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Biological oceanography

Size matters: The power of particles in determining ocean color

Reference: Davies EJ, Basedow SL, Mckee D (2021) The hidden influence of large particles on ocean colour. Sci Rep 11:1–9. DOI: https://doi.org/10.1038/s41598-021-83610-5

Why is the ocean blue?

There are more small particles in the ocean (like bacteria and phytoplankton) than there are larger ones (like zooplankton and fish) (diagram by science.kennesaw.edu).

It’s more complicated than you might think! Most of the sunlight that hits the ocean is absorbed. But sunlight is made of many different wavelengths of light and not all are absorbed equally. Shorter wavelengths of light (like reds) are absorbed faster in the ocean than longer wavelengths of light (like blues). Some sunlight is also absorbed by particles in the ocean. There are billions of particles, ranging from plankton, passive ocean drifters to dissolved material, like dirt, that affect the color of the ocean. When sunlight hits a particle in the ocean, two things can happen: The light is absorbed or the light is scattered.  If light is absorbed, we don’t see those colors, but if the light is scattered it makes its way to our eyes and lends hues to the ocean’s coloration.

Ocean color can even reveal the biological and physical processes occurring beneath its surface. For example, abundant microorganisms (called phytoplankton) perform photosynthesis using green-colored pigment (called chlorophyll). When ocean regions contain an abundance of phytoplankton, the ocean waters appear greener because of the increased amount of chlorophyll in the water.

Scientists have even developed satellites in space and instruments on ships that measure the absorption and scatter of light from different particles in the ocean to determine what could be causing the ocean to appear different colors. These technologies allow oceanographers to “see” ocean properties like how abundant plankton are in an area – they can even detect swarms of plankton from space!

There are more small particles in the ocean (like bacteria and phytoplankton) than there are larger ones (like zooplankton and fish) (from Davies et al. 2021, Creative Commons License 4).

These properties, in turn, can tell scientists how much energy is being generated at the base of the food web through photosynthesis. However, the instruments used to detect particles in the ocean, and determine how they influence ocean color, are actually much better at detecting smaller particles (like phytoplankton) than ‘larger’ particles (like zooplankton (marine animal drifters) and fish eggs) because the ‘larger’ particles are relatively sparse in the ocean, and are therefore, often overlooked by the satellites trying to detect them.

 

 

What can ocean color tell us?

Example image of a Lisst 100X with an external battery as used by the researchers in this study to measure optical properties of larger particles, like zooplankton and fish eggs (from Davis et al. 2021, Creative Commons License 4).

New research published by a team of scientists from Norway and the UK assessed how different sized particles (such as fish eggs or zooplankton) contribute to light absorption and scatter, and therefore influence ocean color. The scientists used two special instruments with cameras that measure ocean color, LISST-100x and the SINTEF SilCam, to determine what wavelengths of light different types of particles (like fish eggs or zooplankton) absorb or scatter. They found that even though there are fewer of these large particles (compared to the abundant but small phytoplankton), they still influence how light scatters through water and can alter the signals scientists measure by satellites. Therefore, they encourage future studies to increase ocean measurements for larger particles or else satellites may underestimate significant numbers of large particles.

Natural particle size distributions in the ocean cover many orders of magnitude, from bacteria to fish. Current ocean color technologies are designed to measure light absorption and scatter (reflectance) from bacteria to phytoplankton, but the researchers in this study argue oceanographers need to increasingly measure optical properties from larger particles, like zooplankton and fish eggs, as well.

Why does it matter?

The study of ocean color helps oceanographers and space scientists gain a better understanding of the impact of plankton on the Earth system. Plankton are important components of the ocean carbon cycle, taking up carbon dioxide from the atmosphere and transporting carbon through vertical migration to depths in the ocean where it can be ‘stored’ for thousands of years. Oceanographers are currently very good at detecting phytoplankton from satellite, but fail to detect larger particles that include zooplankton and fish eggs. To fully understand how all types of plankton contribute to the ocean carbon cycle, instruments measuring ocean color need to be increasingly improved and measure particles from a much broader range than is currently done to successfully incorporate large particles into remote sensing programs.

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