Blackman Lake early test results suggest lakebed fueling algae

SNOHOMISH — The effort to identify what's degrading Blackman Lake is making headway.
An ongoing water sample study will give a better understanding of the conditions in the lake over a full year.
The samples are taken to a lab to measure the levels of phosphorous and nitrates, two key nutrients that feed algae. These are essential nutrients for aquatic life. However, too much can let algae thrive. Blackman Lake has that situation, which is why algae blooms appear in fall.
This winter's test results are giving some early tells.
For one, the greater watershed is not bringing phosphorous-laden chemicals into the lake. The phosphorous levels inside the lake was higher than at the lake inlets.
Instead, aquatic scientist Rob Zisette said, it's looking like phosphorous is embedded in the layers of old sediment, and it's not going away.
Zisette is with the consulting team from Herrera Environmental Consultants which the city hired to make a plan to help the lake. They'll be plotting out a strategy to control the lake's bad algae — the toxic cyanobacteria which can make people sick when it blooms.
When summertime hits, in Blackman Lake the warmth causes the water to stratify into layers. Oxygenated water rises to a layer up top, while a layer of oxygenless water stagnates below.
This layer of oxygenless water has something else going on: Phosphorous is being released from the deep sediment because oxygenless conditions are allowing that.
When temperatures cool in autumn, the lake un-stratifies and a lot of this released phosphorous blends with the whole of the lake, creating a perfect condition for a potential algae bloom.
What to do?
This fall, consultants will have 12 months of sample data in hand to study. They'll use this data to narrow down formal strategies for the lake's future to present to city leaders in spring 2024.
It's very early right now, but a few thoughts are brewing. These center on either pumping more oxygen into it to prevent stratification, or to treat the lake.
One concept Zisette described could be to use annual alum treatments or lanthanum treatments to restore the lake water quality. These interrupt the cycle of algae blooms by holding down the phosphorous under a layer of alum. Lanthanum is an alternative chemical.
Alum sufate treatments successfully cleaned up Lake Ketchum near Stanwood, once one of Washington's most algae-infested lakes.
Another concept he described could be to pump oxygen into the water. Pipes would keep air circulating to prevent the lake from stratifying. It's sort of like refreshing the water in a fish tank by sucking old water out and pumping new water in.
Zisette said oxygenation wouldn't reduce the growth of general algae, it just reduces the conditions that allow blue-green toxic algae.
Dredging the crummy sediment sounds simple, but wouldn't be feasible. The process to remove sediment and dispose of it as waste is "cost-prohibitive," Zisette said. In some spots, there is 20 feet worth of sediment, Zisette said.
Others have suggested speeding up water circulation to essentially flush the lake.
Natural dead plant matter turns into sediment.
If techniques can improve the trophic level of Blackman Lake, it would slow the lake's speed of aging. All lakes age. However, currently the lake is eutrophic, which is a dying state by being too rich in nutrients. Restoration could bring it to the mesotrophic status, which is an intermediate state of having a stable and healthy ecosystem.
Friends of Blackman Lake, a group of lakeside homeowners and stakeholders about the lake's health, hosted a presentation March 26.