Figure 4 Seasonal highflow events appear to coincide with decreased pCO2. From late fall through

1
Seasonal And Episodic Variability Of Carbon In A
Large New England Estuary And Its Delivery To The
Coastal Zone
Chris Hunt1, Joe Salisbury1, Doug Vandemark1,
Janet Campbell1, Wade McGillis2 1 Coastal Ocean
Observing Center, University of New Hampshire
chunt_at_cisunix.unh.edu 2 Lamont Doherty Earth
Observatory, Columbia University
SCI-046 2497

• Objectives
• At present, a thorough understanding of the
  transfer of carbon across the land-ocean
  interface has not been developed. An important
  part of this transfer is the interaction of
  carbon dioxide (CO2) -rich estuarine water with
  the atmosphere. This work represents part of an
  ongoing study addressing several key carbon
  transport issues
• Does the Kennebec Estuary, a temperate
  Northeastern U.S. estuary, act as a source or
  sink of atmospheric CO2?
• Does the interaction between the Kennebec Estuary
  and the atmosphere vary on a spatial or temporal
  basis?
• Do changing climatic conditions (in this case,
  discharge) act as a control upon carbon dioxide
  expression in this estuary?
• Can we estimate how future climate changes might
  affect estuaries of this type?

• Observations
• Median, binned pCO2 shows general decreases with
  decreasing salinity (Figure 4)
• The Kennebec Estuary appears to be a net source
  of CO2 to the atmosphere, which is not unusual
  among other estuary studies found in the
  literature (see Borges 2005).
• However, at certain time of the year pCO2 levels
  drop below mean atmospheric levels, indicating
  CO2 uptake by the estuary

Figure 6- The monthly pCO2 anomaly for each
estuary survey was calculated from the difference
between observed and conservative pCO2. River
and ocean endmember TA and pCO2 were used to
determine pH, and pH and TA were mixed
conservatively across the salinity gradient.
pCO2 anomaly (observed pCO2) (conservative
pCO2)
Figure 7- The median pCO2 anomaly for each
salinity bin Decreases with increasing
river Flow, indicating that greater
river Discharge promotes greater autotrophy and
primary production. This would also coincide
with decreased pCO2 levels in the estuary
(Figure 5).
Figure 3- Relationship between endmember (zero
salinity) TA and discharge
Figure 2- Discharge curve for the Kennebec River,
with survey cruise dates indicated
Figure 4- Seasonal high-flow
events appear to coincide with decreased pCO2.
From late fall through mid-spring the Kennebec
Estuary appears to be a modest sink for
atmospheric CO2, while from late spring through
mid-fall the estuary is apparently a moderate
source of CO2 to the atmosphere.
Figure 1- Map of study area within the Gulf of
Maine (left), with the Kennebec Estuary
circled. our fast-rate CO2 equilibrator (top
left) was deployed aboard the UNH R/V Gulf
Challenger for this work.

Summary

• The Kennebec Estuary acts as a seasonal CO2
  source to the atmosphere in
• spring and summer, and as a small sink of
  atmospheric CO2 in the late fall
• and winter.
• Increased river flow coincides with decreased CO2
  levels in estuarine water.
• We also see seasonal trends in Kennebec Estuary
  metabolism inferred
• autotrophy during the spring and summer, changing
  to inferred heterotrophy
• for the fall and winter.
• Increased precipitation, predicted by several
  regional climate models, if
• translated into increased river runoff, would
  promote lower estuarine CO2
• levels. Coupled with a predicted increase in
  atmospheric CO2, systems
• such as the Kennebec Estuary could change from
  overall sources of CO2
• to overall sinks of atmospheric CO2.

• Study Area and Data Collection
• The Kennebec Estuary in west-central Maine drains
  approximately 25,000 km2, making it one of the
  largest contributors of fresh water to the Gulf
  of Maine system. Peak river flow generally occurs
  during spring snowmelt, and is a strong source of
  fresh water and terrestrial material to the Gulf
  of Maine (Oczkowski et al 2006). We have
  performed monthly surveys of the physical,
  chemical, and optical properties of the Kennebec
  Estuary and nearby coastal waters for three years
  (2005 and 2006 presented here), using both
  continuous-flow and discrete methods, as part of
  UNHs Coastal Ocean Observing Centers monthly
  Coastal Transect cruise.
• The data presented here were gathered from the
  following sources
• Continuous along-track measurements of the
  surface partial pressure of carbon dioxide
  (pCO2), using a fast-rate equilibrator and IR gas
  analyzers, with on-board atmospheric pressure
  measurements. Intake depth 0.7m.
• Along-track salinity and temperature.
• Discrete total alkalinity (TA, measured by Gran
  titration) and pH.
• River flow data from USGS gage 1049265 (Kennebec
  River at North Sidney, Maine)
• Data processing performed
• pCO2 measurements were matched with daily USGS
  discharge, and binned in 5-psu increments. A
  median value for these 5-psu bins was then
  computed.
• Conservative pCO2 was calculated over the
  salinity gradient using the following endpoints
• River endpoints at zero salinity was derived from
  discharge and the presented curve. The lowest
  observed salinty, and pCO2 at this salinity, were
  used to estimate TA and pH.
• The TA and pH at the oceanic endmember were
  estimated from salinity and pCO2.
• TA and pH were allowed to mix conservatively over
  the salinity gradient for each cruise, and
  conservative pCO2 was calculated for each data
  point of corresponding salinity and temperature.
• All these data and much more are freely available
  at our groups website
• cooa.sr.unh.edu

How might Kennebec Estuary pCO2 look in 100 years
according to climate predictions?
Figure 5- The median pCO2 for each salinity bin
decreases with increased river flow. Two models
described in the New England Regional
Assessment of The Potential Consequences of
Climate Variability and Change predict regional
precipitation increases of 10 and 30 in the
next century.
References Borges, A.V. 2005. Do we have enough
pieces of the jigsaw to integrate CO2 fluxes in
the coastal ocean? Estuaries 28 3-27. The New
England Regional Assessment of The Potential
Consequences of Climate Variability and
Change http//www.necci.sr.unh.edu/2001-NERA-Found
ation-Doc.html Oczkowski, A.J. Pellerin, B.A.
Hunt, C.W. Wollheim, W.M. Vorosmarty, C.A.
Loder, T.C. 2006. The role of snowmelt and
spring rainfall in inorganic nutrient fluxes from
a large temperate watershed, the Androscoggin
River basin (Maine and New Hampshire).
Biogeochemistry. 80 191-203. Acknowledgments Th
ank the crew of the Gulf Challenger, and the rest
of the COOA team who brave the seas every month.
Thanks also to the workers in the lab who get our
samples run with care and dedication.

• If we equate a precipitation increase
• with discharge, we predict decreased
• river-borne pCO2.
• Combined with an expected increase
• in atmospheric pCO2 (data from Mauna
• Loa, Hawaii), estuarine pCO2 levels
• would become sub-atmospheric.
• Thus the Kennebec Estuary could shift
• from a CO2 source to a CO2 sink under
• some climate change scenarios.

Support for this work was provided by the NOAA
Coastal Services Center through an award to the
UNH Coastal Ocean Observing Center, NOAA Award
NA16OC2740.