Dolphin poo could be key ingredient in coral reef survival

Marine mammals may be acting as a ‘pump’ to bring nutrients into reefs

There are plenty of things that make for a healthy coral reef, like a diverse set of fish, thriving corals — and maybe dolphin poo.

Researchers recently studied dolphins in coral reefs among the Maldives and Chagos archipelagos, two island chains in the Indian Ocean full of tropical fish, sea turtles, sharks and dolphins.

The scientists determined that by hunting offshore and then returning to the reefs to release their waste, the marine mammals could be bringing in a vital source of nutrients like nitrogen for the reef ecosystem.

The research was published this week in the Journal of Zoology.

Using both visual sightings and sound recordings, the scientists determined that most of the cetaceans — meaning whales and dolphins — in the reefs were spinner dolphins, a common species found in waters around the world.

Often, these dolphins will forage for food like lanternfish in deeper waters outside of the shallow lagoon areas, the researchers note. But then they come back to the reefs to rest and, inevitably, see through the end of their digestive process.

Since much of their eating is taking place outside the lagoon and much of the defecating is taking place inside the lagoon, the researchers speculate that nutrients such as nitrogen are likely flowing into the reef via what they’ve termed the “dolphin pump”.

This kind of nutrient pump to reefs has been recorded before with species like sharks and seabirds, the authors of the new paper notes. And some previous research had documented certain fish species feeding directly on dolphin faeces in another reef, they add.

Nitrogen is one of the fundamental building blocks for life, both on land and in the water, especially for things like plants and some microbes.

Coral reef ecosystems are under siege around the world from development, pollution and overfishing, the US Environmental Protection Agency says.

In addition, the climate crisis is a major threat to reefs. Warming waters can cause “bleaching” events where the algae that live inside the corals and give them their colours disappear, leaving the corals looking pale and ghost-like.

As human-derived carbon from the atmosphere gets sucked up by the ocean, that can also lead to acidification in the water, which endangers coral ecosystems, EPA adds.

Island chains like the Maldives are also threatened by the climate crisis in their own way — as sea levels rise, these low-lying dots in the ocean may have mere years left before falling into the ocean.

Eighty per cent of the Maldivian coral islands are less than one metre (3.2 feet) below sea level, Nasa says.

Under even the best-case scenario, sea levels in the Maldives would rise around half a metre by the end of the century, according to data from the Intergovernmental Panel on Climate Change (IPCC).

Under the worst-case scenario, the island chain could be looking at about 2.2 metres of rise by 2150, the IPCC data says.