Freshwater Fish Depend On Healthy Forests To Survive
June 12, 2014

Freshwater Fish Depend On Healthy Forests To Survive

April Flowers for - Your Universe Online

The phrase "forest loss" or "deforestation" brings to mind negative impacts such as a loss in habitat, the release of greenhouse gases, soil erosion and economic hardships. Such negative outcomes are seen in many of the world's forests. One thing that doesn't come to mind is dying fish. A new study from the University of Cambridge, however, reveals exactly that problem: dying fish populations.

As debris from a healthy forest washes into freshwater lakes, it supplements the diet of microscopic zooplankton and the fish that feed off them. The study, published in Nature Communications, demonstrates how this creates larger and stronger fish. However, the results also show that forest erosion as a result of human activities negatively impacts aquatic food chains as much as terrestrial ones.

The researchers chose a Canadian lake that had already undergone an ecological disaster during the mid-20th century: the local nickel smelting industry caused acid rain. Since then, conservation efforts have reduced the environmental impact, but many areas of vegetation surrounding the lake are still in recovery. Patches of recovered forest interspersed with recovering forest allowed the researchers to study Yellow Perch fish from areas of the lake that have varying degrees of coverage.

The team investigated two sources of carbon in the aquatic food chain: carbon from forest debris and carbon produced by algae. These two types have different elemental masses, allowing the scientists to differentiate them. They analyzed young Perch born that year, finding at least 34 percent of the fish biomass comes from vegetation. That increases to 66 percent in areas with rich forest. Basically, the more forest, the fatter the fish. In areas where the forest cover was scarce, the fish were smaller and less likely to breed or survive.

"We found fish that had almost 70 percent of their biomass made from carbon that came from trees and leaves instead of aquatic food chain sources," Dr Andrew Tanentzap, from Cambridge's Department of Plant Sciences, said in a statement.

"While plankton raised on algal carbon is more nutritious, organic carbon from trees washed into lakes is a hugely important food source for freshwater fish, bolstering their diet to ensure good size and strength," he said.

Daisy Lake at the outskirts of Sudbury, Ontario, Canada, was the site of the study. Sudbury is an industrial town in part of the boreal ecosystem: a vast subarctic climate system that rings round most of the top of the Northern Hemisphere. Such an ecosystem is full of huge, ancient forests that are vital to the planet's carbon cycle.

"More than 60 percent of the world's fresh water is in the boreal areas such as Canada, Scandinavia and large parts of Siberia. These areas are suffering from human disturbance such as logging, mining, and forest fires resulting from climate change – all occurrences predicted to intensify in coming years," said Tanentzap.

Eight different "watersheds" surrounding the lake were examined. A watershed is any given area of land where all the moisture drains into a single stream. The streams are fast moving and full of debris. When they hit the slower waters of the lake, the debris falls out of suspension and sinks, forming layers of sediment which create mini deltas.

Bacteria breaks down the debris, and then is consumed by tiny translucent creatures called zooplankton, which also feed on algae. Fish then eat the zooplankton. Scientists only recently realized that algae was not the only food source for zooplankton. Other recent studies have shown they also feed on the bacteria from forest debris.

The research team collected their samples from the mini deltas. "Where you have more dissolved forest matter you have more bacteria, more bacteria equals more zooplankton; areas with the most zooplankton had the largest 'fattest' fish," said Tanentzap.

The lakeshore nearest the nickel smelt-works remains barren. Only dirt and rock cover what was once lush forest. In this part of the lake, the fish were significantly smaller because of a lack of food. Smaller size leaves them susceptible to poor health and predators as they won't be as strong, and less likely to go on to breed and repopulate.

"It's estimated that freshwater fishes make up more than [six percent] of the world's annual animal protein supplies for humans – and the major and often only source of animal protein for low income families across Bangladesh, Indonesia and the Philippines," added Tanentzap. "While we've only studied boreal regions, these results are likely to bear out globally. Forest loss is damaging aquatic food chains of which many humans are a part."

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