Aquatic vegetation management can be a perplexing problem. The first part of that problem is proper identification. Management of most aquatic plant species depends on properly identifying the desirable or nuisance plant. Many ponds have more than one type of aquatic plant, and care must be taken to identify all the aquatic plants inhabiting the pond. Some pond plants may be beneficial to local or migratory wildlife, and therefore, may want to be encouraged or at least not eliminated.
Planktonic algae are the microscopic, single-celled, and free-floating algae that exist in the top few feet of a pond or lake where the sunlight penetrates. This type of algae is what gives the water a green coloration, a plant mass does not have to be visible. According to Texas A&M AgriLife Extension, planktonic algae are normal and are in fact desirable, because they are essential to the pond’s food chain. Planktonic algae (phytoplankton) provide food for the microscopic animals (zooplankton) that are eaten by fish fry, baitfish, and other pond inhabit-ants, which ultimately support a larger fish population. They display seasonal abundance, with explosions of growth called ‘blooms’ in the spring or summer that often change the color of the pond.
Plank-tonic algae are also important in oxygenation of the pond as they photosynthesize during the day creating oxygen as a byproduct. Rapid die-off due to algae-cide treatments or natural degradation of algal blooms can lead to oxygen depletion and fish kills in the pond. Planktonic algae are the kinds of algae pond owners actually WANT and the fish NEED! It is important to manage and promote planktonic algae to build good fish populations because they provide food and oxygen for fish.
Clear water is not good if pond owners desire a good fishery. Clear water is the equivalent of a disked, fallow field with little vegetation. Just like many cattle can’t be raised well on a fallow field, fish can’t thrive in clear water. Now imagine a lush rye-grass field twelve inches tall. A lot more cattle can exist in this field because it has the food they need. The same is true with green water—many more fish can survive because it contains the food they need.
In order to properly manage planktonic algae, provide nutrients the same as one would for the grass used for cattle forage. Strive to maintain hardness and alkalinity above 30 ppm, which means agricultural limestone (ag lime) may need to be added to the pond every 3 to 7 years. Pond owners might also want to fertilize their bloom, (similar to grass) but with very different nutrients. Nitrogen (N) is the most limiting nutrient for most land-based crops. However, nitrogen is fairly prevalent in water because it diffuses into water from the nitrogen found in the atmosphere. Phosphorous is the limiting nutrient in water, since phosphorous precipitates in water and becomes locked in the pond sediments where planktonic algae cannot reach it.
For new ponds, it is important to start fertilizing shortly after the pond begins to fill. This practice will promote the growth of planktonic algae and limit the growth of nuisance rooted vegetation. Existing ponds also benefit from fertilization, but there is a caveat. Do not fertilize a pond if rooted aquatic vegetation is present or if the pond has extensive areas less than 30 inches deep. Fertilizing when rooted vegetation is present results in as much as four times the amount of rooted vegetation because they will utilize the nutrients before an algal bloom can be-come established. Treating ponds with large areas less than 30” deep promotes the growth of unwanted rooted vegetation.
Instead, first treat existing aquatic vegetation with an aquatically approved herbicide for the species present, and then fertilize. Generally, most recreational ponds receive the most benefit from two fertilizations per year when the management goal is to increase fish density.
However, more frequent fertilizations may be required when the management goal is to prevent the establishment of rooted aquatic vegetation. The first fertilization should be done in the spring when the water temperatures are between 55 and 65°F, before rooted vegetation begins to recover from the winter and start growing. The second fertilization should occur in late spring to mid-summer as needed; i.e., as determined by the state and concentration of the algal bloom.
Oftentimes, ponds require the addition of 5 to 8 pounds of phosphorous per acre at the spring appli-cation while the summer fertilization is made at half to the full rate of the spring fertilization. It takes the addition of 6 pounds of phosphorous per acre to establish a good bloom during the spring fertilization. 3 to 6 pounds of phosphorous per acre is typically adequate for the summer fertilization. Seek fertilizers high in phosphorous content such as ammoniated polyphosphate (9-32-0), polyphosphate (11-37-0), orthophosphate (13-38-0) in liquid forms, diammoni-um phosphate (18-46-0), or triple super phosphate together with ammonium nitrate (0-46-0 + 34-0-0) in granular forms.
An alternative to the inorganic fertilizers (those that come in granular, powdered, or liquid forms) is to add an organic fertilizer. The aquaculture industry has been using cottonseed meal as an organic fertilizer in fish fry ponds for decades. For most farm ponds, 150 – 250 pounds of cottonseed meal spread around the edge of the pond is sufficient to produce a good algal bloom. Organic fertilizers produce blooms more slowly because they release nutrients slowly over time as they decompose.
Remember, the right amount of fertilization is good, but a little extra can be very bad. Too much fertilization can lead to very dense blooms, which consume large amounts of oxygen through respiration during the night when photosynthesis is not occurring. This can lead to early morning fish kills caused by low dissolved oxygen concentrations. Whether they cause a fish kill or not, extremely dense blooms will eventually die-off due to nutrient limitations, and sometimes collapse during massive algae die-offs. Massive algal bloom collapse can also lead to fish kills caused by low dissolved oxygen concentrations.
For more information on this or any other agricultural topic please contact the Hopkins County Extension Office at 903-885-3443 or email me at [email protected]
- Basic Vegetable Gardening, April 24, 2018, 7:00 PM. Cost $10 at the door. Hopkins County Extension Office
- Hopkins County Master Gardeners Annual Plant Swap. April 28, 2018. Bright Star Veterinary Clinic Parking lot. 9:00 to Noon.
- NETBIO Beef Up Cattlemen’s Conference, May 11, 2018 3:00 PM, Regional Civic Center Arena, Free (dinner included). Call 903-449-6079 to secure meal by May 1, 2018.