Article: T. J. Browing; H. A. Bouman; G. M. Henderson; T. A. Mather; D. M. Pyle; C. Schlosser; E, M. S. Woodward; C. M. Moore (2014). Strong responses of Southern Ocean phytoplanton communities to volcanic ash. Geophysical research Letters, Vol. 41, Issue 8. DOI:10.1002/2014GL059364
Phytoplankton, the photosynthesizing base of the marine food web, must take in nutrients in order to live and grow in the ocean. It is just like why you have to add fertilizer to your plants. The nutrient nitrogen is often the limiting nutrient in the ocean (or the nutrient that runs out the fastest and can either slow down or stop phytoplankton growth). However, other nutrients can be limiting too, such as iron. Iron can be hard to come by because it mostly enters into the ocean by winds carrying dust from the continents.
The Southern Ocean is known as a high nitrate low chlorophyll (HNLC) region. For many years this was a mystery: why are the phytoplankton populations so small if there is abundant nitrogen? The answer was that there is very little iron in the Southern Ocean, preventing phytoplankton populations from thriving. This makes sense since the Southern Ocean wraps around ice-covered Antarctica and receives little continental iron-containing dust.
Volcanic ash that is produced during eruptions contains valuable iron and other micronutrients and can be ejected far enough to rain down into the Southern Ocean. Browning et al. investigated the potential for volcanic ash to be a source of nutrients to the HNLC Southern Ocean and help ignite phytoplankton growth.
In 2012 (January to March), Browning et al. went on a cruise to the Southern Ocean to sample surface water in HNLC regions. The collected surface water was incubated for 24 to 48 hours to allow for phytoplankton growth. This means that the surface water was put into similar conditions, such as temperature and sunlight, as the area it was collected from. One third of the incubations were designated as controls, meaning that nothing was added so the phytoplankton could only survive on the existing nutrients. To the other two thirds of the incubations, various treatments were added. Some were spiked with iron, the known limiting nutrient of these regions. The remaining were spiked with two different types of volcanic ash, basaltic ash from the Etna, Sicily eruption in 2002 and rhyolitic ash from the Chaiten, Chile eruption in 2008.
After the 24 to 48 hour incubations, the seawater samples were measured for chlorophyll (a measure of productivity and overall phytoplankton levels), micronutrients (such as iron and manganese), macronutrients (such as nitrogen), and the phytoplankton species present were visually identified.
Browning et al. then compared the control incubations to the amended incubations with iron and volcanic ash to see how phytoplankton communities responded to the additions.
The initial phytoplankton and nutrient concentrations were as expected and previously measured for the Southern Ocean. Seawater showed low levels of iron and other micronutrients.
18 of the 23 incubations spiked with volcanic ash showed significantly more phytoplankton growth and chlorophyll than the control (nothing added) incubations (Figure 1). Additionally, the incubations spiked with volcanic ash also had more enhanced phytoplankton growth and productivity than the samples with only iron added. This suggests that the volcanic ash released other limiting micronutrients to the Southern Ocean phytoplankton. Browning et al. hypothesized that manganese is another limiting micronutrient. The initial seawater manganese concentration was among the lowest ever measured in the ocean and the volcanic ash happens to contain manganese.
Interestingly, the volcanic ash released less iron than the artificial iron solution added to some of the incubations. This indicates that there was a co-limitation of iron and manganese in these Southern Ocean surface waters. So, adding only iron, even if in higher concentrations, was not as effective since there was so little manganese.
The addition of volcanic ash to seawater samples from the Southern Ocean clearly demonstrated that it increased the overall growth and productivity of phytoplankton in this region. This increase in phytoplankton growth could draw down massive amounts of atmospheric carbon dioxide into the deep ocean. This drawdown occurs by the phytoplankton taking in carbon dioxide during photosynthesis, then sinking to the depths of the ocean when they die.
So, during massive volcanic eruptions, the ash ejected high into the atmosphere could ignite a short period of carbon sequestration in HNLC regions. One thing is clear: volcanic ash is fertilizer to the ocean!
Cover Images source: http://villabalisale.com/uncategorized/erruption-in-indonesia