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?