Reports over the last several weeks have suggested that Lake Sawyer currently is home to a toxic blue-green form of algae which poses a significant danger and health risk, particularly to pets. However, each time samples were taken of this "toxic material" by King County and analyzed in the lab, no toxic algae was found in any of these samples at detectable levels. What gives?
Following is a photo of what blue-green algae can look like when in bloom (photo not from Lake Sawyer). Pretty nasty looking stuff.
Though I must admit to not having seen algae like this in the lake, it doesn't mean that it isn't here now or hasn't been sometime in the past. How can we learn more? For those interested, you might start by reading King County's publication Algae and Water Quality. For techies, you might also want to read the paper Download Algae101. One of the things I learned is that it isn't simple. Algae comes in all sizes, shapes and colors and what can cause algae growth and blooms in lakes is both simple yet complicated. It's simple in that algae growth requires photosynthesis and the same general conditions as land based plants. However, like all plants, there are some key variables specific to algae that need to be considered. For example:
- Temperature - warmer temperatures in temperate climates like ours facilitate increased algae growth
- Nutrients - phosphorus is the most critical in lakes providing the food needed for photosynthesis
- Water flows and mixing of nutrients can spur algae growth
- Season of the year - longer days and sunny skies promote growth, while short cloudy days and lower temperatures inhibit photosynthesis
I found the following graphic from King County to be particularly helpful in better understanding these relationships in a seasonal context.
Because the reported Lake Sawyer toxic algae blooms have been in October and early November, one would think that there shouldn't be any algae blooms this time of year. After all, temperatures are declining and there are fewer hours of sunlight. For those lakes with high levels of nutrients (phosphorus), algae blooms begin early and last through the summer, before declining in the fall. Fortunately, Lake Sawyer phosphorus concentrations are low and we should expect a seasonal cycle more similar to the "little P" seasonal graphic. A fall bounce in algae growth can be spurred by increased water flows and mixing.
For perspective, following is how Lake Sawyer compares with other lakes in King County, with monthly data and trends dating back to 1997. First for 2018. The blue buttons are for Lake Sawyer and the black/grey circles are for the other King County Lakes.
The following comparative data shows annual average phosphorus levels from 1997 through 2018. The 2019 report from King County should be available early next year as part of their annual report.
As you can see, Lake Sawyer is at the lower end of phosphorus levels. For more Lake Sawyer data, follow this link to Lake Sawyer Water Quality - August 2019 Update.
So, with fresh water entering the lake this year in September and recent sunshine, it is possible that stirring up of lake nutrients at the bottom along with sunshine and relatively warm water temperatures for this time of year, could result in a fall spike in algae growth like you see in the seasonal chart shown above for "little P" lakes. In any event, it wouldn't necessarily be abnormal. But toxic algae???
Some in the community have suggested that development is the cause of recent algae growth, resulting in higher lake phosphorus levels and therefore algae growth. There has certainly been a lot of development around Lake Sawyer and in other parts of the city. The Oakpointe MPDs have also been developing their site and building new homes. Could they be the culprit? If growth is the problem, its hard to find a smoking gun. Lake Sawyer water quality monitoring reports and data don't support this premise. In addition, none of the stormwater generated by the Oakpointe MPD's are currently discharged into the Rock Creek watershed that flows into Lake Sawyer. The two plats currently being developed are Plat 1-A and Plat 2-C, both of which flow into a stormwater retaining pond that drains south, not north into Rock Creek and Lake Sawyer. You can see this stormwater pond at the bottom center of the following aerial view from King County.
I have been repeatedly told by King County and state DOE staff that you cannot draw conclusions until you can clearly see trends over a significant period. No trends in the vast amount of data that we have either, except perhaps increasing water temperatures and clearer lake water as evidenced by trends in Secchi test results. Ironically, the clearer lake water becomes, and it is getting clearer, the deeper sunlight will penetrate and the further down in the lake photosynthesis can occur. I'm not arguing that this is the case - merely to point out that there are lots of variables and that a smoking gun cause and effect for algae in Lake Sawyer is not at all clear. In fact, no test results to date support claims made that algae in the lake is toxic or even that algae growth has increased in the lake.
What to do? Keep testing and be vigilant for where we can identify contaminated surface water runoff going into the Rock Creek drainage so that it can be remedied. As for me, I will continue kayaking around the lake in search of this elusive toxic algae that I have yet to see any signs of.