Thin layers (oceanography)

Thin layers are concentrated aggregations of phytoplankton and zooplankton in coastal and offshore waters that are vertically compressed to thicknesses ranging from several centimeters up to a few meters and are horizontally extensive, sometimes for kilometers.

The precise values for critical thresholds of thin layers has been debated for a long time due to the vast diversity of plankton, instrumentation, and environmental conditions.

[2] These extraordinary concentrations of plankton have important implications for many aspects of marine ecology (e.g., phytoplankton growth dynamics, zooplankton grazing, behaviour, environmental effects, harmful algal blooms), as well as for ocean optics and acoustics.

[4] A study on the Karenia brevis algae responsible for more recent and increasingly longer red tide blooms shows that the cellular gene expression patterns are extremely diverse which means that this particular species of plankton are more resilient because they adapt well to changing conditions.

[7] Thin layers of non-motile phytoplankton tend to collect at boundaries of strong vertical gradients in salinity (haloclines), temperature (thermoclines), and density (pycnoclines) which often coincide because they are directly proportional.

[7] These particular thin layers are formed by sinking non-motile phytoplankton reaching a neutral buoyancy at a pycnocline, and the stifling of vertical turbulent dispersion at these depths.

Another mechanism specific to dinoflagellates is called helical klinotaxis where the algal cell's ability to respond to both positive and negative chemosensory signals is crucial to their motility.

If dinoflagellates were not capable of both positive and negative chemotaxis, they would not navigate successfully due to the nature of the transverse and longitudinal flagella causing rotating and translating motions, respectively.