In new work published in Nature Communications in July, an international team of scientists from the Massachusetts Institute of Technology and the Institute of Marine Sciences of Barcelona, Spain in collaboration with the National Centre for Scientific Research, France have been using MITgcm to study the balance between phytoplankton diversity and productivity.
How does species richness vary with ecosystem productivity for marine phytoplankton? This question has fascinated biological oceanographers for decades. Phytoplankton communities are composed of many species of unicellular micro-algae. They are at the base of the marine trophic foodweb, growing and surviving by means of photosythesis by fixing CO2; a process known as primary production.
Now a work published in Nature Communications led by an international team of scientists from the Massachusetts Institute of Technology (MIT, US) and the Institute of Marine Sciences of Barcelona (CSIC, Spain) in collaboration with the National Centre for Scientific Research (CNRS, France) has finally solved the mystery using global ecosystem model coupled a general circulation model resolving the global ocean currents (MITgcm).
Recent observational studies have shown that the diversity of phytoplankton species increases with primary production until a peak after which it decreases drastically when primary production reaches unusually high values. However the reason behind this inverted “U” or “hump-shape” was largely unknown. None of the existing theories could provide a complete picture of the mechanisms that were operating.
Sergio Vallina, a former postdoc at the MIT hosted by Mick Follows and Stephanie Dutkiewicz, and currently a research scientist at the Institute of Marine Sciences of Barcelona, is the first author of the study and explains the reasons leading to a hump-shaped relationship between diversity-productivity. First, selective grazing by marine zooplankton targeting the most abundant species of phytoplankton through what is known as “killing-the-winner” predation leads to a positive correlation between primary production and diversity in nutrient starved regimes with lower producitivity. In seasonal regimes, where resources can be plentiful on a sporadic basis, a mismatch between phytoplankton growth and their grazing by zooplankton at the beginning of the growing season (e.g. spring) leads to high productivity but low diversity. Thus, the regimes of highest productivity exhibit a negative correlation. The combination of both mechanisms explains the hump-shaped relationship between diversity and productivity observed in the data.
When phytoplankton growth and zooplankton grazing are tightly coupled, selective killing-the-winner predation prevents the most dominant phytoplankton species from monopolizing all resources, which otherwise would lead to the competitive exclusion of all other less dominant species from the ecosystem. Under such conditions, increasing the nutrient supply of the ecosystem leads to a positive co-variation between primary production and diversity through the stabilizing mechanism known as “predator-mediated coexistence”. This mechanism explains the positive correlation initially observed in the productivity-diversity relationship from low to moderate values of primary production.
However when zooplankton and phytoplankton are decoupled, which usually happens right after winter in high latitude zones with strong seasonality, increasing the nutrient supply implies that the most dominant species of phytoplankton can bloom out of any top-down control, hoarding all resources and thus excluding the less competitive species out the ecosystem. Under these circumstances, primary production and diversity go in opposite directions. This mechanisms explains the negative correlation observed in the productivity-diversity relationship at very high values of primary production which simply reflect those phytoplankton blooms dominated by just a few species of fast-growing diatoms.
The model simulations reproduce the qualitative trends in the field data suggesting that this simple explanation is robust. This is the first time that a marine ecosystem model is able to link the mechanisms of species coexistence and competitive exclusion that are thought to operate simultaneously for marine phytoplankton communities to explain the relationship between diversity and productivity in the global ocean. The broader implications of this basic research study might be useful from the point of view of marine ecosystems’ management because it offers testable hypothesis of the root mechanisms that help sustain species diversity in the oceans.
Read this story by Sergio Vallina at the MIT Darwin project website 
This Month’s Featured Publication:
- S. M. Vallina, M. J. Follows, S. Dutkiewicz, J. M. Montoya, P. Cermeno, and M. Loreau (2014), Global relationship between phytoplankton diversity and productivity in the ocean , Nature Communications5, article number: 4299, doi: 10.1038/ncomms5299
Also new this month:
Helen M. Amos, Daniel J. Jacob, David Kocman, Hannah M. Horowitz, Yanxu Zhang, Stephanie Dutkiewicz, Milena Horvat, Elizabeth S. Corbitt, David P. Krabbenhoft, and Elsie M. Sunderland (2014), Global Biogeochemical Implications of Mercury Discharges from Rivers and Sediment Burial , Environ. Sci. Technol., published online: 9 July 2014, doi: 10.1021/es502134t
Abhisek Chakraborty, Rashmi Sharma, Raj Kumar, and S10.1007/s00382-014-2250-1ujit Basu (2014), A SEEK filter assimilation of sea surface salinity from Aquarius in an OGCM: Implication for surface dynamics and thermohaline structure , Journal of Geophysical Research: Oceans, published online, doi: 10.1002/2014JC009984
Haiyang Cui, , J.D. Pietrzak, and G.S. Stelling (2014), Optimal dispersion with minimized Poisson equations for non-hydrostatic free surface flows , Ocean Modelling, Volume 81, September 2014, Pages 1–12, doi: 10.1016/j.ocemod.2014.06.004
Brian D. Dushaw (2014), Assessing the horizontal refraction of ocean acoustic tomography signals using high-resolution ocean state estimates , J. Acoust. Soc. Am. 136, 122, doi: 10.1121/1.4881928
Fabian Große, Christian Lindemann. Johannes Pätsch, and Jan O. Backhaus (2014), The influence of winter convection on primary production: A parameterisation using a hydrostatic three-dimensional biogeochemical model , Journal of Marine Systems, available online 11 July 2014, doi: 10.1016/j.jmarsys.2014.07.002
Kevin Heng, and Adam P. Showman (2014), Atmospheric Dynamics of Exoplanets , preprint submitted to the Annual Review of Earth and Planetary Science (AREPS), arXiv:1407.4150v1 [astro-ph.EP] 15 Jul 2014
T. Kodaira, T. Waseda, and Miyazawa (2014), Nonlinear internal waves generated and trapped upstream of islands in the Kuroshio , Geophysical Research Letters, article first published online: 23 July 2014, doi: 10.1002/2014GL060113
XiaoMing Li, Lequan Chi, Xueen Chen, YongZheng Ren, and Susanne Lehner (2014), SAR observation and numerical modeling of tidal current wakes at the East China Sea offshore wind farm , Journal of Geophysical Research: Oceans, published online, doi: 10.1002/2014JC009822
Yang, Q., Loza, S. N., Losch, M., Liu, J., Zhang, Z., Nerger, L., and Yang, H. (2015): Assimilating summer sea-ice concentration into a coupled ice–ocean model using a LSEIK filter , Annals of Glaciology, 56 (69), pp. 38-44, doi: 10.3189/2015AoG69A740
John Marshall, Kyle C. Armour, Jeffery R. Scott, Yavor Kostov, Ute Hausmann, David Ferreira, Theodore G. Shepherd and Cecilia M. Bitz (2014), The ocean’s role in polar climate change: asymmetric Arctic and Antarctic responses to greenhouse gas and ozone forcing , Phil. Trans. R. Soc. A 2014 372, 20130040, published 2 June 2014, doi: 10.1098/rsta.2013.0040
Robert M. Moore, Markus Kienast, Michael Fraser, John J. Culle, Curtis Deutsch, Stephanie Dutkiewicz, Michael J. Follows, and Christopher J. Somes (2014), Extensive hydrogen supersaturations in the western South Atlantic Ocean suggest substantial underestimation of nitrogen fixation , Journal of Geophysical Research Oceans, article first published online: 16 July 2014, doi: 10.1002/2014JC010017
C. Naranjoa, J. Garcia-Lafuentea, G. Sanninob, and J.C. Sanchez-Garridoa (2014), How much do tides affect the circulation of the Mediterranean Sea? From local processes in the Strait of Gibraltar to basin-scale effects , In press with Progress in Oceanography, doi: 10.1016/j.pocean.2014.06.005
S. Sammartino, J.C. Sánchez Garrido, J. Delgado, C. Naranjo, F. Criado Aldeanueva, J. García Lafuente (2014), Experimental and numerical characterization of harbor oscillations in the port of Málaga, Spain , Ocean Engineering, Volume 88, 15 September 2014, Pages 110–119, doi: 10.1016/j.oceaneng.2014.06.011
Sözer, Adil; Sannino, Gianmaria; Özsoy, Emin (2014), Bosphorus Strait Exchange Flow Dynamics Focused on Numerical Ocean Models Intercomparison , EGU General Assembly 2014, held 27 April – 2 May, 2014 in Vienna, Austria, id.15374
Sözer, Adil; Sannino, Gianmaria; Özsoy, Emin (2014), Recent advancements on modelling the exchange flow dynamics through the Turkish Strait System , EGU General Assembly 2014, held 27 April – 2 May, 2014 in Vienna, Austria, id.15697
J. Todd and P. Christoffersen (2014), Are seasonal calving dynamics forced by buttressing from ice mélange or undercutting by melting? Outcomes from full-Stokes simulations of Store Gletscher, West Greenland , in review for The Cryosphere Discuss., 8, 3525-3561, doi: 10.5194/tcd-8-3525-2014
Tulloch, R., R. Ferrari, O. Jahn, A. Klocker, J. LaCasce, J.R. Ledwell, J. Marshall, M-J. Messias, K. Speer, and A. Watson (2014), Direct Estimate of Lateral Eddy Diffusivity Upstream of Drake Passage , Journal of Physical Oceanography, published online, doi: 10.1175/JPO-D-13-0120.1
Nadya T. Vinogradova, Rui M. Ponte, Ichiro Fukumori, and Ou Wang (2014), Estimating satellite salinity errors for assimilation of Aquarius and SMOS data into climate models , Journal of Geophysical Research: Oceans, published online, doi: 10.1002/2014JC009906
Jun Wei, Dongxiao Wang, Mingting Li, and Paola Malanotte-Rizzoli (2014), Coupled seasonal and intraseasonal variability in the South China Sea , Clim Dyn, published online 17 July, 2014, doi: 10.1007/s00382-014-2250-1
Xi Zhang and Adam P. Showman (2014), Effects of Bulk Composition on the Atmospheric Dynamics on Close-in Exoplanets , EPSC Abstracts, Vol. 9, EPSC2014-190-1, 2014, European Planetary Science Congress 2014
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