Abstract
The eruption of a submarine volcano south of El Hierro Island (Canary Islands) in October 2011 led to major physical and chemical changes in the local environment. Large amounts of nutrients were found at specific depths in the water column above the volcano associated with suboxic layers resulting from the oxidation of reduced chemical species expelled during the eruptive phase. It has been suggested that the fertilization with these compounds enabled the rapid restoration of the ecosystem in the marine reserve south of the island once the volcanic activity ceased, although no biological evidence for this has been provided yet. To test the biological fertilization hypothesis on the pelagic ecosystem, we studied the evolution and variability in chlorophyll a, from in situ and remote sensing data, combined with information on phytoplankton and bacterial community structure during and after the eruptive episode. Remote sensing and in situ data revealed that no phytoplankton bloom took place neither during nor after the eruptive episode. We hypothesize that the fertilization by the volcano did not have an effect in the phytoplankton community due to the strong dilution of macro- and micronutrients caused by the efficient renewal of ambient waters in the zone.
Highlights
After more than 11,000 earthquakes and anomalous gas emissions, a submarine eruption took place at the leeward side of El Hierro Island (Canary Islands, Fig. 1a), near a marine reserve, on October 10, 2011
In order to test the hypothesis of nutrient fertilization on primary producers –that would lead to a fast ecosystem restoration after the volcanic eruption, we studied the distribution of the phytoplankton community at stations affected and non-affected by the volcanic emissions by looking at the chlorophyll concentration and the phytoplankton community structure, along 10 cruises, spanning both the eruptive and post-eruptive phases, from October 2011 to March 2014 (Fig. 1c; Supplementary Table S1)
Works using remote sensing data, such as the ones carried out following the eruptions of Miyake-jima (Japan)[13] and Soufrière Hills (Montserrat)[14] volcanos, already suggested a fertilization effect by volcanic ash through the addition of ammonium sulfate and Fe
Summary
Bacterial abundances from cruises 1‒7 (previously published data8) show that large and active bacteria (the HNA group) significantly increased (p < 0.001) in OMZ waters and deep waters (DW) above the volcano with respect to other surrounding stations, during the peak of the eruptive phase (Nov., 2011; Fig. 5 upper panels). Derived ocean colour Chl-a time-series (Fig. 7) placed in context the entire process, showing the high anomalous values of Chl-a at station 1 (the volcano; Fig. 1a) during the eruptive phase (Oct., 2011–Feb., 2012) During this period, reported remote sensing Chl-a estimates were higher than the corresponding in situ measurements within plume waters (up to more than ten-fold, see Supplementary Fig. S5 for a match-up). It is worth noting that the winter of 2012 presented relatively high Chl-a values compared to other years, presumably advected from the NW Africa coastal upwelling region (Fig. 7; Supplementary Fig. S6), as discussed below
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