Aquatic Vegetation and Algae

Vegetation Management


This web site cannot provide comprehensive information regarding vegetation management in ponds, since that subject goes well beyond issues of fish management. However, since pond owners are frequently concerned about managing vegetation, a few links to source of information on that subject are provided. More comprehensive information on the management of pond plants can be found on the Cornell Department of Natural Resources web-site.

Plant identification assistance is available from the Ohio Department of Natural Resources at the following site for: 1) submerged, 2) floating, and & 3) emergent plants

In general, it is important to note that many smaller life stages of fish require aquatic vegetation as sources of prey (which are often found in and among the vegetation) and as refuges from predators. Any angler will tell you that fish are frequently found near "weed beds," particularly members of the sunfish family, including largemouth and smallmouth bass. Anglers will also tell you that lakes without any vegetation are generally very poor locations to fish, and that lakes choked with vegetation also contain few fish of desirable size. Therefore, removing all vegetation from a pond is likely to diminish its capacity to sustain fish populations, and the goal is to maintain some sort of balance between too much and too little vegetation. That's difficult to accomplish in small ponds, since these aquatic ecosystems are not large enough to maintain a diversity of habitats -- and the tendency is for the pond to revert to a single habitat type.

Grass Carp

Grass carp (image: Dr. Michael Masser, Texas A&M University)

Grass carp (Ctenopharyngodon idella) are nonnative, herbivorous fish from the minnow family. Their voracious eating habitats make them well suited to controlling overgrown vegetation in ponds. To prevent excessive spread of grass carp in New York waters, only the sterile, triploid form are legal to stock in ponds. As a pond owner, you need to obtain a special permit from the NYDEC to stock triploid grass carp. These fish are available for purchase in New York from special DEC certified triploid grass carp suppliers.

Grass carp are so good at eating aquatic plants that stocking too many of them can easily result in a total loss of vegetation within your pond. To maintain sufficient plant populations covering approximately 20-30% of the pond's surface area, it is important to be conservative when stocking grass carp. Initial recommendations for the first stocking depend on the current plant density within your pond: stock 5 fish/acre for low plant density, 10 for medium and 15 fish/acre for high plant densities.

Observe how the plant population responds, and adjust up or down in future years. In ponds which already contain largemouth bass, it is best to stock grass carp which are at least 8" long. Stocking grass carp during late spring seems to be the most effective time of year. Plants are beginning to grow but the water temperatures and dissolved oxygen content are still favorable. Carp do have feeding preferences and will generally not control emergent vegetation species such as cattail or bulrush. Most grass carp prefer species of Hydrilla, Potamogeton, Ceratophyllum, Najas, Elodea, however carp will consume other plants as well.

Click here for more information on grass carp.
Back to top

Algae Blooms

Algae are an essential part of pond ecosystems- in fact they provide the main source of oxygen for living things in ponds. However, when temperature and nutrient levels are just right- usually during the summer, algal growth can progress uncontrollably forming an "algae bloom." As with most things in excess, too much algae is a bad thing. Algae blooms are characterized by dense mats of green or red colored algae on the pond surface. These mats wreck havoc with oxygen levels within ponds by preventing light from penetrating the deep areas of the pond and thus preventing submerged plants from photosynthesizing and producing oxygen. Because most of the oxygen produced by algae on the pond surface is lost to the atmosphere, most algae blooms lead to an oxygen reduction within ponds. Depending on how long the algae bloom lasts, this oxygen depletion can lead to a fish kill.

Managing pond conditions to prevent algae blooms and resultant fish kills is a challenge all pond owners face during the summer months when blooms most frequently occur. There are two different types of strategies to control blooms: proactive and reactive approaches.

Since an of excess nutrients is the main culprit responsible for uncontrolled algae growth, a proactive (and most effective) approach to preventing algae blooms involves reducing the amount of nutrient input to your pond. Buffer strips and other best management practices (BMP's) can be used to reduce nutrient inputs to ponds which drain agricultural watersheds. Direct inputs such as manure along pond edges and direct, unfiltered drainage to the ponds should be avoided. Since nutrients can accumulate in sediments over time, arranging to have a contractor dredge the pond bottom, may prove successful in older ponds.

Reactive approaches to controlling algae blooms include mechanical and chemical. Removing algae by harvesting is an expensive and time-consuming process. When applied in the spring or fall, barley straw can be effectively used to suppress algae growth. Some chemical algaecides are effective however, some such as copper sulfate compounds can have negative repercussions on fish and invertebrate populations. There are many water dyes available to prevent algae growth within ponds, however these tend to impede light penetration to submerged plants, thus preventing photosynthesis, reducing oxygen levels, and limiting structural plant habitat for fish. Chemical treatments should be used as a last resort as they treat only symptoms, not sources of algae problems.

In general, proactive approaches to reducing nutrient inputs to your pond are the most sustainable, most environmentally sound and in the long run, the most economical way to control algae blooms within your pond.
Back to top

Secchi Disk in Water with High Algal Density

Dense Filamentous Algae Bloom

Using Plankton Net to Collect Algae Samples

Using Plankton Net to Collect Algae Samples

Algae Samples in Jars Arranged From Left(High Algal Density) to Right (Low Density)

Algae Control with Barley Straw

Although the exact manner in which barley straw controls algae is not fully understood, barley straw placed in water begins to decay and during this process, lignins are released from the barley cell walls. Ifdissolved oxygen levels are sufficient, lignins can be oxidized via bacteria to produce humic acids and other humic substances. In the presence of sunlight, hydrogen peroxide in aquatic systems is believed to inhibit the growth of algae. Peroxides are very reactive in solution and will only last for very short periods of time. However, when high levels of dissolved oxygen and sunlight are present, the continuous decay of barley straw may provide a sufficient level of humic substances that are converted to hydrogen peroxide. The use of barley straw does not kill algae, but appears to limit the growth of new algal cells.

For barley to work properly, it must remain near the surface of the water body (e.g. within the photic zone where algal growth and reproduction occurs), and be applied prior to the onset of algae growth. In New York, this generally means that barley straw must be applied by late May or early June, though fall applications may provide some benefit the following spring. The surface waters must contain high concentrations of dissolved oxygen and have good sunlight penetration. It has also been found to be useful to keep the barley straw suspended in the photic zone using floats and some type of device to contain the straw, such as netting. Within the netting, the straw cannot be packed too tightly or it will become anoxic (low oxygen levels).

Back to top