Do: Open Water Descent and the Visible Spectrum

Getting briefed on the first dive on Saturday morning. 7 ml wetsuits, hoods, booties and gloves are worn by students.

Getting briefed on the first dive on Saturday morning. 7 ml wetsuits, hoods, booties and gloves are worn by students.

In the grey-blue waters of the Pacific Northwest not far from the horns of the Washington State Ferry and comfortably surrounded by the piers of the Mukilteo T-Dock I’m starting my first open water dive. I’ve heard, through the rumor mill and from seasoned veterans, that the waters off my coastal home are sought after by divers the world over so I’m curious to do my own exploration. I’m bundled cozy in a 7 mm wetsuit, gloves, hood and booties. Other divers, most instructors, layer fleece under dry suits, donning elaborate systems of lights and air tubes in the parking lot beneath the Silver Cloud Inn. The water temperature sits in the mid 50s and the outside air rises to the low 70s as the day progresses, hindered only by a brisk breeze.

This dive, one of four toward completing my open water certification over the next two days (two dives to 30+ feet Saturday and two to 60 feet Sunday), has evolved into a nonreference descent. My ears refused to equalize at 15 feet, so I abandoned the first attempt and returned to the surface with my instructor. On the second attempt we head straight down and it’s a little disconcerting sinking slowly, directly to the bottom, but certainly a trip to experience the full vertical transition. Until we reach the bottom the visibility is limited to about six feet – the span of my arms – making me aware of how much I’m not aware of how far the bottom is from the tips of my fins. So I settle in, letting small amounts of air from my BCD and equalizing my ears.

At depths past 30 feet the world takes on a green glow. Note the neon yellow fins and neon orange whistle against my shoulder. (Photo courtesy: John Sanders, DiveZen Scuba)

At depths past 30 feet the world takes on a green glow. Note the neon yellow fins and neon orange whistle against my shoulder. (Photo courtesy: John Sanders, DiveZen Scuba)

Just below the surface the view in front of my goggles cancels into a fray of broken green and red algae bits that keep rhythm with the wind and wake. Around 10 feet surface sounds retreat, the water begins to calm and a vibrant green glow sets in from above.

Light after 15 Feet

Water is a selective filter of color and as we sink I watch the saturated red on my instructor’s dry suit fade to a dull, murky green. The color becomes indistinguishable from the surrounding environment at 15 feet. Part of the glow I’m now enmeshed in is due to the organic matter in the waters of the Pacific Northwest but another part is due to the properties of light as it reaches below the surface. A little science review before we head farther down: Imagine a tank of clear water 200 feet deep. If we were to suspend a white light above the tank and descend with six colored blocks – each a color from ROYGBV – the water would filter the colors one by one as we dropped deeper into the tank and further from the light.

Water acts as a selective color filter, absorbing light as depth or distance from the source increases. (Graphic courtesy of:

Water acts as a selective color filter, absorbing light as depth or distance from the source increases. At a depth of 60 feet, only 18 percent of light rays penetrate and only one percent reach a depth of 330 feet. (Graphic courtesy of: University of Maryland Space Systems Laboratory)

The loss of color extends sequentially through the light spectrum – like my instructor’s suit, the red block would be the first to lose color followed by orange then yellow – until, after about 75 feet the last block to lose its hue would be violet. Interestingly, some species of clear jellyfish have a red stomach. Since red is the first color to disappear in the visible spectrum under water, the red (or lack thereof) hides the jelly’s last meal.

If you ever wonder why neons tend to be ubiquitous in water activities, here’s your answer: Ultraviolet, invisible to humans and found after violet at the end of the light spectrum, can travel to extreme depths. When a neon color is struck by invisible ultraviolet, it will glow or fluoresce which is why alternate air sources and other items needing ready accessibility are commonly created in neon.
In our subsequent dives the haze began to clear at a depth of about 30 feet and visibility increased to a distance of nearly 25 feet along the bottom, remaining steady as we descended to 60 feet. Green, blue and violet remained and our instructors used their lights to reveal the nuances of the undersea creatures: Brown mottled flatfish, orange penpoint gunnels, kelp greenlings protecting eggs and dungeness crab (all documented by our class) navigate the sand and pebble-covered bottom, relatively unaware of our presence. And if you look closely, sea stars no larger than the nail of my little finger hide between the pebbles beneath the feet of hermit crabs. At this particular location a large geodome made of PVC pipe has been constructed at 50 feet and become host to a number of small invertebrates including copper rockfish. A short distance away the instructors gently pried apart a set of submerged tires treating us to the rare sight of the resident octopus curled in his den.
Even with the understanding of water’s color selective filtering, I wasn’t expecting the green hue. I admit I’m not really sure what I was expecting – maybe a soft, blue-grey-magenta to match the sunrise or a grey-blue flannel hue to match the morning sheen of the Olympic mountain range. I’ve taken a liking to surfing these waters and grown used to the looks of surprise when I mention it. And I can say honestly that what’s on the bottom is well matched to what we experience on the surface of our Emerald City. Green is the color du jour. It’s dark, it’s sometimes tempestuous but when you stop and look just a little deeper, shapes and then colors are revealed that leave you glad you buttoned up and stepped outside.

As we head back toward shore I’m handed a weightless, bloated object puffed to the size of a football: A sea cucumber, red by a nearby dive light and marked by small yellow nodes. I carry it a short distance and its name dawns on me: hoi sam. The Cantonese translation to “happiness”.

How to get there: Mukilteo T-Dock

The entry to the diving area is found next to the Silver Cloud Inn in Mukilteo, close to the ferry terminal. From I-5, take exit 189 and merge onto WA-526 toward the Mukilteo Ferry Dock.

Follow the signs all the way to the ferry dock – 526 becomes 84th St and you’ll have to take a right to stay on it after the Boeing field, then a right onto 525 – until you get to the waterfront. Take a right on Front Street (directly in front of the ferry terminal). Head 2 blocks and park next to the Silver Cloud Inn (at the intersection of Front St and Park Ave).

View map

Sources:

Complete Diving Manual by Jack Jackson
Shorediving.com > Overview of Mukilteo T-Dock
Pacific Northwest Scuba > Mukilteo T-Dock

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About Jonna

Created for curiosity. View all posts by Jonna

5 responses to “Do: Open Water Descent and the Visible Spectrum

  • Twitter Trackbacks for Open Water Descent and the Visible Spectrum « Vas-y Fille [vasyfille.wordpress.com] on Topsy.com

    […] Open Water Descent and the Visible Spectrum « Vas-y Fille vasyfille.wordpress.com/2009/09/28/dive_visiblespectrum – view page – cached Getting briefed on the first dive on Saturday morning. 7 ml wetsuits, hoods, booties and gloves are worn by students. — From the page […]

  • Lorien

    Interesting!! I never knew that happened when you were in the water. I like your description of hte light! it’s surprisingly informative!

    • Jonna

      Thanks Lorien! I’m hoping to get the next level of certification come spring/summer so I can do a deeper descent and follow up w/ the transition. Safe travels!

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