Data from: Cities can grow without harming lakes: Lake Washington has become less eutrophic despite rapid population growth

As cities grow, lakes are often assumed to suffer from increasing nonpoint
pollution. Many waterbodies have become more eutrophic in recent decades,
as expected, but many others have become less eutrophic, especially in
urban/suburban areas. What policies, practices, and ecosystem processes
have helped some lakes stay stable or become less eutrophic even in a
growing city? Identifying and understanding success stories is important
to continue protecting these lakes and improving other urban/suburban
lakes. We found one such success story when we examined water-quality
trends over the past 25 years (1998–2022) in Lake Washington, a
well-studied large lake in the Seattle metro area. The watershed
population grew rapidly during that time (34% from 2000–2020), yet Lake
Washington became substantially less eutrophic, and indicators of
development impacts stabilized or decreased. Chlorophyll concentrations
during the main spring bloom decreased sharply ( 25%/decade), and water
clarity and near-bottom dissolved oxygen both increased (8.5% and
17%/decade, respectively). Alkalinity and specific conductance had
increased during the 1970s–1990s, but in recent decades they have held
stable. Peak winter/spring nitrogen and phosphorus concentrations
decreased (-4.9% and -5.6%/decade, respectively), indicating decreased
watershed inputs. The type of development during this time was likely a
key contributor: we found no net loss of forest area and little increase
in developed land area (4.7% from 2001–2021). Instead of expanding into
new areas, redevelopment increased density on already-developed land and
likely drove improvements in stormwater treatment and other environmental
protections. Future work comparing stream watersheds could help discern
which specific aspects of redevelopment helped reduce nutrients and other
impacts. However, nutrient reductions were not the only factors
controlling the lake’s trophic state; chlorophyll decreased much more
strongly than phosphorus did. Lake Washington is a complex ecosystem
governed not only by water chemistry but also by interactions with
physical and biological factors such as stratification, warming,
phytoplankton community shifts, or food-web interactions. A better
understanding of all these factors is essential to provide sound
scientific guidance and ensure that Lake Washington and other lakes can
thrive in a growing city.