Simulating motile algae using a mixed Eulerian-Lagrangian approach: does motility promote dinoflagellate persistence or co-existence with diatoms?

Dinoflagellates are characterized by low maximum photosynthetic rates and high respiratory costs.Recent evidence also suggests that dinoflagellates are disproportionately abundant in the diets of many copepods.This suggests that at least some species are preferred prey types.This begs the question: 'How do dinoflagellates co-exist with other, seemingly competitively superior, algal taxa'?Their motility may enable them to maintain position better in the light-rich surface waters, and more successfully make the return journey into deeper waters in order to replenish their internal nutrient stores when surface nutrients become depleted.This theory is examined using a depthresolved model of algal dynamics.Nutrients and organic detrital matter are represented on a Eulerian grid, whilst a Lagrangian approach is used to represent dinoflagellates and diatoms.The model indicates that strictly autotrophic dinoflagellates have difficulty balancing their metabolic budgets.Even in the absence of competition from diatoms, motility is almost essential to dinoflagellate persistence.Only extremely motile dinoflagellates are able to compete successfully with diatoms.In reality, an increasing number of dinoflagellate species are being found to be mixotrophic.This model suggests that mixotrophy may often be an obligatory, rather than optional, behaviour.