Every summer Capitol Lake is blanketed with a thick layer of green algae. While it obviously unsightly, it is also a sign of a sick body of water. The algae growth indicates an unbalanced system, a stagnant, warm lake that has too much phosphorus for its size and shape.
A lot of people think it would be an easy fix to get rid of the algae, just dredge the lake back to where it was in the 1980s. But, the reason the lake was clear of algae back then was because of saltwater flushing. This practice was ended years ago because of obvious impacts to water quality in lower Budd Inlet.
To really get rid of the algae every summer with dredging, you would need to go deeper than 13 feet. Way deeper. Like 300 feet.
Seriously, 300 feet.
This fact is explained in a letter from the state Department of Ecology in 2009 to the then Department of General Administration. A GA staffer asked DOE what the impact would be of dredging to 13 feet on several water quality factors:
What would be the effect of implementing both the shading improvements and the lake dredging on the lake and Budd Inlet relative to the five TMDL water quality factors (dissolved oxygen, temperature, PH, fecal coliform bacteria and find sediment)?
Combining upland improvement and deepening the lake also would not resolve water quality issues within Capitol Lake… Because Capitol Lake currently and under the dredged lake alternative falls well within the eutrophic range, based on available indices, these improvements are unlikely to translate into measurable or significant lake improvements. No changes are expected in Budd Inlet either.
You can read the entire letter here.
Capitol Lake is eutrophic. Meaning, in the case of Capitol Lake, there is extremely low dissolved oxygen and annual algae blooms.
For both the current and dredged lake alternatives, the combination of phosphorus loading and lake geometry plots wells into the eutrphohic zones… The dredged lake alternative would still fall well into the eutrophic range, either for the annual loading or for only the summer loading rate.
Without reductions in the areal phosphorus loading rate, the lake depth would need to be >100 m (300 feet) to fall into the mesotrohpic (or healthy) range… With the propoposed nominal depth of 14 ft (4.0 m), the annual areal loading rate would need to be reduced one to three orders of magnitude to achieve the mesotrophic range.
The problem is then, that 75 percent of the sediment load (which carries algae causing phosphorus into the lake) is natural. So, even under natural conditions, Capitol Lake is too shallow would never be healthy.
But, why isn’t there the same phosphorus problem with an estuary?
Well, simply put, places like Budd Inlet are supposed to get a lot of phosphorus. Most of the phosphorus coming down the watershed is natural and it always used to end up in Budd Inlet with no ill-effects (this is shortly described in a GA document here.
What is unnatural is trapping it in a shallow, stagnant lake. That causes algae blooms and low dissolved oxygen. Letting the natural phosphorus load go naturally out into the saltwater doesn’t cause the same thing to happen.