Blooms at Lower pH Levels Could Upset Ocean’s Acidification Cycle

by Max Breitbarth

Ocean acidification—the absorption of atmospheric CO2 by the ocean—has increased due to anthropogenic emissions of CO2, resulting in growing concentrations of CO2 in our oceans. Flynn et al. (2015) created models based on projections of increasing ocean acidity to explore the effects of algae blooms at decreasing pH levels and the effects of these blooms on phytoplankton populations that keep the ocean’s acidity within a manageable spectrum.

Flynn et al. explain in their 2015 Royal Publishing Society article that the ocean’s acidity is affected by phytoplankton (algae mostly) through two processes. The first, called primary production, is the photosynthetic process by which algae use sunlight and CO2 to create energy and grow. This leads to the reduction of free CO2 in the ocean’s water and “basification,” or reduced acidity. Basification and the addition of inorganic nutrients to the ocean from river sediment and other sources lead to algal blooms, which are explosions in biomass of phytoplankton in the ocean’s water due to the nutrients signaling accelerated production. The second process, respiration, is just like respiration of organisms on land—the consumption of oxygen and the release of CO2 back into the environment (in this case, the ocean), resulting in a higher CO2 concentration. Respiration typically occurs at a high rate under bloom conditions, increasing acidity and decreasing oxygen levels in the ocean. This happens at a massive scale globally: algae produce about 80% of the world’s oxygen (Hall, 2011).

Normally primary production and respiration occur in a balanced system and blooms wax and wane. However, ocean acidification caused by the undue influence of anthropogenic emissions of CO2 has led to a more acidic baseline environment. In highly dynamic coastal regions where inorganic nutrients are more plentiful than in the open ocean, the blooms have a huge effect on the ecology of the surrounding environment. The problem occurs when the nutrients signal algae to begin production that normally wouldn’t occur at such elevated concentrations of CO2. The result is an imbalance that can upset the fragile equilibrium of the ocean’s algae cycle, and have effects across each level of the food chain from the bottom up.

Flynn et al., argue that the short term, dramatic basifications caused by algal “bloom development,” along with the long term acidification projected to continue as a result of human activity, will affect pH-sensitive phytoplankton and have impacts which have been unexplored thus far. “This will fundamentally shift the conditions for competitive interactions between bloom-forming species over a different range of H+ concentrations, with potentially unanticipated impacts on predator-prey dynamics through to higher trophic levels,” say the authors.

Flynn et al. wanted to explore the effects on individual plankton species, so they observed three different ones at three different stages of acidity—present day, future oceanic acidification conditions, and mid-bloom, basic conditions. They ran one set of tests with strict controls on the pH levels by adding negative or positive ions, and another test where there were no added negative or positive ions; the plankton could, in effect, change the conditions of their environment as they would in the natural ocean.

After using the data to create models, Flynn et al. found that the various phytoplankton species had different reactions to the imposed conditions, and that “there is potential for an increase in total bloom size…in coastal or shelf seas. The nutrient balance in shallow waters is also affected by basification during blooms.” The effects of these bloom sizes, along with the reaction of other organisms like the zooplankton that eat the phytoplankton, would lead to adverse effects for the ecosystem. The researchers concluded that the effects of continued oceanic acidification would “likely be associated with increased frequency of deleterious events such as harmful and ecosystem-disruptive algal blooms, and increases in hypoxic and anoxic zones, affecting fisheries and thence food security.” While the effects on the local ecosystems themselves are worth raising alarm about, the fact that these effects may be felt by humans reliant on marine systems for sustenance should have most of the world concerned as a stakeholder on this issue. Unless we slow our own emissions, ocean acidification will continue, and preparing for the effects on phytoplankton on the equilibrium of our oceans will remain a difficult task.

Flynn, KJ., Clark, D.R., Mitra, A., Fabian, H., Hansen, P.J., Glibert, P.M., … Brownlee, C. 25 February 2015. “Ocean acidification with (de)eutrophication will alter future phytoplankton growth and succession.” Proc. Royal Society Publishing, B 282: 20142604.

Hall, Jack. “The Most Important Organism?” The Ecology Global Network. 2011. Web.

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