A mosaic of phytoplankton responses across Patagonia, the southeast Pacific and the southwest Atlantic to ash deposition and trace metal release from the Calbuco volcanic eruption in 2015
Authors: Vergara-Jara Maximiliano J. , Hopwood Mark J. , Browning Thomas J. , Rapp Insa , Torres Rodrigo , Reid Brian , Achterberg Eric P. , Iriarte José Luis 
Published: April 2021

Abstract. Following the eruption of the Calbuco volcano in April 2015, an extensive
ash plume spread across northern Patagonia and into the southeast Pacific
and southwest Atlantic oceans. Here, we report on field surveys conducted in
the coastal region receiving the highest ash load following the eruption
(Reloncaví Fjord). The fortuitous location of a long-term monitoring
station in Reloncaví Fjord provided data to evaluate inshore
phytoplankton bloom dynamics and carbonate chemistry during April–May 2015.
Satellite-derived chlorophyll a measurements over the ocean regions affected
by the ash plume in May 2015 were obtained to determine the spatial–temporal
gradients in the offshore phytoplankton response to ash. Additionally, leaching
experiments were performed to quantify the release from ash into solution of
total alkalinity, trace elements (dissolved Fe, Mn, Pb, Co, Cu, Ni and Cd)
and major ions (F−, Cl−, SO42-, NO3-,
Li+, Na+, NH4+, K+, Mg2+ and Ca2+). Within
Reloncaví Fjord, integrated peak diatom abundances during the May 2015
austral bloom were approximately 2–4 times higher than usual (up to
1.4 × 1011 cells m−2, integrated to 15 m depth), with the
bloom intensity perhaps moderated due to high ash loadings in the 2 weeks
following the eruption. Any mechanistic link between ash deposition and the
Reloncaví diatom bloom can, however, only be speculated on due to the
lack of data immediately preceding and following the eruption. In the
offshore southeast Pacific, a short-duration phytoplankton bloom
corresponded closely in space and time to the maximum observed ash plume,
potentially in response to Fe fertilisation of a region where phytoplankton
growth is typically Fe limited at this time of year. Conversely, no clear
fertilisation on the same timescale was found in the area subject to an ash
plume over the southwest Atlantic where the availability of fixed nitrogen
is thought to limit phytoplankton growth. This was consistent with no
significant release of fixed nitrogen (NOx or NH4) from Calbuco
ash. In addition to the release of nanomolar concentrations of dissolved Fe from ash
suspended in seawater, it was observed that low loadings (< 5 mg L−1) of ash were an unusually prolific source of Fe(II) into chilled
seawater (up to 1.0 µmol Fe g−1), producing a pulse of Fe(II)
typically released mainly during the first minute after addition to
seawater. This release would not be detected (as Fe(II) or dissolved
Fe) following standard leaching protocols at room temperature. A pulse of
Fe(II) release upon addition of Calbuco ash to seawater made it an unusually
efficient dissolved Fe source. The fraction of dissolved Fe released as
Fe(II) from Calbuco ash (∼ 18 %–38 %) was roughly comparable
to literature values for Fe released into seawater from aerosols collected
over the Pacific Ocean following long-range atmospheric transport.