Ocean Ecology: evolution under a changing physical climate

Ocean Ecology

Oceanic ecosystems are being put under extraordinary pressures by human activities, including overfishing, various kinds of pollution, and acidification and warming related to excessive CO₂ emissions. It is therefore expected that the ocean ecosystems will look quite different at the end of this century than they did in the recent past. Our goal is to find out what will be their most likely fate under the anticipated climate change over the coming decades. We attempt this by using the most advanced global climate models, in which ocean life is described by a set of mathematical equations. Our research focus is directed towards phytoplankton, or planktonic algae, which, though being mostly microscopic in size, have a planetary importance: by producing organic chemical compounds from CO₂, water and minerals in photosynthesis, they form the basis of nearly all oceanic food chains and have a great influence on atmospheric CO₂ levels. Once carbon is fixed within organic mater, when organisms excrete it in the waste products of their metabolism or when they die, it can sink and become trapped in the deep ocean. This process, known as biological pump, is responsible for keeping a large portion of CO₂ away from the atmosphere for centuries or even millennia, thus keeping the Earth cooler than it would otherwise be.

(1) Phytoplankton Ecology in latest-generation couple models
In order to discover the likely changes in phytoplankton biomass, photosynthetic productivity, and the strength of biological pump over the 21st century, our group has recently analyzed the output of numerous state-of-the-art global (CMIP5) climate models that include the mathematical descriptions of marine ecology.
We analyze the underlying physical drivers of phytoplankton biomass/productivity changes on various timescales (interannual, interdecadal, centennial), and the links between so-called climate modes (ENSO, NAO, PDO) and ocean phytoplankton ecology.

These climate models generally predict a global reduction of photosynthetic production and an overall weakening of the biological pump. This is largely due to the effect of climate warming on ocean physics in the low and midlatitudes: warming acts to make the water column more stratified, as the top layer of water becomes less dense and harder to mix with the denser layer beneath. This stratification makes it harder for minerals required in photosynthetic productivity to penetrate from the dark deep ocean, where they concentrate, into the sunlit surface ocean, where photosynthesis is possible. With less minerals available, the photosynthetic production of organic materials drops and, in response, less organic matter sinks into the ocean interior.

Maps of recent past (1980–1999 average) and modeled climate change signal (difference between the periods 2080–2099 and 1980–1999), averaged among 16 global climate models for the following variables: (a) sea surface temperature, (b) stratification (defined as the difference in water density between the 200-meter depth and the surface), (c) average concentration of the mineral nitrate in the top 100 meters, and (d) photosynthetic production integrated over the top 100 meters. Crossed masked areas show significant trends at the 8–90 % level, and stripped masked areas significance at more than 90 % level. Note large expected decreases in primary production in low latitudes over the coming century, driven by increased stratification and decreases in nutrients. Sample figure from Cabre et al. 2014

(2) Theoretical Developments
From a theoretical point of view, it is interesting to derive techniques and theories to evaluate and predict (a) the separate impacts of changes in light, nutrients and temperature on phytoplankton biomass and productivity and (b) the outcome of the competition between small and large phytoplankton in a changing physical environment. An example of this work is Marinov et al. 2011.

(3) The inclusion of biological evolution into climate models
Global climate models are presently not capable of simulating the evolution of life in a changed environment through the process of natural selection. Yet, it is likely that some species will not be able to survive in the world of changing climate, while others might evolve traits that will help them cope with the new normal. I am interested in introducing adaptation and evolutionary concepts from the theoretical ecology community into the ecological modules of our coupled ocean-atmosphere models. The goal is to represent many distinct types of marine phytoplankton in cutting-edge climate models (at present big climate models can only do about 3 types), and see how adapting/evolving phytoplankton species respond to 21st century climate warming. I am presently looking for PhD students with a background in theoretical ecology and mathematical ecology for this project.

Climate models are not the only type of research tool that we use. We also analyze observational data, which help us improve our understanding of the natural world and evaluate the model data. We are particularly interested in the observations of marine phytoplankton, microscopic algae that float freely in the sunlit surface layer of the ocean, because they form the basis of most oceanic food chains and play a very important role in the global carbon cycle, as they absorb CO₂ and turn it into organic matter in the process of photosynthesis. A fascinating way to observe phytoplankton is ocean color remote sensing from artificial satellites that orbit the Earth. This is also the only way to get regularly repeated observations of phytoplankton across vast ocean expanses. Phytoplankton pigments (such as the green pigment chlorophyll) and cellular structures interact with sunlight that enters seawater, absorbing some wavelengths of light while scattering other wavelengths. The light that is deflected into space after being in contact with phytoplankton has characteristic properties. When this light meets a sensing device onboard a satellite, its properties are measured and recorded, and can in turn be used to determine various phytoplankton variables, including chlorophyll concentration and biomass.

Most recently, our collaborator Tihomir Kostadinov (University of Richmond) developed a method – based on backscattering of satellite signals – to quantify the biomass of specific groups of phytoplankton that differ in the size of their cells. Low nutrient environments favor smaller phytoplankton, while in high nutrient environments larger phytoplankton dominate, and this affects the organization and functioning of entire regional ecosystems, as well as biogeochemistry.

We are now using the observations of phytoplankton size groups to learn how they vary in response to physical and climate-driven factors.

Percentage contributions of (a) picophytoplankton, (b) nanophytoplankton and (c) microphytoplankton to average total phytoplankton carbon biomass in the surface layer of the ocean over the period September 1997 – December 2010, based on observations from the SeaWiFS satellite. The size ranges in brackets refer to the diameter of phytoplankton cells belonging to a given group, expressed in micrometers (a [µm] is a millionth part of a meter).

Ocean Ecology research team members:

Representative publication:

Kostadinov, T. S., Milutinović, S., Marinov, I., and Cabré, A.: Carbon-based phytoplankton size classes retrieved via ocean color estimates of the particle size distribution, Ocean Sci. Discuss., 12, 573-644, doi:10.5194/osd-12-573-2015, 2015.

Cabré, A., I. Marinov, and S. Leung (2014), Consistent global responses of marine ecosystems to future climate change across the IPCC AR5 earth system models, Clim. Dyn., published online 30 October 2014, 1-28, doi:10.1007/s00382-014-2374-3.

Leung, S., Cabré, A., and Marinov, I.: A latitudinally-banded phytoplankton response to 21st century climate change in the Southern Ocean across the CMIP5 model suite, Biogeosciences Discuss., 12, 5715-5734, doi:10.5194/bg-12-5715-2015, 2015.

Marinov, I., S. Doney and I. Lima: Response of ocean phytoplankton community structure to climate change over the 21st century: partitioning the effects of nutrients, temperature and light, Biogeosciences 7, 3941–3959, doi:10.5149/bg-7-3941-2010, 2010. (PDF)

I. Marinov, S. Doney, I. Lima, K. Lindsay, J. K. Moore, N. Mahowald: North-South asymmetry in the modeled phytoplankton community response to climate change over the 21st century. Global Biogeochemical Cycles, Vol. 27, 1–17, 2013. (PDF) [BROKEN]