So what's up with this green beach gunk?

It seems that every year, as the weather starts to get warm, our Madison Park beaches begin to accumulate quantities of a green odoriferous material that could cause us to wonder if there might be something wrong with the health of Lake Washington. The problem is more apparent at the constricted road end beaches than at the Park beach, but green water is definitely a part of the annual beach scene here from June to August. So what is this stuff?

I contacted Dean Wilson, King County Senior Water Quality Planner, to find out. Here's the executive summary of our conversation: not to worry. It may be stinky, and it may be tough to get out of your dog's fur after a beach swim, but green is good--or at least in this case, not bad. It's simply green algae which is both common to the Lake and non-toxic.

Wilson explained that there are basically two types of green algae that may be contributing to the green build ups in the Lake. One form of these Chlorophytes is Oedogonium (shown at right), and the other is Spirogyra. Unlike some forms of blue-green algae, the green algae that grows in Lake Washington does not produce toxins. Blue-green algae is not unknown in Northwest lakes, however. In 1997 there was a blue-green algae problem in Lake Sammamish, for example. Wilson does not believe that Lake Washington has any toxin producing algae at this time. "We've been monitoring for toxins at the beaches on Lake Washington and we haven't seen anything yet," he said.

So what causes the algae to accumulate and wash up onto the beaches? The answer is that both green algae types grow as filaments which attach to rocks and the bottom of the Lake. When these filaments grow too long they can break off as a result of wind and water action. It is generally in late July when these accumulations are greatest in Lake Washington, just in time for Seafair.

While decomposing piles of washed-up algae can be unsightly and stinky, they pose no health risk and are not evidence of an unhealthy Lake. Ironically, the high water quality of Lake Washington may be the cause of these green-algae accumulations. The higher the water clarity, according to Wilson, the greater the amount of sunlight penetrating lower into the Lake, resulting in increased levels of algae.

Believe it or not, green-algae accumulations have been recorded of up to ten inches deep on some Lake Washington beaches in past years. Not at Madison Park, fortunately.

[Oedogonium photo courtesy of the King County Department of Natural Resources and Parks. Beach photo is of the road end beach at 4299 E. Lee Street in early July.]