Zoom instrumental : C-OPS

Light is essential to the growth of phytoplankton, microscopic plants which form the base of the marine food chain. We have long thought that it was too dark beneath sea ice for algae to grow.

However, the ice-covered Arctic Ocean may no longer be as dark as we thought.

Global warming is thinning the ice sheet and enhancing the snowmelt, so that more sunlight gets transmitted through the Arctic ice cover. One of the objectives of the GreenEdge project is to measure the intensity and the quality of sunlight, how much of it actually makes it through the sea ice, how that light fades with depth in the water column, and how it changes from spring to summer.

This post is about how we measure the intensity and spectrum (colour) of light above and below sea ice.

We use a Compact Optical Profiling System, or C-OPS, which we have customized for use in the Arctic environment, and baptized it “IcePRO”. The IcePRO instrument consists of three irradiance sensors, which capture downward or upward travelling photons. One sensor is installed on a tripod on the ice and looks upward to capture photons travelling downward (incoming sunlight). Two submersible sensors are mounted in a rugged cylindrical case, which we lower to a 100 m depth through a hole in the sea ice. One sensor is looking upward, the other downward. In addition to light, temperature and depth are also measured and transmitted directly to a computer via an electro-optical cable.

Sans titreThe C-OPS profiling radiometer, in its sea-ice version (the so-called IcePRO profiling radiometer), once the wet unit has been deployed in a 10 inches diameter hole in the sea-ice. The bright white circle is the optical diffuser that fits the top of the upward looking light sensor (measuring the light that is travelling downward then!).
Picture credit: Guislain Bécu/Takuvik

Each sensor of the IcePRO is made up of nineteen smaller sensors, each with a different spectral filter that let through only photons with specific frequencies or wavelengths while blocking all others (your sunglasses, for example, are a spectral filter that block harmful ultraviolet radiation). The sensors are really fast, making fifteen measurements per second which allows to resolve the underwater light field at 2 cm vertical resolution.

Sans titre2

The C-OPS profiling radiometer, in its sea-ice version: the so-called IcePRO profiling radiometer. The same wet unit, with the 2 black optical sensors visible from the side (one looking up, the other looking down), hanging in air.
Picture credit: Biospherical Instruments, Inc.

Since May 4th, we deploy the IcePRO instrument in two holes in the sea ice which are forty meters apart. One hole was made in a spot where the ice sheet is 94cm thick with an overlying layer of snow of 35cm, the “high snow” spot, the other hole has 143cm of ice and only 6 cm of snow, the “low snow” spot.

The difference in snow thickness profoundly impacts the amount of sunlight that can reach the underlying seawater.

Our IcePRO measurements show that only about 0.1 percent of the sunlight that sustains phytoplankton growth (PAR, photosynthetically active radiation) gets transmitted through the ice sheet in the high snow spot. In contrast, over 5% of PAR gets transmitted in the low snow spot. That is a difference of a factor 50!

Our IcePRO data show that not only the overall intensity of the light (PAR) is much higher in the low snow spot than in the high snow spot,

The color is also very different: blue under high snow conditions and green under low snow.

Our above and below ice light measurements in the high and low snow spots are shown in the figures below

2015126_icepro_150506_1501_c_data_004_especs

Measurements at High Snow spot. Top: Spectrum of sunlight above the ice. The peak is at blue wavelengths (450-495 nm). Bottom: spectrum of sunlight at various depths in the water column. Almost no red photons (620-750nm) made it through the ice sheet and through the first 3 meters of water (because of strong absorption of red light by ice and water. The peak at all depths is at blue wavelengths.

2015126_icepro_150506_1501_c_data_005_especs

Measurements at Low Snow spot. Top: Spectrum of sunlight above the ice (nearly identical to the High Snow Spot). Bottom: spectrum of sunlight at various depths in the water column. The peak of these curves are shifted towards green wavelengths (495-570nm).

Why? You will find the answer in the GoPRO videos below, which were made in the low snow and high snow spots:

Phytoplankton absorb blue and red light more than green light and use the energy for photosynthesis. The effect of their presence is a green coloring of seawater.

Sans titre3

The IcePRO is being initialized. The tripod on which the atmospheric reference optical sensor is mounted can be seen on the background. Also, the electromechanical cable used to deploy the wet unit can be seen, in the green bucket, an the wet unit with the 2 optical ss can be seen next to the green bucket.
Picture credit: Éric Brossier/Vagabond.

Sans titre4

The IcePRO wet unit is being prepared for its first deployment.
Picture credit: Éric Brossier/Vagabond.

Sans titre5

First deployment of the wet unit. The unit is deployed far away from the logging area (see previous pictures), to avoid as much as possible any optical pollution.
Picture credit: Éric Brossier/Vagabond.

Text : Griet Neukermans and Guislain Becu

Crédit Photos : Sharif Mirshak, Claudie Marec, Éric Brossier

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