Among the blooming white lilies, a carp’s moustache occasionally pokes out above the water’s surface. Moorhens (with yellow beaks and red foreheads) bob around them, while a coot (with a white beak and forehead) sits by her nest. Hovering just above the water’s surface are red-eyed damselflies – light-blue damselflies with deep red eyes – and other small insects. A moth rests on the shore among typical Dutch swamp plants, including the yellow iris. Ecologist Garry Bakker from Bureau Stadsnatuur (the Rotterdam Urban Ecology Unit) breaks open the seed pod – ‘a kind of green bean’ – allowing the seeds to fall onto the green shore.

Juveniel meerkoet vijver UB 10.7.2023_Ronald van den heerik
The coot regularly makes a nest in the pond. Image credit: Ronald van den Heerik

In 2018, the pond at the university library was drained and dozens of fish died. Now, however, the pond attracts a wide variety of animals and plants. Bakker calls the water a “sea of nourishment”, full of plankton, algae (the brown, sludgy layer floating on top) and other micro-organisms. The pond’s fecundity is largely due to a pipe that connects the pond to a nearby ditch: organic material and fish larvae slip through the pipe onto campus, and end up in this “rich reservoir of water.” The carp can grow here in peace and safety, where they have no natural predators.

Jonge slechtvalk juveniel geringd mei 2023 Erasmus MC_Jos van der Geest

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Although the murky brownness may suggest otherwise, the water here is not polluted. The soft humming of the mosaic darners is evidence of the lack of contaminants, as these insects – named after glaziers who once carried glass on their backs – avoid waters used as a dumping ground for unnatural toxins. But don’t panic if they vanish next month: they appear early in the year and disappear in August, unlike many other dragonflies that are still on their way at that time.

Harig wilgenroosje vijver UB natuur campus EUR 10.7.2023_Ronald van den Heerik
Hairy willowherb grows along the pond. Image credit: Ronald van den Heerik

Humans, on the other hand, should probably not swim in the water, says Bakker. Rats are also regular visitors to the pond and use it as a toilet, so it’s probably not very hygienic for humans. Thankfully the moustached carp is not bothered at all.