Phytoplankton are the tiny, one-celled, organisms that make up the base of the marine food web. They are primary producers, meaning that they use sunlight and nutrients, to create their own food in a process called photosynthesis. Macronutrients include dissolved nitrate and phosphate and are required for all living organisms to grow and survive. Most organisms, from zooplankton to fish to whales, depend on phytoplankton to get their nutrients. Thus, phytoplankton productivity and their export to the deep ocean when they die are a way to deliver nutrients throughout the ocean.
Additionally, during photosynthesis, carbon dioxide (CO2) is changed into organic carbon. So primary productivity and export also removes CO2 from the atmosphere, which is the leading culprit in climate change. For all these reasons we desperately need to predict how phytoplankton communities and species will change in the near future.
The ocean is a very large place, so it is impossible for a researcher to measure phytoplankton and nutrient everywhere in the ocean, during all four seasons. In order to predict phytoplankton communities on a global scale, ecosystem models are used. Models use real data from previous investigations (such as phytoplankton levels, growth rates, species, etc.), chemical parameters (macronutrients and micronutrients such as iron), and physical data (temperature, salinity, ocean currents) to predict phytoplankton community changes in areas where no measurements have been taken. Models can also be used to predict future outcomes.
In this study, the authors use an ecosystem model to predict how phytoplankton communities will change in the year 2100 at the current rate of fossil fuel usage.
What you need to know about the ecosystem model
Phytoplankton were divided into two categories: 1) gleaners, which are small and slow growing phytoplankton that are very successful in regions with low nutrient concentrations and 2) opportunists, which are large phytoplankton that grow fast, but require high amounts of nutrients. Based on size, the opportunists will have a higher potential for carbon export.
Direct and indirect effects of climate change were used in the model prediction. Direct effects are the change in atmospheric temperature, which causes ice to melt and the ocean to get warmer; it does not include any changes of nutrient supply. The indirect effect is only the changes in nutrient supply and excludes temperature increases. A third effect, titled “all” includes both increases in temperature and changes to nutrient supply.
The model was first used to generate a control by “predicting” the phytoplankton ecosystem and nutrient supply for the year 2000. This served as a baseline for the 2100 prediction and also tested the sensitivity of the model. The researchers found that the model did a good job, but did have a few “hiccups.” Mainly, the coastal environments had poor resolution and a few spots under-predicted biomass and nutrient concentrations (such as the equatorial Pacific). They hypothesized that it was due to the limited amount of actual iron measurements that exist. However, on a global scale, the model performed well.
Lastly, the “business as usual” scenario for climate change was used to predict changes for 2100. This scenario assumes that CO2 emissions will continue to increase and no reductions will occur. In this 2100 scenario, the mean annual atmospheric temperature will increase by 5°C, the mean surface ocean temperature will increase by 3°C, and the atmospheric concentration of CO2 will be 3100ppmv (currently it is ~396ppmv).
1) On a regional scale, changes in phytoplankton concentrations, primary productivity, and carbon export will change in a way that “winning opportunist” regions in 2000 may become more desolate and poorly populated “losing opportunist” regions in 2100. The same is true for gleanors.
2) Globally, primary production and carbon export will be decreased in the tropics (low latitudes) due to a decrease in the supply of macronutrients. As the surface ocean gets warmer, meridional overturning will decrease. This means that less macronutrients from the deep ocean (which is abundant in macronutrients) will be brought to the surface. This effect is seen the most in the tropics, hence the decrease in production (see image).
3) In the higher, polar latitudes, the lower supply of nutrients is cancelled by the warming and primary productivity actually increases. The warming world will cause for ice to melt (more sunlight penetration into the ocean), the mixed surface layer to get shallower (brings phytoplankton closer to the sunlight), and will actually increase the phytoplankton’s growth rates.
4) The overall picture finds that the gleaners, those smaller and slower growing phytoplankton, will become more abundant in 2100. They will be able to outcompete the opportunists. However, since the gleanors are smaller, they cannot sink as fast, so carbon export rates will decease globally by about 10%.
5) These shifts in phytoplankton populations are dominated directly by the warming temperatures and indirectly by the temperature-caused nutrient decreases. However, both of these processes must be considered to fully understand the biomass changes in the future.
This model highlights that climate change will change the global phytoplankton populations significantly by the year 2100 at the current rate of CO2 emissions. Ultimately, primary productivity and carbon export will decrease and the smaller ‘gleaner’ phytoplankton will become dominant. This means that less macronutrients will be available to the other critters of the ocean and that less CO2 will be trapped in the ocean. This, of course, will just make climate change worse…..
I received a Ph.D. in oceanography in 2014 from the Graduate School of Oceanography (URI) and am finishing up a post-doc at the University of Maryland Center for Environmental Science (Horn Point Laboratory). I am now the Research Coordinator for the Delaware National Estuarine Research Reserve.
Carbon is my favorite element and my past times include cooking new vegetarian foods, running, and dressing up my cat!