Temporal patterns of benthic microalgal migration on a semi-protected beach

Short-term variability in microphytobenthic biomass and production can be attributed primarily to the migratory rhythms of benthic microalgae (BMA). The majority of BMA vertical migration studies have focused on intertidal mudflats, whereas little is known about the effects of grain size or wave energy on migration patterns. The goals of our study were (1) to describe vertical migration patterns of BMA in beach sediments, (2) to test for seasonal variation in migration patterns, and (3) to identify the dominant taxonomic groups involved in these migrations. Sediment cores were collected approximately monthly from February 2002 until January 2003 from a semi-sheltered, sandy beach. On each date replicate cores were taken at three times, corresponding with changing tidal levels (after aerial exposure at ebb tide, at slack low tide, and after tidal immersion), and were vertically sectioned. Environmental parameters including temperature, salinity, wind speed and direction, and light intensity were also recorded. BMA were dominated by diatoms at all times and at all sediment depths. We found three distinct patterns of temporal shifts in BMA: (1) migration, in which a surface chl a maximum was replaced by a subsurface chl a max after tidal immersion, (2) no shift, wherein vertical profiles from low tide through periods of immersion were unchanged, and (3) removal, in which a loss of surficial chl a occurred after tidal flooding, without a corresponding chl a increase at depth. Logistic regression analysis revealed no relationship between patterns of temporal shifts in BMA and temperature or salinity. The relationship with wind direction was significant, with chl a removal associated with an onshore (i.e., east or northeast) wind. There was no correlation between wind speed and vertical distribution pattern. Wind-induced wave energy, dominated by fetch rather than wind speed, appeared to be a major factor in the resuspension process at our study site. Integrated (top 5 mm) chl a concentration also seemed to be related to vertical dynamics of BMA, with the no-shift pattern associated with highest surface chl a levels.