Fertile Ponds and Plankton Blooms

A long time Pond Boss magazine subscriber from Dallas recently called the office and asked a really good question.

“I hear biologists and pond managers say the phrase, read the bloom over and over. Exactly what does that mean?”

Great question.

Complex answer.

Let’s give it a go, though. In different parts of the country, pondmeisters are advised to fertilize their ponds. In some areas of the country, even the thought of fertilizing a pond might be considered a hangin’ offense, compared to the likes of cattle rustling. Pond fertilizers are high in phosphorus, the most limiting nutrient in many southern fishing ponds. In some areas, especially in the north, the waters of a pond already have enough, sometimes too much, phosphorus. By dissolving appropriate amounts of fertilizer into the water column at temperatures above 55 degrees each spring (in areas of the nation where it makes sense) a plankton bloom is started. For those ponds already loaded with nutrients, the plankton bloom often starts by itself.

There are two basic reasons to fertilize where a pond is in need of a boost.

One reason is to create plankton, a microscopic food chain for newly hatched fish. You see, baby fish, called fry, get their food from the yolk of what was their egg. After they absorb the yolk, they must eat. They have no stores of body fat, yet. Think how tiny their mouths are…smaller than the head of a straight pin. Plankton feeds baby fish.

The second reason is to decrease light penetration in the water column. A plankton bloom clouds the water and turns it a pretty shade of green. Decreasing sunlight penetration into a pond effectively stops the growth of rooted submersed aquatic plants. Aquatic plants need sunlight, food and the right temperature. Take away any one of those big three and plants won’t grow.

The caller had a motive for asking his question. During the hottest part of last summer, one of his lakes developed a heavy planktonic algae bloom. It became so dense so quickly, he lost fish. Not once or twice…but three times. He ended up with the wrong kind of plankton bloom at the worst time of the year.

By the time anyone on his ranch figured out there was a problem, it was too late to do anything about it.

Let me describe a good bloom. Clear water is fundamentally sterile for fish production. Fun for swimming, bad for baby fish. Hatcheries need blooms in order to get good survival of spawns each year. When a pond is properly fertilized (heavy on the word properly), a phytoplankton bloom starts. Water takes a bright green color, much like the color of fresh snapped green beans. As the bloom matures in a healthy way, its color changes. It shifts from that bright green tint toward more of an olive-colored shade. From olive it turns to an olive-brown and then to brownish-olive.

What’s happening during this life cycle begins with a healthy population of phytoplankton which, in turn, feeds microscopic animals called zooplankton. As the zooplankton gain a foothold, the water begins its color shift from green to brown.

For the sake of simplicity to learn to read a bloom, the main thing to understand is that a spring time bloom starts with microscopic algae…that’s what creates the green color. As it feeds zooplankton, phytoplankton tries to keep up, but actually diminishes, hence the color change.

Here’s where things get a little complicated.

Most good blooms run their course by mid-summer and the water tends to turn clearer. While that doesn’t hold true for all ponds for all time, it does hold true for most fertilized ponds. Oftentimes, a different bloom springs to life early to late summer. Those blooms can be much more dangerous for pond life. Planktonic algae, with its dense blooms and visibility less than a foot, can wreak havoc on a fishing pond. So can blue-green algae. Those two are the biggest culprits in algae-related summertime fish kills and other related issues.

How do you read those blooms?

There are telltale signs of the symptoms. When good plankton dies off, they sink and often decompose quickly enough to add their nutrients back into the system. But, when planktonic algae and blue-green algae die, their little cells tend to float for some time. It looks just like lime-green paint has blown across the pond, up against the shore. Dead blue-green algae looks like someone spilled an iridescent blue green paint on top of the water.

Here’s more advice how to read a bloom.

Not only must you judge the color, use a secchi disk to read the visibility depth. If your plankton bloom is bright green and density prevents you seeing below14-15 inches in the summer, call a professional pond manager for help. You’re likely gonna need it.

Bright green blooms in water teem with those microscopic algae which photosynthesize during daylight hours and respire after dark.

While planktonic algae and blue-green algae blooms aren’t always considered an automatic problem, they have greater potential for problems than the good plankton blooms, especially in summer months.

Dense planktonic or blue-green algae consume large amounts of oxygen when the sun doesn’t do its magic. Heavy storms, cloudy days or anomalies of weather patterns other than direct sunlight can throw the brakes on oxygen production. When that happens, look out. Those oxygen producing blooms can just as easily become heavy consumers of oxygen…enough so that oxygen levels can tumble below safe levels for fish.

Cloud cover stops photosynthesis, which stops oxygen production from those algae. Another problem occurs when a dense bloom suddenly dies off. All that biomass tumbles from the top few feet of water downward, decomposing rapidly…which consumes oxygen.

Our caller was caught in a catch-22…even had he known about his planktonic algae bloom, it would have been a risky move to try to diminish it via the use of algaecides. The risk would have been creating a domino-effect…dead algae causing more dead algae which cause that dreaded oxygen depletion.

Here are your take-home points from this story.

Learn to read-the-bloom.

Healthy green water each spring leads to olive green shades over a week to ten days. Olive green yields to greenish-brown which goes to brown. This might occur over a period of six weeks…but I’ve seen it happen in five days. As the color shifts to brown, prepare to give your bloom a little bit of a nudge with half a dose of fertilizer, based on recommendations in your region. But, once those hot summer days take the place of spring, back off your fertilization, unless you know what you are doing. Summer blooms can quickly take you off your pace and cause problems with fish.

It happens to someone every year.

Don’t let it be you.

Bob Lusk is a 36-year veteran private fisheries biologist and lake consultant, traveling the nation helping people design, build, stock and manage private fishing waters. He is the editor of Pond Boss magazine, the nation’s leading journal on pond management. He can be reached at [email protected].