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Environmental Measurements Methods
Rapid mapping of dissolved methane and carbon dioxide in coastal ecosystems using the ChemYak Autonomous surface vehicle David P Nicholson, Anna P. M. Michel, Scott David Wankel, Kevin Manganini, Rebecca A. Sugrue, Zoe O Sandwith, and Samuel Andrew Monk Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b04190 • Publication Date (Web): 25 Oct 2018 Downloaded from http://pubs.acs.org on October 31, 2018
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Environmental Science & Technology
Rapid mapping of dissolved methane and carbon dioxide in coastal ecosystems using the ChemYak Autonomous surface vehicle
David P. Nicholson‡,1,*, Anna P.M. Michel‡,2, Scott. D. Wankel1, Kevin Manganini2, Rebecca A. Sugrue2,3, Zoe O. Sandwith1, and Samuel A. Monk2,4 ‡These Authors Contributed Equally 1.
Marine Chemistry and Geochemistry Department, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA
2.
Applied Ocean Physics and Engineering Department, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA
3.
Current Affiliation: Civil and Environmental Engineering Department, University of California, Berkeley, Berkeley, CA 94720, USA
4.
Current Affiliation: National Oceanography Center, University of Southampton, Southampton SO14 3ZH, UK
KEYWORDS (Word Style “BG_Keywords”). Methane, Carbon Dioxide, Estuary, Greenhouse Gas, Autonomous Surface Vehicle, Salt Marsh, Chemical Sensing
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ABSTRACT Coastal ecosystems host high levels of primary productivity leading to exceptionally dynamic elemental cycling in both water
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and sediments. In such environments carbon is rapidly cycled leading to high rates of burial as organic matter and/or high rates of loss to
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the atmosphere and laterally to the coastal ocean as inorganic forms, such as carbon dioxide (CO2) and methane (CH4). To better understand
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carbon cycling across these heterogeneous environments new technologies beyond discrete sample collection and analysis are needed to
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characterize spatial and temporal variability. Here we describe the ChemYak, an autonomous surface vehicle outfitted with a suite of in situ
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sensors, developed to achieve large spatial scale chemical mapping of these environments. Dissolved methane and carbon dioxide are
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measured by a laser spectrometer coupled to a gas extraction unit for continuous quantification during operation. The gas-powered vehicle
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is capable of rapidly surveying coastal system with an endurance of up to 10-hours at operating speeds in excess of 10 km hr-1. Here we
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demonstrate its ability to spatially characterize distributions of CO2, CH4, oxygen, and nitrate throughout a New England salt marsh estuary.
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1.
Introduction
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At the interface between land and ocean, the coastal zone hosts an exceptionally heterogeneous and dynamic spectrum of environmental
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conditions. Largely as the result of physical and geochemical processes controlling nutrient delivery, coastal areas host some of the most
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productive ecosystems on the planet, leading to a tremendous amount of elemental cycling within the waters and sediments of these
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environments. In particular, the turnover of carbon in estuarine sediments and waters can be rapid, leading to high rates of burial as organic
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matter and/or release to the atmosphere and coastal ocean in forms including carbon dioxide (CO2) and methane (CH4). There have been
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strong disciplinary advances in our understanding of the physical, biological and biogeochemical processes that govern the fate of carbon
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in these types of ecosystems. However, as a result of the physical, chemical and biological heterogeneity of these systems, characterizing
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the nature of elemental fluxes to/from these systems remains particularly challenging. These challenges limit our ability to constrain
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atmospheric emission budgets over larger spatial scales and predict how future change will impact these emissions. Overcoming these
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challenges requires the development and implementation of new sensing approaches. Here we describe the “ChemYak,” a JetYak1
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autonomous surface vehicle (ASV) outfit with a suite of chemical sensors, enabling large spatial scale mapping (order 10s km) of key
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chemical parameters and thereby addressing many of the challenges confronting studies of greenhouse gas biogeochemistry in complex,
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heterogeneous coastal systems.
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In coastal ecosystems, there is a particularly important need for understanding the dynamics of greenhouse gas exchange with the
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atmosphere, especially CO2 and CH4, two atmospheric trace gases with large forcing on anthropogenic climate change2. The cycling of
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carbon in coastal environments is exceptionally dynamic, often manifesting as a confluence of starkly contrasting systems. For example,
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rivers laterally transport carbon3 and are often sources of CO2 and CH4 to the atmosphere outgassing of remineralized organic carbon4,5, salt
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marshes are recognized for sequestering significant quantities of carbon, as the result of both high local net autotrophic production and
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sedimentation via physical trapping and burial of particulate carbon5 yet are a source of CH4 to the atmosphere due to pore water production
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and outgassing of laterally advected CH4. Indeed, this net balance of autotrophic primary production and remineralization of organic matter
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by microbial heterotrophy embodies a central feature largely dictating the fate of carbon in these ecosystems6,7. With increasing recognition
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of the importance of the storage of ‘blue carbon’ in coastal sediments, determination of the underlying mechanisms of carbon release from
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the coastal systems is paramount.
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Estuarine emissions of CO2 to the atmosphere are significant in the context of the global carbon cycle8–10, releasing from 0.1 to 0.25 Pg
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C yr-1 6,11,12. While the uptake of dissolved inorganic carbon (DIC; of which CO2 comprises the gas phase) occurs via photosynthetic (and
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chemosynthetic) primary production, the production of DIC occurs via respiration coupled with consumption of the most thermodynamically
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favorable (and available) electron acceptors (e.g., oxygen, nitrate, manganese oxides, iron oxides and sulfate). However, the dynamics of
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these processes, which involve a myriad of microbial and multi-cellular players, are often exceptionally complex and difficult to predict,
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exhibiting substantial patchiness over small spatial and temporal scales. As a result, CO2 emissions often exhibit high variability both within
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and among estuaries, due to the wide diversity in both estuarine biological community composition and hydraulic circulation regimes, and
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their corresponding impacts on carbon cycling processes13. For example, a recent review of CO2 fluxes from 165 estuaries concluded that
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on average the highest CO2 fluxes occur in the upper oligohaline zones, while more saline zones (S > 25) exhibited lower fluxes14. A recent
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comprehensive study of CO2 flux from the Sydney estuary in Australia also noted strong zonation and seasonality underlying areal CO2
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emissions15,16. Despite these overarching patterns, however, data remain too sparse to offer global coverage and there remain large
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uncertainties in the magnitudes of CO2 fluxes from coastal estuarine systems17.
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In contrast to DIC, the production and accumulation of CH4 occurs primarily in zones where major electron acceptors, such as oxygen,
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nitrate or sulfate are unavailable. Abundant sulfate in seawater can lead to more energetically favorable sulfate reduction to outcompete
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methanogenesis. Freshwater systems, in contrast are considered more favorable for methane accumulation due to higher organic carbon
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loading as well as the lack of sulfate.As such, methane accumulation does not occur in marine sediments until below depths of sulfate
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penetration, where the oxidation of CH4 to CO2 typically coupled to sulfate reduction by consortia of bacteria and archaea18–20 becomes
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sulfate limited. For this reason, CH4 typically only escapes marine sediments (e.g., shelf sediments) into the overlying water column when
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transport is enhanced by advection (seepage) or ebullition (bubbles). However, in the coastal zone where tidal flushing/pumping can induce
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strong porewater exchange, the release of methane from subsurface zones of methanogenesis can also readily occur21–23. Moreover, with the
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typically low levels of sulfate in groundwater (that might serve as an oxidant of CH4), inputs of groundwater at the coastal interface often
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contain substantially higher levels of CH4 than the surrounding seawater. In fact, when groundwater fluxes of CH4 are larger than rates of
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subsurface CH4 oxidation, groundwater may represent a substantial source to the coastal zone24,25. A sustained groundwater input exceeding
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rates of CH4 oxidation would be needed to maintain elevated dissolved CH4. CH4 has even been used as a (non-conservative) tracer of
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groundwater inputs in some systems22,25,26.
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Despite a growing understanding of carbon dynamics in coastal and estuarine systems, our perspective remains limited by a paucity of
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data – largely collected through discrete sampling approaches. Thus, in an effort to improve our understanding of carbon cycling in dynamic
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estuarine environments, there is a clear need to expand spatially resolved datasets through large scale, direct sensing approaches. As new
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analytical techniques are developed and improved, the move from sample collection and lab-based analyses to one of direct sensing in these
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types of environments should become more common. Infrared spectroscopic platforms have been developed for gas sensing and are now
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routinely used for atmospheric measurements and monitoring. These techniques are ideal for greenhouse gas quantification as most gases
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have infrared signatures, including CH4 and CO2. Techniques utilized include nondispersive infrared (NDIR) or cavity-enhanced techniques.
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As these approaches require samples to be in the gas phase and thus cannot be directly used for the measurement of dissolved gases,
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dissolved gas must be extracted, using approaches such as showerhead equilibrators27,28, membrane inlets29–33, or membrane contactors33–
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35.
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estuarine environments36, mangrove creeks25,37, and in underway systems shipboard28,35,38–40. In parallel, the advancement of autonomous
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surface vehicles (ASV) for operation in aqueous environments without humans aboard are beginning to enable work in challenging, remote
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and/or rugged environments including, glacier faces, shallow water, and even acidic crater lakes41. Indeed, several groups have used ASVs
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and remote controlled vehicles for studies of coastal biogeochemistry, for example, examining water quality42,43, ocean-glacier interactions44,
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air-sea interactions45 and pH and pCO2 in a coastal upwelling system46.
Several groups have begun using infrared spectroscopic systems coupled with gas equilibration to measure surface water gases in
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Our approach is to couple a membrane based gas extraction system and high-precision long-path laser spectrometer (Los Gatos Research)
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to Jetyak, an ASV recently developed at the Woods Hole Oceanographic Institution. A few commercial sensors were considered for dissolved
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methane measurement, including the Kongsberg CONTROS HydroC CH4 and Pro Oceanus MiniPro CH4. These existing instruments were
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deemed insufficient for our application due to their lower accuracy (±2 μatm and ±20 μatm, respectively) and slower response times.
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To the best of our knowledge, this is the first ASV utilized for dissolved CH4 measurements. Compared to a manned boat, an ASV can
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be much smaller and lighter, thus easier to ship to field locations and able to operate in small areas such as side channels. An ASV has a
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many potential advantages over a manned boat, including the ability to precisely follow pre-planned missions and repeat survey lines, work
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in hazardous environments and more precisely control operating parameters such as speed over ground. An ASV opens up future
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opportunities for adaptive sampling using real time sensor feedback and as coordinated, multi-vehicle operations.
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2. Materials and Methods: Development of the ChemYak
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2.1 JetYak Platform
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The JetYak (Fig. 1) is a small gas-powered ASV, that has been developed for coastal and polar environments at the Woods Hole
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Oceanographic Institution27 using a Mokai jet-powered kayak as the base platform. The JetYak has space for instrumentation, servo-driven
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controls, a Pixhawk autopilot (running Ardupilot, an Arduino-based open source platform), an onboard computer for instrument control and
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data logging, and radios for wireless operation and communications. With a draft of
