Controls on Coral Reef Calcification
Numerous
laboratory and mesocosm experiments have provided valuable information on how
coral calcification responds to changes in CO2 as well as other
parameters such as light, temperature, nutrients, and heterotrophic feeding (Cohen
and Holcomb, 2009). However, results
from controlled laboratory and mesocosm experiments cannot be directly extrapolated
to the coral reef ecosystem level where multiple parameters affect NEC simultaneously. As part of my doctoral research with Richard Feely and Chris
Sabine at NOAA’s Pacific Marine Environmental Laboratory (PMEL), we determined
that a previously published model of coral reef NEC that included CO2
levels, temperature, and coral cover was inadequate for predicting NEC rates on
multiple reefs, indicating that additional parameters must play a critical role
in determining coral reef NEC (Shamberger et al., 2011). Now, as a Postdoctoral Scholar at the Woods Hole
Oceanographic Institution (WHOI), I am building on these initial observations
and ideas working with Anne Cohen to investigate the relationships
between NEC and CO2 on Pacific coral reefs with varying CO2
levels, nutritional status, community composition, and flow regimes. In addition, we are investigating the
relative importance of coral calcification versus coralline algae calcification
in the overall ecosystem response to ocean acidification. Also working with me on this project are chemical
oceanographer Dan McCorkle and physical oceanographer Steve Lentz, both on the
faculty at WHOI. Through this
multidisciplinary collaboration, I am expanding both my field and laboratory
skillset (see below) and my perspectives on the oceanographic, chemical, and
biological processes that influence the coral reef response to climate change.
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Coral Reef Sensitivity to Changing CO2
Coral reefs are highly biogeochemically active ecosystems with high rates of calcification, photosynthesis, and respiration that strongly alter the seawater CO2 system of coral reef waters. For my doctoral research at the University of Washington (UW) and PMEL with Richard Feely and Chris Sabine and our collaborators Fred Mackenzie, Marlin Atkinson, and Eric DeCarlo at the University of Hawaii (UH), I led several field expeditions to measure diel time series of CO2, NEC rates, and organic production rates of the barrier reef in Kaneohe Bay on the island of Oahu in Hawaii. To make these measurements, I monitored spatial and temporal changes in seawater total alkalinity (TA), dissolved inorganic carbon (DIC), salinity, and temperature by manually collecting water samples and deploying CTDs and automated water samplers from a small boat that I operated. I also performed TA and DIC analyses at PMEL and calculated the full seawater CO2 system using CO2SYS. I analyzed TA and DIC on separate systems at UH and PMEL but am now analyzing TA and DIC simultaneously on a VINDTA 3C system at WHOI. We found that NEC rates of the Kaneohe Bay barrier reef were high compared to what has been measured on other coral reefs despite comparatively high CO2 levels. I was also able to utilize the large natural variability in seawater CO2 on the Kaneohe Bay barrier reef to assess the reef’s sensitivity to changing CO2 levels and found that NEC was more highly correlated with CO2 than with temperature, light, organic production, or residence time of water (Shamberger et al., 2011). The relationship between NEC and CO2 has been investigated for only a handful of coral reefs in the world (Shamberger et al., 2011). In order to better quantify the variability that exists in the coral reef NEC-CO2 relationship, I am measuring NEC on coral reefs with a range of CO2 levels as part of my postdoctoral work at WHOI. In Palau we are measuring NEC on the barrier reef where CO2 is low and in restricted bays where the long residence time of water and biochemical processes such as calcification and respiration result in elevated seawater CO2. I utilize box models to calculate NEC and organic production rates in coral reef systems and my postdoctoral work has required modifying these calculations for different reef spatial configurations.
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Natural Cycling of CO2 in Coral Reefs
In order to predict the effects of ocean
acidification on coral reefs, it is necessary to first understand the natural
CO2 cycling within coral reef ecosystems. My doctoral research utilized high frequency
(hourly to every three hours) measurements of air and surface water CO2,
salinity, and temperature over several years by an in situ CO2 mooring to determine the processes
controlling CO2 in Kaneohe Bay lagoon and barrier reef waters on
diel, seasonal, and annual time scales. I
use a combination of Excel and Matlab to analyze these large data sets and can
apply the same methods to data produced by any number of in situ CO2 system sensors. While photosynthesis and respiration control
the diel cycle of CO2, net calcification elevates CO2
levels on seasonal and annual time scales in Kaneohe Bay relative to the open ocean
and atmosphere (Shamberger et al., in
prep). For my M.S. research with Fred
Mackenzie at UH, I performed a bi-monthly time series study of the seawater CO2
system (calculated from TA and DIC data I collected and analyzed) and
determined the processes controlling air-sea CO2 exchange in the
lagoon waters of Kaneohe Bay. In agreement
with my later doctoral work, we found that Kaneohe Bay lagoon waters are a net
annual source of CO2 to the atmosphere due mainly to calcification
(Fagan and Mackenzie, 2007; Massaro et al. 2012). We also found that Kaneohe Bay barrier reef
waters are a net annual source of CO2 to the atmosphere of a similar
magnitude as lagoon waters (Shamberger et al., in prep).
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