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 microorganisms concentrations. During a rising tide, sediment erosion under waves action occurred over the lower and upper part 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 2 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 waters mixing was immediately buffered, probably to the benefit of grazers.

Impact of Chronic and Massive Resuspension Mechanisms on the Microphytobenthos Dynamics in a Temperate Intertidal Mudflat

“…Waves and current BSS can also shape the spatial distribution of P. ulvae individuals on the mudflat (Armonies & Hartke, ) and so the local grazing pressure on MPB developing in the sediment. In the model, the lack of grazing pressure on MPB cells resuspended in the water column may result into an overestimation of the simulated resuspended MPB biomass (Guizien et al, ). The intensive oyster farming in the Brouage mudflat vicinity can result into a substantial removal of the resuspended MPB biomass (Smaal & Zurburg, ).…”

Abundance and Distribution of Enteric Bacteria and Viruses in Coastal and Estuarine Sediments—a Review

“…The input of freshwater into estuarine systems could reduce the efficacy of these bridges, releasing pathogenic bacteria such as V. cholerae from sediment into the water column (Kierek and Watnick, 2003). Guizien et al (2014) found that bacterial numbers decreased due to grazing and that viral titre in the water column did not significantly increase due to re-settlement of virus-clay complex, suggesting a complex story governing resuspension of enteric microorganisms. The risk associated with the sediment of enteric microorganisms depends on concentration, the ease with which bacteria resuspend or release back into the water column, and the frequency with which this will occur (Cox et al, 2005).…”

Microbial interactions in marine water amended by eroded benthic biofilm: A case study from an intertidal mudflat

“…In the water column, the dynamics of benthic viruses may be globally governed by the trade-off between their irreversible and non-infective adsorption onto benthic-suspended matter (Suttle and Chen, 1992) and their multiplication at the expense of more active attached bacteria (Kernegger et al, 2009;Riemann and Grossart, 2008). The abundance of HNF depends directly on the resuspension of their benthic representatives and would also, in the case of muddy sediments, indirectly benefit from the increase of their prey (bacteria and pico-, nanoautotrophs), which benefit from the input of benthic nutrients (Garstecki et al, 2002;Guizien et al, 2013). Thus, the microbial interactions include HNF bacterivory, HNF virivory (Bettarel et al, 2005), viral lysis of bacteria, HNF or viruses and coincidental HNF predation of infected bacteria (Miki and Jacquet, 2008).…”