All the Light We Cannot See: Deploying Optical Sensors to Study the Ocean

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A point source integrating cavity absorption meter has a compact design, long optical path length (and so is highly sensitive), a diffuse light field (which eliminates scattering error) and uses the whole visible light spectrum. It is the manually operated precursor of the hyperspectral absorption sensor, which incorporates a water flow-through system. Image Credit: Jochen Wollschläger

Hyperspectral cavity absorption sensors allow scientists to understand ocean microalgae cycles.

The NeXOS project’s objective was to see, hear, and experience the ocean to broaden human understanding of changes to the marine environment’s biologic, chemistry and physical processes. One of the 10 sensors the project created to monitor the ocean was the hyperspectral cavity absorption sensor, which aids scientists in “seeing” the ocean, and in particular, phytoplankton.

“The anthropogenic input caused by activities in the coastal areas is linked to nutrient influxes and sewage plant influxes, so (the sensor) helps you to trace water masses and look where they originate from,” said Oliver Zielinski, director of the Institute for Chemistry and Biology of the Marine Environment at the University of Oldenburg in Germany. “When you have too many dissolved organics, the water gets dark, light penetration is blocked, primary production is inhibited.”

But why is it so important to understand dissolved organics and phytoplankton concentrations in the oceans? It’s simple, really — humanity needs the oceans to survive.

“Every second breath we take comes from the ocean. Nearly 50 percent of oxygen on this planet is originating from the sea and not the land. Every second breath is owed to the ocean,” Zielinski said. “My interest is in making environmental parameters measurable in real time. If you want to protect something, you need to understand a process, and to understand your monitoring or mitigating effects, you have to measure it.”

While a common method of testing a water sample is to process the sample with a mass spectrometer, which processes the masses of chemical molecules within, frequently scientists need to take measurements in situ. These sensors are capable of measuring their respective parameters highly resolved in time and space, making them the next best thing to an in-lab test.

Researchers developed two versions of the hyperspectral cavity absorption sensor. One uses a FerryBox system, which is a large, automated bench-top instrument often attached to ferries and other ships that traverse the same waters frequently. The other, which is commercially available, is significantly smaller and submersible, but lacks the degree of automation.

The hyperspectral cavity absorption sensor is filled with water and operates in flow-through mode. Light enters the cavity and bounces off the highly reflective walls, which allows the light to travel a long distance — upwards of 10 meters — before it hits the detector. The sensor’s ability to measure organic materials for days at a time makes it an ideal choice to study organic material in the ocean; the oceans have 800 gigatrons of organic matter — the same amount of biomass as is found on land.

The bench-top version of the sensor has the advantage of being able to operate for up to a month at a time without maintenance. Normally, researchers calibrate such an instrument with a liquid dye. The hyperspectral cavity absorption sensor uses a solid black stick instead to create known absorption within the cavity, which fosters automated calibration of the system and lengthens duration of operation significantly.

The smaller version lacks the duration and automation of the larger version, but its size allows it to be easily mounted on board vessels.

Within the cavity, the organic material in the water absorbs light, so the sensor is able to detect the types of organic material in the water spending on light scattered by those materials. Included in the category of organic material are the many types of phytoplankton in the seas.

Phytoplankton are photosynthetic organisms — microalgae — that form the foundation of the ocean’s food chain. It’s the primary food of zooplankton and small fishes. Remarkably, the hyperspectral cavity absorption sensor can differentiate between these tiny creatures.

“The sensors can not only see phytoplankton as a total sum, but can discriminate between different types of phytoplankton,” Zielinski said. “When a phytoplankton is in the water column, it has absorption and scattering properties. The sensor is blind against scattering, but can sensitively measuring absorption, get the absorption fingerprint of the phytoplankton, and with that fingerprint, can identify it.”

Zielinski said the sensor can see the changes to the types of phytoplankton through the year depending on the life cycles of ocean ecosystems. Diatoms, which are chain-forming circular algae, grow the beginning of the season, providing food for fish larvae. As each year progresses, other phytoplankton grow. Unfortunately, anthropogenically caused changes to the oceans can lead to out-of-control phytoplankton growth, which can cause harmful algal blooms.

Currently, the reliability of the phytoplankton detection is closely related to the quality of the database used, said Jochen Wollschläger, also with the Institute for Chemistry and Biology of the Marine Environment.

Absorption measurements, by showing the shape of spectra, provides information for phytoplankton group identification. Image Credit: Jochen Wollschläger

“We measure the absorption spectrum with the instrument. The spectrum has a certain shape. We compare the shape to a database of spectra of which we know what kind of phytoplankton was present,” Wollschläger said. He noted when he and his co-authors publish their data, the database they uses will become available publicly. Until then, researchers rely on their own phytoplankton spectra databases.

The hyperspectral cavity absorption sensor is part of a generation of multifunctional sensors that are web-enabled across platforms, allowing greater communication between sensor systems that allows scientists to improve management of fisheries, better understand marine ecosystem life cycles and monitor contaminants.

Wollschläger said the idea for the hyperspectral cavity absorption sensor was developed from researchers who also were interested in harmful algal blooms. As long as harmful algal species have significant features in their absorption spectra and are included in the phytoplankton database, the sensor can detect them.

“The instrument can identify certain phytoplankton characterized by certain pigments which influence the absorption spectrum,” Wollschläger said.

Wollschläger said until now phytoplankton investigations have often been restricted to biomass measurements. New sensors give researchers more information about the succession of the phytoplankton during the year. As scientists accumulate time series data, they could potentially better understand the relationship between harmful algal blooms and climate change.

Finally, the hyperspectral cavity absorption sensor allows researchers who are working on identifying phytoplankton from satellite imagery validate their measurements.

“To do something 24/7 you need in situ sensors that are robust and reliable,” Zielinski said. “Optics is a very robust 24/7 tool to address key parameters in the marine ecosystem. It’s still a young technology. I’m pretty sure there will be more technological steps within the next 10 to 20 years. This is just the beginning.”

This is part of a series of NeXOS articles. The others are below:


This article was funded in part by the NeXOS project by Grant Agreement No. 614102 under the call FP7-OCEAN-2013.2 from the European Commission.

Kelley Christensen is Earthzine’s science editor. Follow her on Twitter @kjhchristensen.