Article: Kearney, K.A., D. Tommasi, and C. Stock. 2015. Simulated ecosystem response to volcanic iron fertilization in the subarctic Pacific ocean. Fisheries Oceanography 24(5): 395-413. doi:10.1111/fog.12118.
Phytoplankton are the base of many marine food webs, and thus their success can often dictate that of higher food chain organisms, such as zooplankton and fish populations. Phytoplankton require a number of resources in order to grow, including temperature, light, and nutrients. While nitrogen, phosphorous, and silica are some of the most common nutrients responsible for initiating phytoplankton blooms, iron can also be a necessity, and sometimes the limiting factor that keeps blooms from growing. This concept has been observed in laboratory and field experiments, where the addition of iron (commonly referred to as iron fertilization) has allowed phytoplankton blooms to exceed their normal potential.
Iron is typically provided to marine systems from dust clouds. These clouds originate in dry terrestrial systems, traveling across continents and oceans, to later fall out of the atmosphere into the ocean. Another source of iron, although less frequent, is from volcanos’ ash emissions during explosions, similarly being deposited from the atmosphere into the ocean. In August 2008, the Kasatochi volcano erupted and released an ash-plume that drifted and deposited over the Gulf of Alaska (Figure 1).
This ash was found to stimulate phytoplankton growth, resulting in blooms well above averages for the time of year. Coincidently, the returning Fraser River sockeye salmon population in 2010, the same population that was at sea in the Gulf of Alaska in 2008, was well above the normal annual population size. Thus, it is speculated that the elevated returning sockeye salmon population in 2010 is the result of the Kasatochi stimulated iron fertilization and increased phytoplankton blooms in 2008, providing increased food for the salmon while they were at sea and allowing the population to exceed normal levels.
Questions and Methods
Two major questions were to investigate the ecosystem response resulting from a Kasatochi-like iron fertilization event: (1) did the iron-ash emissions and increased phytoplankton growth result in higher biomass for sockeye salmon and other fish, and (2) what would have happened if the volcano erupted at a different time during the year? To solve these questions, Kearney and her colleagues used a holistic ecosystem model that incorporates physics, biogeochemistry and food web dynamics for the Gulf of Alaska. The physics are accounted for by modeling the water column structure over depth and the associated conditions that are important for initiating phytoplankton blooms (temperature, salinity, and water density). The biological modeling component connects small scale biological processes (capturing nutrient, phytoplankton, zooplankton dynamics), with both biogeochemical cycling functions and a food web model that incorporating the predatory-prey interactions in the ecosystem.
Modeling the Kasatochi eruption event resulted in an immediate increase in surface iron concentration. Over the next two months, the surface iron concentrations were drawn down to pre-eruption levels due to phytoplankton uptake and the iron sticking to other particulates in the water. The drawdown in iron concentration was concurrent with decreases in nitrogen and silica that were also utilized for phytoplankton growth (Figure 2). Small and large zooplankton (including copepods and gelatinous zooplankton) and primary production increased as much as twice the normal conditions in response to the volcanic iron deposition. The increased production propagated up the food web to higher trophic levels, particularly for those groups that feed on gelatinous zooplankton: large jellyfish and chum salmon. However, these increases were modest in comparison to phytoplankton (~30%). Fraser River sockeye salmon biomass only increased by 10%. This low increase in sockeye biomass indicates that the iron fertilization from Kasatochi could not solely be responsible for the successful 2010 sockeye returning population. The sockeye increase is likely small compared to the lower trophic levels because there is little time where the sockeye salmon was allowed to feed without predatory pressure and low competition for food.
The timing of the bloom from iron fertilization was also found to impact the type of zooplankton to prevail and the amount of production to make it to higher trophic levels. In the spring time, crustaceous zooplankton species persist, providing an efficient energy transfer pathway to higher trophic levels. In the summer, gelatinous zooplankton populations begin to dominate as the crustaceous zooplankton species begin to decline. The volcano eruption happened late in the summer which allowed gelatinous zooplankton predators, such as chum salmon and other jellyfish, to positively respond to the iron fertilization, while other species like the sockeye salmon did not benefit as much with the increased phytoplankton production.
This study reminds us that the ocean is not a closed system, and continuously interacts with the land and atmosphere. The authors of the study illustrate how changes in other disciplines (such as geology and physics) can have implications for local marine species and ecosystem dynamics. While energy transfer through the food web is important for fish populations’ success, the results for sockeye salmon remind us that it’s just one of the many forces that control fish populations. The other factors (e.g. fishing pressure, ecological relationships, and environmental conditions) also need to be evaluated when investigating what controls fish populations. As the planet continues to change in response to natural and human-induced perturbations, it is important now more than ever to remember how each system interacts with one another.