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 h igh respiratory costs. Recent evidence also suggests that dinoflagellates a re disproportionately abundant in the diets of many copepods. This suggests that at least some species are preferred prey types. This begs the questio n: 'How do dinoflagellates co-exist with other, seemingly competitively sup erior, algal taxa'? Their motility may enable them to maintain position bet ter in the light-rich surface waters, and more successfully make the return journey into deeper waters in order to replenish their internal nutrient s tores when surface nutrients become depleted. This theory is examined using a depth-resolved model of algal dynamics. Nutrients and organic detrital m atter are represented on a Eulerian grid, whilst a Lagrangian approach is u sed to represent dinoflagellates and diatoms. The model indicates that stri ctly autotrophic dinoflagellates have difficulty balancing their metabolic budgets. Even in the absence of competition from diatoms, motility is almos t essential to dinofiagellate persistence. Only extremely motile dinoflagel lates are able to compete successfully with diatoms. In reality, an increas ing number of dinoflagellate species are being found to be mixotrophic. Thi s model suggests that mixotrophy may often be an obligatory, rather than op tional, behaviour.