Bottom-up Ecological Processes Better For Clear Waters, Study Finds

January 17, 1997

DAVIS, Calif. -- Weighing in on the ecological debate about how best to control human-induced algae growth in naturally clear lakes, University of California, Davis, researchers have made a bottom-line discovery.

Tinkering with food webs may not help restore most lakes to their previous clarity. This is because in most cases the upper levels of a lake's food web cannot exert enough influence to control the algae feeding on polluting nutrients coming into the lake.

Instead, limiting the supply of nutrients in a lake appears to be a stronger mechanism for controlling algae blooms, according to a meta-analysis of eight lake studies involving 11 experiments in Canada, Czechslovakia, Norway and the United States, said Michael Brett, a staff research associate in the UC Davis environmental studies division.

Brett and UC Davis professor Charles Goldman, director of the UC Davis Castle Lake and Lake Tahoe research groups, published the results of the analysis in the Jan. 17 issue of the weekly journal Science.

The analysis also casts doubt on a popular model of how energy moves through food webs in general and of how many plants and animals can be found at each level of the web. The way energy moves through food webs plays a key role in determining fisheries production in lakes and oceans.

In most lakes around the world, the most common pollutants causing murky water are actually nutrients that fuel the growth and number of tiny plants at the bottom of the food chain. Limnologists have long recognized that nitrogen (typically from sewage and fertilizer) and phosphorus (from nearby construction or logging) feed the tiny marine plants known more commonly as algae.

Algae thrive on these pollutants. The quantity of algae directly determines water clarity. In a process called eutrophication, algae can deplete oxygen, killing fish, and severe algae blooms can water to smell and taste foul.

Unnaturally murky lakes are more commonly found in the middle and eastern United States and Europe. Eutrophication was responsible for the dramatic decline in water quality seen in several of the Great Lakes during the 1960s and 1970s and is responsible for the often abysmal water clarity of urban lakes and ponds. Lake Tahoe in the Sierra Nevada, for example, is losing about one foot of its famous clarity a year due to algae.

But algae also are a critical part of a lake's food web. They are eaten by slightly larger shrimp-like creatures called zooplankton. In turn these tiny crustaceans are preyed upon by small fish, such as minnows, which in turn are often devoured by bigger fish, such as pike or bass.

In the mid-1980s, ecologists had gathered enough evidence to suggest that the clarity of a lake could be regulated by manipulating this structure of the food web. For example, putting more large fish in the lake would mean more smaller fish would be eaten. With fewer smaller fish swimming around looking for dinner, the zooplankton would increase and eat more algae. The algae, in turn, would decrease and water clarity would increase.

This idea of "top-down" control of the food web is known as the "trophic cascade hypothesis," named and popularized by Steven Carpenter, with the Center for Limnology at the University of Wisconsin, Madison. The professor of limnology is considered among of the world's most influential lake ecologists.

Yet, his top-down hypothesis has not been without its detractors. The trophic cascade hypothesis was sharply criticized several years ago as being "unsoundly based on many half-truths and much hand-waving and overextrapolation of the data," according to a 1992 paper in the journal Limnology and Oceanography by Rita DeMelo and colleagues.

DeMelo used a statistical technique called "vote counting," a common method of summarizing information from large bodies of research literature in the ecological sciences. Vote counting consists of tallying all studies that detect statistically significant effects and nonsignificant effects and inferring the generality of the phenomenon, Brett explains. Vote counting is likely to find only large effects -- especially given the nature of ecological studies, which typically use small samples sizes -- and ecological systems can vary greatly.

So Brett decided to take another look at the trophic cascade hypothesis using a newer statistical technique called "meta-analysis." More sensitive to smaller effects and to common patterns among different lake ecosystems, this method ignores the original statistical work of each study and goes back instead to the raw data to look for general patterns. The leading proponent of applying meta-analysis to ecological problems is considered to be Jessica Gurevitch, an associate professor of ecology and evolution at the State University of New York, Stony Brook, whom Brett credits as a direct influence in the work.

In the first step toward tackling this problem last year, Brett and Goldman published a meta-analysis of 54 studies, providing unequivocal support for the trophic cascade hypothesis in the Proceedings of the National Academy of Sciences.

Next, the pair set out to compare this top-down hypothesis with the "bottom-up" effect of nutrients upon the quantity of algae.

The UC Davis researchers looked at data reported in eight papers from 11 experiments testing the impact of adding fish versus nutrients to food webs and comparing them with controls. Overall, adding small fish, such as minnows, to the top of the small food webs in the studies caused a 75 percent decrease in zooplankton biomass and an 80 percent increase in algae biomass. Adding nutrients to the bottom of the food webs resulted in a 180 percent increase in algae and a 24 percent increase in zooplankton. The bottom-up processes had a greater impact on algae growth than the top-down processes.

"Both played a role, but nutrients had a bigger impact on algal biomass," Brett said.

The science meta-analysis paper by Brett and Goldman also casts doubt upon the universal nature of a popular model developed by Lauri Oksanen, a theoretical ecologist at the University of Umeå in northern Sweden. The model predicts that food webs will respond to increased growth at the plant level, such as algae, in different ways depending upon the number of levels in the food web.

In the experiments summarized in the UC Davis analysis in Science, the response of phytoplankton and zooplankton to adding nutrients was not related to the number of trophic levels in the system, Brett said. In food webs with two levels (plants and herbivores), Oksanen's model predicts that increased plant production will fuel growth of herbivores to the extent that the plant eaters munch all the extra food, resulting in no net gain at the plant level.

In food webs with three levels (plants, herbivores and predators), Oksanen's model predicts that increased plant production will result in more predators and plants, but fewer herbivores, because the predators will eat all the extra herbivores, allowing the plants to grow and thrive.

The UC Davis researchers found opposite results from the model predictions for the two-level food webs. "The crux of the model is that the number of levels determines how energy moves through the food web, and we found the response was not related to the number of trophic levels," Brett said.

Some of the most important unanswered questions in food web ecology still remain, according to Brett and Goldman. What determines how fast and how much energy moves through food webs? How can lakes or oceans with similar algae production vary so dramatically in fisheries production?


University of California - Davis

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