Summer Algae Control

Water Quality - 8/2/2018

I must apologize that it has been a while since I have updated you on the state of the lake. This has been a very busy spring as we have focused on the marina remodel, but the lake does not care about that and will do what it wants to regardless of our schedule.

It has been a long and hot July, as predicted by the Oregon Department of Agriculture but things are supposed to cool off the latter part of August as summer winds down. We are in an El Niño Southern Oscillation (ENSO) neutral phase this summer after being in La Niña last fall. However, this fall we are predicted to be back in El Niño this winter which means warmer than normal conditions.

Our July weather was unusually warm but not because of a high temperature, but because there were so many of them. The warmest day was 100 degrees, but there were 16 days over 90 degrees and no rain. I remember not so long ago that we would be lucky to have a dry July 4th holiday but that has not been an issue for several years.

Chart showing daily air temperature range (grey area) lake temperature (red circles) and rainfall (blue bars) for June and July.
The horizontal orange bar denotes the 90 degree temperature, showing graphically the 16 days of exceedance in July



Considering the weather, cyanobacteria populations in the lake were not that high during July, with a slight Microcystis bloom early that shifted to Gloeotrichia later in the month. Phosphorus was trending up as it usually does in June and July as the weather warms and water is brought in from the Tualatin river. We like to keep phosphorus at or below .02 mg/L in order to keep cyanobacteria growth under control, so as our July 23rd sample showed a high concentration a decision was made to carry out the pre-planned alum treatment.

You notice I have not used the word algae in this post, and that is by design because Oswego Lake does not have an algae problem it has a cyanobacteria problem. There are parts of the lake that have abundant algae – West Bay and Blue Heron Canal – but that is not a bad thing. Yes, algae prevents you from seeing the lake bottom and makes the water more brown than blue but with the amount of phosphorus we have in this lake we are going to grow algae, cyanobacteria or plants.

I remember when I first started at LOC West Bay and Lakewood Bay were choked with plants that impeded boating and swimming for most of the summer. Plants in West Bay were treated with herbicide, which shifted the dominance to cyanobacteria, most likely fed by what blew in from the main lake. You remember what that can be like, with the entire west end of the lake smelling and a cyanobacteria film over the bay. 

The good news is several years ago West Bay shifted to diatoms and brown algae with very little plants or cyanobacteria. Although it does make the bay tan in color there are no plants to fight when you are swimming and no cyanobacteria films on the surface. Yes there are occasional migrations of cyanobacteria blown in from the main lake, fortunately they are short lived and do not lead to a bloom. Blue Heron Bay and Lakewood Bay also have their unique algae populations that have become dominant over plants and cyanobacteria. 

Remember that algae are at the base of the food chain and cyanobacteria are not. Aquatic insects cannot eat cyanobacteria so a lake dominated by them is not a healthy lake. The areas that do have a lot of algae should be celebrated because the alternative situation where cyanobacteria or plants take over would not be desirable.