What are plankton?
The word “plankton” comes from the Greek word “planktos” which means drifter or wanderer. Plankton are organisms that are unable to swim against ocean currents. They are an incredibly diverse group of organisms. Plankton vary in size from single-celled organisms 1/1000th of a millimeter, to multicellular organisms as large as a meter.
Plankton are often divided into two main types: phytoplankton and zooplankton. Phytoplankton are the primary producers – the base of the food web. Like plants on land, phytoplankton use sunlight to photosynthesize, converting carbon dioxide into organic compounds that other organisms eat. This process helps to regulate the planet’s climate by taking up large amounts of carbon dioxide from the atmosphere and facilitating its storage in the ocean.
There are many different types of single-celled phytoplankton including “diatoms” which have delicate glass shells, “coccolithophorids” which are covered in tiny “plates” made of calcium carbonate, and “dinoflagellates” which have two whip-like flagella they use to maneuver.
Unlike phytoplankton, zooplankton cannot photosynthesize. Instead, like us, they must eat other plankton. Zooplankton include many different types of organisms, such as krill, fish and crab larvae, and jellies. Krill, which look like small shrimp, are important food for whales. Copepods, another zooplankton, are crustaceans that are among the most abundant animals on the planet, and are prolific grazers on phytoplankton.
Planktonic food webs are complex, with many steps. Studying plankton ecology is important for understanding all life in the ocean and how carbon cycles through it.
Our global oceans are not static bodies of water – they are constantly in motion. Because plankton are small and unable to swim against currents, studying how waters move is essential to understanding how plankton live.
What is turbulence?
Turbulence is around us all the time and occurs in both air and fluids on earth as well as on other planets (think of Jupiter’s swirling storms). If you were to illuminate the air in a room with a laser in just the right way, you would see the air swirling around you. We experience it in our everyday lives when we turn a faucet on high or light an incense stick: the smooth straight part of the smoke plume is an example of laminar flow, and then it starts to wave, turning into swirling turbulence at the end.
Mathematically speaking, turbulence is extremely complicated. In fact, there is a million-dollar prize to whoever can solve the Navier-Stokes equations, the set of equations that analytically describe turbulent flow! Because it is so important for many fields of study, simplified equations are used to describe turbulent flow in science and engineering.
Turbulence is a type of fluid flow where both velocity and vorticity are irregularly distributed in both space and time. Turbulent flow is characterized by eddies and vortices of irregular velocity fluctuations, with varying speeds and changes in direction and size. Swirls and eddies get smaller and smaller as energy is dissipated as heat.
Turbulence is found throughout the ocean, from millimeters to hundreds of kilometers. Measurements of turbulence are collected by physical oceanographers and used for many applications in oceanography.
Biological oceanographers study life in the ocean – the distribution, abundance, and production of marine organisms – as well as how the physics, chemistry, and geology of ocean systems interact with the ocean’s life.
The movement and mixing of water in the ocean, and the life in these waters is deeply intertwined with our global climate, carbon cycle, and atmospheric conditions. The most abundant and important type of marine life are the tiny plankton; they play a central role in supporting life on earth.
Imagine you are a plankter...
For an individual plankton, encountering turbulence in the ocean may be one of the most significant events in its life. Plankton are primarily concerned with eating, not being eaten, and mating. All these significant life events require contact with another organism. The turbulent movement of water around the plankton could impact any of these life changing events.
Let’s consider the copepod. A female copepod leaves a chemical trail for a male copepod to find her and mate – turbulence could break up this trail and make it impossible for the male to find the female. Larval fish feed on copepods. Turbulence may “blow” a copepod away from the fish as it’s preparing for its attack. A copepod may also mistake turbulence for a predator and jump away from it, expending unnecessary energy.
There has been a great deal of research investigating the impacts of oceanic turbulence on plankton reproduction, grazing, predation, and other interactions. Many of these studies have been done in laboratory experiments that subject the plankton to experimentally generated turbulence.
A recent paper, “Oceanic Turbulence from a Planktonic Perspective” co-authored by Scripps scientists Prof. Peter Franks, Dr. Bryce Inman, Prof. Jennifer MacKinnon, Prof. Matthew Alford and Dr. Amy Waterhouse takes a different approach. They took a planktonic perspective to understanding oceanic turbulence. By comparing real turbulence measured in the ocean to turbulence in laboratory experiments, they found that turbulence in the laboratory experiments was far, far stronger than found in the ocean. Furthermore, the authors argue that the methods of measuring and simulating turbulence does not accurately capture the structures of turbulent flows experienced by a plankter.
This disparity opens the questions of how, and if, plankton experience turbulence in the ocean. The authors suggest, “[t]o better understand the importance of turbulent processes to planktonic dynamics, we must develop a new framework for understanding how oceanic turbulence is experienced by plankton.”
Taking a planktonic perspective allowed these biological oceanographers to ask different questions and gain new perspectives their research.