Light & Color in the Ocean

The penetration of sunlight into the sea and its interaction with water and dissolved and suspended materials is an important physical phenomenon in the ocean.

Introduction to Light: Khan Academy

By Dr. Sean Chamberlin

Evaporation and precipitation determine the salinity of the ocean in any given region and both of these processes depend on energy from the sun. Heat from the sun drives the ocean currents and modifies our climate. Sunlight also provides an energy source for the photosynthetic processes of phytoplankton, on which most life in the sea depends. Thus, it behooves us to know a little about sunlight in the sea.

The study of light in the sea is a recent subdiscipline of oceanography called optical oceanography. Oceanographers who study light in the sea and its interactions with the flora and fauna are called bio-optical oceanographers.

When you look at a blue piece of fabric, a red car, the blue sky, have you ever stopped to consider why you see that particular color? It’s very simple, really. Objects of a particular color cannot absorb that color; thus, what we see are reflected or scattered wavelengths of light that are not absorbed.

Rainbow showing that sunlight is actually composed of different wavelengths (colors)

In the case of the fabric, it absorbs all the greens and yellows and oranges and reds, and what’s left is blue. Same with the car and the sky. They absorb all the wavelengths of light except the one you see. Gradations in color or combinations of colors work the same way, only a spectrum of colors is not absorbed, or different segments of the visible spectrum are absorbed differentially.

The same is true for the oceans. Water (and seawater) is a very good absorber of all wavelengths of light except blue. Because water makes up a significant percentage of the atmosphere, our skies are also blue. But what about when the ocean is green or blue green?

Pink beach in Bonaire

Changes in ocean color are primarily due to changes in the type and concentration of organisms suspended in the water, namely phytoplankton (which include photosynthetic bacteria, such as the cyanobacteria). Areas of river outflow, sewage outfall, or intense land runoff, near the coasts, may contain large amounts of suspended sediments, which give seawater a milky or dirty color.

Galveston windsurfer
Galveston, Texas windsurfer

In some areas, such as near pulp mills, discharges of dissolved organic material can cause changes in the color of the ocean. For the most part, however, it is the phytoplankton that cause variability in ocean color.

Before we can appreciate the changes in the color of the ocean, we need to know something about how light in general changes as it penetrates into the ocean. When sunlight hits the ocean surface, some of it is reflected (around 5% on average) and the rest is transmitted through the water where it is eventually absorbed by water and the chemical and particulate components (salts, plankton, etc.) in the water. The zone of penetration of light into the water column is called the euphotic zone.

Sunset on Red SeaIn general, the euphotic zone is defined as the area between the sea surface and the depth where light diminishes to 1% of its surface value. The depth of the euphotic zone depends largely on the concentration of organic and inorganic materials dissolved or suspended in the water column. Thus, with more materials, such as in coastal waters, the depth of the euphotic zone will be shallow, perhaps only a few feet. In waters like the open ocean or tropical waters where terrigenous influences are negligible and concentrations of plankton are sparse, the euphotic zone may be quite deep, perhaps 150 m (~450 ft) or more.