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Top Down vs. Bottom Up Top Down vs. Bottom Up Each group, such as the algae, zooplankton or piscivores, has a functional role in the ecosystem. When you attempt to reduce nutrients to control algae, you are working from the “bottom-up,” the method that most consistently works for controlling algae. However, it is possible to approach the problem from the other end and work from the “top-down.” Increasing or decreasing the numbers of consumers (organisms that eat other organisms) to control the numbers of lower level organisms is called biomanipulation. One way to reduce the numbers of zooplanktivores is to poison the lake. While this option is generally unappealing, it is the most reliable biomanipulation technique for increasing zooplankton and reducing algae. Another method is to drag a trawl net through the lake and sort the undesirable from the desirable fish; however, this is a labor intensive approach. Yet another method of bolstering the zooplankton community is to add bigger fish (i.e. piscivores) to eat the zooplanktivores. The approach of adding more piscivores is very appealing to lake managers, because of the benefit of additional fishing opportunities. However, the success rate of this management approach as an algae control technique is questionable. One of the issues is that nearly all fish eat zooplankton at some point in their lives. Even largemouth bass fry will eat zooplankton until they grow enough to eat larger food items. Another concern is that the inherent variability in natural systems is amplified with each trophic level involved in the process. As a result, the effect of biomanipulation decreases with each trophic level that exists between the intended target and the manipulated species. For example, if you want to decrease algae via biomanipulation, you will likely have a stronger response if you directly alter zooplankton rather than piscivores. In some cases, increased zooplankton abundance can result in reduced phosphorus concentrations. Zooplankton consume algae and the nutrients contained within the algae. As the zooplankton die, they sink to the bottom, taking whatever nutrients they have in their bodies with them. However, the impact of these nutrient reductions is negligible in most cases. Top-down efforts to control algae aren’t always successful when used alone, and when they are successful, the results aren’t reliably predictable. This is especially true in the eutrophic (nutrient rich) lakes typically found in Missouri. In the presence of both excess nutrients and numerous zooplankton, the algal community is likely to be restructured rather than reduced. As zooplankton consume green algae and diatoms, less desirable blue green algae may begin to dominate the algae community. The overall consensus seems to be that biomanipulation efforts are most successful when executed in conjunction with “bottom up” controls, such as reducing the inputs of nutrients to the lake. It always seems to come back to the nutrients! References: Brett, M.T., and Goldman, C.R. 1996. A meta-analysis of the freshwater trophic cascade. Proc. Natl. Acad. Sci. USA,. 93(7723-7726). Back to Page 2 <==== Back to the Fall 2005 Waterline |
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