Wave effects on the vertical migration of two benthic microalgae: Hantzschia virgata var. intermedia and Euglena proxima

Two benthic microalgal species, Euglena proxima (Dangeard) and the diatom Hantzschia virgata var. intermedia (Grun) Round, congregated at the sediment surface of intertidal sand flats surrounding the Duke University Marine Laboratory in Beaufort, North Carolina, during low tide and migrated below the surface prior to tidal flooding. E. proxima was found down to a depth of 12 cm below the surface during high tide while H. virgata was not found below 8 cm. One potential advantage of vertical migration for benthic microalgae is avoidance of the sediment surface during high tide when the winnowing action of waves can remove cells from the intertidal habitat. This hypothesis was tested by comparing the mean depths of E. proxima and H. virgata exposed to calm water conditions with their mean depths under wave-exposed conditions in shallow outdoor tidal tanks equipped with wave generators. Two sediment treatments were tested in each tank to evaluate the effect of a subsurface layer of black sediment (subsurface sediments, presumably reduced) on vertical migration: a uniform column of light brown sand (surface sediments; presumably oxygenated), and a layered column with light brown sand overlying a layer of black sediment. One-way and two-way ANOVAs indicated that wave action increased the mean depth of both species under certain conditions related to the presence, absence, and depth of a subsurface layer of black sediment. In an experiment with a shallow black layer (5 mm), wave action did not result in a significant increase in the mean depth of H. virgata as it did in a previous experiment with a deeper black layer (3 cm). Unlike H. virgata, E. proxima exhibited a wave effect only in the absence of a subsurface black layer. A subsurface black layer seemed to reinforce downward migration in E. proxima under both calm water and wave conditions. The absence of a wave effect in the presence of a subsurface black layer suggests that the mean depth of E. proxima during high tide was more tightly controlled by the depth of the black layer than wave energy. *** DIRECT SUPPORT *** A01BY085 00009