Conventional wisdom has held that additional nutrients flowing into stream systems causes an increase in production all along the food chain, from the tiniest organisms up to the largest predators. But a long-term, ecosystem-scale study by a team of University of Georgia researchers has now thrown this assumption into question. The researchers found, unexpectedly, that while nutrient enrichment did indeed cause a steady increase in the production of organisms lower on the food chain, organisms at the top of the food chain did not benefit.
Nutrient inputs to lakes and streams come from fertilized agricultural and suburban lands and from human and animal wastes that enter aquatic systems from treated sewage, septic tanks, or run-off from land.
The study, published in the Proceedings of the National Academy of Sciences, documents the effects of long-term nutrient enrichment of a headwater stream in a forested area at the Coweeta Hydrologic Laboratory in North Carolina.
For the first two years of the study, the results were as expected: the production of both prey (the organisms low on the food chain) and predators (in this case salamanders and macroinvertebrates) increased. But with continued addition of nutrients, things began to change. While the prey continued to increase at the same rate, the production of predators leveled off, signifying a "decoupling" of the typical relationship between predators and prey.
"What we found was a dead end in the food chain," said researcher Amy Rosemond. "This is the first time we've seen this kind of trophic decoupling, or break in the food chain, between the levels of prey and predator on this scale. This kind of disruption of the food web wasn't on anyone's radar screen before now."
Rosemond explained that the break was driven by the traits of the various prey species that inhabit the stream system. Some of these species were better able to take advantage of the extra nutrition than were others. After the first two years, the nutrient enrichment began to favor the growth of large-bodied prey, such as the caddisfly, over smaller organisms. These large-bodied prey were simply too big for the stream's predators to consume; hence, they were unable to capitalize on the increase in available food.
Co-researcher John Davis said that the work has global implications. "Nutrient enrichment is a global threat to the health of freshwater ecosystems," he explained. "However, our understanding of its effects is limited. Our experimental results varied substantially from the few other large-scale experiments, which suggests that ecosystem-level responses to nutrient enrichment are largely context-dependent. This is important because humans are increasing nutrient loading rates to a diversity of ecosystems, but our understanding of their effects is based on only a small number of ecosystem types."
The need to understand the effects of nutrient enrichment continues to grow more important. According the EPA, the health of 47 percent of lakes and 45 percent of streams in the U.S. is impaired, with excessive nutrients a significant source of that impairment.
The researchers said more large-scale, long-term studies in a variety of ecosystems were needed. "It took over four years for nutrient enrichment to decouple predator and prey production within these headwater streams," Davis pointed out. "But most ecological experiments are limited to time scales of weeks to months."
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Source: University of Georgia
