A mosaic of phytoplankton responses across Patagonia, the SE Pacific and SW Atlantic Ocean to ash deposition and trace metal release from the Calbuco 2015 volcanic eruption

Abstract. Following the April 2015 eruption of the Calbuco volcano, an extensive ash plume spread across northern Patagonia and into the SE Pacific and SW Atlantic Ocean. Here we report the results of field surveys conducted in the marine 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 gradient in offshore phytoplankton response to ash. Additionally, leaching experiments were performed to quantify the release of total alkalinity, trace elements (Fe, Mn, Pb, Co, Cu, Ni and Cd) and major ions (Fl, Cl, SO₄, NO₃, Li, Na, NH₄, K, Mg, Ca) from ash into solution. Within Reloncaví Fjord, integrated peak diatom abundances during the May 2015 austral bloom were 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 two weeks following the eruption. In the offshore SE Pacific, a short duration phytoplankton bloom corresponded closely in space and time to the maximum observed ash plume, potentially in response to Fe-fertilization of a region where phytoplankton growth is typically Fe-limited at this time of year. Conversely, no clear fertilization was found in the area subject to an ash plume over the SW Atlantic where the availability of fixed nitrogen is thought to limit phytoplankton growth which was consistent with no significant release of fixed nitrogen from ash.

In addition to release of nanomolar concentrations of dissolved Fe from ash suspended in seawater, it was observed that low loadings (< 5 mg L⁻¹) of freshly deposited ash were an unusually prolific source of Fe(II) into solution (up to 1.0 µmol Fe g⁻¹), suggesting that the release of bioaccessible Fe from ash sources may generally be under-estimated when quantified from aged ash. This release of Fe(II) may make freshly deposited ash an unusually efficient dissolved Fe source with the 18–38 % fraction of dissolved Fe released as Fe(II) from Calbuco ash roughly comparable to literature values for Fe released into seawater from aerosols collected over the Pacific Ocean.