Evaluating the Sensor-Equipped Autonomous Surface Vehicle C-Worker 4 as a Tool for Identifying Coastal Ocean Acidification and Changes in Carbonate Chemistry

Sarah Cryer,Terry Wood,James Asa Strong,Filipa Carvalho,Richard Sanders,Arlene Young,Claire Evans,Socratis Loucaides,Peter Brown

doi:10.3390/jmse8110939

Abstract

The interface between land and sea is a key environment for biogeochemical carbon cycling, yet these dynamic environments are traditionally under sampled. Logistical limitations have historically precluded a comprehensive understanding of coastal zone processes, including ocean acidification. Using sensors on autonomous platforms is a promising approach to enhance data collection in these environments. Here, we evaluate the use of an autonomous surface vehicle (ASV), the C-Worker 4 (CW4), equipped with pH and pCO2 sensors and with the capacity to mount additional sensors for up to 10 other parameters, for the collection of high-resolution data in shallow coastal environments. We deployed the CW4 on two occasions in Belizean coastal waters for 2.5 and 4 days, demonstrating its capability for high-resolution spatial mapping of surface coastal biogeochemistry. This enabled the characterisation of small-scale variability and the identification of sources of low pH/high pCO2 waters as well as identifying potential controls on coastal pH. We demonstrated the capabilities of the CW4 in both pre-planned “autonomous” mission mode and remote “manually” operated mode. After documenting platform behaviour, we provide recommendations for further usage, such as the ideal mode of operation for better quality pH data, e.g., using constant speed. The CW4 has a high power supply capacity, which permits the deployment of multiple sensors sampling concurrently, a shallow draught, and is highly controllable and manoeuvrable. This makes it a highly suitable tool for observing and characterising the carbonate system alongside identifying potential drivers and controls in shallow coastal regions.

Highlights

The ocean has taken up approximately 25% of all CO2 emitted to the atmosphere by human activity [1], resulting in a decrease in ocean pH of 0.1 units since 1750 [2], a phenomenon known as ocean acidification
We evaluate the use of an autonomous surface vehicle (ASV), the C-Worker 4 (CW4), equipped with pH and pCO2 sensors and with the capacity to mount additional sensors for up to 10 other parameters, for the collection of high-resolution data in shallow coastal environments
The CW4 is a platform that has the capability of being fitted with commercially available sensors and collect high-resolution biogeochemical and oceanographic data that can begin to mitigate this data gap

Summary

Introduction

The ocean has taken up approximately 25% of all CO2 emitted to the atmosphere by human activity [1], resulting in a decrease in ocean pH of 0.1 units since 1750 [2], a phenomenon known as ocean acidification. Oceanic uptake is directly proportional to the gradient in pCO2 between the surface ocean and the atmosphere [3]. Current models indicate that projected emission rates will cause 1000 μatm pCO2 to be reached by 2100, with ocean pH decreasing by a further 0.3 units [4]. It is becoming increasingly apparent that ocean acidification driven by the uptake of anthropogenic CO2 from the atmosphere alone is mainly an open ocean syndrome and that coastal ocean acidification has far more complex causes [6]. Full characterisation of coastal ocean acidification requires highly resolved measurements of multiple parameters over both spatial and temporal scales [10,11], with autonomous platforms providing a scalable solution to this challenge [10]

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Conclusion