Lake biology is slow to recover from acid rain

Two decades of research at the University of Wisconsin-Madison have revealed that while the chemistry of a lake can recover from acid rain relatively quickly under natural conditions, biological conditions take much longer to return to normal.

The research was conducted on Little Rock Lake, in northern Wisconsin, which was separated into two basins with a mesh curtain, so that one side could be kept in its natural state, and the other slowly acidified.

Between 1984 and 1990, the test basin was taken from its natural pH of 6.1 down in two-year intervals to 5.6, 5.2 and 4.7, and was then allowed to recover without intervention.

According to Thomas Frost, Director of the research, the lake showed a remarkable resilience by returning to its pre-disturbance conditions. Due to the complexity of the ecological communities, however, the biological changes lagged behind the chemistry, taking several years longer to reach their original balance.

Adult fish in the lake, such as bass and perch, endured the pH decrease, but their offspring were unable to survive, with mercury levels in fish also increasing with acidity, possibly due to higher processing of metal mercury by organisms within the lake. Combined with the current improving pH conditions, mercury deposition in the area has also declined, probably due to a change in the coal used in power plants, explained Frost.

The study also revealed that the decreasing pH also brought about a complete revolution within the zooplankton populations, with rare species taking over the lake, and once-dominant species almost vanishing.

“We found that the pH levels had a controlling but indirect influence for nearly every biological factor in the lake,” said Frost. “The nature of the food web changed completely.”

“The entire ecosystem of the lake is much more resilient than individual species. Some species were decimated and others thrived, but the sum-total of life in the lake stayed the same,” added Frost.

The acidified half of the lake also became almost crystal clear, increasing ultraviolet light penetration, says Frost. Chemical changes helped a filamentous algae nicknamed ‘elephant snot’ to spread across the lake bottom.

In the northeast US, where lakes have been most vulnerable to acid rain, the average pH of rainfall went from the worst cases of about 4.0 in the 1980’s to around 4.8 today. This slight increase has been due in part to the Clean Air Act’s capping of sulphur dioxide emissions, though, says Frost, the effects of nitric acid warrant further study.

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