Abstract
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.
Highlights
Volcanic ash has long been considered a large, intermittent source of trace metals to the ocean (Frogner et al, 2001; Sarmiento, 1993; Watson, 1997), and its deposition is deemed a sporadic generally low-macronutrient, highmicronutrient supply mechanism (Ayris and Delmelle, 2012; Jones and Gislason, 2008; Lin et al, 2011)
As micronutrient (e.g. Fe and Mn) availability is expected to be the main chemical mechanism via which phytoplankton dynamics in the offshore marine environment could be affected, we primarily focus our investigation on the release of dissolved trace metals from ash in seawater
Any changes in pCO2 or pH occurring as a direct result of the eruptions are challenging to distinguish from background variation due to short-term or seasonal shifts in the carbonate system which are pronounced in this dynamic and strongly freshwater-influenced environment (Fig. 3)
Summary
Volcanic ash has long been considered a large, intermittent source of trace metals to the ocean (Frogner et al, 2001; Sarmiento, 1993; Watson, 1997), and its deposition is deemed a sporadic generally low-macronutrient, highmicronutrient supply mechanism (Ayris and Delmelle, 2012; Jones and Gislason, 2008; Lin et al, 2011). As volcanic ash can be a regionally significant source of allochthonous inorganic material to affected water bodies, volcanic eruptions have the potential to dramatically change light availability, the carbonate system, properties of sinking particles and ecosystem dynamics (Hoffmann et al, 2012; Newcomb and Flagg, 1983; Stewart et al, 2006). The greatest potential positive effect of ash deposition on marine productivity would generally be expected in high-nitrate, low-chlorophyll (HNLC) areas of the ocean (Hamme et al, 2010; Mélançon et al, 2014), where low Fe concentrations are a major factor limiting primary production (Martin et al, 1990; Moore et al, 2013). The greatest negative impact of ash on primary producers would be expected closest to the source, where the ash loading is highest, and in areas where macronutrients or light, rather than trace elements, limit primary production
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