Phosphorus is an essential nutrient for sustaining life on Earth, influencing entire food webs by fueling photosynthetic organisms. However, in agricultural regions like the Western Lake Erie Basin, it’s possible to have too much of a good thing: too much phosphorus can cause an overgrowth of algae, creating harmful algal blooms (HABs).
Scientists know that phosphorus from the Maumee River watershed is the primary driver of a HABs’ size in western Lake Erie, but they are unsure how rivers and streams affect the transport of phosphorus into the lake. New interdisciplinary research funded by Ohio Sea Grant recently provided more insights about this phenomenon.
“We really don’t know what happens to phosphorus in river systems between when it leaves agricultural fields and makes it into the lake,” said Dr. Jim Hood, an associate professor within the Department of Evolution, Ecology and Organismal Biology at The Ohio State University, who led the project. “It’s likely that rivers are influencing how much phosphorus enters Lake Erie and how available it is to organisms. What we don’t know is exactly how.”
To address this knowledge gap, Hood’s team worked to develop a new modeling approach to characterize how phosphorus cycles in the Maumee River watershed over time. To accomplish this, the project took advantage of high-frequency nutrient monitoring systems from the National Center for Water Quality Research (NCWQR) at Heidelberg University.
“The idea was to use that high-frequency data to build models that would actually help us understand how the phosphorus is cycling,” said Hood, who also serves as co-director of Ohio State’s Aquatic Ecology Laboratory. “The broader goal is to understand whether the streams are taking up or releasing phosphorus and how this changes over time.”
Many widespread watershed models assume that rivers transport nutrients downstream like a pipe, yet that’s not how streams work, Hood said. Streams are often reactive and changing, sometimes serving as a “sink” that stores phosphorus or a “source” that adds phosphorus to the water. Meanwhile, because phosphorus cycles are complex, measuring them in the field can be time-consuming and expensive.
Through the project, Hood collaborated with Laura Johnson, former NCWQR director, to use data from two sites at Potato Run and Shallow Run in northwest Ohio equipped with monitoring technology called NuLAB. The devices measure forms of phosphorus and nitrogen every two hours.
“Those are really promising,” Hood said. “They give us really good high-frequency data on phosphorus dynamics through time.”
For the duration of the project, Hood’s team also collected samples from the two agricultural streams on a monthly basis, gathering on-site data about how fast phosphorus is entering and leaving sediment and how nutrients are affecting the aquatic ecosystem.
Researchers then worked with Dr. Oksana Chkrebtii, associate professor of statistics at Ohio State, and Emerson Webb, statistics Ph.D. student, to develop the new model. The team used NuLAB data from a 10-day period during the summer to create a model that can estimate phosphorus uptake or release, taking algae and plants, bacteria, and sediment into account.
“This sort of work is a huge, interdisciplinary, team effort.” Hood said. “And what’s really great about this model is that it allows us to sort of break apart that complexity and estimate what the sediment is doing, what the algae are doing, and what the bacteria are doing.”
One important component in the model’s estimations is called zero equilibrium phosphorus concentration, which is the dissolved phosphorus concentration at which phosphorus goes into the sediment or is released.
“This sort of work is a huge, interdisciplinary, team effort. And what’s really great about this model is that it allows us to sort of break apart that complexity and estimate what the sediment is doing, what the algae are doing, what the bacteria are doing.”
Dr. Jim Hood
“It’s a really critical parameter to estimate,” Hood said. “If the stream water phosphorus concentration is high, then phosphorus is going to move from the water to the sediment and the stream is going to be a sink for phosphorus. If the stream phosphorus concentration is really low, then the sediment is going to release its phosphorus out into the stream and the stream will become a source.”
From the model, researchers found that as a whole, the streams are mostly a net sink for phosphorus available to algal blooms. In addition, the values predicted by the model over time were comparable to on-site measurements, meaning that the model was a “good fit,” Hood said.
More specifically, results showed that streambed sediment is most frequently a net phosphorus sink, with algae taking up slightly less phosphorus and bacteria releasing a lot of phosphorus back into the water. The model captured daily fluctuations in phosphorus levels alongside photosynthetic activity as well.
“I think what we ended up getting out of this was a good proof of concept for this model,” Hood said. “It’s doing a pretty good job of estimating different components of the phosphorus cycle, considering all the complexity involved.”
Results will help improve understanding of how phosphorous cycles in rivers and streams so that decision-makers can better respond to harmful algal blooms. Notably, Hood’s research found that streams are likely mitigating blooms by storing phosphorus, suggesting that streams could play a role in conservation efforts. The model also proved successful in estimating phosphorus over daily, monthly, and seasonal timeframes.
“Estimating how stream systems are influencing phosphorus exports over scales relevant to management is very complicated,” Hood said. “I’d like to see these models used more widely so that we can get a better system-wide understanding of this.”
Ohio Sea Grant is supported by The Ohio State University College of Food, Agricultural, and Environmental Sciences (CFAES) School of Environment and Natural Resources, Ohio State University Extension, and NOAA Sea Grant, a network of 34 Sea Grant programs nation-wide dedicated to the protection and sustainable use of marine and Great Lakes resources. Stone Laboratory is Ohio State’s island campus on Lake Erie and is the research, education, and outreach facility of Ohio Sea Grant and part of CFAES School of Environment and Natural Resources.