Centre researcher James Watson explains how particles in the oceans move from one part of the planet to another in a significantly short time. CORRECTION: In the video, James intended to say "radionuclide" instead of "radio-nucleotide". Video animation by: Bror Jönsson, Princeton university
Microscopic particles in oceans
The distance between Fukushima and the west coast of the United States is about 8700 kilometres. If microscopic particles – like phytoplankton or radioactive isotopes – were to travel that fifth of the world’s circumference, it seems like that would take ages.
However, the world is not so big after all, since that is not actually the case.
A new study co-authored by centre researcher, James Watson, recently published in Nature Communications found that the earth’s global surfaces are highly connected.
By investigating the largely underexplored and rarely quantified mechanisms of global surface connectivity, Watson and his co-author Bror Jonsson from Princeton University found that microscopic particles can reach all regions of the ocean in only a decade.
This study emerged from contrasting camps of ideas about planktonic community dispersal in ocean ecosystems: one suggesting that everything is connected and environmental conditions decide where species live; another proposing that spatial isolation leads to genetically distinct species; and another suggesting that both of those ideas fail to tell the whole story.
On top of that, the time it takes for planktonic communities to travel around the ocean surface is a question that is still largely unresolved.
“These short surface-connection times are relevant to anyone studying dispersion in the surface ocean beyond planktonic species, including radioactive materials, plastics and other forms of pollution"
James Watson, co-author
Modeling global surface current connectivity
To tackle these inconsistencies in understanding and questions about time, Watson and his co-author create a model to track particles moving across the global ocean surface. To do this they use a number of different concepts and techniques.
This study uses minimum connection time, the fastest time that particles can travel from one location to another, instead of the commonly used expected connectivity time, which uses mean travel time. Watson notes there are two advantages to this approach.
"Minimum connection time is a more appropriate metric for phytoplankton and bacterial connectivity since asexually reproducing organisms have high reproductive output that attenuates low dispersal probabilities. Additionally, mean transit times in the global ocean are not well deﬁned, as water can recirculate eternally and, hence, every particle seeded in a given patch eventually will reach all other patches if enough time is provided," explains Watson.
Calculating minimum connection times from Lagrangian particle tracking, a method for understanding computational fluid dynamics, the authors described the global ocean as a network “with patches in the ocean as nodes and minimum connection times as edges connecting the nodes."
The authors then considered each patch pair and multi-step connections, or in other words particles traveling along a number of patches, and applied Dijkstra’s algorithm, commonly used for finding the shortest path between nodes, to create a network of minimum connection times between every region of the ocean’s surface.
The authors point out that while this global network does account for timescales of physical connectivity, they do not account for environmental factors which undoubtedly play a role in connectivity.
While the idea for this study emerged from tiny plankton, the results have blue whale-sized relevance for other ocean surface traveling objects.
Furthermore, these results could in the future help us understand and prepare for how long it takes harmful particles to connect across the globe – like from Fukushima to western United States, or plastics aggregating along the coasts.
“A real example is the 2011 Fukushima disaster, in which a Japanese nuclear reactor released a large quantity of radioactive isotopes into the Pacific Ocean. Traces of radioactivity were detected on the Pacific Coast of the US in November of 2014 – 3.6 years later. Our estimated minimum connectivity time between the Fukushima release site and its detection site of the US west coast is 3.5 years,” explains Watson, an indirect verification of their method.
From a planktonic perspective, the results suggest that planktonic communities may be able to keep pace with climate change by changing locations to better suit their preferred environmental niche.
In a bigger global perspective, Watson concludes that these results, “quantify the effects of global-scale dispersal on how marine communities can adapt to their changing ocean environment.”
The model above shows how phytoplankton traveling on ocean currents would spread over a three-year period. The researchers "released" thousands of particles representing phytoplankton from a starting point (green) stretching north to south from Greenland to the Antarctic Peninsula. The colours to the right indicate low (blue) or high (red) concentration of particles. Over time, the particles spiral out to reach the North and South Pacific, Europe, Africa and the Indian Ocean. The researchers eventually tracked more than 50 billion particles — a fraction of the phytoplankton in the ocean. Video: Bror Jönsson, Princeton university
James Watson's research aims to improve governance of marine systems understanding crucial feedbacks between physical, ecological and social processes.
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