Tenth Year Annual Report:
Carbon Science: Impacts of Anthropogenic Change
Impacts of Ocean Acidification on Phytoplankton
A fraction of the anthropogenic CO2 released to the atmosphere dissolves into the surface ocean; the resulting change in chemistry, called "ocean acidification," will have a host of effects on the ocean biota -- some subtle, some perhaps dramatic. The work of the Francois Morel and colleagues concerns the effects of ocean acidification on the growth of marine phytoplankton, the microscopic plants that form the basis of the marine food chain and are responsible for nearly half of primary production on Earth. A major focus of that work is the effect of acidification on the bioavailability of elements that are essential to the growth of phytoplankton: inorganic carbon (C), nitrogen (N), phosphorus (P) and a variety of trace elements including iron and zinc.
To convert inorganic carbon into organic biomass, phytoplankton must concentrate CO2 intracellularly, so the ongoing CO2 increase is likely to affect the efficiency of photosynthesis. Indeed a number of laboratory and field experiments, including those conducted by the Morel Group, show an increase in growth efficiency at elevated CO2, possibly due to down-regulation of the carbon concentrating mechanism. But this effect is not always seen and the cellular and biochemical mechanisms involved in concentrating CO2 are poorly known. The team recently quantified the fluxes and concentrations of inorganic carbon species in sub-cellular compartments in marine diatoms, a significant first step in quantifying the relationship between photosynthesis and the ambient CO2 concentration.
Through laboratory and field experiments, Morel and colleagues have shown that the acidification of seawater caused by the increase in CO2 results in a decrease in the bioavailability of iron, one of the most important limiting nutrients in the ocean. New laboratory and field experiments show that, contrary to expectations, acidification of the water can also decrease the bioavailability of zinc, and probably that of other essential metals. These results point at the possibility of very intricate and substantial effects of global change on the bioavailability of essential trace metals and, hence, on ocean productivity and ecology.
In regions where the available inorganic P and N are very low, some phytoplankton can acquire these nutrients from organic compounds. Such nutrient acquisition is often achieved by cleaving phosphate or ammonium moieties from organic molecules using external enzymes whose activity is affected by pH. These results how a significant decrease at low pH in the activity of alkaline phosphatase, the enzyme most responsible for P acquisition from organic compounds.
National Academies Report on Ocean Acidification
Francois Morel chaired a congressionally requested study on ocean acidification that was published this year - "Ocean Acidification: A National Strategy to Meet the Challenges of a Changing Ocean." The National Academies committee reported that, since the beginning of the industrial revolution, the average pH of ocean surface waters has decreased approximately 0.1 unit -- from about 8.2 to 8.1 -- making them more acidic. Models project an additional 0.2 to 0.3 drop by the end of the century, a rate of change that exceeds any known to have occurred in hundreds of thousands of years.
The committee found that congressional legislation mandating a National Ocean Acidification Program has laid the foundation for a program that will advance our understanding and improve our response to ocean acidification, and recommended that the program contain six key elements:
- an integrated ocean acidification observation network that includes the development of new tools, methods, and techniques to improve measurements
- research to fulfill critical information gaps
- assessments to identify stakeholder concerns and a process to provide relevant information for decision support
- a data management office that would ensure data quality, access, and archiving, plus an information exchange that would provide research results, syntheses, and assessments to managers, policymakers, and the general public
- facilities to support high-quality research and training of ocean acidification researchers
- an effective 10-year strategic plan for the program that will identify key goals, set priorities, and allow for community input, in addition to a detailed implementation plan
Anthropogenic Impacts on Ocean Oxygen
The Sarmiento Group has recently focused on the impacts that increased anthropogenic CO2 and resulting climate change have on oxygen in the ocean. The biological carbon pump depletes oxygen as sinking organic matter is transformed back into carbon molecules by bacteria. Some areas of the ocean, called oxygen minimum zones or OMZs, have oxygen levels so low that they affect the distribution and abundance of fish and other macroorganisms which may negatively affect economically important fisheries. Following recent analyses of observations that indicate the size of OMZs is increasing, Sarmiento and colleagues are using a combination of databases and earth system models to look at the future of carbon and oxygen in the ocean.
Estimates of the extension of OMZs commonly rely on global data-based products such as the gridded World Ocean Atlas (WOA). However, gridded datasets appear to overestimate the oxygen concentration in OMZs compared to in situ observations. By applying an empirical correction to the WOA dataset based on in situ measurements, Sarmiento and colleagues refined the estimates of open ocean OMZ volumes. The new estimates for suboxic volumes (O2<20 mmol/m3) are approximately one third higher than the uncorrected WOA. This figure increases when considering fully anoxic waters.
Global general circulation models (GCM) of low and intermediate resolution used for biogeochemical and climate studies can produce very different patterns of dissolved oxygen ranging from relatively small OMZs localized in the thermocline, to OMZs extending over most of the deep ocean. With the updated picture of OMZ extension, the researchers turned to investigating the factors that control OMZs in GCMs by using a novel tracer decomposition that separates oxygen and nutrients into different components that respond separately to specific physical and biogeochemical processes. This separation will allow researchers to track the contribution of each process affecting oxygen, and will guide the development of better models of oxygen cycling.