In Missouri, reducing nutrient inputs is central to our efforts dealing with nonpoint source pollution. Though it seems counterintuitive, some lake managers in the U.S. intentionally put nutrients into lakes, a process known as lake fertilization. By fertilizing, lake managers hope to increase the harvest of sport fish that lake users spend so much time and money to catch.
Fish yield and lake trophic state are highly correlated, meaning that greater nutrient concentrations in a lake equate to greater fish biomass. Adding nutrients to a lake will directly increase the amount of algae present, and the algae in turn provide food for zooplankton, many of which become food for fish (see the article Bottom Up—Top Down).
People love to catch big fish, but before you decide to pour your lawn fertilizers into the lake, read on. Lake fertilization is generally only recommended in a handful of U.S. states where soil fertility is very low and fishing pressure is high. In addition to fertilization, some U.S. lakes are so acidic that lime must be added to increase both the pH and the chemical availability of the phosphorus. In acidic water, any added phosphorus will quickly bind with sediments on the lake bottom. Typically, Missouri lakes aren’t blessed with the problem of too little algae, and our abundance of limestone generally ensures that the pH is high and phosphorus is readily available to algae.
Fertilization efforts are expensive and labor intensive. Fertilized lakes must be monitored frequently and fertilizer needs to be applied multiple times per growing season to provide a stable environment for fish populations. The heavy biological loads associated with fertilization require careful attention to dissolved oxygen concentrations. To avoid fish kills in cases of very high fertility, aeration may be required.
Fertilization is really only a viable option for lakes where fishing is the primary use. Swimmers, divers and municipal water users would certainly be upset to find that considerable money was spent to turn their lake into a green algae soup! Considering a lake’s existing uses is important for any management plan, and favoring one lake use (fishing) to the exclusion of another (swimming, water supply) may not be legal.
The level of fish biomass that a lake can support is ultimately determined by the lake’s fertility. The concentration of chlorophyll (the algal pigment measured by the LMVP), is one measure of lake fertility. In a study of Missouri and Iowa lakes, Jones and Hoyer (1982) showed the concentration of chlorophyll explained about 83% of the variation in fish harvest.
A 2004 study of Iowa lakes by Egertson and Downing shows that Catch Per Unit Effort (measured as weight of fish caught in a net each night) increases as chlorophyll concentration increases. However, with the increase in algae, the species of the fish captured shifts from traditional sport fish to benthivorous (i.e. “bottom feeding” see page 5) species like the common carp and the black bullhead. So, a lake with more algae can support more fish than a lake with less algae, but those fish may not be the desired species.
Ironically, in some Northwest U.S. waters, human activities have resulted in too few nutrients. Salmon fry hatch in fresh waters and grow as they move downstream to the sea. After a few years, the salmon have grown large by feeding in the ocean, and they return to freshwater to spawn and die. Historically, most of those salmon bodies would rot in the stream or lake, depositing nutrients in the process. As human fishing pressure has increased, more and more nutrients are taken out of the water as fish meat, leaving inadequate nutrients in the stream to support the flora and fauna upon which the salmon depend. For this reason, fertilization programs have begun in some northwestern lakes.
Jones, J.R. and Hoyer, M.V. 1982. Sportfish harvest predicted by summer chlorophyll-a concentration in Midwestern lakes and reservoirs. Trans. Am. Fish. Soc. 111: 176-179.
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