March was indeed in like a lion this spring and seems to be out like a lamb if the current trend holds. (Actually it was February that gave us the cold weather, but close enough.) After a cold February it is nice to have some nice sunny days this past week, and just in time for the first day of spring! Rainfall this winter was pretty typical in contrast to very wet months of past years (Fig. 1). The lake temperature is starting to warm and as of March 18th it is 43 degrees near the surface, up three degrees from the previous week.
Figure 1. Bar chart showing total monthly rainfall in inches for the past four water years.
Current water year is depicted by a yellow bar.
Red horizontal bars reperesent historical monthly average rainfall from 1941 to 2005.
Our lake is also in spring phase of activity with water clarity slowly increasing through May. One of our weekly monitoring tasks is to take Secchi readings to measure clarity, which is a simple indicator of lake conditions. As of March 13th visibility is just over nine feet into the water and will likely increase throughout April. The highest Secchi, or the deepest we have been able to lower the disc and still see it, was over 30 feet several years ago. It typically is not that high but will regularly reach 15 feet. Using water clarity is a good first step in assessing how much suspended sediment or algae is in the water.
The spring biological cycle that affects Secchi readings starts with a diatom (good algae) bloom in the spring, followed by zooplankton (aquatic insects) grazing down the diatoms in March and April. The species of diatom (Stephanodiscus niagarae) that bloomed in February 2018 is depicted in the blog title heading (Source). By the time May arrives the zooplankton have done their work, leaving little algae in the water. This is followed by a zooplankton die-off since they have eaten through their food source (Fig. 2). The result is a few weeks of not much in the water to block visibility. This is called the “clear water phase” and marks the time of year with highest Secchi readings. The specific dates vary, but generally that is how this and most other lakes work.
Figure 2. Phytoplankton population in 2018. A large population of diatoms in January declined rapidly until mid-April. This was followed by a brief green algae (Chlorophyta) bloom in June and a cyanobacteria bloom in August. Our first alum treatment July 24-26, near the peak of the summer bloom helped reduce severity of the bloom later in the fall. Grey horizontal line near the top shows Secchi readings throughout the year. Depth on the right axis is depicted with the surface at top so higher Secchi readings go down in the chart.
As summer progresses the lake warms and becomes dominated by cyanobacteria (aka bluegreen algae), which are better adapted to the warm water than diatoms are. In addition, cyanobacteria are not very good food for zooplankton so they do not get grazed down like the diatoms. Cyanobacteria also have the ability to regulate their buoyancy, seeking the optimum level in the lake to maximize phosphorus and sunlight harvesting. The result is a late summer cyanobacteria bloom that usually forms surface layers easily visible on the lake.
Our summer alum applications do reduce the cyanobacteria population, and if timed to precede a bloom it can greatly reduce bloom intensity. The trick is knowing when a bloom will happen, and it depends primarily on the amount of sunlight and food that are available for algae. A dry spring means we have to use Tualatin river water earlier, which means we are bringing in food. A warm and sunny summer means algae and cyanobacteria are fed by lots of sunlight. A warm summer also means the golf course and lakeside residents irrigate from the lake more, increasing the amount of Tualatin river water we use. Since the river has four times the concentration of phosphorus as the lake it is to our advantage to limit its use.
We have taken several steps to improve water quality during summer. We cannot control the weather so all our steps are focused on reducing phosphorus, which is the primary cause of our blooms. I will discuss these efforts in future posts.
I have been elected as regional representative for the NALMS Board of Directors and will be helping direct the organization for the next three years. NALMS is an international lake advocacy group with great programs to promote lake science and support students. We are working to bring the national conference to the Pacific NW in fall 2022, which likely means either the Portland or Vancouver WA area depending on cost and availability. I will be at a Board meeting in Denver the weekend of March 23rd.