2023-07-12 06:30:06
Scientists have highlighted differences in the timing of phytoplankton blooms in the Gulf Stream region. This shift is linked to the presence of oceanic fronts which cause this biomass to bloom earlier. This phenomenon could have an impact on the ability of the oceans to sequester carbon.
Ocean fronts are meeting places between colder waters and warmer waters and can be compared to atmospheric fronts. At their interface, the properties of the water (temperature, salinity, nutrients) are modified and this has the effect of affecting the growth of phytoplankton. Despite numerous observations of this phenomenon locally, it remains largely unknown on a large scale. Ephemeral fronts indeed move and dissipate continuously on time scales ranging from days to weeks and are therefore particularly difficult to sample. A research team from CNRS-INSU (Institut national des sciences de l’Univers) has succeeded in studying the impact of these oceanic fronts on the growth of phytoplankton in the North Atlantic region surrounding the Gulf Stream. Their study was published in the journal Biogeosciences.
To carry out this work, the scientists used satellite images to criss-cross the ocean, like pixels in a photograph. Since phytoplankton carry chlorophyll, a green pigment, it is possible to spot it when it proliferates thanks to the change in the color of the water. Although satellite estimates of chlorophyll are limited to the ocean surface, they currently represent the only way to assess the impact of ocean fronts on this biomass, over large areas. The use of 20 years of satellite data made it possible to consolidate the statistical processing model developed within the framework of this study.
Scientists have found that the increase in surface phytoplankton associated with ocean fronts is rather modest and does not exceed 5%. But thanks to the space data, they managed to identify, each day, the frontal zones, non-frontal zones, and to compare the growth statistics of the phytoplankton in these two environments of ephemeral location. They thus discovered that the blooming of phytoplankton is far from homogeneous, since differences in the start of the growth of these microscopic plants were observed less than 10 km away. Phytoplankton first begin to bloom in the fronts, with a one to two week earlier start to flowering compared to non-frontal areas, and bloom there is also two to three times more intense. In the end, this flowering period extends over several months due to the geographical extent of the area studied.
A competition between the different species of phytoplankton
These differences in flowering could have cascading consequences for ocean ecosystems, biodiversity and the ocean’s ability to fix carbon. Because during its development, phytoplankton is consumed by zooplankton as well as by a multitude of marine organisms. Since it develops earlier in frontal areas, predators arrive on site much earlier, and therefore may not allow time for phytoplankton to develop in nearby non-frontal areas. Such a phenomenon could disturb the balance between the different species of phytoplankton that grow in the frontal zones and outside.
In a context of climate change, the development of this biomass presents great uncertainties. No one currently knows precisely how the ocean fronts will evolve over time in the face of rising temperatures. This research provides a proven methodology that could allow the study of the links between ocean physics and phytoplankton response in other ocean regions of the world. Phytoplankton form the basis of marine food webs and are a key player in the ocean carbon cycle.
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