Last week’s solstice had me thinking about the impacts of light in the ocean. It is always a relief when our shortening days instead becoming lengthening days and we can enjoy a bit more sunlight even in the coldest weather. Light is important for everything living from causing flowers to bloom to generating more happy-making serotonin in us humans. Light is also important in the ocean as well – and its effects are more complex than you might think.

The food chain in the ocean begins with phytoplankton, or plant plankton. Because they need to photosynthesis, they need sunlight. That means that they live up in the sunlit shallows. They take the sun’s “food” and turn it into a form that animals in the ocean can eat – themselves. Some of the animals that eat them are giants like filter feeding whales, but most are tiny zooplankton (animal plankton). While zooplankton don’t need light to survive, they do need to go where the phytoplankton are so that they can eat them. The complicating factor is that there are a lot of things in the ocean that want to eat the zooplankton, so they don’t spend all of their time up in the bright sunlight where they are easily visible. This leads to a magical sun-driven dance up and down in the water by these tiny creatures. They travel up to the sunlit waters to eat and then back down to the depths where they can hide. Typically they go up just as the sun sets and back down as the sun comes up. This is known as a diel, or daily migration.

This gets to another interesting aspect of light in the ocean. Light doesn’t work the same way in water as it does in air. It doesn’t travel at 671,000 miles per hour, the speed of light, but instead travels at a 503,000 miles per hour. That’s because a lot of light gets refracted in the water, bouncing off of other molecules as it tries to travel forward. That means that as you get deeper, it gets darker quickly. In fact, there is rarely any significant light beyond 200 meters. The water is separated into zones going from the light photic zone to the slightly spookier twilight zone to the pitch-black midnight zone.

Animals that live in the midnight zone are quite weird. The anglerfish dangles a bioluminescent lure from a barbel on its head that stands out in the darkness. The goblin shark, one of the oldest species on the planet, has been around for 125 million years. Giant isopods (ocean pill bugs) go for five years without eating – good thing since there isn’t much food around. You can learn much more about these zones and the creatures that live there in a neat animation at:

In addition, many of these deep-sea creatures have giant eyes to help them see with little or no light. But, not having any light isn’t the only challenge of seeing under water. To the human eye things look blurry underwater. That’s because your eyeball has to deal with light coming in from many directions and it isn’t designed for that – unless you’re a fish. If you’ve ever had the pleasure of dissecting a fish eye, you will find that it has a round lens like a little hard clear ball. This is quite different from a human eye lens that looks much like a contact lens – curved but certainly not spherical. This sphere shape allows a fish eye to focus light coming in from many directions.

The change in the amount of light that happens each year at the solstice isn’t likely to change much in the ocean at this time of year. It will undoubtedly have more impact on humans who aren’t as perfectly designed for constant darkness as deep-sea creatures. And, perhaps it will give us a little more daylight to get out to the coast even on the coldest days of the year.

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