A mathematical expression is proposed to describe the relationship between the photosynthetic capacity (P,,,,) of natural assemblages of motile benthic diatoms and temperature. Experiments were performed in 2 different seasons to document the response of P,,,, to a rapid increase in temperature (3°C h-'). In both cases, there was a progressive increase in P,,,,, up to an optimum temperature (Top,). beyond which P,,,,, declined rapidly. Top, did not change from September to December 1995, but the maximum photosynthetic capacity (PMAx, i.e. P,,,,, at Top,) was twice as high in September as in December

No abstract

In the present paper, we list and document the relevant behavioral and physiological processes controlling primary productivity of epipelic microalgae on intertidal mudflats in order to develop a simplified model. We first propose, in an attempt to characterize the 'photosynthetically active biomass' of the epipelic community, a new approach to describe the photic environment at the sediment surface, by substituting a discrete 2-layer model in place of continuous vertical light distribution. This concept thus allows us to build a functional representation of the distribution of the photosynthetically active biomass in the sediment and, by then integrating the light and temperature forcing of the latter biomass, to predict the dynamics of the whole epipelic community on short-term time scales. The model then clearly reveals an oscillatory behavior of the total biomass at the surface of the sediment, consistent with field biomass daily measurements performed during a spring-neap tidal cycle. Biomass increases occur during the diurnal emersion phase, in alternation with decreases during the submersion and nocturnal emersion phases; the dynamics of intertidal microphytobenthos is thus controlled by the night/day cycle and tidal hydrodynamic forcing, which determines the fast changes in environmental conditions (light and nutrient availability, temperature) which epipelic microalgae experience. The similarity between model simulations and field observations leads us to conclude that the conceptual framework of the model is valid.

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.