In April, we talked about nitrogen in regards to water quality. This month, we’ll be talking about a related nutrient, phosphorus. Like nitrogen, phosphorus tends to be a limiting nutrient for plants, which is why it is part of fertilizer. It also has a natural cycle as it passes through different forms in the environment.
Some phosphorus is cycled through rocks and other sediments, while other forms of phosphorus cycle through living organisms. The living cycle is relatively simple; plants take up phosphorus from sediments, animals eat those plants and use their phosphorus, then animals die and decompose, returning that phosphorus to the sediment for other plants to take it up. The non-living side of things is a little slower; some types of rock have phosphorus in them, which release phosphorus slowly over time as the rock ages and weathers. This phosphorus can then enter sediments and waterways to be used by plants. We also see some sediments and decaying organic matter becoming rock over time, if conditions are right. The two parts of the cycle are connected but the formation of new rock and weathering of old rock is slow, while the movement of phosphorus through living organisms is relatively quick. This is why humans have learned how to mine phosphorus from rock to produce fertilizer and speed up plant growth.
This means we’ve increased the amount of phosphorus in the living side of the system. While this can help us grow more food, it also tends to lead to more phosphorus in waste, which means it’s ending up in natural systems, not just in agricultural systems where we want it. This ties into our discussion of turbidity back in February, along with our discussion about dissolved oxygen and eutrophication in April and June. It’s all coming together now!
So, too much nitrogen and phosphorus enter the Chesapeake Bay through runoff and other waste water, which affects the tiny plants in the Bay (phytoplankton), causing them to bloom like crazy, die off and use up all the oxygen, leading to fish kills. When we see high levels of nitrogen and/or phosphorus in the Bay, that’s a good warning that we may see a phytoplankton bloom and subsequent hypoxia. This is why we monitor these different water quality parameters; nitrogen, phosphorus, turbidity, and dissolved oxygen.
When it comes to measuring phosphorus in the water, we typically measure orthophosphate (PO43-) since this is the form that is most common in nature. Like with many of the water quality parameters we’ve discussed this year, the amount of phosphorus in a sample is measured using a colorimetric assay. This means the phosphorus in a sample is reacted with another reagent, which causes the sample to change color and the intensity of the color change tells us how much phosphorus is present. For less accurate but quick results, you can use a test strip that is dipped in the water you wish to sample. For more accurate but slower results, a water sample is taken to a lab for analysis. Portable and accurate phosphorus sampling isn’t really possible, such as with a YSI, though there are sampling units that can be set up to continually monitor phosphorus on site. These units are often set up at wastewater treatment facilities and even for environmental monitoring at locations of concern and they basically run the colorimetric reaction inside the unit and the reagents for the reaction need to be regularly replaced.
So, how are we doing on the Chesapeake Bay Watershed Agreement phosphorus goals? It is estimated that we achieved 67% of the phosphorus reduction goal by 2023 but that means we are not on track to reach the 2025 goal. A significant reduction in phosphorus in wastewater is where much of this has come from, thanks to the removal of phosphorus from some items like dish soap and laundry detergent and improved removal technology at wastewater treatment facilities. We’ve also seen some reduction in the amount of phosphorus coming from agriculture, thanks to increases in practices like winter cover crops and testing requirements before fertilizer can be applied. However, we are seeing population increases in the watershed, which means increases in developed land to provide infrastructure for those people, which has led to increases in phosphorus coming from developed land.
If you’re wondering how you can help, major sources of phosphorus into the Chesapeake Bay include fertilizer, animal and human waste, organic material, and wastewater. As mentioned in the previous paragraph, our region has worked hard to upgrade wastewater treatment facilities to remove more phosphorus than ever but for folks not on municipal sewer, be sure to maintain your septic tanks. Fertilizer is not just used on agricultural land but also to maintain lawns and other developed landscapes, so reducing or stopping fertilizer use at home can also help. As we learned earlier, living things release phosphorus when they die, so keeping excess organic material, such as leaves and grass clippings out of the water can also help. If available, give these materials to your local yard waste collection or consider composting them on your property. Leaving the leaves on your property is fine too, especially if you have natural areas that will use the nutrients but what we don’t want is to rake up leaves and grass clippings and pile them where they can blow into storm drains and other waterways that lead to the Chesapeake Bay. Finally, pick up after your pets and ensure that their waste is not contributing to phosphorus (and nitrogen and E. coli) in the Bay as well!