GARDENS July 2001

Liquid Refreshment

Ponds and other water features can be lovely—but they can be stinking, murky eyesores. A primer on what makes the difference

I have always been a firm believer in taking a bold approach to gardening, sort of a horticultural equivalent to what the military calls reconnaissance-by-fire: just get out there, blast away, and see what happens. This is generally both instructive and fun. Occasionally it produces astonishing successes, such as the four diminutive but irrefutable artichokes I managed to eke out of my sultry Virginia garden last summer.

Gardening is as much art and craft as it is science, and there is so much that has to be adapted through individual experience to individual climate, soil, tools, temperament, and tolerance for tedium that studying handbooks and following expert advice can take a person only so far in any case. The failures that come from just getting out there and trying something are no worse than any other failures one faces when confronting the notoriously fickle gods of agriculture.

That is what I thought, at least, until I tried water gardening and a fine late-winter day earlier this year found me, shin-deep, bailing out 200 gallons of something that smelled like a cross between pond scum and an open sewer. It smelled that way mainly because, technically speaking, that's what it was. Floating in the murk was one semi-intact dead goldfish; at the very bottom was a forlorn fish skeleton that reminded me of old Sylvester the Cat cartoons. The rest of the many fish that had been there last summer, before things went really bad, had vanished without a trace. By the time I was finished bailing it all out, my hands were dyed algae-green, a vivid color that came off only after five vigorous scrubbings. The odor was not so easily eradicated. And thus ended my just-get-out-there-and-do-something attempt at "natural" water gardening.

The idea of a garden pond dotted with lily pads and colorful fish, alive at night with the chirping of frogs, has proved irresistibly alluring to a growing number of home gardeners in recent years—and understandably so. Richard Koogle, of Lilypons Water Gardens, a major supplier of equipment and accessories, says that business has grown about 20 percent a year for most of the past fifteen years. He attributes this growth in large part to the availability of inexpensive, durable, and convenient rubber or PVC pond liners, which have taken the place of expensive and difficult concrete. A typical backyard do-it-yourself water garden, he says, costs $500 to $1,000.

When I jumped on the bandwagon, a few summers back, I found it quite easy to dig out a small pond in the center of my formal garden; install the flexible liner; add a few goldfish, snails, and water lilies in pots; and stand back and let 'er rip. The fish spawned with remarkable celerity, the lilies blossomed, and a variety of wild animal life showed up in no time: dragonflies, frogs, even a turtle. It was a lovely, dynamic centerpiece to the staid square lawn and boxwoods, and part of the pleasure was that it really did seem to be a complete ecosystem in miniature, with its own internal rhythms and sense of purpose.

In their sales literature the purveyors of water-garden supplies take a wonderfully inconsistent attitude toward what's involved in making a garden pond work. On the one hand, they make it sound like a snap. On the other hand, they prominently advertise a frightening armamentarium—electric pumps, biological and mechanical filters, UV sterilizers, chemical disinfectants, underwater vacuum cleaners, skimmers, algicides, and a vast pharmacopoeia for fish—that strongly hints at disasters awaiting anyone not equipped with everything modern technology has to offer. The idea of running a high-tech outdoor aquarium did not appeal to me; neither did turning my garden into some sort of water extravaganza, with jetting fountains, waterfalls, and, who knows, colored lights and mood music. Much of the "it's a snap" advice, though, suggested that a water garden, even a small one, could be made to function as a balanced and natural ecosystem without all these gizmos. That sounded like the ticket.

Things worked well until last summer, when the water in my pond started turning darker and darker, first green and then brown. Fish sightings became rarer and finally ceased altogether. I admit that I was guilty of gross wishful thinking ("The fish are probably just hiding"; "It will clear up by itself"). I finally faced reality on that late-winter day. If this was an ecosystem in miniature, it was now a dead ecosystem in miniature. And so I bailed, and prepared to begin anew. The question I wanted answered before I started again was whether a water garden really can be self-regulating, or whether that is just propaganda aimed at the lazy and gullible. I still didn't want to install pumps and filters, with their noise and plumbing and wiring. But I knew that even big-time zoologists and botanists who try to re-create balanced ecosystems on a large scale usually have to mess around with them constantly to keep things from going haywire. Furthermore, I have seen plenty of perfectly natural small ponds out there in the perfectly natural world covered with perfectly natural pond scum, and reeking.

Michael Masser is an expert on aquaculture and the ecology of ponds; he is an associate professor at Texas A&M University and a fisheries specialist who advises both commercial fish growers and amateur water gardeners. When I phoned him, not long ago, the first thing he told me was that although many of the key ecological cycles found in nature can indeed be reproduced in a small garden pond, such ponds differ from real ones in important ways. Natural biological and chemical processes can operate even in a rain barrel, but by themselves they generally won't carry the burden of keeping things in balance. In a small water garden it's more likely that they will have to be tricked into doing the dirty work.

One big difference between most real ponds and most garden ponds is the density of fish. Even a few small goldfish in a several-hundred-gallon water garden result in a huge injection of nutrients, especially when the fish are fed pellets or other food beyond what they would forage, thereby increasing their natural output. "The fish are excreting nitrogen and phosphorus, the same ingredients in fertilizer," Masser explained. The growth of microscopic algae that float is held in check in most ponds only by the limited availability of these nutrients, which all plants require for growth. Masser said, "People tend to overstock with fish and tend to overfeed the fish they have. And then they get all bent out of shape about green water."

A second big difference between gardens and the real world is that real ponds do a much better job of tying up phosphorus. In a real pond most of the phosphorus compounds contained in fish and other animal droppings bind chemically with the soil at the bottom of the pond, and then are slowly recycled by rooted underwater plants or processed by microbes before the algae ever get to them. Garden ponds built with artificial liners don't have a soil layer to perform this essential filtering chore. (And given that the water gardener should clean accumulated debris off the bottom every few years, it's generally not practical to cover the liner with a thick layer of soil.)

When the excess nutrients end up in solution, they fuel an explosive growth in algae. Single-celled algae can multiply at an astronomical rate, so they respond far more quickly to an injection of nitrogen and phosphorus than do vascular plants. If things get really bad, eutrophication ensues: the algae use up so much oxygen from the water for metabolism—especially at night, when photosynthesis is not occurring to replenish the oxygen supply—that the fish and other organisms start to choke and die. Decay microorganisms then undergo a burst of metabolic activity to decompose all that rotting stuff, and they in turn start rapidly depleting what's left of the pond's oxygen supply. When all the oxygen is gone, the rotten stuff stops breaking down and starts stinking. This was apparently the fate of my water garden.

When the water-garden suppliers are in their don't-worry-be-happy mode, they assure us that fish keep algae in check by eating them—which sounds like the perfect cycle of balance: what the fish sow, they reap. The trouble, Masser says, is that they don't. Some species of fish may eat a bit of the long, filamentous algae that grow in some ponds, but fish don't eat the microscopic algae that make the real trouble. Suppliers also often promote snails as a natural means of controlling algae, but Masser is skeptical that they do very much either. Worse, he says, the snails sold to water gardeners are invariably non-native species, and several species have already gotten loose and caused serious problems. A notorious troublemaker in the southern United States and Hawaii is the apple snail. What do apple snails do? I asked. "They get huge and eat everything they want to," Masser told me. How huge? "As big as your fist." Apple snails are apparently a particular menace in rice paddies.

Beyond reducing the number of fish, feeding them less, and tolerating a bit of green murkiness in the water, the best bet, Masser says, is to "overwhelm" the natural nitrogen cycle in the pond so that it cannot end up fueling algae growth. One way to do this is to have plenty of rooted plants in submerged pots, so that nitrogen from fish waste is quickly taken up. Rooted underwater plants also oxygenate the water, helping to counteract any tendency toward eutrophication. Making sure that a water garden is inoculated with naturally occurring bacteria that will process nitrogen and decompose organic matter helps to ensure that these key parts of the nitrogen cycle are operating; a bucket of (real) pond water will do the trick. The nitrogen from fish waste is mostly ammonia, which is highly soluble and thus readily available to the floating algae. Ammonia is also toxic to fish. The bacteria do the good deed of converting ammonia to less soluble compounds such as nitrates and nitrites, and even to nitrogen gas, which exits the system altogether.

Another less than perfectly natural compromise is to blanket the water with floating plants, keeping at least 80 percent of the surface area covered at all times. The idea is not only to create competition for the nutrients but also to block out sunlight. That's cheating in a way, and not many real ponds look like this, but it is a way to manipulate and harness natural energy and nutrient flows to achieve the desired end.

If worse comes to worst, there is filtration. Apparently, if you want to keep koi, those large, flashy fish that are the essence of Japanese water gardens, you're pretty much stuck running your pond like an aquarium, because koi are extremely messy and like to root around in the plants, stirring up dirt and detritus, which hopelessly clouds the water absent mechanical pumps and filters. The least bothersome form of filtration is biofiltration, which gives the nitrogen cycle a boost by pumping the pond water through a medium that harbors nitrifying bacteria; the resulting nitrites and nitrates get left behind in the filter, and the clean water is returned to the pond.

The lesson in all of this, I concluded, is that water gardeners need to think of natural processes as a tool to be manipulated and exploited, not an end in themselves. There is too much that's artificial about a water garden, especially a small one, to expect it to maintain a state of equilibrium without some firm guidance. I had shied away from introducing floating plants—non-native tropicals such as water lettuce (Pistia stratiotes) and water hyacinth (Eichhornia crassipes), which are actually illegal in some parts of the country because of their invasive habits and which in any case look more as if they belong on the Amazon than in a "natural" Virginia pond. But I'm beginning to think they look a lot more natural than fish skeletons or electric pumps and tubes running every which way.

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