Swimming helps phytoplankton avoid being carried by currents



Observations of an upwelling event in Monterey Bay, California, show that non-bioluminescent phytoplankton from the northern part of the bay are carried with the southward flowing currents along the entrance to the bay, but many bioluminescent dinoflagellates avoid such advection. It has been suggested that the vertical swimming ability of bioluminescent dinoflagellates to descend to deeper layers helped them avoid being carried away by currents. Shulman et al. tested that hypothesis using a tracer model in which the dinoflagellates population is modeled as a concentration, and vertical swimming speeds are introduced into the model. They show that a swimming speed of 20 meters per day, which is in the middle of the observed swimming velocity ranges for observed species of dinoflagellates, leads to only 40% of the population being advected with the current in comparison to the case without vertical swimming. This is in agreement with the observed ratio of mean bioluminescence intensity at the entrance to the bay to the observed mean intensity in the northern part of the bay. Thus, the model confirms that dinoflagellates with moderate vertical swimming speed could avoid being carried away by currents. (Journal of Geophysical Research-Oceans, doi:10.1029/ 2011JC007480, 2012)