Taxon-specific tidal resuspension of protists into the subtidal benthic boundary layer of a coastal embayment

Abstract

Sediment resuspension has widespread effects on microbial processes, primary and secondary production, and nutrient cycles, but its influence on protists other than microalgae is largely unknown. Distributions and abundances of protists in subtidal benthic boundary layers (BBL), in particular, are poorly known. We measured vertical profiles of protists in the BBL and underlying sediment at a subtidal silty site in Buzzards Bay, Massachusetts USA, to determine cell-resuspension patterns. Tidal flow produced maximal bottom shear velocities of 1.4 to 2.2 cm s-1. Near-bottom turbidity increased during each slack tide, when the suspended load settled, and it decreased during tidal exchange, presumably after a thin veneer of sediment resuspended from the sediment-water interface (SWI) and mixed into the upper water column. Tidal periodicities in protistan vertical profiles were taxon- and functional-group specific. Heterotrophic nanoflagellates (HNan) and ciliates, including scuticociliates, oligotrichs, and hypotrichs of the genera Euplotes and Urostrongylum, showed periodicities in distribution consistent with cycles of resuspension and deposition. BBL concentrations of HNan and scuticociliates were elevated during tidal exchange by factors of ≤2.1 and 4.6, respectively, within 5 cm of the SWI; oligotrichs were found consistently in the BBL but were in the sediment only during slack tide; Euplotes was present consistently in the sediment but was in the BBL only during tidal exchange. Total resuspended cells in the bottom 1 m were of the order 108 to 109 HNan m-2 and 105 to 106 ciliates m-2, and in some cases the measured cell disappearance from surficial sediment during tidal exchange balanced the increase in the BBL. In contrast, pigmented nanoflagellates, pennate diatoms, and ciliates, including karyorelictids and other hypotrichs, maintained constant profiles throughout tidal cycles. Specificity of results among protistan groups might be due to behavioral adaptations such as depth zonation in the sediment, associations with particles, and vertical migration. We know of no other documentation in the field of cyclical emergence of heterotrophic protists and re-entry into sediment. Our data suggest complex taxon-specific linkages between sedimentary and water-column protistan communities that may be controlled by flow in the BBL, potentially influencing food-web dynamics.