I was recently flying into Portland over Casco Bay on a very calm wintry day and was struck by the color patterns in the still water. I could see the mouths of rivers pushing out a line of sediment and stirring up a lighter green-ish blue and also the pale steely hue of the shallows along the shore. While I have understood the color of ocean water to varying degrees in the past, I have never fully delved into why the ocean is the color that it is at that moment in that place.
The explanation is multi-faceted. But, it can be broken down into three key components – light absorption, depth, and particles in the water. The first one is probably the most familiar piece of the puzzle. The ocean absorbs a lot of sunlight and sunlight is composed of many different wavelengths that represent many colors. The longer wavelengths – red, yellow and green – are absorbed by water molecules. Many of these are absorbed by phytoplankton in the water, which use the sunlight to produce oxygen. The shorter wavelengths – blue and violet – travel further through the water and are the color that we most often see.
The depth of the water determines the color. This is quite obvious from the sky, a vantage point from which you can see the gentle fade from shallow to deep. Part of this is light penetration and the fact that those blue and violet wavelengths are able to travel deeper, thus showing off a deeper hue in deeper water. But, much of the color also has to do not just with how deep the bottom is, but what the bottom is like. Think about a seafloor of fine sand or coral like that in the Caribbean versus one covered in plants or of dark rock such as much of our seafloor in Maine. The first allows for lots of reflection and the second for a lot of absorption.
The last piece of the explanation is trickier. There can be a lot of stuff in the water. It could be sand or silt that has been stirred up from the bottom by the tides or by waves or a storm. Or it could be washed off the shore by rains or down a river. These tiny particles reflect and scatter light, which can make the water look lighter in color. Scientists use what is called the Forel-Ule color scale to measure this ocean color. Some of these particles are dissolved materials that have washed off the land. These can make the water look brown or yellow. Then, there are the living particles — phytoplankton. These are the teeny tiny ocean plants that form the basis of the food chain and provide half of the Oxygen that we breathe. The chlorophyll in phytoplankton absorbs red and blue wavelengths and reflects the green light, making the water look greener. At certain times of year or under certain conditions, phytoplankton can multiply, or “bloom” so that this color change can even be seen from space. There are some stunning images taken by satellite-based remote sensing technology on NASA’s OceanColor Web site: oceancolor.gsfc.nasa.gov. They look more like something in a lava lamp than what you’d think is happening in the ocean.
The scale of that level of ocean observation is impressive and gives scientists a good overall sense of ocean productivity. But, there are a lot of things going on at a much finer scale that help us to understand our local ecosystems. One local non-profit, Friends of Casco Bay (FOCB), realized the lack of near shore data and organized a network of volunteers to help document ocean color. Their “Color by Numbers” initiative empowered people with the ability to record ocean color using their smart phone by taking a picture of the water and comparing it to known Forel-Ule value levels and colors. They were instructed to take the photo at a certain time and tide to get the best results. The information was tagged with a date and location and put into a worldwide database of ocean color. You can view the results on a worldwide map at: www.eyeonwater.org or sign up to volunteer on FOCB’s website www.cascobay.org.
There is a lot more to the answer of why the ocean is blue — or why it isn’t it blue — than you may have thought, and that answer can help us to understand more about the health of our oceans.