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Biological oceanography

Small currents influence small creatures

Lévy, M., Franks, P. J., & Smith, K. S. (2018). The role of submesoscale currents in structuring marine ecosystems. Nature communications, 9(1), 4758.

Marine phytoplankton, the microorganisms that photosynthesize in the ocean and form the base of the marine food web, live in an extremely complex, dynamic environment. Most of these tiny creatures are at the whim of the water they live in:  they might sink, float, or swim small distances, but generally drift with currents. How and why these organisms grow is directly influenced by the movement of the ocean. Scientists have long sought to understand the physical and environmental influences currents have on plankton.

Figure 1 – An aerial image of submesoscale plankton filaments in the Gulf of Aden (adapted from Lévy et al., 2018)

There is increasing evidence that planktonic life is shaped by submesoscale currents, features about 1-10 kilometers in size. These small vortices and fronts stir the water locally, unlike the major ocean currents that form the global overturning circulation. Submesoscale structures can move nutrients toward the surface causing blooms of plankton. They can also stretch and stir existing plankton populations, sometimes driving them out of the sunny upper ocean (fig. 1). But these features are extremely hard to study; they are barely visible from satellites and below the resolving power of existing computational models. Frustratingly, they also form and dissipate quickly and unpredictably, also making them a challenge to study from a vessel.

The oceanographic community is actively debating how influential the submesoscale is on plankton populations. Scientists are approaching the problem with a range of techniques and a variety of backgrounds. In an effort to collate existing research and get everyone speaking the same language, Dr. Marina Lévy and her colleagues reviewed the state of the field. Moreover, they proposed a framework for conceptually sorting the ways in which plankton might be influenced by submesoscale features.

Lévy argued that the effect of submesoscale features on plankton can be broken down into active, passive, and reactive processes (fig. 2). An active process alters the environment in some way favorable or detrimental to the growth of a plankton population. A small front might bring nutrients closer to the surface, allowing more plankton to grow. The same front might suck a different population away from the surface, making it hard for a population to thrive.

Figure 2 – A schematic of the three processes. On the left, two small vortices passively stretch and mix two populations. In the middle, a front brings the nutrient rich purple water closer to the surface. The right most image shows different species collecting at a patch of plankton (Adapted from Lévy et al., 2018).

A passive process does not alter the biomass of plankton, instead rearranging existing populations. Small vortices on the surface might stretch a patch of plankton into filaments. Likewise, processes in the ocean interior could stretch a population vertically, forming layered regions of high biological activity.

Changes in the plankton population have a cascading effect on other parts of the ecosystem. These reactive processes could include changes in the predator-prey relationships between larger zooplankton and their phytoplanktonic food. Larger organisms like fish and whales have also been shown to collect at plankton patches on these smaller size scales.

Many studies have been done to examine these processes. But the influence of submesoscale ocean movement on plankton remains relatively obscured – even with powerful computational models, satellite imaging, and advanced in situ observation methods. Indeed, the authors stress that it is hard to even estimate the effect such dynamics have on global scale biogeochemical cycles. Lévy hopes that this conceptual framework can help guide scientists as try to tease apart these complex questions.

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