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

Abstract

Abstract. Following the eruption of the Calbuco volcano in April 2015, an extensive\nash plume spread across northern Patagonia and into the southeast Pacific\nand southwest Atlantic oceans. Here, we report on field surveys conducted in\nthe coastal region receiving the highest ash load following the eruption\n(Reloncavi Fjord). The fortuitous location of a long-term monitoring\nstation in Reloncavi Fjord provided data to evaluate inshore\nphytoplankton bloom dynamics and carbonate chemistry during April–May\xa02015.\nSatellite-derived chlorophyll\xa0 a measurements over the ocean regions affected\nby the ash plume in May\xa02015 were obtained to determine the spatial–temporal\ngradients in the offshore phytoplankton response to ash. Additionally, leaching\nexperiments were performed to quantify the release from ash into solution of\ntotal alkalinity, trace elements (dissolved Fe, Mn, Pb, Co, Cu, Ni and Cd)\nand major ions (F − , Cl − , SO 4 2 - , NO 3 - ,\nLi + , Na + , NH 4 + , K + , Mg 2+ and Ca 2+ ). Within\nReloncavi Fjord, integrated peak diatom abundances during the May\xa02015\naustral bloom were approximately\xa02–4 times higher than usual (up to\n1.4\u2009 × \u200910 11 \u2009cells\u2009m −2 , integrated to 15\u2009m depth), with the\nbloom intensity perhaps moderated due to high ash loadings in the 2\xa0weeks\nfollowing the eruption. Any mechanistic link between ash deposition and the\nReloncavi diatom bloom can, however, only be speculated on due to the\nlack of data immediately preceding and following the eruption. In the\noffshore southeast Pacific, a short-duration phytoplankton bloom\ncorresponded closely in space and time to the maximum observed ash plume,\npotentially in response to Fe fertilisation of a region where phytoplankton\ngrowth is typically Fe limited at this time of year. Conversely, no clear\nfertilisation on the same timescale was found in the area subject to an ash\nplume over the southwest Atlantic where the availability of fixed nitrogen\nis thought to limit phytoplankton growth. This was consistent with no\nsignificant release of fixed nitrogen (NO x or NH 4 ) from Calbuco\nash. In addition to the release of nanomolar concentrations of dissolved Fe from ash\nsuspended in seawater, it was observed that low loadings ( \u20095\u2009mg\u2009L −1 ) of ash were an unusually prolific source of Fe(II) into chilled\nseawater (up to 1.0\u2009 µ mol\u2009Fe\u2009g −1 ), producing a pulse of Fe(II)\ntypically released mainly during the first minute after addition to\nseawater. This release would not be detected (as Fe(II) or dissolved\nFe) following standard leaching protocols at room temperature. A pulse of\nFe(II) release upon addition of Calbuco ash to seawater made it an unusually\nefficient dissolved Fe source. The fraction of dissolved Fe released as\nFe(II) from Calbuco ash ( ∼ \u200918\u2009%–38\u2009%) was roughly comparable\nto literature values for Fe released into seawater from aerosols collected\nover the Pacific Ocean following long-range atmospheric transport.