Microorganism dynamics in a mudflat during rising tide Microorganism dynamics during a rising tide: Disentangling effects of resuspension and mixing with offshore waters above an intertidal mudflat
Abstract
Resuspension of microphytobenthic biomass that builds up during low tide has been acknowledged as a
major driver of the highly productive food web of intertidal mudflats. Yet, little is known about the contribution
to pelagic food web of the resuspension of other microorganisms such as viruses, picoeukaryotes,
cyanobacteria, bacteria, nanoflagellates, and ciliates, living in biofilms associated with microphytobenthos
and surficial sediment. In the present study, a novel approach that involves simultaneous Lagrangian and
Eulerian surveys enabled to disentangle the effects of resuspension and mixing with offshore waters on the
dynamics of water column microorganisms during a rising tide in the presence of waves. Temporal changes
in the concentration of microorganisms present in the water column were recorded along a 3 km cross-shore
transect and at a fixed subtidal location. In both surveys, physical and biological processes were separated by
comparing the time-evolution of sedimentary particles and microorganism concentrations. During a rising
tide, sediment erosion under wave action occurred over the lower and upper parts of the mudflat, where
erodibility was highest. Although erosion was expected to enrich the water column with the most abundant
benthic microorganisms, such as diatoms, bacteria and viruses, enrichment was only observed for
nanoflagellates and ciliates. Grazing probably overwhelmed erosion transfer for diatoms and bacteria,
while adsorption on clayed particles may have masked the expected water column enrichment in free viruses
due to resuspension. Ciliate enrichment could not be attributed to resuspension as those organisms were
absent from the sediment. Wave agitation during the water flow on the mudflat likely dispersed gregarious
ciliates over the entire water column. During the rising tide, offshore waters imported more autotrophic,
mainly cyanobacteria genus Synechococcus sp. than heterotrophic microorganisms, but this import was also
heavily grazed. Finally, the water column became a less heterotrophic structure in the subtidal part of the
semi-enclosed bay, where mixing with offshore waters occurs (50% decrease), compared to the intertidal
mudflat, when resuspension occurs. The present study suggests that this differential evolution resulted predominantly
from dilution with offshore waters less rich in heterotrophic microorganisms. Indeed, any input
of microorganisms accompanying physical transfers due to bed erosion or offshore water mixing was immediately
buffered, probably to the benefit of grazers.
Origin | Files produced by the author(s) |
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