Sediment from rivers correlates with marine biodiversity

In a 2011 paper, a team of scientists tackled one of the most fundamental questions in biology: “How many species are there on Earth and in the ocean?” Their estimate, based on analysis of the taxonomic classification of currently catalogued species, was 8.7 million – give or take 1.3 million.

But an equally important question relates to what drove the evolution of all of this biodiversity.

According to Tristan Salles, Associate Professor in the School of Geosciences at the University of Sydney, the answer lay in the complex relationship between the landscape, tectonic activity and climate – and the billions of tonnes of sediment that have flowed across the land and into the oceans over millennia as a result.

This theory has its roots in the work of 19^th century German naturalist Alexander von Humboldt. But it gained weight recently, when Salles and colleagues published a paper in Nature, which presented the results of a state-of-the-art global landscape model they had spent 3 years developing.

“We wanted to build a tool that would represent the entire evolution of Earth’s landscape over hundreds of millions of years,” Salles tells Cosmos in the leadup to this weekend’s Ocean Lovers Festival in Bondi, where he will be discussing his research.

Seeking to avoid the pitfalls of siloed science, the model combines physical, chemical and biological systems. With a spatial resolution of five kilometres and a time interval of five million years over half a billion years, it was very expensive to run and required the supercomputing power of Australia’s National Computational Infrastructure.

The combined simulations, which were equivalent to ten years of computational time, revealed not just how the landscape evolved over the last 540 million years, but also when and where “sedimentary pulses” occurred around the globe as a result of erosion.

“What our model is able to do is to represent the different processes of erosion and track how much sediment is eroded and deposited over time,” Salles says, adding that the timing of the pulses “varied quite significantly”.

“For example, in the initial phase, most of the sediment filled the ocean, because the geography of the landscapes at that time was characterised by steep mountain ranges really close to the ocean, like the present-day Andes.

“Then, as we move forward in time, from 500 to 300 million years ago, we start to see more continental or alluvial plains, where sediments can be deposited before they reach the ocean.”

How does sediment flow contribute to marine biodiversity?

Salles’s team then compared this information with datasets about the evolution of Earth’s biodiversity from the fossil record. This revealed that there was a positive correlation between the amount of sediment rivers carried into the oceans and marine biodiversity.

“When sediment is flowing, there are a lot of nutrients like carbon, nitrogen and phosphorous that flow with it and fuel the development of marine life,” Salles says.

But what “impressed” Salles even more was that when there was a plateau or drop in sediment flux, there was a corresponding drop in marine diversification. “We really were not expecting that kind of correlation. It really shows that there is a strong relationship between sediment and biodiversity.”

The same was also true on land. “When sediments start to deposit on land, they form soil, and this soil helps plants to diversify. The landscape will basically drive the diversification of plants by providing fresh soil to grow.”

But there is one counter argument to this hypothesis which Salles has considered closely. “It’s a possibility that our model is just reflecting the fact there are more fossil records where sediment has been deposited.” If this is the case, “it means our model could be used to look for other fossil records in places that haven’t been examined before.”

A wide range of scientists will be appearing alongside Salles at the Ocean Lovers Festival, including Dr James Hunter, who will discuss virtual reality shipwreck dives; Associate Professor Ziggy Marzinelli, who will explain the world-first approach he and his colleagues are using to restore degraded marine environments like Sydney Harbour; and Dr. Vanessa Pirotta, who will guide people through the mysterious world of whales. Adventurer and scientist Tim Jarvis is also on the guest list.

This is a world Pirotta has come to know well thanks to her extensive work as a wildlife scientist, which has included using innovative research methods like drones to observe whale behaviour, measure their body size and even collect a sample of their snot which can later be analysed to assess their lung health.

“It’s basically a PCR test for whales,” Pirotta says. “And the good thing is that we don’t have to kill whales anymore to learn more about their health; new technology likes drones has really made conducting this sort of research a lot safer for everyone.”

But, as she points out, whales still face many other threats from humans, including entanglement in shark nets – a topic which is also set to be discussed at this weekend’s festival by environmental policy expert Dr Chris Pepin-Neff, marine ecologist Lawrence Chlebeck, CEO of Surf Life Saving NSW Steven Pearce and CEO of Action for Dolphins Hannah Trait.

“We need to work towards more targeted ways of mitigation to ensure marine mammals are safe,” Pirotta says.

But even though some populations of humpback whales have shrunk significantly in recent years, Pirotta is confident they will “continue to persist despite us making the ocean a noisier, more polluted, and warmer environment…Fortunately, whales are quite robust to human activities.”

And, with their annual migration from Antarctica to the warmer tropical waters off Queensland now underway, there’s a good chance they might also make a fleeting appearance at Bondi this weekend.