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Earth & Marine Sciences - Seeing the Light

Marine optics research has the potential to tell us more about what lies beneath

by Aimee Dolloff

Emmanuel Boss

Emmanuel Boss

For oceanographer Emmanuel Boss, uncovering secrets underwater isn’t about finding buried treasure or unearthing sunken ships.

Using specialized sensors, Boss studies the effects of particles on the underwater light to understand what lies beneath. His research focuses on what marine optics can tell us about what’s in the ocean and how those particles — most no bigger than a few micrometers — impact processes in the water, as well as in the global system.

Knowing what’s below the surface throughout the oceans is needed in researching the role of the water in cycling elements such as carbon that are fundamental in understanding Earth’s climate and monitoring climate change. With marine optics, Boss and other scientists use satellite ocean color imagery to study ecological processes, such as what species of phytoplankton dominate, and biogeochemical processes, including how fast carbon is fixed by phytoplankton into organic material.

“There are many events in the ocean that are happening that we’re completely missing using current sampling techniques,” says Boss, an associate professor in the University of Maine School of Marine Sciences. “With data from new systems, potentially we can start accounting for those.”

Most recently, by using novel robotic technology he equipped with optical sensors, Boss has been able to probe beneath the ocean’s surface to layers not observable from space. At first, when he began recording data with the sensors on profiling floats, there was nothing unusual in the data.

“Then in 2006, a float got stuck in an eddy and suddenly, at 1,000-meter depths, we saw an elevated amount of particles we had not seen during the whole mission,” he says. “The sensor had been stable over three years. There’s nothing at the surface that could explain how these particles were produced. I still have no idea where they came from. That’s what makes this research exciting.”

Scientists are particularly interested in particles sinking to great depths — an important process in carbon sequestration from the atmosphere. Boss is focused on the connection between the ocean physics and the biosphere — the part of the Earth and its atmosphere that is capable of supporting living organisms.

During photosynthesis, plants convert carbon dioxide into organic particles that are distributed throughout the upper water column. Some remain suspended while others are carried to depth.

“It’s crucial we understand their dynamics if we want to understand the role of the ocean in the carbon cycle,” Boss says.

For Boss, that starts by determining what is in the ocean and why it’s there. Using passive sensors that take advantage of the sun’s light as a source, and active optical sensors that produce their own light source, he quantifies particle concentration, composition and size distribution. Optical properties of suspension reveal changes in concentration of material in the water.

Boss is also beginning to study acoustical properties of the water column by measuring sound intensity scattered by particles at different frequencies.

“Acoustical and optical properties of material reflect on their size, composition and, to a lesser degree, on their shape and internal structure. Different wavelengths of light or frequencies of sound interact with the material differently, meaning they are sensitive differently to the properties of the particles,” says Boss.

Those properties were primarily measured by sensors on stationary buoys or aboard ships. Boss has pioneered the use of profiling floats designed to collect ocean temperature and salinity profiles to measure biogeochemical properties. The battery-powered devices drift with the currents, descending and ascending by taking in or releasing oil in an external bladder. At the surface, the float relays data via a satellite to users, as well as international databases.

“Oceanographers have collected a lot of data in the open ocean, but during spotty campaigns and on moorings lasting only a few years,” says Boss. “The floats allow you to have a persistent presence.”

While more than 3,000 profiling floats now measure ocean temperature and salinity, only a handful have optical sensors to measure properties such as fluorescence, scattering and attenuation. Boss hopes to change that with national and international collaborative programs to equip floats with sensors with expanded capabilities.

Last fall, he was awarded $1.5 million from NASA under the National Oceanographic Partnership Program for the development, assessment and commercialization of biogeochemical profiling floats for calibration and validation of ocean color and carbon studies. The project is a collaboration between UMaine and global leaders in sensor, platform and communication technologies. Satlantic Inc., of Halifax, Nova Scotia, and WET Labs of Philomath, Ore., are manufacturing the sensors; Teledyne Webb Research of East Falmouth, Mass., the floats; Maryland-based CLS America, tools for the floats’ satellite communication and data dissemination.

“What we are pioneering is a new end-to-end float-sensor system that measures physical, chemical and biological parameters,” says Boss.

The sensors Boss previously deployed on floats measured bio-optical properties — scattering (the process of shooting a beam of light and measuring what comes back), and the fluorescence of chlorophyll (shining a blue beam of light and looking at only what returns in the red light). The goal of the new project is to develop an advanced biogeochemical profiling float.

The float work will focus on organic carbon dynamics in the upper ocean, including measurement of the color of available light, as well as proxies of particulate organic carbon, chlorophyll and colored dissolved material. Oxygen sensor data will be compared with the optics-based estimated productivity in water. The sensors will be powered by and attached to a Teledyne Webb Research APEX float with conductivity temperature and depth sensors.

A major thrust of the effort is to establish the potential of this technology for calibration and validation of satellite-based ocean-color measurements. The new floats are enabled with a two-way communication system that allows researchers to control when the floats descend and ascend, and when they take measurements.

“Radiometers allow us to do a better job in modeling primary production,” says Boss. “We’re trying to see if we can use them to calibrate satellites, and plan on having other sensors measure for scattering. That allows us to get more information on what’s in the water.”

Most of the existing floats are programmed to descend and ascend for specific periods of time to take a predetermined number of measurements. Using wireless communication and data dissemination created by CLS America, researchers will provide the floats with commands during missions, including changes in response to events such as hurricanes.

The data collected will be sent to a centralized Web site for all researchers to analyze and for future input into assimilating ocean ecosystem models.

With more advanced communications systems, it may also be possible to increase the life of profiling floats. Currently, researchers can record about 300 profiles from one float. The devices are limited by battery life, and once the batteries die, it’s not possible to recapture the devices. One of Boss’ goals is to test recovery possibilities, so that floats can be reused.

Scientists from the NASA Goddard Earth Sciences Data and Information Services Center, partners in this project, are building a tool that will provide crucial remotely sensed information around the float surfacing location for measurement context. Every time a float reports its location, NASA will provide real-time data on weather, temperature and events in a 50-kilometer radius.

“We have the opportunity to make a huge difference in the future of our field and its ability to provide much-needed information on how carbon and other material are processed globally,” says Boss.

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