A winter storm recently made its’ way through Ohio bringing with it colder temperatures, ice and some snow.
Continuing SENR’s exploration of winter habitats, we met up with Eugene Braig, Extension program director for Aquatic Ecosystems in the College of Food, Agricultural, and Environmental Sciences at The Ohio State University to learn more about the functions of ponds, how winter weather may impact ponds and aquatic life and ways to mitigate for any disruptions.
The School of Environment and Natural Resources (SENR) had the opportunity to visit with Eugene Braig, Extension program director for Aquatic Ecosystems in the College of Food, Agricultural, and Environmental Sciences at The Ohio State University to learn a bit more about the impact of winter conditions on ponds.
SENR: What are ponds, how do they differ from say a lake?
EB: That’s a trickier question than seems on the surface. The obvious answer is “size”: lakes are bigger than ponds, but it’s not that easy, and definitions still sometimes vary by region, even among individuals. I like a functional definition. Lakes are big enough that wind has enough of an open stretch (called “fetch” on the water) to move water: create substantial wave action, possibly generate currents, etc. Ponds are small enough to ordinarily not. The state used a somewhat arbitrary cut-off of 5 acres in the past, smaller being ponds. However, that number is a little arbitrary; the shape of a basin, its alignment with prevailing winds, its setting within the contours of the landscape, etc. can all play into whether a basin functionally is a pond or lake.
SENR: Are there lots of ponds in Ohio?
EB: The easy answer is “yes.” When I refer to “ponds,” I usually have small, human-made basins in mind. Those may be excavated on private, rural property to serve recreational or aesthetic purposes, as a source for domestic or agricultural water, etc. They may be constructed in densely populated suburban or urban places to manage stormwater running off from residential neighborhoods. And there are literally thousands in Ohio.
SENR: Do all ponds have fish, or other aquatic life in them? What about plant life and other animal life?
EB: Yes, all ponds have some forms of aquatic life in them. Plants with diverse dispersal mechanisms (like seeds on wind or wildlife) will find their way to ponds. Aquatic microorganisms always find a way to colonize as do aquatic insects (adults can often fly), reptiles and amphibians (that can cross land from nearby waters), etc.
Larger, strictly aquatic organisms, like fish, will need some pathway to introduce them to artificial, human-made ponds from elsewhere. That’s usually via direct stocking by human interests, but can also be via temporary floodwater connections to nearby surface waters.
Ponds and their aquatic life in the winter months
SENR: What can happen to ponds and their aquatic life in the winter months? Are there ways to mitigate any adverse impacts?
EB: Strategies vary greatly. Annual plants senesce (i.e., deteriorate), seeds often overwintering. Some aquatic plants also regenerate from vegetative structures like tubers or turions (rigid/turgid shoots that can regenerate an entire plant).
Females producing egg clones of themselves is the most common order of business for waterfleas (a process called parthenogenesis). However, as winter approaches, females produce some eggs that generate males. Sexual reproduction then results in “resting” eggs that survive the winter.
Larger animals (leeches, amphibians, etc.) tend to hibernate in burrows into pond substrates below the frost line (called “hibernacula”).
Ohio’s fishes, however, depend on open water beneath the ice. Water is unique in that solid water (ice) is less dense than cold water (ordinarily densest at 39°F/4°C), so ice floats on cool water creating a kind of protective skin between cold water and the below-freezing temperature of winter air. Even though fish metabolism is much slower in the cold, they also still need some dissolved oxygen beneath the ice to respire and survive. Problems for fish arise when (1) ice completely seals the surface of the pond cutting off the dissolution of oxygen from the atmosphere and (2) snow accumulates on ice, cutting off sunlight and ending the production of oxygen as a byproduct of photosynthesis by whatever plants and algae remain. If that condition continues for long enough, oxygen can be depleted to result in winter kill, a mass-mortality event (i.e., lots of fish die). The spring thaw to follow then reveals the unfortunate situation.
strategies to reduce risks to fish
EB: Winter kill is a much greater risk for small and shallow ponds that are either choked with excessive aquatic vegetation or completely devoid of any vegetation; risks are lower for large, deep, and moderately vegetated ponds. The owner of a pond that routinely freezes for long periods of winter can take some actions to reduce risks.
- Manage the pond in the long term to only have moderate coverages of aquatic plants. Don’t apply herbicides/algaecides to large areas of water too late in the season. Dead plant material that is still decomposing has a substantial demand for oxygen.
- Ponds that use diffusers to aerate from deep water in warm months can repurpose their aeration to reduce risk. Move a diffuser to shallow water (maybe half the pond’s maximum depth) or keep a diffuser in shallow water (and turn off those in deeper water during winter) to erode a hole in ice and allow some exchange with the atmosphere.
There are a couple caveats.
(1) If open water isn’t common in your area in winter, any bit of open water may attract unusually high concentrations of waterfowl. Be aware of that possibility.
(2) Don’t run diffusers from deep water under ice for long periods of time. That deepest water will be that dense 39°F/4°C, obviously warmer than the 32°F/0°C of surface ice, and an important warmer-water refuge for fish (if there’s still oxygen down there). A diffuser operating from that deep water will mix it, pushing it to the surface to interact with ice and returning it to depth at nearer-to-freezing temperatures. If you’re successful in eroding a hole in ice, that moving water will directly interact with even colder air, will be less likely to freeze in moving, and can be returned to the deep super-chilled, potentially stressing or killing some fish.
- Shovel snow from the pond’s surface to allow sunlight to stimulate some photosynthesis and thus oxygen production under ice. Generally, plan to remove snow from at least 1/4 (ideally even more) of the pond’s surface. Perhaps obviously, that’s a much easier job on a half-acre pond than on 5 acres.
- But don’t do both! If you’re actively eroding holes in pond ice, I don’t want to catch you walking around out there with a shovel. Deliberately eroded ice isn’t safe.
Resources to learn more
SENR: Where are some resources for individuals to check out to learn more about ponds and how to manage for seasonal changes?
EB: As you might expect, I collect handbooks on pond management, and there are lots of them.
The Ohio Pond Management Handbook (Austin et al. 1996, 2015) is one of the good ones, and hard copies are available for free from Ohio Department of Natural Resources, Division of Wildlife district offices and sometimes from local Soil and Water or Extension offices (if available; it’s popular and disappears quickly). Electronic copies are also available here.
Or drop me a line directly. I’m happy to chat about your pond concerns and interests.
SENR: Thank you Eugene for sharing about ponds and their aquatic life with our audiences and strategies to reduce risks to fish.