, to develop simple 2-box models that
Cotner et al. 2004, Urban et al. in focus on the impacts of NPP, mixing
OCB NEWS • Spring/Summer 2011 4
Science
Table 3: Lake properties input to box models.
Lake Property Superior Michigan Huron Erie Ontario
Mean depth (m) 149 85 59 19 86
Thermocline (m) 20 20 20 15 20
DIC (mmol m-3) 860 2165 1569 1782 1817
Alkalinity (meq m-3) 838 2181 1561 1817 1836
Annual NPP (gC m-2 yr-1) 116 130.5 86 174 178
Surface Area (km2) 82,000 57,800 59,600 25,700 18,960
Net Input (Tg C yr-1) 0.8 20 10 0.7 5
Lake-air CO flux (Tg C yr-1) 0.8 20 10 0.7 5
2
of the water column, and net inputs sumed for other lakes due to lack of up pCO . In August, NPP reaches its
2
from the watershed on air-lake CO better information). Remineralization peak at the same time temperature
2
fluxes. These models estimate a lake- of organic matter returns carbon to peaks, and these have opposite effects
wide seasonal cycle of pCO and CO its dissolved inorganic form. The lake on pCO . In fall, cooling occurs as
2 2 2
exchange with the atmosphere. Our exchanges carbon dioxide with the at- NPP falls off, again compensating,
validation data are the EPA bi-annual mosphere, using regional atmospheric and then in December winter mixing
survey in spring and summer for years concentrations of CO from Park raises pCO .
2 2
1986-2009 (1996-2009 for Lake Supe- Falls, WI. Climatology of ice cover For Lake Superior (Figure 4a), the
rior). Sampling locations are shown from observations is applied to block 2-box model is able to reasonably cap-
in Figure 3 for Superior, and for other air-sea gas fluxes in proportion to the ture the EPA-based pCO estimates as
2
lakes can be found at http://www.epa. fractional coverage. well as the lake-wide integrated results
gov/glnpo/monitoring/guard/sam- The 2-box models without external from MITgcm.Superior with either
pling_stations.html. supplies of carbon come into equilib- zero Net Input or Net Input of 0.8 Tg C
The 2-box models have two layers, rium with the atmosphere after a few yr-1 (Table 2), though the seasonal vari-
surface (epilimnion) and deep, with years of integration and there is no net ability is muted in the 2-box model. In
the sum being the observed mean lake influx or efflux because loss of C to Lake Michigan (Figure 4b), a Net Input
depth. The surface layer has a con- the sediments is not included. A term two orders of magnitude larger than
stant thickness equal to the summer for the net input of carbon from up- suggested from the literature (20 TgC/
maximum thermocline depth (Table stream and from the watershed (Net yr as opposed to 0.25 Tg C yr-1, Table
3). Temperature is set at a constant Input) is added, distributed in time 2) allows the 2-box model to capture
value of 3.91ºC in the bottom layer, according to the fact that the spring observed summer pCO , and still does
2
and surface layer temperatures follow melt drives a significant fraction of to- not quite capture spring pCO . In Lake
2
the 1992-2010 climatology, derived tal runoff into the lakes (Bennington Huron (Figure 4c), we also must in-
by the Great Lakes Surface Envi- 2010). The annual Net Input is tuned crease the Net Input by approximately
ronmental Analysis using satellite until results are within the 1 standard two orders of magnitude (10 Tg C yr-1
observations (GLSEA2, http://coast- deviation uncertainty estimate for as opposed to 0.07 Tg C yr-1, Table 2)
watch.glerl.noaa.gov/statistic/statistic. EPA-based estimates of pCO . Because in order to approach the lower bound
2
html). Tracers within the two model the lakes are in equilibrium without of the observed pCO in both seasons.
2
layers convectively mix when the epi- river input, the Net Input directly de- In Lake Erie (Figure 4d), the range of
limnion temperature is within 1.0ºC termines the lake-air CO flux (Table 3). observed pCO in both seasons is quite
2 2
of the bottom. All biological produc- For each lake (Figure 4), the box large, and the 2-box model is able to
tion occurs within the model surface model has elevated pCO in January as capture these observations within the
2
layer, and a prescribed annual cycle respired DIC is returned to the surface uncertainty with zero Net Input. This
of net primary production reduces box by mixing. Late winter cooling is as close as the 2-box model can get
surface concentrations of dissolved and NPP then begins to draw down to the net sink suggested from the
inorganic carbon (DIC) (Sterner, 2010 pCO . In April and May, Net Input is literature review (-0.47 Tg C yr-1, Table
2
for Superior, with a similar shape as- maximal (if turned on) and this drives 2). In Lake Ontario (Figure 4e), the
OCB NEWS • Spring/Summer 2011 5
Science
2-box model is able to capture the low
500
summer observed pCO with zero Net
2 (a) Superior
Input, similar to the literature review
sink (-0.53 Tg C yr-1, Table 2). Howev-
er, we are only able to begin to capture 250
the spring observed pCO by including
2
a Net Input that is ten times larger
than this (5 Tg C yr-1), and with this,
500
the summer pCO is no longer consis-
2 (b) Michigan
tent with the observations.
Though the 2-box models include
the processes that are quantitatively 250
dominant to carbon cycling on the
lake-wide, annually integrated scale
(NPP, respiration, Net Inputs, Fig-
ure 2), they are unable to capture m) 500
at (c) Huron
bi-annual observations of lake-wide µ
(
pCO with Net Input estimates that 2
2 O
are consistent with the literature C
p 250
review (Table 2). The exception is Lake
Superior, the lake for which the 2-box
model is best parameterized. A critical
uncertainty whose resolution might 500
(d) Erie
improve this approach is the seasonal
cycle of NPP, for which we do not have
a good lake-wide description for any
250
lake except Superior. Recent coupled
physical-ecosystem modeling studies
of Lake Michigan (Pauer et al. 2011)
and Lake Erie (Leon et al. 2011) might 500
be sources for improved lake-wide (e) Ontario
estimates. Better characterization of
the seasonal cycle of inputs from the
catchment and of lake-wide pCO2 is 250J F M A M J J A S O N D J
also needed. Month
The lack of success with a single-
Figure 4: Climatological mean surface lake pCO cycle from 2-box models for (a) Lake
column, 2-box model approach 2
Superior, (b) Lake Michigan, (c) Lake Huron, (d) Lake Erie, and (e) Lake Ontario. Mean lake
outside of Superior also suggests that
(with 1 standard deviation) EPA bi-annual survey data for April and August is shown with x
spatial heterogeneity needs better
and vertical bars. Solid black line has zero Net Input; dashed black line has Net Inputs of
characterization. Figure 3 illustrates (a) 0.8 Tg C yr-1, (b) 0.25 Tg C yr-1 and (c) 0.07 Tg C yr-1, consistent with Table 2; and red
the significant spatial variability of dashed line is for enhanced inputs of (b) 20 Tg C yr-1, (c) 10 Tg C yr-1, and (e) 5 Tg C yr-1. In
surface lake pCO in April 1997 in (a), the blue line is lake-mean pCO for 1996-2001 from MITgcm.Superior, with river inputs
2 2
Lake Superior from MITgcm.Superior. of 0.15 Tg C yr-1.
Here, we see the localized elevation
more variable than the pCO cycle bound estimate for the net carbon
of pCO due to river inputs, as well as 2
2 from the 2-box model (Figure 4a), efflux from the Great Lakes, based
open-lake variability driven by spatial
further suggesting that spatial vari- only on respiration of allochthonous
heterogeneity in local NPP and tem-
ability needs to be taken into account organic carbon, degassing of DIC
perature, as well as redistribution of
in order to understand and quantify inputs (Superior only), and burial of
tracers by the circulation (Bennington
Great Lake carbon budgets. autochthonous organic carbon, is 0.12
et al. 2010). The climatological mean
4. Conclusions Tg C yr-1 (Table 2). When literature
lake-wide seasonal cycle of pCO from
2 values for internal cycling of carbon
MITgcm.Superior is significantly From a literature review, a lower-
OCB NEWS • Spring/Summer 2011 6
Science
are included and an alternative budget of acidification in conjunction with MCDONALD, C.P., et al. Global Biogeo-
is created, the net carbon efflux esti- other anthropogenic influences is chem. Cycles, in review (2011).
mate is 2.3 Tg C yr-1 (Figure 2), which needed for all the lakes. MEYERS, P. A., and R. ISHIWATARI.
may still be an underestimate. For Org. Geochem. 20: 867-900 (1993).
References:
a series of simple 2-box models that
NOAA. Ocean and Great Lakes
approximately capture the climatol- ATILLA, N. et al. Limnol. Ocean-
Acidification Research Plan. 142pp.
ogy of bi-annual surface lake pCO ogr. 56(3), 775-786, doi:10.4319/
2 (2010).
observations, the Great Lakes efflux lo.2011.56.3.0775 (2011).
PAUER, J. et al. J. Great Lakes Res. 37
needs to be at least 35.8 Tg C yr-1, AUSTIN, J. A., and S. M. COL-
(1), 26-32 (2011).
which is most likely an overestimate. MAN. Geophys. Res. Lett. 34:
These estimates range across two doi:10.1029/2006GL029021 (2007). SHIH, J.-S., et al. U.S. Geological Sur-
orders of magnitude and thus indicate vey Open-File Report 2010–1276, p.
BARBIERO, R. P. et al. J. Great Lakes
the poor state of knowledge regarding 22. (2010).
Res. 32: 131-141 (2006).
carbon budgets of the Great Lakes. STERNER, R.W. J. Great Lakes Res. 36,
BATTIN, T. J., et al. Nature Geoscience
The discrepancies between the three 139–149 (2010).
2: 598-600 (2009).
approaches highlight the areas most
TRANVIK, L. J. et al. Limnol. Oceanogr.
BENNINGTON, V. PhD Thesis,
in need of further work.
54: 2298-2314 (2009).
University of Wisconsin-Madison
Critical unknowns that are ripe
(2010). TRUMPICKAS, J., et al. J. Great Lakes
for future research include lake-
Res. 35: 454-463 (2009).
wide spatial heterogeneity of carbon BENNINGTON, V., et al. J.
processing, in particular NPP and Geophys. Res., 115, C12015, URBAN, N. R. et al. J. Geophys. Res.
respiration. Direct observations with doi:10.1029/2010JC006261 (2010). 110: doi: 10.1029/2003JC002230
better temporal resolution of surface BUFFAM, I. et al. Global Change Biology (2005).
lake pCO2 observations are needed. 17: 1193-1211 (2011).
The effects of circulation on biogeo-
CHRISTENSEN, T. R. et al. Philosophi-
chemistry and carbon cycling are
cal Transactions Of The Royal Society
beginning to be addressed in Lake Su-
A-Mathematical Physical And Engineer-
perior (Bennington 2010, McKinley et
ing Sciences 365: 1643-1656 (2007).
al in prep), and need to be studied in
COLE, J. J. et al. Ecosystems 10: 172–185
the other lakes. The impact of tempo-
(2007).
ral variability in response to climate
forcing is also poorly characterized. COTNER, J. et al. Ecosystem Health &
To address these issues, field studies Mangement 7(4), 451-464 (2004).
and numerical modeling efforts will DEL GIORGIO, P. A. et al. Nature 385:
be required. In addition, the develop- 148-151 (1997).
ment of well-validated algorithms for
DOLAN, D. M., and K. P. MCGU-
space-based retrievals of biogeochemi-
NAGLE. J. Great Lakes Res. 31: 11-22
cal parameters for the Great Lakes is
(2005).
critically needed (Mouw et al. in prep).
IPCC. Climate Change 2007: The
In all studies of the Great Lakes, the
Physical Science Basis. Intergov-
myriad of anthropogenic influences,
ernmental Panel on Climate Change
such as invasive species and cultural
(2007).
eutrophication, must be considered.
An additional anthropogenic influ- KARIM, A., et al. Chemical Geology
ence that has received little attention 281: 133-141 (2011).
so far is the impact of acidification KARLSSON, J. et al. J. Geophys.
due to increased atmospheric pCO Res.-Biogeosciences 115 G03006,
2
(NOAA, 2010). Lakes that are ap- doi:10.1029/2010JG001305 (2010).
proximately neutral with respect
LEON, L., et al. J. Great Lakes Res. 37
to atmospheric pCO or a net sink
2 (1), 41-53 (2011).
should be particularly susceptible.
An assessment of the likely impacts
OCB NEWS • Spring/Summer 2011 7
Science
The Impact of MOC Variability on Marine Productivity and
Carbon Uptake and Storage in the North Atlantic
by Apurva Dave & Susan Lozier (Division of Earth and Ocean Sciences Nicholas School of the
Environment, Duke University)
1. The Meridional Overturn- the dominant mechanistic view of the 2.2 Recent insights into the MOC
ing Circulation (MOC) global MOC has been that it operates For decades, the ocean conveyer
like an oceanic ‘conveyer belt’ consist- belt has been the dominant paradigm
Our planet receives a greater flux of
ing of upper and deep limbs, within for describing the MOC; as such
solar radiation at the equator than it
which the transport of water masses its description of the structure and
does at the poles. Arising in response
and their properties occurs via a mechanics of ocean overturning
to this meridional imbalance, the
system of continuous, linked currents. has shaped our ideas of how MOC
large-scale fluid motions of the ocean
Bulk transports in the lower limb variability might impact marine
and atmosphere act to redistribute
pass through the relatively quiescent ecosystems and ocean biogeochem-
excess heat from low to high latitudes.
deep ocean, while transports in the istry. Yet the ocean conveyor “model”
A significant portion of the oceanic
upper limb must traverse the highly was developed during a time when
contribution to this global poleward
energetic, wind-driven gyres. The there was considerably less informa-
heat flux occurs in the North Atlan-
conveyer belt model includes two basic tion about the ocean’s flow field than
tic, where observations of the bulk
assumptions about the overturning there is today. Recent findings from
movement in the upper ocean reveal
circulation: 1) that MOC variability observational and modeling studies
that warm waters are transported
is primarily driven by variability in have forced the ocean science commu-
northward to regions where they
the rate of deep water formation in nity to reconsider some fundamental
release their heat (becoming colder
the high-latitude North Atlantic, and aspects of the MOC’s structure and
and denser in the process), sink and
2) that MOC variability is coherently functioning:
return towards the equator as deep
transmitted downstream from one Transport pathways in the upper and
water masses. This overturning cir-
point on the conveyer to another. lower limbs — In the deep ocean, the
culation is understood to be part of a
In keeping with this physical model, conveyer belt model describes the
larger, global Meridional Overturning
discussions of the impact of MOC continuous advection of water masses
Circulation (MOC) in which recently
variability on marine ecosystems and along deep western boundary currents
ventilated deep waters are exported
ocean biogeochemistry have custom- (DWBCs) that link together across
from their source regions in the high-
arily focused on the downstream basins to create a single pathway along
latitude North Atlantic into the rest
effects of changes in deep water mass the deep limb of the MOC. Over the
of the global ocean, where they are
production. For example, from a ma- past decade, however, studies have
eventually upwelled and transported
rine ecosystems perspective, changes demonstrated the presence of energetic
back to the deep water formation sites.
in overturning in the North Atlan- eddy fields at depth that can not only
In addition to transporting heat, the
tic are expected to impact primary disrupt the DWBCs, but also produce
MOC also plays an important role
productivity in the surface ocean by large-scale recirculations that trans-
in the cycling of dissolved constitu-
altering the upwelling of nutrient- port water masses and their properties
ents such as carbon, oxygen and
rich deep waters elsewhere around away from the western boundary and
nutrients through the marine reser-
the globe. From a biogeochemical through the ocean interior. Similarly,
voir. Thus, changes in the strength
perspective, changes in overturning transports in the upper limb of the
of the MOC could be expected to
are expected to impact the transport MOC may vary strongly as a function
impact marine ecosystems and
of dissolved carbon and oxygen from of wind-driven gyre dynamics and
ocean biogeochemistry.
the surface ocean into the deep ocean. as a result, not provide continuous
2. Conceptualizing In the case of carbon, this would alter throughput from one ocean basin to
the MOC the oceanic storage of an important the next.
greenhouse gas. In the case of oxygen, MOC coherence and temporal vari-
2.1 The ocean conveyer belt
this would affect the biochemistry of ability — The disruption of lower and
For much of the last half-century,
organic matter respiration at depth. upper limb pathways by wind and
OCB NEWS • Spring/Summer 2011 8
Science
eddy activity clearly demonstrates the
Boundary
impact of local physical variability on
Current
MOC transports. Buoyancy changes
in the high-latitude North Atlantic
can thus no longer be considered to be Figure 1: A schematic Interior
the sole determinant of the strength of the Labrador Sea
shows that convection
of the global MOC. Indeed, studies
is confined to an interior
have shown that MOC variability
region that is surrounded
itself is highly spatially variable, h1
by a boundary current
having a strong gyre-scale structure,
that flows into and out of
H
with transports at one location often
the basin. The proper- D
having little correlation to those at
ties of Labrador Sea
another location. The main driver of Water are set by the h
2
the observed spatial patterns of MOC exchange of properties
variability on interannual time scales between the interior and
appears to be basin-scale wind forc- the boundary current.
ing. Further study is needed, however, (from Straneo, 2006) (c)
to understand whether meridional American Meteorological Boundary Current (unwrapped)
Society. Reprinted with
coherence is recovered when the over-
permission.
turning is estimated in density rather V
1 ρ
than depth space. 1
Overturning and deep water mass ρ2 V2
properties — Recent studies have also
shown that deep water mass property
changes from one year to the next may
spatial structure also imprints itself to
3. The impact of MOC
not always match changes in actual
some degree on productivity variabil-
variability on marine
overturning in the source region from
ity. Complicating matters, however, is
productivity and carbon
which the water mass is exported. This
the fact that the productivity respons-
uptake and storage
discrepancy arises because the trans-
es to a given hydrographic change can
fer of properties from surface to depth The dismantling of the ocean
be very different in the light-limited
likely depends on other factors besides conveyer challenges us to consider
subpolar gyre than in the nutrient-
the strength of overturning. For interann6u3al° vNariability in marine eco-
limited subtropical gyre. Moreover,
example, the transport and properties systems and biogeochemistry within a
just as recent research has expanded
defining the deep water mass exported new physical framework. This article
our understanding of the mechanics
from the Labrador Sea basin (Labra- focuses on two processes of particu-
60°N of the MOC, so too have recent find-
dor Sea Water, or LSW) are believed to lar current interest: marine primary
ings challenged accepted notions of
be a function of the property exchange productivity and the oceanic uptake )
the response of mar¹ine productivity to
between the convectively-produced and storage of carbon. Our focus is on s
57°N physical forcing in tChe subtropical and
interior waters and the surrounding the North Atlantic, the basin with the °
subpolar gyres. m
boundary current that flows into and most studied MOC. Productivity respon(se to imported
out of that basin (see Figure 1). With T’
3.1 MOC5 4an°Nd marine productivity nutrients — Surface Mv’OC transports
such a model, eddy activity within
in the North Atlantic crossing the equator into the sub-
the basin, as well as the strength and
Primary productivity in the North tropical North Atlantic provide a mass
properties of the boundary current,
Atlantic 5is1 e°xNpected to be sensitive to balance fo0.r0 0t6he export of deep water
can impact the properties and trans- m°Cs ¹
changes in surface hydrography that out of the basin. A recent study has
port of LSW to the same degree as 65°W 60°W 55°W 50°W 45°W 40°W
result from transport variability in demonstrated that these transports
the local buoyancy forcing that sets
the MOC’s upper limb. As mentioned also act as a conduit for nutrients,
the interior water properties. Thus, it
earlier, this transport variability has a advecting them into the subtropi-
is possible that LSW exported from
strong gyre-sca3l.e2 struc3t.u4re w3.it6h lit3tl.e8 4cal gy4r.e2 from4. s4ource regions in the
the subpolar basin may have varying
coherence between gyres. It is re aTso(°nC- ) equatorial and southern Atlantic
properties, even over intervals where
able to suppose, therefore, that this (Figure 2). The influx of nutrients by
convection in the basin is unvarying.
OCB NEWS • Spring/Summer 2011 9
Science
that other dynamics besides vertical
mixing are controlling the supply
of nutrients to the surface. Among
the possibilities are: 1) wind-forced
changes in the size and intensity of
the downwelling subtropical regime,
2) wind-forced changes in horizontal
Ekman nutrient fluxes into the gyre,
and 3) wind- and buoyancy-forced
changes in the formation and spread
of mode waters, which would impact
the sub-surface nutrient reservoir in
the subtropics.
Productivity response to MOC changes
in the subpolar North Atlantic — In the
light-limited regime of the subpo-
lar gyre, the relationship between
stratification, vertical mixing, and
productivity has traditionally been
treated as the reverse of that in the
subtropical gyre, with the expecta-
tion that increases in vertical mixing
would suppress photosynthesis by
decreasing the exposure of phyto-
Figure 2: A map of climatological phos-
plankton to light. However, recent
phate concentrations in the North Atlantic Productivity response to MOC changes
research has suggested that increases
along the 1026.45 σ density surface. The in the subtropical North Atlantic — In
θ in mixing might also ultimately in-
elevated levels observed in the Gulf Stream addition to altering advective fluxes
crease net phytoplankton community
current reflect the advection of nutrients of nutrients into the subtropics, vari-
into the North Atlantic via the surface trans- ability in surface MOC transports growth rates by decreasing encounter
ports of the MOC. Thus variability in the rates between phytoplankton and graz-
exerts a ‘second-order’ influence
MOC is expected to impact nutrient supply ers. Thus, the response of subpolar
on nutrient supply by altering the
and productivity in the basin. (from Palter productivity to interannual stratifica-
advective fluxes of heat (or freshwa-
and Lozier, 2008) tion variability created by changes in
ter) into the gyre. Thus, a weakening
MOC heat fluxes remains unclear.
surface transports would thus consti- (strengthening) of MOC heat fluxes
tute a ‘first-order’ MOC impact on the would be expected to reduce (increase) 3.2 MOC variability and carbon
nutrient supply into the gyre, with the sea surface temperatures and weaken uptake and storage
expectation that increases (decreases) (strengthen) the regional stratifica- Carbon uptake response to MOC
in the strength of the MOC trans- tion. Changes in stratification within variability — The drawdown of atmo-
ports would contribute to increases the nutrient-limited subtropical gyres spheric carbon into the upper ocean
(decreases) in productivity within the have traditionally been expected to be is largely controlled by the disequi-
gyre. Subtropical productivity vari- negatively correlated with productiv- librium between the partial pressure
ability would also likely be impacted ity variability, with the expectation of carbon dioxide (pCO ) in the
2
by variability in upstream processes that weakened stratification would atmosphere and that below the sea
in the source regions for these nutri- enhance productivity by increasing surface. Within the water, the pCO
2
ents. The impact of a variable MOC the mixing of deeper, nutrient-rich is a function of temperature, salinity,
on the advection of nutrients into waters towards the surface. A recent alkalinity, and dissolved inorganic
the subpolar gyre, however, remains observational study, however, suggests carbon (T, S, ALK, DIC). Thus, the
unclear, primarily because the nutri- that productivity and stratification uptake of CO would be expected to
2
ent pathways between the subtropical variability in the subtropics are not be sensitive to all of the types of MOC
and subpolar gyres have yet to be strongly correlated. A growing body of variability discussed in the previous
elucidated. evidence instead supports the notion section, insofar as this variability is
OCB NEWS • Spring/Summer 2011 10