Illustrations adapted by Betty Swift from King County Department of Natural Resources and Parks website.
Figure 1: Lake Union
Lake Union / Portage Bay
Water Quality Monitoring Stations
Ever wonder about the quality of the water you are floating on or swimming in? There is actually a wealth of historical and current data available, brought to you by your local King County Department of Natural Resources and Parks. Surf over to this mouthful of an address and away you go:
http://green.kingcounty.gov/lakes/LakeUnion.aspx
Samples of water are collected monthly from two sites in the lake and two at its edges, as shown on the map (Figure 1); for the two deeper sites, samples are collected both at the surface and from near the bottom. These samples are analyzed for a variety of “conventional” things of interest to lake water quality, everything from temperature to nutrients to algae levels to bacteria. This routine water quality monitoring has been going on for decades, so there is a large historical data set to look back on to see how things are going.
Temperature
Figure 2: South Lake Union temperature Cycle
Let’s explore this a bit. Take the site down in the southwest part of the lake (#A522 on the map). Click on that and, voila, a variety of parameters are listed. Try surface temperature, and you will get a graph (copied here as Figure 2) that plots out five years of results, through 2002. This shows the general trends: that the water is typically around 7 degrees Celsius (about 45 degrees Fahrenheit) in the winter, warming slowly to a tolerable 16 or 17 degrees C. (low 60s F.) by mid June, peaking in the low 20s C. (a little above 70 degrees F.) at the first of August, staying pleasant through September, then plummeting back to winter cold in October.
These temperature data are very similar to my own personal log book from my place at 2019 Fairview, where I have sporadically dipped a thermometer and kept a record since 1997. The coldest I’ve ever recorded is 7.0 C (about 45 F.) in January; the warmest was 24.4 C (76 F.!) on July 30th, 2003. In fact, by my records, last summer was the warmest water we’ve ever had. Sure was good for swimming, but I get ahead of myself.
If you look at the deep lake temperatures on the Web, you see that in the winter the lake water is about the same temperature from top to bottom, while in the summer, the deep water never gets above about 18 C. (mid 60s F.). This stratification is common in almost all lakes in the summer and is important for the fish (another story). Typically the first big storms in the fall, coupled with the cooling temperature at the surface, cause “turn-over” to happen in the lake, and after that the water column is uniform in temperature (and many other parameters) until spring.
Conductivity and Nutrients
Measures of conductivity illustrate the phenomenon of salt water intrusion into the bottom of the lake from the locks, which has actually improved quite a bit in recent years with changes put in by the Army Corps of Engineers. On the Web, notice the higher levels of conductivity at depth at the sampling site by the Fremont Bridge compared to the main lake.
Figure 3: South Lake Union chlorophyll cycle
Nutrients (nitrogen, nitrates, ammonia, total- and ortho-phosphorus) are all measures watched closely by limnologists (lake specialists), but I won’t go into the gory details here. These nutrients are key to the small algae that thrive in the lake water, which are measured by both chlorophyll-a levels and the general transparency of the water. As many of you have observed from your excellent perspective living a few inches above the water surface, Lake Union and Portage Bay see boom-and-bust cycles of algae beginning in abandon in the spring and tapering off in mid to late summer. The chlorophyll-a counts (Figure 3) reflect these cycles, showing greatest ups and downs in the spring and continuing, albeit with not quite as wide fluctuations, through October.
Figure 4: South Lake Union transparency cycle
Transparency (Figure 4) goes up and down as well, although it tends to be greatest in late summer and early fall. Being able to see on occasion the little measuring disk at depths of more than 20 feet is really quite remarkable for an urban lake! Even the average of around 3 meters (about 10 feet) deep is quite amazing. Of course this clear water allows for more aquatic plants to grow, but that is another story.
Bacteria
The parameters probably of most interest to you are the bacteria measures. Routine analyses are done for fecal coliform bacteria and for enterococcus bacteria, shown in Figures 5 and 6, respectively.
Figure 5: South Lake Union fecal coliform levels
We do still have some combined sewer overflows as well as many storm drain discharges into the lake, although the former have been greatly reduced in recent years (yet another future article). We see lots of scatter for bacterial measures, which is to be expected, yet general trends emerge of higher levels in the winter, when CSO and stormwater discharges occur, and thean occasional fecal coliform spike in the summer (although rare). In general, fecal coliform levels up to 200 (colony-forming-units per 100 milliliters) are considered within standards, even for swimming beaches.
Figure 6: South Lake Union enterococcus levels
You can see that the Enterococcus levels, which are another measure of potential human pathogens and human sewage sources, are negligible in the summer. On the Web, compare the bacterial levels at the Montlake Cut (negligible - good news for you'all in Portage Bay) with those down in the main lake. For more detailed and interesting info on bacteria and temperatures at designated public swimming spots on Lake Washington and beyond, check out:
http://green.kingcounty.gov/swimbeach/
Look in the near future for reporting on E.coli levels, a new monitoring parameter around which a federal water quality standard will soon be issued.
I watch these data. I swim in the lake. I let my kids swim in the lake. While there are always some risks, it is my opinion that the levels of bacteria we see in the typical summer months are O.K. and not a significant issue. I won’t swim after a big storm, which is rare in the summer around here but does happen on occasion. I don’t swim in the winter, when the CSOs and stormdrains still release high bacterial levels into our neighborhood.
I am concerned about these untreated discharges to our waters, as well as the occasional oil slick, the floating trash and debris, the legacy of the gas works, ship yards and contaminated sediments, and other environmental insults. My comments above deal with the general picture of the quality of our water, which is surprisingly good for so urban a place. I live on the water because I care about it and want a close connection to this environment. Our lake is alive with algae, zooplankton such as Daphnia, little and big fish, fish-eating birds, aquatic mammals and plenty of those Homo sapiens types. It is neat to see the boom-and-bust cycles of algae and plankton, the salmon jumping and the beaver splashing. We should celebrate what we have. For such a large urban lake, we have amazingly good water quality. (The sediments on the bottom are another story entirely – I wouldn’t muck around down there if I were you.)
I hope that some of this ramble has been of interest to you. I encourage you to jump onto the Web and check out these water quality data collected and plotted by King County scientists. Feel free to send your questions to me at Dave.Galvin@KingCounty.gov. I will log them, consult with local experts, and answer in later issues of this newsletter.