A curious phenomenon found in phytoplankton communities is the forming of so-called thin layers, wherein phytoplankton biomass can stretch out kilometres in the horizontal but only a few metres in the vertical. These layers are typically found at the pycnocline, just below the surface mixed layer. Thin layers are usually attributed to a range of complex environmental and species-dependent factors. However, we believe that, given the frequency at which this phenomenon is observed, a simpler mechanism is at play. In this study, we found that phytoplankton thin layers can be attributed simply to a decreasing light availability with depth, when there is an abundance of nutrients in the euphotic zone and below the mixed layer. This mechanism was ascertained using a number of modelling approaches ranging in complexity from analytical solutions of a simple 1-dimensional plankton model to a 3-dimensional biophysical model incorporating large-eddy simulation. The conditions which, according to the results of our study, allow thin layers to form are ubiquitous in the coastal ocean and are therefore a likely candidate explanation as to why planktonic thin layers are so frequently observed.
Bibliographical noteFunding Information:
Acknowledgements. This work was supported by the Korea Meteorological Administration Research and Development Program under grant KMI2018-07210. We are also grateful for support from the PycnMix project (NE/L003325/1). This work used the ARCHER UK National Supercomputing Service. We are extremely grateful to Prof. Peter Franks for important criticisms on an earlier version of the model, which guided the present study considerably. We are also thankful to Dr David Lewis for valuable discussions concerning the modeling work and to the 2 anonymous reviewers who provided constructive feedback, which helped to improve the manuscript significantly.
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All Science Journal Classification (ASJC) codes
- Ecology, Evolution, Behavior and Systematics
- Aquatic Science