Abstract: Lake Washington, in the Seattle metro area, became a well-known environmental success story with the lake’s rapid, dramatic recovery after sewage was diverted from the lake in the 1960s. But the story took a turn as the lake’s ecosystem showed impacts from continuing development in the watershed. Events such as a large bloom of potentially toxic cyanobacteria in 1988 foreshadowed worsening problems in the future. When the most recent set of studies were published around 2000, these impacts were expected to increase as the watershed developed further.
In this presentation, we add the latest chapter to Lake Washington’s story. But instead of increasing the dramatic tension as expected, the last 25 years have found Lake Washington surprisingly stable despite substantial population growth and development in the watershed. We use data from King County’s long-term lake monitoring to explore this “new normal” state of the lake. This program measured depth profiles of physical and chemical parameters 1-2x/month year-round since 1993. The monitoring station was the same mid-lake station used by researchers since the 1960s, continuing an important scientific record.
Typical seasonal dynamics in Lake Washington over the last decade (2013-2022): The main phytoplankton peak was a spring diatom bloom beginning as early as late February. As the lake stratified beginning in April, the diatom bloom was gradually confined to surface waters, then interrupted briefly as nutrient depletion, zooplankton grazing, and sinking of diatoms below the thermocline (~10 m) caused the bloom to decline in late May. The lake was mesotrophic in summer, with a smaller secondary phytoplankton bloom. In late summer and fall, areas of low dissolved oxygen developed in the metalimnion (copepod respiration) and near the bottom (decomposition). In the fall, stratification weakened and waters began mixing, until a large storm in late fall mixed the lake completely for the winter.
Trends over the last 25 years (1998-2022) were stable or opposite of those expected from development impacts. Summer average surface concentrations (June-September, 0-2 m) of phosphorus, nitrogen, and chlorophyll have all remained stable, as has total alkalinity (an indicator of construction and/or land conversion that had been increasing until the mid-1990s). The spring diatom bloom’s average and peak surface chlorophyll concentrations have decreased, which in turn drove increases in surface silica, whole-lake dissolved oxygen, and hypolimnetic pH. Understanding how Lake Washington achieved this surprisingly stable state, despite ongoing development, could help future work to protect this and other lakes.
