“The great mystery of plankton is what controls its distribution and abundance, and what conditions lead to large plankton blooms,” said Dennis McGillicuddy, senior scientist and director of the Department of Applied Ocean Physics and Engineering at the Woods Hole Oceanographic Institution (WHOI).
Two new papers explore this question and provide examples of conditions that lead to massive plankton blooms with very different potential impacts on the ecosystem, according to McGillicuddy, co-author of the two papers. Both articles also stress the importance of using cutting-edge technology – including plankton video recorders, autonomous underwater vehicles and the Coastal Pioneer Array of the Ocean Observatories Initiative – to find and monitor these blooms.
In an article, Diatom Hotspots Driven by Western Boundary Current Instability, published in Geophysical Research Letters (GRL), scientists have discovered surprisingly productive subsurface hotspot blooms of diatom phytoplankton.
In the GRL document, researchers studied the dynamics controlling primary productivity in a region of Mid-Atlantic Bight (MAB), one of the most productive marine ecosystems in the world. In 2019, they observed unexpected diatom hotspots in the slopes region of the euphotic zone of the bay, the ocean layer that receives enough light for photosynthesis to occur. Phytoplankton is a photosynthetic microorganism that forms the basis of the aquatic food web.
It was surprising to the researchers that the hot spots occurred in high salinity water from the Gulf Stream. “Although these low nutrient Gulf Stream water intrusions were considered likely to decrease biological productivity, we present evidence of an unexpected subsurface diatom bloom resulting from the direct intrusion of a meander. from the Gulf Stream to the continental shelf “, note the authors. They hypothesize that the hot spots were not fed by surface water from the Gulf Stream, which is generally poor in nutrients and chlorophyll, but rather that the hot spots were fed by nutrients that had returned to the area of the sea. sunshine from the deeper water of the Gulf Stream.
As the stability of the Gulf Stream has changed, Gulf Stream intrusions have become more frequent in recent decades, researchers say. “These results suggest that the large-scale change in circulation has regional productivity consequences that are not detectable by satellites due to their appearance well below the surface,” note the authors.
“In this particular case, climate change has led to an increase in productivity in this particular region, due to a subtle and somewhat unexpected interaction between the physics and biology of the ocean. This same dynamic may not necessarily apply elsewhere in the ocean, and it is highly likely that other areas of the ocean will become less productive over time. This is of great concern, ”said McGillicuddy.“ There are going to be regional differences. in how the ocean responds to climate change. And the company must be able to manage intelligently from a regional perspective, not just from a global perspective. “
The research results demonstrated “a cool and counterintuitive biological impact of this shifting large-scale circulation,” said the GRL the main author of the article, Hilde Oliver, postdoctoral researcher in applied ocean physics and engineering at OMSI. She remembers watching the instrument data come in. With typical summer values of around 1 to 1.5 micrograms of chlorophyll per liter of seawater, the researchers recorded “unprecedented concentrations of chlorophyll in this region in summer”, reaching 12 or 13 micrograms per liter. , says Olivier.
Oliver, whose doctorate. focused on modeling, said the cruise had helped her examine phytoplankton blooms from a more than theoretical perspective. “Going into the ocean and seeing how ocean physics can manifest these blooms in the real world has opened my eyes,” she said.
Another article, A Regional, Early Spring Bloom of Phaeocystis pocketetii on the New England Continental Shelf, published in the Journal of Geophysical Research: Oceans (JGR: Oceans), has also opened our eyes. Researchers investigating the biological dynamics of the New England continental shelf in 2018 found a huge overgrowth of the haptophytic phytoplankton Phaeocystis pocketetii.
However, unlike the diatom hotspots described in GRL article, Phaeocystis is “unpleasant to many different organisms and disrupts the entire food web,” said Walker Smith, retired professor at the Virginia Institute of Marine Science William and Mary, who is the lead author of the article JGR: Oceans. The phytoplankton forms gelatinous colonies a few millimeters in diameter.
When Phaeocystis flowers, it uses nutrients like any other form of phytoplankton. However, unlike the diatoms noted in the GRL paper, Phaeocystis converts biomass into something that doesn’t tend to pass through the rest of the food chain, McGillicuddy said.
“Understanding the physico-biological interactions in the coastal system provides a basis for predicting these potentially harmful algal blooms and may lead to better prediction of their impacts on coastal systems,” the authors said.
Massive colonial-stage blooms of this and similar species have been reported in many systems in different parts of the world, which Smith studied. These types of blooms probably occur every three years or so on the New England continental shelf and likely have a fairly large impact on New England waters, food webs and fisheries, Smith said. Coastal managers should be made aware of these blooms as they can have economic impacts on aquaculture in coastal areas, he said.
“Despite the fact that the Mid-Atlantic Bight has been well researched and extensively sampled, there are things going on that we still don’t really like,” Smith said. “An example are these Phaeocystis flowers that are deep in the water and that you will never see unless you are there because the satellites cannot show them. So the more we look, the more we find.”