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Environmental experts are still searching for the reasons for a massive fish kill in Rehoboth Beach’s Silver Lake on Saturday, April 26.
The Department of Natural Resources and Environmental Control (DNREC) and other environmental experts say the cause of the kill was low dissolved-oxygen levels. But, no consensus has been reached on why the oxygen was so low.
Todd Fritchman of Envirotech and also the captain of the Dewey Beach Patrol said he reported the fish kill on the morning of Saturday, April 26 while doing a temperature analysis of Silver Lake. Fritchman said the water temperature at the time was 64.4 degrees, which didn’t indicate any problems with dissolved oxygen. He said some other variable caused the loss of oxygen in the water.
Whatever the cause, Fritchman said there must be a collaborative effort between the city, DNREC, homeowners and specialists in managing lakes and ponds in order to assure large fish kills do not happen. He said a comprehensive lake management plan is needed to address issues such as removal of invasive species, purifying the soil and water in and around the lake, providing methods to filter and remove contaminants from stormwater runoff drains and pipes and extensive educational outreach for homeowners around the lake.
Craig Shirey of DNREC’s Division of Fish and Wildlife said the oxygen levels in the lake have still not returned to normal. He said oxygen should be around 7 to 8 parts per million but is now down to 3 or 4 parts per million now. Oxygen remains particularly low in the narrow, most inland part of the lake, but in most of the lake, oxygen levels were high enough to avoid another massive fish kill.
Robin Tyler of DNREC’s Division of Water Resources said the department still did not know for sure what caused the dissolved oxygen levels to drop so low. Tyler observed conditions on Monday, April 28. He said the lack of oxygen was caused by an accumulation of organic matter. He said there are a variety of sources for this: an algae bloom, droppings from the various water birds who use the lake for resting and food and nutrients that get into the stormwater runoff that goes into the lake.
“Any kind of human activity that introduces additional nutrients to a water body is going to contribute to the type of algal growth in the water,” he said. “Any activity that reduces the nutrients is progress in the right direction. The only thing that can drive oxygen levels down to where they were on Saturday is a large amount of organic matter in the water.”
DNREC Secretary John Hughes speculated that high concentrations of pollen might be a cause. Shirey said high concentrations of pollen could act as a barrier to the oxygen getting through the water if it were thick enough.
Tyler said the likelihood that pollen played a role in the kill is remote.
“Pollen is distributed on the surface of water all over the state,” he said. “We’re not having fish kills all over the state. If it’s fresh, pollen gets the attention of someone walking by. The kind of organic matter that causes oxygen levels to go down is starting to rot.”
Tyler said the low oxygen levels at Silver Lake could have been caused by the crash of a seasonal algae bloom. He said it is possible that the first algae bloom of the spring developed, because of the dry and warm weather throughout mid-April, reached its maximum capacity and crashed.
“When an algae bloom crashes, bacteria multiply to break it down into simple forms of nutrients to be used in the next bloom,” Tyler said. “Bacteria use oxygen. When there is lots of organic matter, the bacteria digest it. If there is enough (bacteria), the oxygen levels can get low for animals such as fish.”
The most prominent and largest fish killed last Saturday were large mouth bass, many of which were females carrying eggs. Shirey said estimated about 100 large mouth bass were killed, and Tyler said the fact that the bass were spawning caused extra stress on the fish besides the low oxygen.
Tyler said fish kills like last Saturday’s are very unusual for this time of year. When they do occur, it is typically in the summer when the water heats up. He said the oxygen levels at Silver Lake on Saturday must have gotten extremely low because carp were gasping for air at the surface. He said carp can tolerate low oxygen levels, as can large mouth bass, although not as well.
Fritchman said the problem runs deeper than just dissolved oxygen. Rapid development, introduction of invasive plant species, the lack of a buffer zone around the lake and contaminants from stormwater runoff and runoff from impervious surfaces all played some part in the fish kill.
Shirey also said the low oxygen levels could be a result of a cumulative effect of different contaminants being introduced into the water but without a program to monitor that, there was no way of knowing it.
Tyler said it was doubtful chemical inflow caused the low oxygen; he agreed that human activity certainly plays a role in algae growth in Silver Lake and that intercepting and filtering the stormwater that runs into the lake will help remove the nutrients that come with it. He said while this wouldn’t totally eliminate the problem, it would cause some reduction.
Sallie Forman, director of Save Our Lakes Alliance3, said the fish kill was “a wake up call,” and “a tragedy waiting to happen because of the benign neglect.” She said she was organizing a meeting with Silver Lake homeowners on Saturday, May 31, to discuss the conditions of Silver Lake and what they can do to be better stewards of the lake. She said Hughes would be among the speakers at the meeting.
“We can’t keep going the way we’re going,” Fritchman said. “Until the land-use management is done more responsibly, you’re going to see more and more of these.”
Contact Ryan Mavity at ryanm@capegazette.com
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