Robotic Analysis of the Total Carbon System in a Coral Reef Environment

Bill Martin, Marine Chemistry & Geochemistry
Anne Cohen, Geology & Geophysics


Proposal Summary

Ocean acidification (OA) will alter the carbonate budget of coral reef ecosystems by slowing biogenic calcium carbonate production and accelerating dissolution.  However, efforts to scale up results of laboratory-based C02 manipulation experiments to quantitative predictions of OA impacts on specific reef systems, are constrained by the complexities of biogeochemical, hydrodynamic and ecological processes and feedbacks that occur at the ecosystem scale.  To address this issue, we are using Lagrangian techniques to quantify Net (calcification minus dissolution) Ecosystem Calcification (NEC) of multiple Pacific reef systems.  Calcification utilizes carbonate alkalinity and dissolution returns alkalinity to the overlying seawater.  Thus, NEC can be calculated from the measured change in alkalinity of a parcel of seawater as it moves across the reef.

On the Palauan barrier reef, our NEC estimates in 2012/2013 are 50% lower than they were in 1994, when a Japanese group made the same measurements along the same stretch of reef.  This implies that the barrier reef is net producing 50% less calcium carbonate now than it was 20 years ago, underscoring both the need to establish early 21st century NEC rates for as many coral reef ecosystems as is possible, and to gain a better understanding of the drivers of ecosystem scale change.  Achieving this goal is impossible with our current instrumentation.  Even at the sites we study most intensely, we're data limited because we rely on water samples collected by a Remote Access Sampler (RAS) that have to be decanted, preserved and shipped home for analysis.  The longest continuous sampling we can do is 4 days with a frequency of 2 hours.  Commercially available pC02 and pH sensor combinations cannot give us the precision we need to calculate NEC from alkalinity anomaly, which is why we're still collecting discrete water samples.  What we need are highly accurate and precise autonomous instruments that can be deployed in a reef environment for extended (weeks-months-years) periods.

There are two autonomous instruments currently developed at WHOI that can generate the data (pH, Te02) with the precision we need to measure NEC from alkalinity change - the DMAS and the RATS (Robotic Analyzer for the Total CO2 System).  Aleck Wang's DMAS is a beautiful instrument - accurate, precise, compact with high throughput - but in its current format it needs an external power source, so it's not ready for reef work.  Bill Martin and Fred Sayles RATS is much closer to being ready for reef deployment - it's on battery power and deployments at home show high precision and accuracy for extended periods at a sampling frequency of 2.5 hours.  RATS measures pH (by spectrophotometric method) and TC02 in situ single-deployment longevity of 6 months.  Data are downloaded to a laptop computer by a scientist or technician sitting on a boat out on the reef.  Data can be downloaded at any time without the RATS being retrieved and returned to the lab.  This obviates the need for preservation, decanting, shipping and analysis of water samples.

RATS has never been shipped away from home or deployed in a reef environment.  I have NSF funds for NEC work at 4 Pacific reef sites, including Palau.  I propose to take the RATS to Palau this fall and deploy it for a period of 6 weeks.  This expedition provides an opportunity for us to co-deploy the RATS and the RAS over a period of 1-2 weeks, allowing us to compare data.  The continuous RATS deployment I propose will get us a significantly longer data set of reef metabolism under changing climate and hydrodynamic conditions than we or anyone has ever collected before.