MEPS 509:113-126 (2014)  -  DOI:

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

R. G. Macdonald1,*, D. G. Bowers1, D. McKee2, G. W. Graham3, W. A. M. Nimmo-Smith3

1School of Ocean Sciences, Bangor University, Menai Bridge, Anglesey LL57 1TF, UK
2Department of Physics, University of Strathclyde, Glasgow G4 0NG, UK
3School of Marine Science and Engineering, Plymouth University, Plymouth PL4 8AA, UK
*Corresponding author:

ABSTRACT: 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.

KEY WORDS: Chlorophyll a · Diel vertical migration · Tidal channel · Phytoplankton particle size · Menai Strait

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Cite this article as: Macdonald RG, Bowers DG, McKee D, Graham GW, Nimmo-Smith WAM (2014) Vertical migration maintains phytoplankton position in a tidal channel with residual flow. Mar Ecol Prog Ser 509:113-126.

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