Angle Lake Algal Bloom

As many residents around Angle Lake have noticed in the last few weeks, the clarity of the water is not as good as we like to see it. In fact, the lake has taken on a murky brown color which has reduced visibility in the water to less than one meter, so it is difficult to see the lake bottom from the shoreline or docks.

The City, through a contract with King County, has had a water sample tested and it confirmed our suspicion that the cause of murky water is an algal bloom. The good news is that none of the species found produce toxins, like the infamous blue green algae, and that algal blooms have a limited life cycle. They eat up the nutrients in the lake; then die off. So the clarity of the lake should return to normal. We are hopeful that this will occur by mid-spring. The bad news is that this type of algae can give off a strong fishy odor during the final stages of its life. Conditions on the lake may get worse in the short term before they get better. It is important to remember that while murkiness of the water and the potential odors that may appear are a nuisance, they are a natural phenomena and do not represent any health concerns.

What causes an algal bloom?  Like land plants, algae need certain things to grow – water, carbon dioxide, sunlight, and nutrients. All lakes have a plentiful source of water and carbon (from decaying vegetation). In our case the likely culprits are warmer than usual winter weather and increased level of nutrients in the lake.

Has this happened before and will it happen again?  According to King County records, this is the first occurrence of this algal bloom. While it is the first occurrence, it does not mean it will happen again. In fact it may never happen again, or it may re-appear periodically. All of the right conditions have to align for this bloom to reoccur.

What can we do to reduce or prevent algal blooms?  To reduce or prevent algal blooms, we have to limit the elements that make it thrive. Since we cannot limit water, carbon, or sunshine, the only way we can limit the growth of algal is to reduce the volume of nutrients flowing to our lake. Sources of these nutrients include: duck and geese feces, pet feces, and fertilizers from lawns and gardens.  Simple practices that Angle Lake watershed residents can adopt to help reduce nutrient loading to the lake are:

• Don’t feed the ducks or geese. It is unhealthy for the birds and it leads to an unhealthy build up of fecal matter on our shores and docks, which eventually gets washed into our lake. 
• Scoop up daily after pet, bag it and put it in the garbage, don’t bury it. When it rains, pet waste left in the yard will dissolve and send nutrients and bacteria into the lake. 
• Limit the use of fertilizers. If you must use them, use natural slow release fertilizers that are phosphorus free.

The preceding information was obtained from the City of SeaTac - City Manager's Weekly Update dated March 5th.  For information and answers to questions on lake ecology, or the City’s lake monitoring program, please contact Sally Abella with King County’s Water and Land Resources Division at (206) 296-8382.