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A full cyanobacterial (algae) bloom where all the water is green and is spilling over the dam into the Murderkill River. The UD researchers have their sensors located next to this dam.

Pea soup pond

Aerial Photos by Stephanie Dohner and Art Trembanis

UD study looks at how low streamflow can increase cyanobacterial blooms in Coursey Pond

Around the same time every summer, the water in Coursey Pond turns thick, smelly and pea soup green.

This happens because the pond, which is located between Felton and Frederica, Delaware — just downstream from Killens Pond — is overrun by noxious cyanobacterial blooms that disrupt the ecology of the water.

Cyanobacterial blooms have increased significantly in recent years in a number of settings worldwide. Locally, 14 of Delaware’s 43 state-managed ponds have historically suffered from seasonal or episodic algal blooms that include potentially toxic cyanobacterial species.

Not only do the blooms discolor surface waters and form smelly scums that discourage recreational use, but they also limit light penetration in ponds, deplete the oxygen in the water and affect the abundance and survival of other pond populations.

In addition, there are concerns about the health impacts of cyanobacterial blooms as a number of bloom-forming species and strains can produce toxic compounds under some conditions. While there have been no known cases of human sickness in Delaware yet, sicknesses in humans and animals have been reported in other states. The town of Sandusky, Ohio, for instance, had to turn off their water intakes from Lake Erie because of cyanobacterial blooms and the toxins released by the blooms.

New research from the University of Delaware recently published in the Journal of Environmental Quality suggests that in addition to the rising water temperature and other factors that occur naturally during the summer, the blooms in Coursey Pond are triggered by a lack of water flow and circulation within and through the pond. This, in large part, is due to pumping of irrigation wells in the watershed. Water diverted to irrigation is not available to flush the pond.

The study was led by Scott Andres, a senior scientist and hydrogeologist with the Delaware Geological Survey; Bill Ullman, professor of marine and geological sciences in UD’s College of Earth, Ocean and Environment (CEOE); Tye Pettay, a post-doctoral researcher in CEOE; and Christopher Main from the Delaware Department of Natural Resources and Environmental Control (DNREC).

The researchers used continuous and automated biogeochemical sensors to study the pond, which has been the subject of research since 2014 when monitoring began as part of a Delaware EPSCoR North East Water Resources (NEWRnet) project.

Andres said that Coursey Pond is hit with the “perfect storm” of conditions in the summer to allow for these harmful blooms to occur.

These conditions include lots of sunlight, warm water and adequate amounts of nitrogen and phosphorous. There also has to be conditions where the cyanobacteria can accumulate rather than be flushed from the pond with stream flow, which happens during the summer when flow decreases.

“When you heat up the landscape, trees and plants are consuming a lot of water in the watershed and farmers turn on their irrigation systems so the amount of water coming into Coursey Pond decreases markedly,” said Andres. “When that happens, the amount of time that a given molecule of water spends in the pond goes up tremendously from just a couple of days to weeks. When you put those things together, that means you can accumulate cyanobacteria and they will take over the whole pond ecology.”

Agricultural land around Coursey Pond in the Murderkill River watershed is heavily irrigated, which draws water away from the Murderkill River and, ultimately, Coursey Pond.

“The water taken out of the ground for irrigation is intercepting the water in the shallowest aquifer feeding the Murderkill River,” said Andres. “The amount of irrigation in the summer time in a dry year exceeds the streamflow in the Murderkill River. If we can manage how we use water, maybe we can maintain stream flow to the point where we discourage cyanobacterial blooms.”

An early cyanobacterial bloom in Coursey Pond is shown with streaks of green water as the cyanobacteria increase in abundance and disperse.

By monitoring these bloom-forming conditions regularly, the researchers said that while they might not be able to stop the blooms from forming, they can at least predict when they are going to occur.  

“We know that during a hot summer, within a few weeks of the start of the irrigation season, the flow rates go down leading to a stagnant water body and cyanobacteria blooms,” said Ullman. “Killens Pond, which is a recreational facility upstream, has the same problems. The last 5 or 8 years, they’ve had to close swimming beaches because of cyanobacteria. Prior to that, they didn’t have this problem. They would have an occasional few days-worth and that was it. Now, it’s pea green most of the summer.”

Delaware Sea Grant, in collaboration with researcher Pettay, created a bookmark showing Coursey Pond and explaining blooms, from cyanobacteria and harmful algae, as part of its series of educational bookmarks. Members of the public can request a copy of the bookmark by contacting Pam Donnelly at pdonnell@udel.edu.

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