Change in water pH appears to disable chemical predator alert
In a bleak, ground-floor laboratory stacked high with algae-covered aquariums, Grant Brown sets fish in motion.
After just a few drops of a chemical naturally stored within some fishes' skin are added to the water, the four fathead minnows in each aquarium dart skittishly about, nosedive to the bottom of the tank, then huddle together motionless.
“About a half a gram is enough to scare everything in an Olympic-size swimming pool. It doesn't take a whole lot of this stuff,” said Brown, an assistant professor of biology at Union College.
The strange dance staged in Brown's laboratory is similar to behavior seen in nature. When a yellow perch or other predator pierces the flesh of a smaller fish during an attack, some species release chemicals known as alarm pheromones into the water to warn other fish in the area that it's dangerous to hang around.
Picking up the “scent” causes some species to duck behind aquatic plants. Others freeze in mid-swim. Some plunge to the bottom to get out of harm's way.
But what Brown has discovered — in hundreds of trials like the one described above — is that the chemical signals get crossed when the pheromone is introduced to fish living in slightly acidic waters. The same amount of pheromone released into a tank treated with sulfuric acid induces no response from the minnows.
What that means, according to Brown, is that declines in fish populations could be happening in acidic lakes well before acid rain really begins to take its toll, killing off food supplies and destroying reproduction.
Scientists have typically focused on lakes that have higher concentrations of acid, studying such phenomenon as the leaching of aluminum and the formation of methyl mercury, a toxic metal that makes eating the fish risky. But in the Adirondacks, about 26 percent of the 2,800 lakes are at the same pH — a measure of the amount of acid in water — at which Brown has performed his experiments. Some are that way naturally, others are at that pH because of power plant pollution blown east from Midwestern plants.
Even more troublesome is that 30 fish species living in Adirondack waters rely on alarm pheromones to avoid being eaten.
“The alarm pheromone is a survival mechanism. When the pH changes, the fish lose that valuable survival mechanism, and become more vulnerable to predation,” said Bob Daniels, a fisheries biologist at the State Museum familiar with Brown's work.
Daniels was excited about the research.
“People have done research on alarm pheromones, but what happens to those chemicals when they enter waters with a low pH is new,” Daniels said.
Brown, who worked with Union College organic chemist Jim Adrian on the experiments, has discovered that acidic water conditions were changing the chemistry of the alarm pheromone, making it unrecognizable to the fish. The linchpin in the pheromone's chemical structure — which Adrian has isolated and can produce en masse in the lab — is a bond connecting nitrogen to oxygen. Without that link, the compound is useless as a warning signal. The idea that acid rain could tinker with that bond came up when Brown was perusing the scientific literature.
“The idea came up from this funky little nitrogen oxide group,” Brown said pointing to a diagram of the chemical structure. “The literature says it's a relatively unstable compound that's rock solid in neutral, or basic conditions” but susceptible to breakage in acidic waters.
“When you lose the nitrogen oxide, you lose the function of the alarm pheromone,” he added.
Brown plans to submit a paper on his work to the journal Environmental Biology of Fishes. This summer, he will venture into the Adirondacks to test his theory, placing traps — one surrounded by sponges soaked with the warning chemical, another treated only with distilled water — in streams and lakes at various acid concentrations. If his theory proves correct, in acid waters fish will swim right past the pheromone and into his trap.
“In neutral water, the chemical scares the bejesus out of them,” Brown said.