Category Archives: Algal bloom

Algal blooms can be dramatic and are a result of excess nutrients occur naturally or induced by human activities in both freshwater and marine (salt) water bodies.

Turbulence causes swimming algae to congregate in dense patches, say physicists

[UK, Germany] Mild turbulence on the millimetre scale causes swimming phytoplankton to form dense patches, according to researchers in Germany and the UK. The research could boost our understanding of the spatial distribution of phytoplankton in oceans and lakes and help predict – and possibly prevent — the occurrence of toxic algal blooms.

Phytoplankton are microscopic algae that live near the surface of oceans and lakes. Some phytoplankton are motile, which means that they can propel themselves forwards. Like plants, they derive energy from the Sun by photosynthesis and produce more than 50% of oxygen we breathe. The organisms also play a key role in regulating atmospheric carbon dioxide levels.

An algal bloom is created when environmental factors such as nutrient availability and water temperature are favourable for the rapid reproduction of algae. Blooms can be destructive to wildlife, fisheries and tourism by blocking sunlight and by the release of toxic compounds into the water by the algae.

Mixing effect

Blooms can stretch over hundreds of square kilometres of ocean. Tidal currents and winds will often create uneven spatial distributions of algae, resulting in kilometre-sized patches. At length scales shorter than a centimetre, the mixing effect of fluid turbulence was expected to result in a uniform density of microorganisms. However, recent observations surprised scientists by showing that algal blooms have patchy distributions even at the millimetre scale. This patchiness could affect how fast a bloom develops because phytoplankton can reproduce sexually and therefore reproduction rates could be higher in regions of high density.

“The fact that there is a patchy distribution of phytoplankton on a kilometre scale can be easily understood and explained by oceanic currents and strong winds. Nevertheless, at the small scale, hydrodynamics should make things very homogeneous. And yet it doesn’t,” explains Marco Mazza, of Loughborough University.

Mazza and colleagues at Loughborough and the Max Planck Institute for Dynamics and Self-Organization in Göttingen used computer simulations and calculations to explore the origins of this unexpected millimetre-scale patchiness. They have shown that that an interplay between phytoplankton motility and interactions between individual organisms could play a role in creating patchiness on short length scales.

Navier–Stokes equations

The team looked at how motile phytoplankton are affected by hydrodynamic flows. To assess the phytoplankton behaviour on a sub-centimetre scale, the researchers combined 3D particle dynamics simulations with Navier–Stokes equations. The latter are the governing equations of fluid flows and define the relationship between the pressure, temperature, density, and velocity.

“We are solving the full Navier–Stokes equations without any further approximations and coupling this to a particle model describing the position of the particle within the flow, speed and direction of its swimming, it’s orientation, and thermal and biological noise,” Mazza explains, adding “Moreover, we consider the particle-particle interactions”.

Researchers found that the coupling of interactions between individuals to the small-scale flow features strongly favours the creation of dense patches of the organisms.

Prediction tool

These patches have implications to ecology and even human health, as algae will be more likely to mate in dense areas, leading to even faster growth. This research could ultimately contribute to creation of a tool for predicting algal blooms, which would be very useful to those involved in fishing and tourism.

“We are doing fundamental physical research and bridging the research to biology is a challenge,” says Michael Wilczekat the Max Planck Institute. For example, what we did not include in our study at all is population dynamics. Another interesting future step would be to include the shapes and sizes of the individual phytoplankton species. Alternatively, we could extend these studies to include zooplankton, where small scale patchiness is even more important, since zooplankton wants to prey on phytoplankton.”

Raymond Goldstein of the University of Cambridge, who was not involved in the work, told Physics World “the general issue is extremely interesting, but I don’t think the researchers have yet explained exactly the mechanism by which the clustering occurs. I do not see a clear physical picture of what exactly the interactions and alignments of cell are that lead to cluster formation. Having said that, this [research] will certainly motivate further investigations, precisely because it raises a nice set of questions and a way to analyse them.”

 

Photo: Turbulent times: turbulence could cause motile phytoplankton to come together. (Courtesy: Michael Wilczek, Marco Mazza and colleagues)

View original article at: Turbulence causes swimming algae to congregate in dense patches, say physicists

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A dramatic seaweed invasion has hit coastlines across Florida and the Caribbean, killing wildlife

Shorelines around Florida and the Caribbean have been choked with invaders over the past month. No, it’s not tourists — it’s seaweed.

Sargassum seaweed, which originates in the Gulf of Mexico and is actually a type of algae, has been washing up on beaches and coastlines in vacation-heavy hotspots like Miami and Cancun since July.

The weeds have wreaked havoc on local fauna, choking coral reefs and destroying habitats for birds, sea turtles, and fish. The seaweed deluge has also made life difficult for fisherman, since it is capable of wrecking boat propellers, fishing nets, and engines.

Sargassum seaweed is usually pushed by currents into the Sargasso Sea — a large gyre off the coast of North America — where the floating mats serve as an important habitat for marine organisms.

Researchers are struggling to figure out why the weeds have started washing up on Caribbean coastlines. Some experts say the influx of Sargassum could be fueled by a combination of increased nitrogen pollution from agricultural runoff and rising ocean temperatures, according to The New Republic.

The first Sargassum invasion in the Caribbean was recorded in 2011, according to the BBC.

In some extreme cases, resorts have had to close beaches during the busy summer season to remove the seaweed. Here’s what the invasion looks like:

Sargassum algae is pictured along Punta Piedra beach in Tulum, Mexico on August 11, 2018. REUTERS/Israel LealThe most recent invasions began in July, and experts say they may last through September.

A boat floats on the water, surrounded by Sargassum in Bahia La Media Luna, Mexico on August 5, 2018. AP Photo/Eduardo Verdugo

The island of Barbados declared a national emergency in August because of the seaweed invasion.

Workers clear Sargassum algae along Punta Piedra beach in Tulum, Mexico on August 11, 2018. REUTERS/Israel Leal

The seaweed can pile up to 7 meters thick (over 22 feet) on coastlines.

Sargassum washes along the shores of Sunny Isles Beach in South Florida on July 11, 2018. AP Photo/Brynn Anderson

“We’ve had mass mortality of sea turtles that have gotten trapped under ever-thickening piles,” Hazel Oxenford, a Barbados-based fisheries biologist at the University of the West Indies, told The New Republic. “When the turtles try to come up for air, they drown.”

In its natural habitat in the Sargasso Sea, the floating algae provides a habitat for fish and crustaceans, which seabirds and sharks then feed on.

Beachgoers swim with seaweed on July 11, 2018, in Sunny Isles Beach, Florida. AP Photo/Brynn Anderson

Researchers struggling to understand these Sargassum blooms have said more research is needed, especially into the role of nitrogen pollution and ocean acidification.

Beachgoers make their way through seaweed on Wednesday, July 11, 2018, in Sunny Isles Beach, Florida. AP Photo/Brynn Anderson

“The issue is that we never know what it’s going to be like — we can have a week or two weeks where it’s very clear and then all of a sudden overnight it washes in,” Larry Basham, chief operating officer of Elite Island Resorts, told the BBC.

Children play on the beach full of Sargassum in Bahia La Media Luna, Mexico on August 5, 2018. AP Photo/Eduardo Verdugo

“It’s yet another man-made problem that’s been thrown at the Caribbean that isn’t our doing,” Oxenford told The New Republic.

Beachgoers pick their way past seaweed on July 11, 2018, in Sunny Isles Beach, Florida. AP Photo/Brynn Anderson

 

Photo: In this Sunday, Aug. 5, 2018 photo, children play on the beach full of sargassum in Bahia La Media Luna, near Akumal in Quintana Roo state, Mexico. AP Photo/Eduardo Verdugo

View original article at: A dramatic seaweed invasion has hit coastlines across Florida and the Caribbean, killing wildlife

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