Water Quality - June

Water Quality - June
Water Quality - 6/18/2020
First I want to say thank you to all residents and easement members who participated in the survey by our intern Laura Costadone. There were almost 500 responses but unfortunately we heard from only three easement members. Over half the respondents have lived on the lake for at least 15 years and 67% consider the water quality good or excellent. Something that stood out for me is 89% of people are familiar with harmful algal blooms and 58% are concerned about the potential for toxins to be present in the lake. I think this is a good opportunity to explain our cyanobacteria control strategy ­– which for our lake means a phosphorus control strategy.
  • I want to acknowledge that residents in West Bay and the canals may not feel their water quality is as good as in the Main Lake, which was reflected in the comments. This does not mean we neglect the shallow areas of the lake, but there is a tradeoff between algae and plants in shallow water locations. West Bay and Blue Heron Canal grow a lot of diatoms, which are very beneficial algae and a primary food source at the base of the food chain, but they also affect water clarity. 
  • Historically these areas have been dominated by either plants or cyanobacteria, which made it difficult to swim or boat in the past (see image from a 2004 bloom in West Bay at the head of this post). Diatoms shade the water to keep plants and cyanobacteria from proliferating, and we do inject alum into these locations in an effort to limit how extensively they grow. When we were using water from the river to generate power,  Oswego Canal was more of a river than lake. This made conditions in Oswego Canal good to the detriment of the rest of the lake because all the bad stuff flowed downstream. Both canals are surrounded by trees that drop a lot of debris on the water, especially in spring and fall. We have added skimmers to collect this material and that has helped a lot, but due to the amount of vegetation there will always be some debris on the water.
It's All About Phosphorus
Oswego Lake has always been dominated by cyanobacteria every summer and in the past the LOC would apply copper sulfate every week to control the blooms. These applications killed cyanobacteria, but they also killed beneficial algae and planktonic organisms that eat algae. This was not a sustainable practice because it did not treat the cause of the blooms, only temporarily reduced the bloom intensity. 

Editorial comic from the July 2, 1987 Lake Oswego Review

In order to control cyanobacteria in a lake the best strategy is to reduce nutrients that feed the blooms. In our lake phosphorus is what limits cyanobacteria growth, and Oswego Lake has had an abundance of phosphorus, but it has been declining over the years. The reductions started after the TMDL on the Tualatin River was established that limited phosphorus to 100 µg/L, and prior to that the decision to restrict how much water was imported from the river.
  • Phosphorus loading to lakes come in internal and external forms. Internal loading is phosphorus that originates within the lake and is released into the water column as a result of organic material decomposing or breaking of bonds between minerals and phosphorus. External loading is phosphorus that originates from outside the lake, such as leaves falling into the water, storm drain and stream flow into the lake.

Reducing flow from the Tualatin River was a significant step in lowering phosphorus. Our historic use of the river was for power generation so we brought in water year around and sent it through our hydroelectric generator. This had the effect of providing treatment for the Tualatin River since we brought in silt and phosphorus from the river, allowed it to settle in the lake, and pushed relatively clean water through the powerhouse. As an example, in the 1987 water year 2,650 cubic yards of dry sediment (~6,000 yards of wet sediment) and 30,000 pounds of phosphorus were imported to the lake from the river. Now we only use the river to keep the lake full during summer.

Decades of importing phosphorus rich sediment from the river built up a layer of nutrient rich material on the bottom of the lake. During summer when the lake stratifies this sediment becomes anoxic, causing phosphorus to be released into the water column. In 2001 the LOC installed a hypolimnetic aeration system, which immediately reduced phosphorus coming from the sediment. This provided some relief, but we were still importing a lot of phosphorus from the river during summer.

In 2004 there was a severe Microcystis bloom despite two years of aeration that reduced internal loading. This occurred in part due to heavy rain in late August and early September washing nutrients from the watershed into an already blooming lake. Conditions got so bad the LOC had to restrict water contact activities until the bloom subsided. 

The West End of Oswego Lake during the summer 2014 bloom.
Microcystis was so bad you could not tell where the lake ends and the lawn begins.

After the 2004 bloom the LOC visited DEQ to discuss our options for reducing future blooms. We mentioned we did not want to continue using copper sulfate since that did not address the cause of the problem, only temporarily reduced the bloom intensity. We wanted to use alum but were concerned it would not be permitted, but it was actually recommended by DEQ during our visit. This turned out to be the best option to treat our high phosphorus concentration.

In 2005 we applied a cap of alum over sediment in both canals and West Bay to reduce internal loading. Aluminum provides a binding site for phosphorus that is more robust than the iron prevalent in the lake bed and will hold onto phosphorus longer than iron will. The following year we constructed a liquid alum application barge to strip phosphorus from the water column during summer and have been using that method for 15 years. These ongoing liquid alum applications help with both internal and external loading because even after liquid alum strips phosphorus from the water column the aluminum settles to the lake bed to continue intercepting phosphorus that may come out of the sediment.

We also inject alum in the canals, West Bay and the West End near the mouth of Oswego canal. These treatments are important during spring to capture phosphorus coming from storm drains. We also inject alum at the headgate to intercept phosphorus coming from the river during summer. Finally, we inject alum into Lakewood Bay because that is a flooded wetland with over 25 feet of peat at the bottom of the bay.

Additional Projects
Another significant step was a project in 2005 to reduce water leakage through our powerhouse. Any water that leaves the lake during summer must be replaced by river water so we upgraded our powerhouse to reduce that water loss. The effect was to delay by at least three weeks the date we open the headgate during spring. We are working on other steps to stop leakage and further reduce external loading. We continually monitor and adjust alum injection to increase efficiency, and our operations crew removes tons of leaves and material from the lake surface that would eventually settle on the lake bed and contribute to internal loading. Also, during spring we keep the lake level a bit higher than the normal to delay when we open the headgate. These are all steps we take to control phosphorus in Oswego Lake, and in turn control the amount of cyanobacteria the lake grows during summer. 

Back to our nemesis cyanobacteria. We still have cyanobacteria in the lake and this includes species that can become toxic. However, concentration is the key, and our management goal is to not let cyanobacteria get too numerous. The Oregon DEQ Harmful Algal Bloom Strategy states that a cyanobacteria population of 100,000 cells/ml and above can cause adverse health effects and the lake should be posted to notify users of the risks. The highest number we have seen in Oswego Lake for the past five years during our normal sampling activity is around 50,000 cells/ml, during summer 2017. The past two years they have been very low and our goal is to not have surface films. However, that is difficult to achieve due to the unique properties of cyanobacteria.

Our policy for notifying residents of is not tied to the cell counts from samples we collect on a regular basis, but if there are visible scums on the water surface. That is what makes managing cyanobacteria so challenging. Cyanobacteria have the ability to adjust their buoyancy to maximize nutrient uptake, and during certain weather conditions can collect on the surface at much higher concentrations than deeper in the water column. 

Our routine sampling collects water from below the surface so this does not reflect surface accumulations that may have much higher concentrations of cyanobacteria cells. These surface concentrations can move around with the wind so a concentration at one location in the morning may end up at another location in the afternoon. The overall concentration of cyanobacteria in the lake may be small, but the surface accumulations can still cause health problems. In those instances we send a notice to be alert of cyanobacteria films and avoid the areas where they collect. The concentration is typically not high enough to affect overall lake health, and you can use the lake as normal while avoiding the isolated areas where cyanobacteria may concentrate. Our goal is to not experience cyanobacteria films, but occasionally weather conditions cause some concentrations.

Our nutrient reduction program has greatly improved the lake over the past 15 years and we continue to look for ways to further improve conditions. For instance, our upcoming dredge activity will remove sediment deposits and clean Oswego canal near the headgate so internal loading is reduced and water from the river can be treated before reaching the lake. As mentioned we are actively working on slowing another leak so we can further reduce external loading from the Tualatin River. These are areas where we can control phosphorus sources, but climate change will bring warmer summers and more intense rain that washes more nutrients into the lake. These are ideal conditions for cyanobacteria so our work will continue.