Strong responses of Southern Ocean phytoplankton communities to volcanic ash

Abstract

Volcanic eruptions have been hypothesized as an iron supply mechanism for phytoplankton blooms; however, little direct evidence of stimulatory responses has been obtained in the field. Here we present the results of twenty-one 1-2 day bottle enrichment experiments from cruises in the South Atlantic and Southern Ocean which conclusively demonstrated a photophysiological and biomass stimulation of phytoplankton communities following supply of basaltic or rhyolitic volcanic ash. Furthermore, experiments in the Southern Ocean demonstrated significant phytoplankton community responses to volcanic ash supply in the absence of responses to addition of dissolved iron alone. At these sites, dissolved manganese concentrations were among the lowest ever measured in seawater, and we therefore suggest that the enhanced response to ash may have been a result of the relief of manganese (co)limitation. Our results imply that volcanic ash deposition events could trigger extensive phytoplankton blooms, potentially capable of significant impacts on regional carbon cycling.

Diagnosing oceanic nutrient deficiency

“…Phytoplankton have an absolute requirement for the micronutrient Mn, in part due to the presence of this element within the water splitting complex [2,27,45]. There is some direct [110,111] and indirect [112,113] evidence for the (co-)limiting potential of Mn within oceanic surface waters. Calculating the relative deficiency of both Fe and Mn relative to N and P (figure 7c,d) reveals that Fe largely remains the most deficient nutrient at high latitudes within the upper ocean (approx.…”

Volcanic ash supply to the surface ocean—remote sensing of biological responses and their wider biogeochemical significance

“…Arguably the most convincing evidence that natural phytoplankton communities can access Fe supplied from volcanic ash has come from shipboard bioassay experiments that have detected changes in phytoplankton physiology (Achterberg et al, 2013;Browning et al, 2014a), biomass (Achterberg et al, 2013;Browning et al, 2014a;Mélançon et al, 2014), and community structure (Mélançon et al, 2014) that are consistent with relief of phytoplankton Fe limitation (Boyd et al, 2007). However, observed responses have not been consistent in either the magnitude of phytoplankton response to a given ash loading, or the relative response compared to parallel Fe-only treatment experiments.…”

“…The Sierra Negra and La Cumbre volcanoes referred to in the text are located in the Galapagos Islands. Also shown are sites of ash addition experiments (blue circles) performed in the North Atlantic (Achterberg et al, 2013), North Pacific (Mélançon et al, 2014) and South Atlantic and Southern Ocean (Browning et al, 2014a), and the location of Ocean Station Papa (OSP, black square). PCC, Puyehue-Cordón Caulle. to originate from large scale Fe fertilization of the remote highnitrate Southern Ocean after the eruption of Mt Pinatubo in 1991 (Figure 1; Sarmiento, 1993;Watson, 1997;after Spirakis, 1991).…”

“…Unfortunately, whilst presenting an intriguing hypothesis, direct evidence for marine phytoplankton responding to ash fallout from the eruption was lacking. More recently, multiple studies have reported significant phytoplankton responses to volcanic ash supply; either following a natural deposition event (Hamme et al, 2010;Langmann et al, 2010a;Lin et al, 2011;Achterberg et al, 2013), or through controlled supply of ash to phytoplankton communities in shipboard bottle enrichment experiments (Achterberg et al, 2013;Browning et al, 2014a;Mélançon et al, 2014).…”

Mechanisms of dissolved and labile particulate iron supply to shelf waters and phytoplankton blooms off South Georgia, Southern Ocean

“…The island is situated between the Antarctic Polar Front (PF) and the Southern ACC Front (SACCF), within the general northeast flow of the ACC (Meredith et al, 2005;Whitehouse et al, 2008). The ACC surface waters are enriched in nitrate, phosphate, and silicic acid, but strongly depleted in most trace elements, notably Fe and manganese (Mn) (Browning et al, 2014). The large seasonal phytoplankton blooms downstream of South Georgia are thought to be supplied with Fe from the island during the passage of ACC waters (Borrione et al, 2014;Nielsdóttir et al, 2012).…”