12.2019 | THE SCIENTIST 23© ISTOCK.COM, JIM WILTSCHKO
bia University biological oceanographer
Hugh Ducklow, who was not involved in
the work and coauthored an accompany-
ing perspective article in Science. “There
had been work on the chemistry and phys-
ics when all that hot lava goes into the
ocean, but actual influences on the biol-
ogy were not considered.”
At first, though, the results didn’t
seem to jibe with Letelier’s and Karl’s
previous predictions. Te n years earlier,
the two had published a hypothesis that
an influx of iron and phosphorus would
set the stage for a bloom of nitrogen-
fixing organisms (MEPS, 364:257–68,
2008). Because Kīlauea lava is rich
in these elements, the eruption had
seemed like the perfect natural experi-
ment to test the idea—and the fact that
a bloom had formed initially seemed to
support their theory. But it turned out
that the phytoplankton they’d found
weren’t nitrogen fixers at all. On the
contrary, the microorganisms were usu-
ally found in environments where bio-
available nitrogen is plentiful.Chemical analyses showed that the
water around Hawaii’s Big Island did con-
tain high concentrations of usable nitrogen
in compounds such as nitrates to feed these
microbes, but the lava itself was poor in
such nutrients. It seemed that the research-
ers were missing something. “When you’re
on board, it’s hard to think straight,” Wil-
son says. “You’re so tired, sometimes you’re
sick, sometimes you’re trying to coordinate
eighteen scientists—logical and clear think-
ing is real tough to do. But... when we
returned to land and caught up on sleep,
we had a number of possibilities.”
High nitrate levels could have been
the result of chemical fixation, if the
lava was hot enough to split dinitro-
gen molecules (N 2 ) in the water. But
the team soon homed in on another
possibility: that nitrates were com-
ing from an upwelling of nutrient-rich
waters deep in the Pacific. According to
this hypothesis, hot lava that’s sunk to
the seafloor heats deep, nutrient-filled
water, causing it to rise. Meanwhile,
underwater explosions caused by lavacooling further mix up different layers
of ocean water, allowing more nutrients
to come to the surface.
This idea was supported by mea-
surements of the water’s oxygen con-
centrations and nitrate isotopes, which
revealed levels more like those in deep
water than those in normal surface
water. And that September, when the
eruption had died down, a Woods Hole
Oceanographic Institution submersible
identified lava 700 meters deep in the
sea around Kīlauea, Wilson adds. “That
was proof that lava had accessed these
nutrient-rich deeper waters.”
The findings suggest that the lava is
not directly responsible for the phyto-
plankton bloom; it merely set the stage,
says Pierre Delmelle, a geochemist at Uni-
versité Catholique de Louvain in Belgium
who was not involved in the study. “It’s
really this physical effect of upwelling gen-
erated by the heat.... This is the first study
that documents this.” He adds that Letelier
and his team carried out the study thor-
oughly and carefully, so their speculation
that past influxes of lava into the ocean
could have similarly altered local biology
sounds reasonable. “It’s something we’ve
never thought about.”
Letelier says he still thinks that his and
Karl’s hypothesis that iron and phospho-
rus deposition could promote the growth
of nitrogen-fixing bacteria is a sensible one.
The lava-affected waters around Kīlauea
just had a different chemistry than they had
anticipated, he says. “We did not consider
that, while volcanic ashes being deposited
are cold, the lava entering the ocean is very
hot.” In addition to triggering deep-water
upwelling, he notes, the heat kickstarted
chemical reactions that depleted the water
of incoming lava-borne phosphorus.
But on the other side of the Hawaiian
Islands, ash, rather than lava, was blanket-
ing the ocean, and satellite images revealed
widespread increases in chlorophyll. With
the ash’s iron and phosphorus, and with-
out the influx of lava sapping phosphorus
out of the water, Letelier speculates, “that
region could be a region where nitrogen
fixation is becoming active.”
—Jef AkstOCEAN INPUT: Lava erupting from
Kīlauea Volcano influences the growth
of plankton in the surrounding sea,
researchers find.