SOURCE: Nicholson, D.; Michael, A. Manganini, K.; Sugrue, R.; Sandwith, Z.; Monk, S. (2018). Rapid Mapping of Dissolved Methane and Carbon Dioxide in Coastal Ecosystems Using the ChemYak Autonomous Surface Vehicle. Environmental Science & Technology, 52 (22), 13314-13324. DOI: 10.1021/acs.est.8b04190
CARBON CYCLING IN COASTAL ECOSYSTEMS
Coastal ecosystems are areas where land and water meet, such as salt marshes, estuaries, and bays. One important characteristic of coastal ecosystems is high nutrient levels, which results in high productivity and element cycling in the water and sediment. One of these elements is carbon. Carbon is an important element for several reasons. First, it is one of the essential building blocks of life. In fact, 18% of the human body is carbon. Carbon is also a fossil fuel used for energy. Unfortunately, when carbon is burned for energy it is released to the atmosphere as carbon dioxide. Carbon dioxide is a greenhouse gas that traps heat in the Earth’s atmosphere contributing to climate change.
The cycling of carbon on Earth is complex and coastal ecosystems play an important role (Figure 1). In coastal ecosystems, carbon can be buried as organic matter in sediments. Carbon may also be released to the atmosphere and ocean as carbon dioxide and methane. Because coastal ecosystems themselves are complex with many inputs and outputs, it is difficult to know their exact contribution to carbon cycling. It is important that scientists develop a better understanding of the movement of carbon through coastal ecosystems because it will help determine carbon budgets on a large scale and predict how it will change in the future.
Traditionally, carbon measurements in coastal ecosystems were made by collecting samples and analyzing them in a laboratory. Because coastal ecosystems are complex measurements taken near each other may produce very different results. This requires collection of many samples, which is both expensive and time consuming. As a result, scientists have been looking for new approaches to measure carbon in coastal ecosystems. In the study highlighted here, David Nicholson and his colleagues developed an autonomous (driver-less) surface vehicle they think will change the way carbon measurements are collected.
David Nicholson and his colleagues called their new autonomous surface vehicle the ChemYak (Figure 2). The ChemYak is a modified JetYak, a small gas-powered kayak developed by the Woods Hole Oceanographic Institution. David and his fellow researchers attached an instrument to the JetYak that can measure carbon dioxide and methane in the water using infrared signals. This is the first autonomous surface vehicle capable of measuring methane dissolved in water. In addition, the ChemYak can also measure conductivity (salinity), temperature, water depth, dissolved oxygen, and nitrate (a type of nutrient). Above the water, the ChemYak has a weather station that measures GPS position, air temperature, wind velocity, and relative humidity. The ChemYak can run for 8-10 hours on one battery pack and has a max speed of 3.5 m/s (~11.5 ft/s).
Autonomous vehicles have several advantages compared to manned vehicles. Autonomous vehicles are smaller and lighter making them easier to transport to field locations, and are capable of operating in small shallow channels. For example, the ChemYak lies 20 cm (~7.9 in.) in the water, allowing it to take measurements in shallow waters that are normally inaccessible with traditional boats. In addition, autonomous vehicles can follow preplanned missions, repeat previous missions, and work in environments that are hazardous to humans.
The researchers tested the ChemYak in the North River Estuary in Marshfield Massachusetts. On day one they deployed the ChemYak in the main estuary channel and a small tidal creek called Cove Brook. During the deployment the ChemYak successfully traveled about 12 km (~7.5 miles). With the data collected, the scientists determined there was a net release of carbon dioxide and methane from the river to the atmosphere. In addition, the amount of methane and carbon dioxide were greater in Cove Brook than in the main river channel.
PROS AND CONS
The field test allowed the scientists to identify several pros of the ChemYak. For starters, the chemical analysis of carbon dioxide and methane is done in the environment rather than at a laboratory. This expedites the generation of results. Also, the ChemYak can collect data at a much faster rate than humans collecting water samples during a given time period. Finally, the ChemYak can collect data on a larger spatial scale.
The researchers also identified some areas in need of improvement. Sometimes air bubbles got into the sensors causing a break in measurements. David and his colleagues also want to add additional sensors for pH and nitrous oxide (another greenhouse gas). However, the system can quickly become too heavy from battery packs and instrumentation. In addition, the researchers would like to run the system on an energy source other than gasoline.
In the end, the ChemYak offers the potential to enhance our knowledge of carbon cycling in coastal ecosystems. This will allow scientists to better predict how coastal ecosystems will impact carbon cycling in the future.