No abstract

Resuspension of the top few sediment layers of tidal mud flats is known to enhance planktonic biomass of microbiota (benthic dlatoms and bacteria). This process is mainly controlled by tidal shear stress and cohesiveness of mud, and 1s also influcnced by bioturbation activities. Laboratory experiments in a race track flume were performed to test the interactive effects of these factors on both the critical entrainment and resuspension kinetlcs of microbiota from sllt-clay sediments from the hlarennes-01eron Bay, France. The marine snail Hydrobia ulvae was used to mimic surface bioturbation activities. As expected, the klnetics of microbial resuspension versus shear stress were largely controlled by the cohesiveness of s~lt-clay sediments. However, our results indicate that the rffect of surface tracking by H. ulvae on microbial resuspension was clearly dependent on the Interaction between sediment cohesiveness and shear velocity. Evidence was also found that microphytobenthos dnd bacteria are not simultaneously resuspended from slit-clay bioturbated sediments. Thls supports the theory that diatoms within the easily eroded mucus matrix behave actively and bacteria a d h e r~n g to fine silt particles eroded a t higher critical shear velocities behave passively.

Chloropigments and carotenoids were measured by HPLC in an intertidal muddy sediment of Marennes-Oleron Bay (France). Concentrations were determined as a function of s-ent depth at low tide. The analyses were carried out at monthly intervals over l yr. Pigment analindicated that the microphytobenthic community was dominated by diatoms throughout the sampling year. Chlorophyll b was not encountered at any time or depth, indicating that no input from macmphytic detritus had occurred at the sampling site. There was a large pool of phaeopigments a. d whlch phaeophorbides were the major forms (75%). A microphytobenthic bloom occurred between March and June during which phaeopigments significantly increased with a high proportion of phaeophorb~d e s likely due to intense grazing activ~ty of benthic invertebrates. P~gment concentrat~ons were still high at 5 cm depth and changes occurred simultaneously in the aphotic (<5 mm sediment depLh] and photic (>5 mm sediment depth) layers of the sediment, reflecting an impact of bioturbation and physical mixing (resuspension/redeposition) of the upper sediment layer. There was a decrease of the pigment content of the sediment between June and July. It was attributed to grazing and to a resuspension event. Overall, the pigment analysis of this intertidal sediment indicates a predominance of t h e diatom community, a close coupling between the dynamics of microphytobenthos biomass and the grazing activity of benthic invertebrates, and the influence of sediment mixing.

The distribution of chlorins and carotenoids in 22 sapropel samples from Ocean Drilling Program Leg 160, Holes 966D and 969C, have been examined by combined liquid chromatography-mass spectrometry under atmospheric pressure ionization conditions. Up to 42 components were detected over the range of samples (including chlorophyllone a, pyrophaeophorbides, pyrophaeophytins, steryl chlorin esters a and b, as well as carotenes). The samples showed two basic types of pigment distribution, with no obvious relationship between these distributions and the sedimentological parameters.