Vertical migration maintains phytoplankton position in a tidal channel with residual flow

R. G. MacDonald, D. G. Bowers, D. McKee, G. W. Graham, W. A M Nimmo-Smith

Research output: Contribution to journalArticlepeer-review

103 Downloads (Pure)


A tidal channel can retain phytoplankton, despite a residual flow, if the phytoplankton migrate vertically with a daily rhythm. Tidal currents are slowed down by bed friction and so plankton experience faster flow when higher in the water column. The lateral movement of the plankton depends on the nature of the vertical migration, particularly the time spent near the surface and the phase of the tide. A model of this process accorded with observations of chlorophyll derived from in situ fluorescence at a mooring in a tidal channel. Peaks in chlorophyll at the end of the flood tide indicated the presence of a phytoplankton bloom downstream of the mooring. Peaks in chlorophyll at the ends of the morning flood tides were 3 to 4 times larger than at the ends of the evening floods, over several days. In contrast, well-mixed particulates were removed from the channel by the residual flow in just 2 d. Both the day-night asymmetry and the sustained presence of chlorophyll were explained by allowing for vertical migration of the phytoplankton and constraining the period during which they were near the surface. Tidal channels retaining phytoplankton that migrate vertically can be ecologically more diverse and yield higher commercial output of farmed bivalves. The natural timings of some phytoplankton blooms in tidal channels are controlled by the nature of the migration. Although a by-product of vertical migration, longer residence in the tidal channel affords the phytoplankton more nutrients than phytoplankton that advect offshore.

Original languageEnglish
Pages (from-to)113-126
Number of pages14
JournalMarine Ecology Progress Series
Publication statusPublished - 27 Aug 2014


  • chlorophyll a
  • diel vertical migration
  • Menai strait
  • phytoplankton particle size
  • tidal channel


Dive into the research topics of 'Vertical migration maintains phytoplankton position in a tidal channel with residual flow'. Together they form a unique fingerprint.

Cite this