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Coral Reef Biogeochemistry

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People in scuba gear taking samples from ocean bottom

Coral Reef Biogeochemistry

Research Area: Carbon Cycling in Modern Systems
Related People: David Mucciarone

We seek to understand the dynamics of carbon budgets on different marine ecosystems (e.g. kelp forests, coral reefs, sea grass beds) and how they become more or less vulnerable to ocean acidification.

Research questions we address
Coral reef ecosystems – marine equivalents in richness and value as rainforests – are particularly vulnerable to climate change patterns in warming surface temperatures and ocean acidification. Since coral reefs have unparalleled intrinsic ecological value as well as significant economic and cultural value to millions of people dependent on reef resources, we need to know how sensitive coral reefs are to global warming and rising acidity levels. What confers resilience to coral reefs? What types of reef structures are likely to be more or less vulnerable to declining pH levels in the future? How does the hydrodynamic environment (flow and residence time) interact with different ecological communities comprising reefs to affect coral metabolism and reef health metrics (e.g. net community calcification rates and net community photosynthesis rates)? We know that the biological activity of coral reefs (calcification/dissolution and photosynthesis/respiration) imprints a signal on the overlying seawater. Does the natural diurnal variability in carbon budgets observed by measuring seawater above reef communities directly scale with vulnerability to ocean acidification?  Global climate models (GCMs) tell us the surface ocean will experience a pH decline of ~0.3-0.5 units by 2100 (Caldeira and Wickett 2005). Yet, on many reefs, we observe such a change daily, which raises more questions. Are reefs experiencing greater diurnal variability in pH likely to be more resistant and resilient to future ocean acidification? Or could the lower boundary of that diurnal pH range be already pushing the physiological limits for some of the reef’s organisms? Some of these questions and field observations lead us to believe that different reefs would be affected differently by ocean acidification, to the extent that the rate of acidification on reefs could be different and determined by a variety of factors, including ecological community composition and reef water circulation.

Over the years we have been developing and deploying custom-made carbon chemistry measurement system, which allows us to resolve carbon budgets on a reef on a 5-minute time-scale. Our main objectives are: 1) to record natural diurnal variability in carbonate system parameters such as pH, total dissolved inorganic carbon (TDIC), total alkalinity, dissolved oxygen, CO2, aragonite saturation state. We have recently developed autonomous pumping systems to collect water for measurement in the laboratory and pumping systems to deploy BEAMS systems developed by Takeshita et al. (2016).

In addition to a custom-made high-throughput automated continuous-flow system we employ, the field deployment design involves the ‘Control Volume’ approach. The “control volume” is an engineering protocol that combines point measurements (Eulerian approach) at a series of points that define a volume over a reef patch. When analyzed, this series of measurements allows us to determine the residence time of a water parcel in the control volume, as well as the evolution of the water mass with respect to physical and biogeochemical properties as it moves over the reef patch due to its interaction with the reef. Here we use a control volume to determine drawdown or increase in the seawater Dissolved Inorganic Carbon (DIC) and Total Alkalinity (TA) pools as the water mass travels over a coral reef patch. Seawater samples from different depths at the corners of a control volume are measured for DIC, TA, pH, temperature, and salinity via collection by a pump-driven continuous flow system. Water parcel tracking is achieved by high-resolution (space and time) Acoustic Doppler Current Profiles and Acoustic Doppler Velocimeters.

Field sites
We have successfully deployed this system in the Republic of Palau (Western Pacific), Palmyra Atoll (Northern Line Islands, Central Equatorial Pacific), and American Samoa (South Pacific).