Warmer Seas Blocking Nature’s Carbon Pump

Diatoms are one of the most common types of phytoplankton.

Climate change isn’t just warming the atmosphere, it’s also warming the ocean’s surface and deeper levels of the water column. This is known as the pelagic ocean and it just so happens to harbor the most productive ecosystem on planet Earth (the “pelagic zone” is any part of the water column other than that at the sea floor). The pelagic ocean is responsible for an estimated half of the world’s primary production (i.e., the basic food or nutrient making needed to sustain other life), and sustains most of the world’s natural fisheries.

The pelagic zone also plays a very complex but important role in the global carbon cycle. Inorganic carbon (mostly in the form of CO2) can be “drawn down” from the atmosphere by two main processes: the respiration of photo-synthetic algae and plankton (which produce oxygen and serve as a food source as well), and, secondly, the sedimentation of carbon (in the form of sinking, dead marine matter) onto the sea floor. Most algae and phytoplankton have chlorophyll and live in the upper most layer of the water column where there is sufficient sunlight penetration (this is called the euphotic zone; from the surface down to 200 meters is the epipelagic zone). Although carbon is also removed via “outgassing” (the exporting of carbon and carbon-based molecules into the atmosphere via ocean-air circulation), these two processes keep carbon out of the atmosphere. And of the two, bottom accumulation (via sinking) is the predominant means by which carbon is removed from the water column.

This absorption of carbon and its eventual sinking to and accumulating on the sea bottom (also described as “vertical exporting”) is known generally as the “carbon pump”, and is perhaps the single most important engine of global carbon cycling. CO2 build up in the atmosphere tends to increase warming in the biosphere and atmosphere, and, in also contributing to warming of the sea surface, some of this heat is circulated by wave action and absorbed, making its way down to greater depths. This impacts marine creatures, large and small, with consequences for long-term carbon cycling stability (this is in addition to acidification due to CO2 combining with hydrogen ions in water to form carbonic acid). Over the past several years, climatologists and oceanographers have been accumulating sufficient evidence to assert that both sea-surface and pelagic ocean temperatures are increasing. The scientists showed that the numerous marine creatures that inhabit this zone respond variably to temperature increases, but the primary focus of a recent study (Wohlers et al, Changes in biogenic carbon flow in response to sea surface warming,) was the role of phytoplanktons (such as diatoms) in the “draw down” of inorganic carbon. Using an indoor mesocosm model (a scaled up, enclosed model of planktonic habitat) to cause algal blooms, the scientists showed that rising temperatures increase the respiration of organic carbon (carbon composing or coming from living tissue). They also showed that a temperature increase from 2° to 6°  C. causes a decrease in the biological draw down of inorganic carbon (such as by plants during photosynthesis) by up to 31%. Further, the loss of organic carbon through sinking (a major part of that carbon “pump”) was significantly reduced. Ocean-air mixing, the absorption of CO2 through phytoplankton respiration, and the eventual sinking (or vertical exporting) of carbon are all crucial for keeping CO2 build up in the atmosphere in check

(Left) Diagram of the carbon cycle. The black numbers indicate how much carbon is stored in various reservoirs, in billions of tons (“GtC” stands for GigaTons of Carbon and figures are circa 2004). The dark blue numbers indicate how much carbon moves between reservoirs each year. The sediments, as defined in this diagram, do not include the ~70 million GtC of carbonate rock and kerogen (organic chemcal compounds found in sedimentary rocks).

Marines fauna sensitivity to warming is highly variable. Some creatures in the ocean (such as the light-harvesting planktons and cyanobacteria), generate their own nutrients (autotrophs), while other organisms (like dinoflagellates, generally zooplankton) are able to generate some of their nutrient needs (heterotrophs), but also depend on other organisms (like sugar-producing planktons) to provide some of these.  The scientists expect that this variance in sensitivity to increasing temperatures will “cause major shifts” in the carbon and energy flow (including the Nitrogen cycle) of the pelagic system. Warmer oceans mean less dissolved oxygen (O2, what most, non-planktonic marine creatures require for normal respiration) which means that larger creatures like fish will have less O2 (and may shift to breathing less efficient CO2) and less energy to pursue smaller fish to eat, and possibly, reproduce less, further contributing to the decline in carbon sequestering via dead fish sinking to the bottom. This outcome illustrates the general findings that show increasing temperatures leading to both an increase in organic carbon respiration and decreased sedimentation. The researchers further warn that increasing temperatures in this vital, globally-extended ecosystem could “reduce the transfer of primary produced organic matter to higher trophic levels” (e.g., such as those that sustain corals and the many species that use them as habitat), interfere with the global carbon pump, and possibly set up a positive feedback mechanism, further increasing atmospheric carbon dioxide.

top photo credit: Prof. Gordon T. Taylor, Stony Brook University, USA