How Migrating Swimmers Generate Eddies in Stratified Water

Countless tiny aquatic organisms in oceans and lakes undertake synchronized daily movements that create significant water disturbances. These collective journeys, involving trillions of animals, can mix layered waters, influencing the distribution of nutrients and the foundation of aquatic food webs. This exploration delves into the lives of these migrating swimmers, the structured aquatic environments they inhabit, and the process by which their passage generates swirling eddies.

The Daily Journey of Aquatic Swimmers

Every day, in the world’s oceans and deep lakes, one of the largest migrations on the planet occurs, largely unseen. This phenomenon, known as Diel Vertical Migration (DVM), involves a massive movement of animal life from the deep, dark waters up to the surface and back again over a 24-hour cycle. The participants are incredibly diverse, ranging from microscopic zooplankton like copepods and krill to larger organisms such as small fish and jellyfish.

The primary motivation for this commute is a trade-off between feeding and safety. The sunlit surface waters are rich with phytoplankton, the base of the aquatic food web. At night, under the cover of darkness, these migrating swimmers ascend to this bountiful layer to feed when the risk from visual predators is low. As dawn approaches, they descend hundreds of meters into the darker, safer depths.

This daily trek is a highly synchronized, collective behavior involving immense populations. The sheer biomass on the move creates what is sometimes called a “deep scattering layer” that can be detected by sonar. This coordinated movement between distinct water layers is a fundamental process that sets the stage for significant physical changes in their environment.

Layered Waters: The Stratified Column

The aquatic environments where these migrations occur are often structured into distinct layers, a condition known as stratification. This layering acts like invisible barriers within the water, preventing easy mixing between different levels. The formation of these layers is governed by differences in water density, driven by temperature or salinity.

In many lakes and oceans, sunlight warms the surface, creating a warmer, less dense top layer called the epilimnion. Below this lies a transitional zone, the metalimnion, where temperature drops rapidly with depth. The region of steepest temperature change is known as the thermocline. The deep, cold, and dense water at the bottom is the hypolimnion.

In estuaries and parts of the ocean, salinity plays a major role. Freshwater flowing from rivers is less dense than salty ocean water, creating a distinct layering effect known as a halocline. Any sharp change in density is called a pycnocline, and these boundaries inhibit the transfer of nutrients and dissolved gases between layers.

Stirring the Waters: How Swimmers Generate Eddies

As vast aggregations of swimmers move upwards or downwards, they displace water and create turbulence in their wake. This collective disturbance generates swirling, rotating bodies of water known as eddies. The process is a direct consequence of fluid dynamics, where the movement of a dense group of many small bodies through a fluid initiates motion in that fluid.

The scale of these eddies is directly related to the size of the migrating group. An “aggregation-scale” eddy means the vortex created is proportional to the dimensions of the swarm. When millions of these organisms move in unison, their individual wakes combine and amplify, creating a larger rotational flow than any single creature could produce. This biologically generated turbulence is a form of “biomixing.”

The pre-existing stratification of the water column heavily influences these eddies. The sharp density boundaries of a thermocline or pycnocline can act as a surface, causing the eddies to flatten and spread horizontally. This interaction can make the eddies more persistent, allowing them to last longer and travel farther than they would in unstratified water. The swimmers’ movement injects energy into the water, and stratification helps organize that energy into coherent, swirling structures.

The Ripple Effect: Significance of Biologically Generated Eddies

The eddies generated by migrating swimmers have significant ecological consequences. By creating localized turbulence, this biomixing plays a part in breaking down the barriers of stratification. This process facilitates the transport of materials between layers that would otherwise remain separate. For instance, eddies can help move nutrient-rich water from the deep up into the sunlit surface zone.

This upward transport of nutrients, such as nitrates and phosphates, can fuel the growth of phytoplankton, the foundation of most aquatic food webs. Enhanced productivity supports a greater abundance of zooplankton and the animals that prey on them. The mixing also works in reverse, carrying dissolved oxygen from the surface down to deeper waters, including oxygen minimum zones (OMZs).

Scientists study this phenomenon using a combination of methods, including acoustic instruments that track zooplankton aggregations and sensitive probes that measure micro-scale turbulence. These observations are often paired with computer models to simulate the fluid dynamics and biogeochemical impacts. Understanding this biologically driven mixing is important for creating accurate models of ocean productivity and global carbon cycles.