Swirling rafts of algae paint the ocean in vivid greens and blues, a natural painting—lamentably temporary. While their presence is aesthetically pleasing, phytoplankton blooms make drastic changes to water chemistry. Microbes feeding on the sudden abundance of tiny phytoplankton cause a decrease in dissolved oxygen levels. Outbreaks of algae are normally driven by a large influx of cold, deep, nutrient rich water drawn up to the surface waters, but they can also be triggered by the flow of nutrient-bearing runoff from agricultural regions as can be seen in the above image of the Mississippi River’s mouth draining into the Gulf of Mexico.
New research in the November issue of Nature Geoscience, however, shows that the consequences of fertilizer-induced algae blooms can reach far beyond temporary oxygen depletion. The spike in microbial activity drives up local dissolved carbon dioxide levels, which in turn break apart into carbonate ions and a hydrogen ions, increasing the ocean’s acidity. The study’s authors found that this effect accounted for a 0.29 drop in pH, adding to the observed 0.11 pH decline due to traditional ocean acidification.
Most importantly, the researchers found that phytoplankton blooms reduce the ability of the ocean waters to buffer changes in pH, a process which would normally keep any sudden variations in acidity in check. This reduced buffering capacity was worth another 0.05 unit drop in pH, meaning that coastal regions under the effect of human-induced phytoplankton blooms experience ocean acidification even more strongly than you’d expect from either process alone—a divergence that will only increase as atmospheric carbon dioxide levels continue to swell.
Photo: NASA SeaWiFS
Study: Acidification of subsurface coastal waters enhanced by eutrophication, Nature Geoscience