Story by Rachael Rourke
Fort Stewart DPW Environmental Fisheries Biologist
The term turnover can have multiple meanings. When talking about a vehicle, it might mean that your engine started; in football, your team lost possession of the ball.
In pond management, a turnover is a natural occurrence that describes vertical movement of water of differing temperatures and oxygen levels. This occurs normally in spring and fall as air temperatures warm or cool. This gradual mixing of the water layers has little effect on fish. However, if certain conditions during the summer cause a rapid turnover, depleted oxygen levels in the water can result in a fish kill.
Two ponds at Hunter Army Airfield recently experienced turnovers, with varying levels of fish mortality.
For a better understanding of why these fish kills occurred, let's discuss a little more about pond dynamics and what happens during a turnover.
The water in a pond is divided into horizontal layers. The top layer is the most fertile and oxygen-rich. This is the layer where phytoplankton, microscopic plant life, grows.
Phytoplankton is beneficial to ponds and like other plants, phytoplankton produces oxygen. When there is a good phytoplankton bloom, this upper water layer will be highly oxygenated and will take on a green appearance due to all the phytoplankton.
This green color blocks sunlight from reaching the bottom of the pond and helps prevent the growth of unwanted plants. Phytoplankton is also important in the food chain of a pond. If more phytoplankton is present, there is more food for fish to eat.
Thus, more oxygen and more food mean more fish in the top layer of the pond.
The bottom layer of a pond is colder and less fertile. The phytoplankton in the top layer block light from reaching the lower layers, reducing the temperature and restricting plant growth.
The cooler water is heavier than the warmer water, keeping it in the lower portion of the water column. With few or no plants to carry on photosynthesis, there is little oxygen available in this layer. Also, as organisms in the pond die, they sink to the bottom and decompose. The decomposition process uses what little oxygen remains.
A turnover occurs when the top layer (warm, fertile, highly oxygenated) and the bottom layer (cold, infertile, and poorly oxygenated) are suddenly mixed together. This typically happens when a heavy, cold rain event blows through in the summer. The rain quickly cools the water in the upper layer, and with the help of winds, this top layer sinks and mixes with the bottom layer.
When these layers mix, the dissolved oxygen is depleted in the upper layer of the pond where fish generally occur. With little dissolved oxygen, the fish become stressed and often move to the surface to gulp air. If nothing happens to improve oxygen levels, a fish kill can occur.
The percentage of fish that actually die varies greatly depending on how extreme the turnover is, the amount of dissolved oxygen present in each layer, how quickly the pond is able to recover, and if the fish are experiencing stress from another source. Sometimes, less serious turnovers kill only a handful of fish. Severe turnovers will kill every fish in the pond.
In some summers, conditions that cause pond turnover never occur. In other summers, they may happen repeatedly.
In August 2019, Oglethorpe Pond, Pond 29, at Hunter, suffered a pond turnover after a heavy rain event, but the Fish and Wildlife Branch was able to react quickly and aerate the pond with a tractor-operated pond aerator.
The pond quickly recovered and only a handful of fish died. Anyone that went near Wilson Gate Pond, Pond 35, toward the end of July 2019 noticed that it did not fare as well. Pond 35 experienced a heavy fish kill and the pond will remain closed for the foreseeable future. It will be restocked this upcoming fall and spring.
Even though pond turnovers and resulting fish kills are always a concern, there is still plenty of great fishing on the installation. Anglers have been reporting great catches all summer long from Stewart-Hunter's 20 managed ponds and miles of rivers and creeks.
Halstrom Lake, Pond 24 at Hunter, was stocked the last week of September 2019 for a children's fishing event.
The season's final Marne Bass Tournament is scheduled for 6 a.m. Oct. 12 at Metz Pond, on Fort Stewart. The cost is $30 for active duty Soldiers, retirees and their Family members, $40 for the civilian community. There is a $5 boat entry fee. Register at StewartHunter.ArmyMWR.com.
For more information on fishing on the installation, contact the Fort Stewart-Hunter Army Airfield Directorate of Family Morale, Welfare and Recreation Outdoor Recreation Office on Fort Stewart at 435-8205 or 315-9354. at Hunter.