news from the waterfront by Tim Matson
ADAPTING YOUR POND TO CLIMATE CHANGE
Climate change is here: just ask a pond owner who’s been watching a pond for years. Ponds are barometers of the weather, putting pond owners up close and personal with significant climate changes. A pond that used to be algae free blooms with pond scum, that stringy filamentous algae. Where trout used to grow, they die during a few hot, summer days. A pond that once maintained its waterlevel looks like a dusty moon crater.
What to do?
Adapting your pond to climate change involves
responding to three primary weather effects: increasing temperatures, shifts in the seasons, and extremes in precipitation.
Let’s start with increased warmth.
Ponds with higher than average summer water temperatures can threaten fish, especially in northern waters where cold water fish normally live all year. People who used to be able to raise trout are discovering that higher temperatures kill their fish. Brook trout prefer especially cool temperatures, and they are first to succumb; rainbows can tolerate somewhat higher temperatures, but may also die in the increased heat.
What’s the solution?
Many pond owners, out of necessity, have switched species. Some gave up on brook trout and switched to rainbows or brown trout. If temperatures are still too high, even for just a few days, these northern purist pondmeisters may switch to smallmouth bass, just to have some fish that can tolerate those warmest summer days.
Another way to adapt your fish pond to higher
temperatures is to add oxygen. One of the main
reasons higher water temperatures kill fish is
due to decreased dissolved oxygen levels. Cold water holds more dissolved oxygen than warm water. That’s why fish kills often occur during the warmest stretches of weather. A big bloom of algae can also point to oxygen depletion. How to add oxygen to water? Aeration.
While aeration has gained a foothold in the south, out of necessity, it’s making its way into northeastern ponds, too. Aeration can be done naturally or mechanically. Either way, the effect you want is increased dissolved oxygen in the water. Nature aerates water in several ways. A waterfall in a brook creates a splash pool full of air bubbles – dissolved oxygen is added from better contact with the atmosphere. Rain splashing in a pond aerates the water. Wind-churned waves across the water
add dissolved oxygen horizontally. Since these natural effects may not be present in a pond when needed, people mimic nature in various ways.
One of the simplest ways to boost oxygen levels
is to aerate your inflow sources. Perhaps you have
a small stream flowing into the pond which can
feature a waterfall at the entry so that water hits the pond with a splash. Presto, dissolved oxygen. However, be sure that the waterfall you create doesn’t expose the water to enough sunlight or heated rock to warm up the water and offset
the effects of aeration.
The same goes for a water source piped in from a spring, well, or other source. Instead of piping it
in underwater, splash it in. Well water is usually low in oxygen, so splashing it in is even more important.
If natural aeration doesn’t do the trick, try mechanical aeration. There are many types of motorized aeration, with bottom-diffused oxygen at the top of most lists.
Diffuser systems pump air to the bottom of the pond, where it is released through an “air stone” or other diffuser. Air then rises in a column of bubbles to the surface. As the air passes through the water it adds dissolved oxygen. It also causes some circulation which helps mix the warmest surface areas with the colder, deeper layers of water. Since the cold lower layer is often short on oxygen, the mixing further enhances oxygenation. (Circulation is usually a good thing unless it warms up the bottom strata too much for cold water fish.)
Other types of aeration include mechanical splashers and fountains, which work on the top layer of water. Paddle wheels also churn up surface water, and pumps which re-circulate water by splashing and aerating the returning water back into the pond.
Aeration also helps supress algae in some cases. Algae feeds on nutrients throughout the water column and on the pond bottom. When a pond is low in oxygen, those nutrients tend to accumulate faster. Oxygen assists with rapid decomposition of those
nutrients. The circulating effect of aeration also helps mix the lowest oxygen layers at the pond bottom with the richest oxygen layers at the surface, which mean the nutrients on the bottom get a better chance to decompose.
Another downside of climate change is calendar confusion. That is, the seasons aren’t happening when they used to.
Let’s start with summer, the season most popular with pond users. Summer has gotten longer. It starts earlier and ends later. One of the most pronounced effects of longer summers is warmer waters. We’ve gone over many of the things you can do about offsetting the effects of higher temperatures with aeration. Another offset is adding supplemental water to keep temperatures down and prevent waterlevel drops. In my experiences in New England, the healthiest ponds are overflowing most of the time.
Supplemental water can come from a stream diverted by channeling or pipe, springs sourced upstream and stored in a pool or cistern, or well water. Extra water can also come from roof catchment systems and foundation drains. The ideal location for extra water is uphill from the pond, so it can flow to the pond by gravity. Otherwise pumping will be required.
In addition to offsetting the warming effects of
longer summers with extra water, proper pond
design and maintenance will help. Ponds should be deep enough to keep a reserve of cool water down below. The sides should sloped enough to discourage plant and algae growth, and unwanted aquatic plants kept in check.
A pond sitting all day in the hot sun will be vulnerable to the negative effects of overheating. Siting a new pond so it will catch some tree shade can be a good strategy for the changing climate, although you may have to deal with leaf deposition. I worked on one new pond this summer where the site was shifted to a shaded location to get off an exposed open field.
As summer gets longer, winter shortens. What are the effects on ponds? I remember being able to start
skating on Thanksgiving. Last year there was open water on Thanksgiving, and it wasn’t safe to skate until Christmas. On bigger lakes ice fishing shanties don’t appear on the ice until mid-January. And as you might imagine, the truncated season ends sooner.
I feel lucky to have a pond that is shaded in winter. Compared to many neighboring ponds, it ices up early and remains locked up until mid April. Even with the shorter winters, it remains a good skating pond.
If I were building a pond from scratch today,
I’d consider siting it with enough trees nearby
to cool it in summer and winter. And if the best site is tree free, plant some. Keep in mind that the most effective shade year round is from conifers. And if shade is needed for an existing pond, planting trees is a good option (but not on the dam!).
With winter shrinking and summer expanding, what’s up with spring and fall? As far as ponds go, spring and fall are in-between seasons. Fish are often stocked in spring and if raised for food, harvested in fall. You might do maintenance in the spring, like repair erosion in the inflows and spillways, get equipment such as aeration ready to go, or put in docks or floats. Come fall, put things away.
Whatever the season, changes in precipitation are
We’re getting more, or less, rain and snow than we’re used to. In some regions, like the northeast, that has often meant flooding. In the south and west, drought. And everyone seems to be on the receiving end of more high energy storms, like Sandy, Katrina, Irene, and raging thunderstorms with tornadoes across much of the nation. The kind of rainfall we used to expect at 25 to 100 year intervals comes more frequently. What does this mean for pond owners?
Unusually high rainfall and flooding means more strain on spillway systems. When there’s more rainfall, or snowmelt combined with rain, discharge systems have to handle more volume or the pond will
flood. Sometimes the main spillway needs replacing with a larger pipe. If you have a piped system (standpipe, culvert) sized for “old fashioned” precipitation, make sure you have an emergency spillway that can handle excess overflow. Sometimes emergency spillways need cleaning out or enlarging.
If your main discharge is a natural earthen spillway, keep it cleaned out. I’ve seen more than one pond breach the dam when an overgrown and sediment clogged spillway channel could not handle overflow.
What if a pond is flooded? Not good. Excavated ponds may be especially vulnerable to flood plain runoff. Be sure the pond is bermed up around the perimeter to divert flood water. The upslope end is usually most vulnerable.
If you have an inflow stream feeding the pond, you may want to construct a system to divert the stream during high runoff periods. Silt pools can also be effective at trapping sediment carried to the pond by a flooding stream.
If you can drop the pond waterlevel using a drain, or a splash board in the spillway, the pond should handle high inflow.
More rain and runoff can often cause erosion, especially in the inflow and outflow channels. These channels can be reinforced with stone, and perhaps a foundation layer of construction fabric.
Any steep exterior slopes, like the backslope of
a dam, can be vulnerable to erosion. It’s a good idea to make a habit of inspecting the pond
Another effect of increased precipitation is high nutrient runoff. In addition to water, runoff contains sediments, minerals, and organic matter that can add to the nutrient load in ponds. The result is often large blooms of algae after summer storms (which is why lake beaches may be closed after high runoff storms due to toxic algae blooms and high bacteria levels).
Where high nutrient sources can be pinpointed,
such as a farm field or municipal storm drainage,
it may be possible to use ditches or drainage systems to divert runoff away from the pond.
In streams feeding a pond, sediment pools can
help trap some of these nutrients. Pools will need periodic cleanouts.
In cases where increased rainfall and sediment
loading cause high nutrient loads and consequent algae blooms and other water problems, the pond may need to be dredged. Aeration can also help decompose nutrients. And certain fish species are sometimes used to take up the growth triggered by nutrients, such as grass carp, tilapia, and crayfish – where they are legal to stock.
Some Interesting Conclusions
In my region of the northeast, primarily Vermont,
a dramatic example of climate change was tropical storm Irene, in August 2011. More recently, the devastating effects of Hurricane Sandy, ripping into New York and New Jersey, causing billions of dollars of damage and untld ecological havoc are at the forefront of our minds. With Irene, rainfall and high water exceeded levels set back in the flood of 1927, which had been the worst on record. Irene caused nearly a billion dollars in damage to roads, bridges, buildings, and farms. But in my discussions with pond contractors and owners, the concensus is that our ponds held up well. There were very few breached dams, destroyed ponds, or flooding damage. It was a big test of our pond building standards and regulations, and we passed. I see no reason to cut back on pond building in fear of climate change. In fact, for decades, one of the primary uses for ponds has been as retention impoundments. They hold back flood water for slower release during high precipitation events. The main lesson of climate change is to maintain your pond, and make sure inflows and outflows can handle the increasing rains.
For those drought prone areas, deeper is better. Design a pond that can withstand those severe droughts.
A final note: this summer I visited more ponds and pond sites in Chittenden County than ever before. These are the towns and villages near Lake Champlain, where there are many beaches and opportunties for boating and fishing. Who needs ponds? After about my fifth visit, I was standing near a potential pond site and asked the land owner, “Isn’t Lake Champlain just over that hill?” He nodded and anticipated my question. “It’s dirty,” he explained. I recalled many stories I’d read about big rains causing nutrient overload from farm and municipal runoff, leading to toxic algae blooms and beach closings. He was building a pond to get clean recreational water. Climate change: another good reason to build ponds.
This article was originally published by Pond Boss
magazine. Visit their website at www.pondboss.com
May 1, 2013