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Marine Biogeochemistry in 2025

Kenneth S. Johnson*


Developments in the past five years have enabled a remarkable shift in measurement capabilities that will revolutionize our approach to observing ocean biogeochemistry on a global scale. A suite of chemical and biological sensors can now be deployed for years in the ocean on profiling floats and return data with no detectable drift in sensor response. These systems are becoming sufficiently affordable that it is possible to envision biogeochemical sensor networks with hundreds of nodes or more, similar to the current Argo network of 3000 profiling floats. This will allow the development of basin-scale and, ultimately, global-scale observing systems. These sensor networks will permit ocean scientists to quantitatively observe fundamental biogeochemical processes such as rates of nutrient supply, net community production, physical controls on bloom development (e.g., the Sverdrup Hypothesis), dynamics of oxygen minimum zones and their impacts on denitrification, and carbon export throughout the ocean with a level of detail hitherto impossible. The spatial and temporal responses of these processes to climate oscillations and greenhouse gas forcing will be observed with a resolution that is simply not possible when observations are limited to ships. An integrated observing system that combines in situ sensors deployed on long endurance platforms with satellite sensors and data-assimilating, biogeochemical-

*

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Marine Biogeochemistry in 2025 Kenneth S. Johnson* Developments in the past five years have enabled a remarkable shift in measurement capabilities that will revolutionize our approach to observing ocean biogeochemistry on a global scale. A suite of chemical and biological sensors can now be deployed for years in the ocean on pro- filing floats and return data with no detectable drift in sensor response. These systems are becoming sufficiently affordable that it is possible to envision biogeochemical sensor networks with hundreds of nodes or more, similar to the current Argo network of 3000 profiling floats. This will allow the development of basin-scale and, ultimately, global-scale observing systems. These sensor networks will permit ocean scientists to quantitatively observe fundamental biogeochemical processes such as rates of nutrient supply, net community production, physical controls on bloom development (e.g., the Sverdrup Hypothesis), dynamics of oxygen minimum zones and their impacts on denitrification, and carbon export throughout the ocean with a level of detail hitherto impossible. The spa- tial and temporal responses of these processes to climate oscillations and greenhouse gas forcing will be observed with a resolution that is simply not possible when observations are limited to ships. An integrated observ- ing system that combines in situ sensors deployed on long endurance platforms with satellite sensors and data-assimilating, biogeochemical- * Monterey Bay Aquarium Research Institute 130

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131 KENNETH S. JOHNSON ecological models would provide previously unachievable constraints on the carbon cycle and its sensitivity to a changing climate. It would transform ocean biogeochemistry. Today, our primary sources of information on temporal variability of biogeochemical processes within the ocean come from a few, ship-based time series programs at single points (e.g., Hawaii Ocean Time-series [HOT], Bermuda Atlantic Time-series Study [BATS], Carbon Retention In A Colored Ocean [CARIACO], European Station for Time-series in the Ocean, Canary Islands [ESTOC]) and from satellite ocean color measure- ments. In addition, a few programs, such as the Atlantic Meridional Tran- sect (AMT), involve repeat transects over broad regions at near annual scales. Ship-based sampling at time series sites is generally monthly, at best, which misses high frequency processes. Satellites excel at provid- ing global coverage at higher frequencies in cloud-free areas, but ocean color data is generally limited to one optical depth (<30 m in much of the ocean), the data do not resolve vertical structure and many high latitude areas are not cloud-free. The result is that we have little understanding of how ocean biogeochemistry is changing in response to natural climate oscillations such as El Niño and PDO or to anthropogenic climate changes driven by burning fossil fuels. Even the processes that control regular, annual events such as the spring bloom at high latitudes are not always well understood. Oxygen sensors are now being deployed on profiling floats for multi- year periods with little or no drift in sensor response (Kortzinger et al. 2005; Johnson et al. 2007). These sensors are being used to study ocean ventilation (Kortzinger et al. 2004), the balance of net community produc- tion in oligotrophic regions (Riser and Johnson 2008), and carbon export (Martz et al. 2008). Remarkable precision has been attained with oxygen sensors deployed for years on profiling floats (Kortzinger et al. 2005; Riser and Johnson 2008). Bio-optical sensor technologies have also advanced rapidly. There have now been a number of studies using sensors on profiling floats (Mitchell et al. 2000; Bishop et al. 2002; 2004). The data reported by Boss et al. (2008a; 2008b) show measurements of chlorophyll fluorescence from 400 m depth to the surface in the North Atlantic for three years (Figure 1). These observations clearly resolve the annual cycle with no apparent drift in sensor response at depth and the data show remarkable events driven by mesoscale processes. Optical nitrate sensors (Johnson and Coletti 2002) are currently deployed on profiling floats and have operated successfully for more than one year with little sensor drift (Figure 2). The power budget implies that they can operate for 4 years with 60 nitrate measurements from 1000 m to the surface at a cycle time of 5 days. Plans to deploy pCO2 sensors on floats are under way (Kortz- inger, personal communication). Long-endurance pH sensors based on

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132 OCEANOGRAPHY IN 2025 FIGURE 1 Chlorophyll fluorescence observed over three years with sensors on an Apex profiling float in the North Atlantic. The spring bloom is clearly resolved each year and a remarkable export event is seen in mid- to late-2006. Data from Boss et al. (2008a). Ion Selective Field Effect Transistor technology are being adapted for use on profiling floats. Optical particulate inorganic carbon sensors are in development with an eye towards deployment on profiling floats (Guay and Bishop 2002). By 2025, we can expect that the ocean will be populated with a dense array of biogeochemical sensors on platforms that have evolved from the current design of profiling floats. These sensors will allow ocean scien- tists to monitor significant components of the carbon cycle, ranging from primary production to carbon export, without leaving their office. The availability of this array will greatly shift the way biogeochemistry of the ocean is studied. Numerical models will continuously assimilate this biogeochemical data and offer real time assessments of biogeochemical processes throughout the ocean. Shipboard research will focus primar- ily on process studies that are conducted within the framework of the background sensor array. These studies will refine our understanding of detailed environmental impacts on processes observed with the global array. Development of the array will enable scientists from around the world, who do not have direct access to the ocean, to participate in bio- geochemical science that is on the leading edge.

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133 KENNETH S. JOHNSON Nitrate[ M] 0 12 10 50 8 100 Depth[m] 6 150 4 200 2 250 Ocean Data View 0 300 0 100 200 300 400 Days Since 1/1/2008 FIGURE 2 One year of nitrate measurements made with an ISUS nitrate sensor (Johnson and Coletti, 2002) deployed on an Apex profiling float near Hawaii. Real Johnson_Figure2.eps time data is available at http://www.mbari.org/chemsensor/floatviz.htm. Data from K. Johnson and S. Riser. REFERENCES Bishop, J.K.B., R.E. Davis, and J.T. Sherman. 2002. Robotic Observations of Dust Storm En- hancement of Carbon Biomass in the North Pacific. Science. 298: 817-821. Bishop, J.K.B., T.J. Wood, R.E. Davis, and J.T. Sherman. 2004. Robotic Observations of En- hanced Carbon Biomass and Export at 55S. Science. 304: 417-420. Boss, E., M.J. Perry, D. Swift, L. Taylor, P. Brickley, J.R. Zaneveld, and S. Riser. 2008a. Three Years of Ocean Data from a Bio-optical Profiling Float. Eos. 89(2): 209-210. Boss, E., D. Swift, L. Taylor, P. Brickley, R. Zaneveld, S. Riser, M.J. Perry, and P.G. Strutton. 2008b. Observations of Pigment and Particle Distributions in the Western North Atlan- tic from an Autonomous Float and Ocean Color Satellite. Limnology and Oceanography. in press. Gruber, N., S.C. Doney, S.R. Emerson, D. Gilbert, T. Kobayashi, A. Körtzinger, G.C. Johnson, K.S. Johnson, S.C. Riser, and O. Ulloa. 2006. The Argo-Oxygen Program: A White Paper to Promote the Addition of Oxygen Sensors to the International Argo Float Program. [Online]. Available: http://www-Argo.ucsd.edu/o2_white_paper_web.pdf. Guay, C.K.H. and J.K.B. Bishop. 2002. A Rapid Birefringence Method for Measuring Sus- pended CaCO3 Concentrations in Seawater. Deep-Sea Research I. 49: 197-210. Johnson, K.S. and L.J. Coletti. 2002. In situ Ultraviolet Spectrophotometry for High Resolu- tion and Long Term Monitoring of Nitrate, Bromide and Bisulfide in the Ocean. Deep- Sea Research I. 49: 1291-1305. Johnson, K.S., J.A. Needoba, S.C. Riser, and W.J. Showers. 2007. Chemical Sensor Networks for the Aquatic Environment. Chemical Reiews. 107: 623-640, doi 10.1021/cr050354e.

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13 OCEANOGRAPHY IN 2025 Körtzinger, A., J. Schimanski, U. Send, and D. Wallace. 2004. The Ocean Takes a Deep Breath. Science. 306: 1337. Körtzinger, A., J. Schimanski, and U. Send. 2005. High Quality Oxygen Measurements from Profiling Floats: A Promising New Technique. Journal of Atmospheric and Oceanic Tech- nology. 22: 302-308. Martz, T.R, K.S. Johnson, and S.C. Riser. 2008. Ocean Metabolism Observed with Oxygen Sensors on Profiling Floats in the Pacific. Limnology and Oceanography, in press. Mitchell, B.G., M. Kahru, and J. Sherman. 2000. Autonomous Temperature-irradiance Pro- filer Resolves the Spring Bloom in the Sea of Japan. Proceedings, Ocean Optics XV. Monaco, October 2000. Riser, S.C. and K.S. Johnson. 2008. Net Production of Oxygen in the Subtropical Ocean. Nature. 451: 323-325.