by Sergio Vallina
Dept of Marine Biology and Oceanography, Institute of Marine Sciences, Barcelona
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.
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.
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).
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.