The speed of diatom migration through natural and artificial substrata

The speed of diatom locomotion during cell migration was investigated in laboratory experiments using natural assemblages of intertidal diatoms inhabiting the upper intertidal mudflat at Portishead on the Severn Estuary in winter. Persistent vertical migration rhythms were verified for the epipelic diatom assemblages in the field and the migratory behaviour was successfully maintained in a tidal laboratory experimental system. No migratory behaviour was apparent for salt marsh assemblages at the same site and a brief investigation indicated that this behaviour was not overtly related to the water content of the sediments. The dominant diatom on the intertidal mudflat at the time of the investigation was Gyrosigma spencerii (W. Smith) Cleve which was used for the investigation of locomotion through sediments. Initial measurements of surface progression were made which compared well with values taken from the literature. G. spencerii was found to move over an artificial sediment (kaolin) at an average speed of 4.7 μms⁻¹. Migration speed through, rather than over, kaolin and natural sediment was examined by deposition of kaolin and sediment, free from diatoms, onto the surface of diatom assemblages inhabiting cores of natural sediment maintained in the laboratory. Determination of the thickness of the deposited layer by freeze-fracture techniques allowed the calculation of the rate of upward movement. The results suggest much lower locomotion speeds for diatoms moving through substrata than on horizontal surfaces. Vertical migration of G. spencerii was found to be 25 times slower than surface speeds over kaolin and natural sediment with average values of 0.17 and 0.19 μms⁻¹, respectively. Diatom species showed a synchronous emergence at the surface of the sediments with G. spencerii first to appear. The significance of these findings to diatom microspatial vertical distribution within sediment, endogenous rhythmic behaviour, frustule streamlining and energy budgets are discussed.