Ponds around the world--Holland

Marten Scheffer, a biologist at Wageningen University in the Netherlands, grew up swimming in the clear lowland ponds of his country. In the 1980s, many of these ponds turned turbid. The plants would die, algae would cover the surface, and only bottom-feeding fish remained. The cause — fertilizer runoff from nearby farms — was well known, but even after you stopped the runoff, replanted the lilies and restocked the trout, the ponds would stay dark and scummy.

Mr. Scheffer solved this problem with a key insight: the ponds behaved according to a branch of mathematics called “dynamical systems,” which deals with sudden changes. Once you reach a tipping point, it’s very difficult to return things to how they used to be. It’s easy to roll a boulder off a cliff, for instance, but much harder to roll it back. Once the ponds turned turbid, it wasn’t enough to just replant and restock. You had get them back to their original, clear state.

Science is a graveyard of grand principles that fail in the end to explain the real world. So it is all the more surprising that Mr. Scheffer’s idea worked.

By applying the principles of dynamical systems, Mr. Scheffer was able to figure out that to fix the ponds, he had to remove the fish that thrive in the turbid water. They stir up sediment, which blocks sunlight from plants, and eat the zooplankton that keep the water clear. His program of fixing the Netherlands’ ponds and lakes is legendary in ecology.  Source