Eleventh Year Annual Report: Impacts Studies
The Morel, Pacala, and Sarmiento Groups are conducting research on the impacts of changing atmospheric and ocean chemistry on marine and terrestrial ecosystems.
Impacts of ocean acidification on phytoplankton
A third of the anthropogenic CO2 released to the atmosphere dissolves into the surface ocean. What effects the resulting changes in chemistry (called ocean acidification) will have on phytoplankton, the base of the ocean's food chain, is a focus of research in the Morel Group.
To fix inorganic carbon into organic biomass, phytoplankton must concentrate CO2 via a system known as the Carbon Concentrating Mechanism (CCM). The material and energetic costs of the CCM, which are poorly known, will determine in part the response of phytoplankton to the ongoing CO2 increase. To provide a quantitative assessment of the CCM efficiency in marine diatoms, the Morel Group has been quantifying the fluxes and concentrations of inorganic carbon species in sub-cellular compartments by mass spectrometry.
Previous work showed that the bioavailability of iron, a key limiting nutrient in the ocean, decreases at the lower pH caused by increasing CO2. This year, the extension of this work to other essential trace metals has demonstrated that the bioavailability of zinc and cadmium also decreases with pH. This surprising result is explained by the bioavailability of weak organic complexes of the metals whose concentration decreases at low pH.
A focus of continuing work is the effect of ocean acidification on nitrogen fixation by the marine cyanobacterium Trichodesmium, the dominant N2 fixer in the oceans. Because N2 fixation requires large quantities of iron, its response to ocean acidification is made complicated by the dual (and likely opposite) effects of low pH on iron availability and high CO2 on photosynthesis.
Impacts of climate change on ocean productivity and marine fisheries
Changes in ocean biogeochemistry and phytoplankton community structure may also affect fish and invertebrate distribution and productivity. In a collaborative study with the group of Daniel Pauly and William Cheung of the University of British Columbia, the Sarmiento Group assessed the sensitivity of projected changes in the distribution and fisheries catch potential to biogeochemical changes in the ocean.
A dynamic bioclimatic envelope model was used that incorporates predictions of physical as well as biogeochemical fields from a fully coupled Earth System Model. The model projects a distribution-centroid shift towards higher latitudes and deeper depths in the Northeast Atlantic (Figure 14). Ocean acidification and reduction in oxygen content reduce growth performance, increase the rate of range shift, and lower the estimated catch potential by 20-30% from year 2005 to 2050 relative to simulations without considering these factors. Consideration of phytoplankton community structure may further reduce projected catch potentials by ~10%.
These results highlight the sensitivity of marine ecosystems to biogeochemical changes and the need to incorporate likely hypotheses of their biological and ecological effects in assessing climate change impacts. The full global response in a large number of such Earth System Model simulations will be assessed in a future study. The goal is to determine the range in responses of the properties used to drive the dynamic bioclimatic envelope model, and to estimate how much uncertainty this variability between models introduces into the maximum catch potential estimates.
Impacts of ocean acidification and deoxygenation on fish gill function
Changes in ocean biogeochemistry in response to climate change may also impact fish directly through changes in gill function. Fish are aerobic organisms that uptake oxygen and release carbon dioxide in their gills using diffusion gradients. Gas exchange in the gill requires seawater oxygen to be higher than blood oxygen and seawater carbon dioxide to be lower than blood carbon dioxide. The rising levels of carbon in the seawater environment due to atmospheric absorption of carbon dioxide may therefore have consequences for gas exchange. Oxygen availability is also a concern because oxygen minimum zones appear to be increasing worldwide. The increase in carbon dioxide and decrease in oxygen in the ocean environment may result in a reduction in habitat, and the effect it will have on pelagic fisheries and ecosystems is of increasing concern.
Predicting habitat reduction is difficult because the physiological responses to hypoxic and suboxic levels of oxygen are highly variable among species. To quantify this effect, the Sarmiento Group developed a model that simulates gas exchange along a lamella in the gill of a teleost fish. This model incorporates physics, blood physiology, and gill morphology and can be adapted for different fish species. It is forced with temperature, oxygen, and carbon dioxide, and preliminary results indicate that temperature and carbon dioxide are important factors controlling oxygen uptake. In addition, the researchers are working on quantifying oxygen uptake and predicting habitat thickness for a range of physiological and morphological traits in the global ocean for the present and exploring the potential impacts of climate change over the next century.
In the next year, the Sarmiento Group plans to continue the development of the model and determine the sensitivity of oxygen uptake in fish gills to variations in temperature, O2, and CO2 in the environment. Potential adaptation will also be examined by manipulating physiological and morphological parameters in the model.
Terrestrial floral species' response to climate change
Postdoctoral research fellow Adam Wolf of the Pacala Group has developed a set of statistical tools to investigate the response of diverse terrestrial floral species to climate change, and used these tools to investigate change in two main arenas. The first area of investigation was species shifts across the entire California flora, some 3500 taxa, most of which are native or endemic. He developed a statistical method to estimate species shifts using herbarium specimens, which are intrinsically confounded by sampling biases. Subsequently, he applied this method to estimate species shifts across the flora, and found a strong tendency for taxa to move upslope, which reduced experienced warming for these species. Nevertheless, many endemics with small ranges could not move up at a fast enough pace to avoid significant warming over the 20th century.