MEPS 259:103-115 (2003)  -  doi:10.3354/meps259103

ABSTRACT: Mechanisms and consequences of sediment resuspension are well studied in coastal environments, but the associated resuspension of micro-organisms is poorly understood. Using an in situ flume, and by sampling the benthic boundary-layer during tidal cycles, we measured resuspension of protists and sediment at a subtidal, sandy site in Buzzards Bay, Massachusetts, USA, and compared it to previous studies at a silty site in the bay with similar current magnitude. At the sandy site, where the bottom shear stress typically is greater on flooding than on ebbing tides, resuspension occurred only with the flooding current, whereas at the silty site, where the bottom stress is relatively similar on flooding and ebbing tides, resuspension occurred during both flood and ebb. For sediment and most protistan taxa at the sandy site, erosion thresholds were higher and the total material resuspended in strong flow was lower than at the silty site. At both sites, organic-rich, flocculent surficial sediment resuspended in weak flow, but the composition of the protistan community associated with it differed. At the sandy site it included heterotrophic nanoflagellates (HNan), euglenoid flagellates, oligotrich ciliates, and scuticociliates, whereas at the silty site it included HNan, oligotrich ciliates, and the diatom Navicula distans. Differences in the resuspension thresholds of ciliate groups between sites were due largely to differences in taxonomic composition, whereas differences for diatoms were linked to sediment dynamics. At both sites, protistan taxa resuspended in sequence as flow accelerated, implying that the assemblage of species exchanged between benthic and planktonic communities depends on the maximal bottom stress. At the silty site, however, the composition of this group should be more sensitive to variations in tidal strength (e.g. during the spring-neap cycle) than at the sandy site. Resuspension of protists may have strong impacts on microbial food-web ecology in the water column and sediments by linking the communities and by creating strong fluctuations of cell concentrations (measured up to 24x in the boundary-layer in the case of oligotrichs). Tidal resuspension of protists is common in the subtidal of Buzzards Bay, but its dynamics depend on local sediment properties, hydrography, and taxonomic composition.

KEY WORDS: Resuspension · Protists · Benthic flume · Erosion threshold · Buzzards Bay