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

Nidzgorski, Daniel1; DeGasperi, Curtis1

Published Oct 17, 2025 on Dryad. https://doi.org/10.5061/dryad.dfn2z35fj

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Abstract

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.