A team of researchers from the University of Miami have returned from an expedition in the Atlantic, where they studied the impact of volcanic ash and aerosolized desert sand on the ocean ecosystem, starting with the smallest form of life in the sea, microscopic algae, which are at the base of the ocean food web. The team found these two components combine to provide the base nutrients and ingredients that allow life in the oceans to flourish.

“Both desert dust and volcanic ash  – says Hope Elliot, Ph.D. student in ocean sciences at the Rosenstiel School of Marine, Atmospheric, and Earth Science – contain phosphorus, a key element found in the world’s oceans that essentially acts as a fertilizer for the sea.  In sufficient amounts, phosphorus stimulates the production of phytoplankton, microscopic plants that reduce atmospheric carbon dioxide concentrations by performing photosynthesis and form the base of the marine food chain.”

But in some ocean regions, phosphorus exists in low concentrations, raising concerns about the health of the marine ecosystem in those waters.  In the surface waters of the North Atlantic, for example, phosphorous levels are so low that they approach amounts that have been documented in the phosphorus-starved eastern Mediterranean Sea.

“We’ve long known – continues Elliot – that dust from desert regions can boost phosphorus levels in the ocean. But little is known about how much of a role phosphorus from volcanic ash can play in boosting phytoplankton growth.”  In order to answer this question, the team first analysed the marine microbial community from the docks at the Rosenstiel School in Florida but, following that it was clear that there was a need for further testing in the open ocean, under real environmental conditions.

And so the expedition to the Atlantic was organised, using aerosol samples from the Godzilla dust storm, a massive Saharan dust cloud that travelled across the Atlantic to North America in June 2020, as well as samples of ash from the La Soufrière volcanic eruption of 2021. These were mixed in seawater, then analysed using a spectrophotometer to measure how much phosphorus was released and could potentially be used as fertilizer by phytoplankton. “We discovered – explained Elliot – that they both raised concentrations of phosphorus in the water. And by tracking the movement of the phosphorus, we found that the phytoplankton do actually digest it.”

In January, during five days aboard the Woods Hole Oceanographic Institution’s R/V Neil Armstrong, Elliott and five other graduate and undergraduate students collected a multitude of samples from the waters surrounding Bermuda, exposing the samples to Saharan dust and volcanic ash that had been collected from filters at the Rosenstiel School’s renowned Barbados Atmospheric Chemistry Observatory.

Kim Popendorf, associate professor of ocean sciences, who led the team of students, conducted other research determining the drivers of variation in microbial growth efficiency said, “It was very much a collaborative effort. For that experiment, we conducted multiple depth profiles to 500 meters measuring microbial metabolic energy turnover rates along with a suite of chemical and microbial community measures.”

“Due to climate change – concludes Elliot – it’s predicted that the emission of natural aerosols, anything that’s not anthropogenic pollution, will increase.

 That includes mineral dust emissions and wildfire smoke, which can get deposited into ocean regions or lakes.  We’re expected to see more extreme events and more dust in general going into the ocean, which will impact the nutrient concentrations and the phytoplankton.  So, for the future, I hope to look at how atmospheric deposition impacts harmful algal blooms.”

Photo source: University of Miami

By Paolo Ponga