Do tiny colloid particles found naturally in Ohio rivers and streams make up one of the key nutrients that contribute to harmful algal blooms, potentially disrupting scientists’ ability to forecast harmful algal blooms on Lake Erie? A team of researchers at The Ohio State University, funded by the Harmful Algal Bloom Research Initiative (HABRI), tested this claim through a recent project.
Researchers know that phosphorus flowing into Lake Erie in its dissolved form, which is available to be taken up by organisms, drives harmful algal blooms. However, a 2019 study found that phosphorus bound to small, natural particles called colloids could make up half of “dissolved” reactive phosphorus, or DRP.
Colloids are extremely small — 1 to 450 nanometers, which are a billionth of a meter — and differ widely in their chemical composition. Notably, compared to dissolved phosphorus, colloidal phosphorus often has lower bioavailability, meaning it may not be as readily available to support algal growth.
Researchers collected water samples from streams in northwest Ohio during high flow events to study colloidal phosphorus concentrations.
If colloidal phosphorus accounts for a large fraction of dissolved phosphorus being measured by scientists, then this lack of understanding could impede how they monitor phosphorus loading and manage harmful blooms on Lake Erie.
In response, a team of researchers led by Dr. James Hood of The Ohio State University aimed to find out how much colloidal phosphorus contributes to dissolved phosphorus loads and how bioavailable it is. Their project sought to better understand the source, composition, and chemical and ecological reactions involving colloidal phosphorus in waters ranging from agricultural edge-of-field sites to wetlands and rivers to Lake Erie.
“This is a story about a bunch of scientists who, for good reason, evaluated one of the core simplifying assumptions in their field, about dissolved phosphorus,” Hood said. “The key part of this work was really challenging this assumption that colloidal-DRP is zero and/or not bioavailable.”
Specifically, the team measured the concentration of colloidal phosphorus and planned to determine how rapidly it was being created or lost within the streams and rivers flowing into Lake Erie. They also planned to use algal assays to determine the bioavailability of colloidal-DRP in Lake Erie.
After the first year of sampling, however, researchers found that colloidal-DRP concentrations were much lower than expected, so the team switched gears. They focused mostly on several goals: determining whether colloidal phosphorus was ever a large portion of DRP in the Lake Erie watershed, studying the bioavailability of colloidal phosphorus, and understanding why previous studies had found high concentrations of colloidal phosphorus.
Researchers conducted a survey of Lake Erie tributaries focusing on high flow events, which have been shown to carry the highest colloidal phosphorus concentrations. They also conducted several experiments focusing on understanding how freezing, a technique used to preserve samples in previous studies, influenced colloidal-DRP concentrations.
Ultimately, the study showed that colloidal-DRP concentrations and loading to Lake Erie were relatively low.
“During these storm events, we’re not seeing high colloidal-DRP concentrations,” Hood said.
Lake Erie managers currently operate on the understanding that loading of DRP, which is nearly 100% bioavailable, into the Maumee River is the main cause of harmful algal blooms. If results had shown that most of the DRP loading was associated with colloids, which are likely not as bioavailable, then the way scientists monitor, forecast, and manage Lake Erie HABs might have needed to be modified, causing an expensive disruption to management efforts.
Instead, based on the study’s findings, the team recommends that colloidal phosphorus does not need to be monitored or managed. Researchers also found that freezing alters colloidal phosphorus concentrations and are preparing a paper that would recommend against freezing colloidal-DRP samples.
“What our results suggest is that managers and harmful algal bloom forecasters focused on Lake Erie do not really need to worry about colloidal-DRP, and there’s no need to change monitoring or management to account for it,” Hood said. “However, given how dry things were during the two years of the study, we might want to keep an eye on colloidal-DRP in the future.”
To learn more about this project, contact Dr. Hood at hood.211@osu.edu or watch his recent Freshwater Science webinar.
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.