By Pilar Deniston, for work done at the College of the Environment, Western Washington University
Have you ever noticed a lake with bright green algae or a weird-looking scum? These issues could be linked to phosphorus. This nutrient naturally originates from rocks and soil and enters our waterways via erosion. However, phosphorus is also introduced by humans living near waterbodies, since human and animal waste, as well as many fertilizers, contain phosphorus. While a small amount of phosphorus is necessary for plants, too much can be an issue. When there is an excess of nutrients, a process called eutrophication is triggered, which can lead to harmful algae blooms. To tackle this problem, Washington implemented a statewide ban on phosphorus-containing turf fertilizers in 2013.
Figure 1: Map of the 46 lakes that met all filtering criteria in Snohomish, King, and Thurston counties
WALPA was the key advocate for this ban and, without their passion and dedication, the ban likely would not have passed. Other laws in Washington have been implemented with similar goals, such as one that limits the amount of phosphorus in dishwashing and laundry detergents. Many other states have also passed phosphorus-containing fertilizer bans. Thanks to these efforts, some fertilizer companies and dishwashing detergent companies have removed phosphorus from their national formulas, helping to reduce eutrophication nationwide.
Despite the intention behind Washington’s phosphorus-containing fertilizer ban, there has been no comprehensive assessment of its effectiveness. To fill this gap, I decided to investigate the effectiveness of this ban as my Honors College capstone project at Western Washington University.
A call for data from Snohomish, King, Pierce, and Thurston counties led to almost 25,000 total phosphorus measurements from 84 lakes! Slowly but surely, I cleaned, organized, and analyzed the data. For data consistency, the data were culled to the years 2002 to 2023 from June to September. Lakes were only included if they had at least ten samples with at least two years of samples on either side of the ban. Outliers — such as lakes with restoration or lakes less than 2.5 m deep — were eliminated. Ultimately, I was left with 46 lakes and just under 5,000 data points.
Figure 2: Total phosphorus (μg/L) was averaged for each year for each lake. The blue line indicates the mean total phosphorus across all lakes. The vertical line indicates the 2013 phosphorus-containing fertilizer ban.
Using statistical analyses and data visualization techniques, I uncovered some interesting results. The data suggest that the ban may have reduced phosphorus concentrations in lakes in the years following its implementation. However, this effect appears to have diminished over time (Figure 2, Figure 3). For instance, a comparison of data from four years before and after the ban (2009-2016) reveals a decreasing trend in phosphorus levels. However, over a longer period (11 years before and after implementation), the decrease is still visible but is much less pronounced.
It’s important to note that this ban was not designed to remove existing phosphorus in the lakes, but to reduce future phosphorus loading in our lakes. In this way, the ban is protective and not restorative. Thus, while the initial impact of the ban seemed promising, the absence of an increase in overall phosphorus levels over time still represents a positive outcome.
Figure 3: The results of a statistical test (paired t-test) for all the lakes (df = 45, α = 0.05). The negative signs indicate a decrease, and the bolded lines indicate a significant decrease in total phosphorus.
Several factors can influence phosphorus concentrations in lakes: rainfall, land use, septic systems, geology, and potentially even large climatic phenomena like El Niño and La Niña cycles. These variables can confound the results and should be considered in future analyses and predictive models. The availability of control groups also constrained the scope of my study. Although I was able to find and analyze data from remote lakes in Washington, which are expected to be minimally affected by the ban, and lakes in Oregon, which lack similar regulations, the data were insufficient and yielded inconclusive results. Furthermore, Snohomish County’s LakeWise program, launched in 2013, aimed to improve lake health through workshops, incentive programs, and increased public awareness. Since many of the lakes in my study are located in Snohomish County, the concurrent implementation of the LakeWise program could have further reinforced the results. Thus, the combined effects of the phosphorus ban and LakeWise initiatives make it challenging to isolate the impact of each intervention individually.
In conclusion, while the effectiveness of the phosphorus ban appears to have waned over time, the lack of an increase in phosphorus levels suggests that the policy has helped preserve lake water quality. The combined efforts of individuals, counties, WALPA, and the state to reduce phosphorus pollution and protect our lakes have led to positive results, but much work remains to be done.
I would like to express my gratitude to Angela Strecker for her guidance throughout the project. I would also like to thank Marisa Burghdoff, Jennifer Oden, Wafa Tafesh, Tim Clark, John Haberlin, and Bryan Mohlman from Snohomish, King, Pierce, and Thurston counties who shared the data to make this project possible.
