Gérard F. Blanchard

Effect of production and biomass of intertidal microphytobenthos on meiofaunal grazing rates

Abstract Microphytobenthos dominate primary production in unvegetated intertidal habitats. Meiofauna are likely to be important consumers of this productivity, yet little is known about meiofauna herbivory in these environments. We studied meiofaunal grazing on microphytobenthos from an intertidal mudflat on the southwest coast of France. Meiofaunal grazing rates were measured in response to manipulations that varied microphytobenthos production and biomass to determine if meiofauna have functional responses to changes in food quality and quantity. Harpacticoid and ostracod grazing rates did not change in response to changes of microphytobenthos production, but nematode grazing rates did increase with increasing production. As microphytobenthos biomass increased, meiofauna responded with greater grazing rates, removing more biomass per unit time, yet taking longer periods of time to deplete (or clear) the microbial population. Therefore, meiofauna taxa exhibited functional responses to microphytobenthos as food, although different taxa had different responses. The impact of these responses in the laboratory could be seen in the field. For example, when chlorophyll a (Chl a) concentration in the top 1 cm of sediment increased from 31.7 mg · m−2 to 124 mg · m−2 2 wk later, the total community grazing rates increased from 32.9 ng Chl a · h−1 to 176 ng Chl a · h−1. Harpacticoid densities also increased 2.7 times, but nematode and ostracod densities remained the same. The results of this study support the idea that intertidal meiofauna, particularly harpacticoids, have a dependent relationship with their autotrophic food resources in intertidal habitats and can regulate their behavior to maximize intake of food.

The effect of geomorphological structures on potential biostabilisation by microphytobenthos on intertidal mudflats

The chlorophyll a and colloidal carbohydrate content of sediments were measured at Skeffling mud-hat in the Humber estuary, UK, in July 1997 as part of a fieldwork experiment carried out within the framework of the INTRMUD project. The aim was to analyse the spatial variations of Chi a and colloidal carbohydrate concentrations within the surface 1 cm of sediment (together with physical variables) in the different macroscopic sedimentary structures found at four stations along a cross-shore transect. The underlying assumption was that epipelic microalgae (Chl a) produce extra cellular polymeric substances (EPS), largely comprised carbohydrates, when migrating vertically at the sediment surface. This organic material binds sediment particles and thus contributes to enhance sediment cohesiveness/stability. Therefore, the shape and the strength of the relationship between Chi a and colloidal carbohydrates are fundamental for assessing the role of autotrophic microbial communities in biostabilisation processes. At station A, the highest level of the mudflat, there were no obvious sedimentary features, while a ridge (crest) and runnel (trough) system was present at mid-tidal stations (B and C), At station D, the sediment was sandier; crests and troughs were obvious but did not form a ridge and runnel system as at stations B and C, Taking all data together, a significant positive linear relationship between colloidal carbohydrates and Chi a was found, but analysing data separately by station indicated that there was no relationship between variables at the sandy station (D). At stations B and C, there was a difference in the Chi a-carbohydrate relationship between ridges and runnels: (i) there was no relationship in runnels, i.e. carbohydrates concentration was roughly constant whatever the mud Chi a content, and (ii) there was a positive linear relationship in ridges. This indicates that the increase of epipelic biomass on ridges increases the amount of EPS, which is likely to stabilise the sediment surface of these features. The biomass level in runnels is lower and does not enhance the amount of EPS. Therefore, the activity of epipelic microalgae in runnels does not contribute to sediment stability. This observed difference between ridges and runnels does not mean that epipelic microalgae from these two features necessarily behave in a different way; carbohydrates produced by microalgae in runnels are very likely to be dissolved because of the higher water content. Thus epipelic algae cannot build up a pool of carbohydrates in runnels. As a conclusion, it is clear that geomorphological features of intertidal mudflats influence biological processes in a way which exacerbates the physical processes: (i) ridges are regularly exposed and the sediment surface is stabilised, which apparently favours microphytobenthos growth and carbohydrates production with a further increase in sediment stability (according to our initial assumption); (ii) runnels are drainage structures with a high water content, which prevents microphytobenthos from building up a carbohydrate pool. Therefore, there seems to be a synergistic effect between physical and biological processes on ridges to stabilise the sediment surface. [KEYWORDS: intertidal mud flat; microphytobenthos; epipelic diatoms biostabilisation; Extra cellular Polymeric Substances (EPS); chlorophyll alpha Ems estuary; sediment; diatoms]

SEASONAL EFFECT ON THE RELATIONSHIP BETWEEN THE PHOTOSYNTHETIC CAPACITY OF INTERTIDAL MICROPHYTOBENTHOS AND TEMPERATURE1

The response of the photosynthetic capacity (Pmax) of microphytobenthos to short‐term variations of temperature (in the range 5–35° C) was assessed on a seasonal basis. The relationship is described mathematically, and relevant physiological parameters are identified: PMAX, the maximum value of Pmax achieved at Topl, the optimum temperature. Estimated values of Topt do not change significantly throughout the year and remain close to 25° C. It is thus concluded that Topt is not influenced by seasonal variations in the daily range of mud surface temperature. Identical conclusions hold for Tmax (ca. 38° C), the thermal threshold beyond which no photosynthesis occurs. Conversely, PMA estimates exhibit substantial variability: PMAX (mean ± root mean square error) is highest in April (11.18 ± 0.42 [μg C · [μg Chl a]−1· h−1) during the beginning of the annual increase in temperature, photoperiod, and maximum irradiance and is lowest in December (3.04 ± 0.16 μg C · [μg Chl a]−1· h−l). From an ecological point of view, the short‐term and seasonal variations of PMAX suggest that the microphytobenthic community takes advantage of the abiotic spring environmental conditions, allowing the onset of the bloom. Nevertheless, no “acclimation strategy” (i.e. shifts in Topt and Tmax that prevent temperature inhibition in summer or improve photosynthetic rates in winter) is apparent from our results.

Measurement of meiofauna grazing rates on microphytobenthos: is primary production a limiting factor?

Nematode and harpacticoid copepod grazing rates on microphytobenthos were measured in an oyster pond on the French Atlantic coast using a three-compartment model (sediment, microalgae and meiofauna). Undisturbed natural sediments were used in simulated in situ conditions. Total meiofauna grazing pressure (33.94 μg C·10 cm−2·h−1) slightly exceeded primary production (29.53 μg C· 10 cm−2·h−1). Microbiota-meiofauna interactions (e.g., feeding preference and the assimilation: ingestion ratio) might explain such results and could be an alternative to the meiofaunal food limitation hypothesis. Despite a lower biomass, the grazing rate of nematodes (4.6 × 10−3 · h−1) was almost twice as high as copepods (2.42 × 10−3 · h−1). Other food sources are probably involved in C fluxes to meiofauna, particularly in copepod nutrition.

Photo synthetic characteristics of microphytobenthos in Marennes-Oléron Bay, France: Preliminary results

Abstract The photosynthetic characteristics of microphytobenthos inhabiting an intertidal mudflat in Marennes-Oleron Bay (France) have been studied on samples collected at the air-sediment interface during low tide. Photosynthesis-Irradiance (P-I) parameters were estimated at different times of low tide in March and in July. In summer, two different sediment depths were also compared. The results point out different kinds of changes in the photophysiological characteristics of microphytobenthos, according to the time scale under consideration. At a seasonal scale, there was an increase of I k from March to July, thus indicating photoacclimation of microphytobenthos. At an hourly scale there was no change in I k . In March, there was no change in P-I parameters, but there was a significant change of α B and P m B in July with the highest values at T 3 (middle of low tide) at the surface of the sediment. It is speculated that this change could be due to an endogenous photosynthetic rhythm. There was no difference in the photosynthetic characteristics of microphytobenthos from the photic and the aphotic layers, suggesting sediment mixing by physical and/or biological processes (or microalgal migration) at a rate compatible with the persistence of the photosynthetic response in the aphotic layer. Although benthic microalgae did experience photoinhibition in experimental conditions, there was no indication of a photoinhibitory effect in situ. This suggests that microalgae were not exposed to strong illumination for extended periods of time during low tide in spite of the direct exposure of the mudflat to incident irradiance. We suggest that the strong attenuation of light within the sediment, selfshading within algal mats, the motility of microalgae, and bioturbation can explain this apparent paradox.

Temporal and spatial distributions of moisture and organic contents across a macro-tidal mudflat

Abstract The purpose of this study was to make comprehensive surface measurements of the changes in moisture and organic contents across the Skeffling mudflats (Humber Estuary, UK) and to examine the effects of exposure and location on the sediment properties. Sediment cores were collected using modified 50xa0ml syringes, and the data compared with measurements of chlorophyll a and colloidal carbohydrate concentrations. The temporal and spatial relationships between these surface sediment properties were quantified and related to the measured critical erosion shear stress. Moisture and organic contents correlate significantly with respect to morphology, both across the mudflat and with depth below the sediment surface. These variations affected the distributions of bulk density and sediment erodibility, although these two variables were not directly related. The relatively coarse vertical resolution of the cores prevented clear relationships being identified between critical erosion shear stress, moisture and organic contents, and the chlorophyll a and colloidal carbohydrate concentrations. Analysis of mean values removed some of the spatial variability and a reasonable exponential relationship was obtained between erosion shear stress and colloidal carbohydrate. This paper highlights the influence of the physical, morphological and biological characteristics of the surface sediment upon its erosion resistance. The implications for the development of the surface morphology and mudflat are considered.

Quantification of the microphytobenthic primary production in European intertidal mudflats – a modelling approach

Abstract A deterministic model quantifying the dynamics of the microphytobenthic biomass in the first centimeter of the mud was formulated as part of the MAST-III/INTRMUD project. The main modelled processes are production by photosynthesis, active vertical migration of the microphytobenthos and global biomass losses, encompassing grazing, mortality and resuspension during immersion periods. The model emphasizes the role of the interface between mud and air, which is crucial for the production. The microalgal biofilm present at the mud surface during day-time emersion periods induces most of the differences between microphytobenthos and phytoplankton communities dynamics. The study of the mathematical properties of the system, under some simplifying assumptions, shows the convergence of the solutions to a stable cyclic equilibrium, in the whole observed range of the frequencies of the physical synchronizers of the production. Resilience time estimates suggest that microphytobenthic biomass attains an equilibrium rapidly, even after a strong perturbation. Around the equilibrium, the local model solutions agree fairly with the in situ observed dynamics of the total biomass. The sensitivity analysis suggests investigating the processes governing biomass losses, which are so far uncertain, and may further vary in space and time.

Spatial distribution of sediment particulate organic matter on two estuarine intertidal mudflats: a comparison between Marennes-Oléron Bay (France) and the Humber Estuary (UK)

Abstract A comparison based on the bulk composition of surficial sediment particulate organic matter (POM) has been carried out on mudflats representative of two large European macrotidal estuarine zones: the Marennes-Oleron Bay and the Humber Estuary. Samples were collected in early summer, at low tide, during medium spring tide. Six sites were investigated along a 2500xa0m cross-shore transect with 12 cores collected at each site. Elemental analyses (C, N) and proximate analyses (chloropigments, carbohydrates and lipids) were performed on the fraction

Contrôle de la dynamique à court terme du microphytobenthos intertidal par le cycle exondation-submersion

Abstract We test herein the hypothesis that the succession of low tides — during which biomass increases — and high tides — during which biomass decreases — controls the dynamic behaviour of the microphytobenthic compartment. Results indeed show that during a complete lunar cycle (14 d) biomass exhibits a series of oscillations due to biomass increases during low tides and decreases during high tides. Moreover, the datasets show that the production level is in the range of previously recorded values and they indicate that the major part of the photosynthetically produced biomass during low tide can be exported into the water column during high tide. This clearly suggests that microphytobenthos is the major autochtonous source of organic matter production in the littoral zone and that it directly supplies the associated pelagic trophic web.

La dynamique à court terme de la biomasse du microphytobenthos intertidal. Formalisation mathématique

Abstract We formulate a deterministic mathematical model to describe the dynamics of the microphytobenthos of intertidal mudflats. It is ‘minimal’ because it only takes into account the essential processes governing the functioning of the system: the autotrophic production, the active upward and downward migrations of epipelic microalgae, the saturation of the mud surface by a bio film of diatoms and the global net loss rates of biomass. According to the photic environment of the benthic diatoms inhabiting intertidal mudflats, and to their migration rhythm, the model is composed of two sub-systems of ordinary differential equations; they describe the simultaneous evolution of the biomass ‘S’ concentrated in the mud surface biofilm — the photic layer — and of the biomass ‘F’ diluted in the topmost centimetre of the mud — the aphotic layer. Qualitatively, the model solutions agree fairly well with the in situ observed dynamics of the S + F biomass. The study of the mathematical properties of the model, under some simplifying assumptions, shows the convergence of solutions to a stable cyclic equilibrium, whatever the frequencies of the physical synchronizers of the production. The sensitivity analysis reveals the necessity of a better knowledge of the processes of biomass losses, which so far are uncertain, and may further vary in space and time.