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The influence of external factors other than nutrients on biofilm development and composition was studied with a combination of optical (Confocal Laser Scanning Microscopy, PAM fluorometry) and chemical methods (EPS extraction, HPLC, TOC determination). The development of algal-bacterial biofilms was followed from bare surfaces to mature biofilms in two water reservoirs on Dutch filtration dunes. Biofilms developed under the influence of grazing, light limitation or undisturbed by either of these two factors. Biofilms appeared similar at the beginning of the colonization period at the three sites and laser microscopy observations revealed the predominance of bacteria and capsular EPS (extracellular polymeric substances) in young biofilms. After 3 weeks development, the biofilms had a higher number of taxa; undisturbed biofilms presented the highest biomass, the longest developmental period and showed a significant correlation between organic carbon, chlorophyll and EPS production, indicating a close coupling between autotrophic carbon production and EPS. In light-limited biofilms, this coupling was weaker (although the organic carbon content was similar to that of the undisturbed biofilms) and a lower algal biomass was reached. Light-limited biofilms were mostly composed of diatoms, which are more efficient in low irradiances than green microalgae. Biofilms grazed by the snail Potamopyrgus antipodarum presented the lowest biomass level, but the highest proportion of EPS. Grazing seemed to favour the predominance of EPS-rich algae, as well as firmly attached diatoms. Although filamentous cyanobacteria were found in mature biofilms at the three locations, they were more abundant in the grazed biofilms. The differences in carbon uptake with respect to its allocation indicated that external factors influencing biofilm development affect the cycling and transport of carbon in biofilms and hence influence the effect of biofilm metabolism on the overlying water quality. [K, The influence of external factors other than nutrients on biofilm development and composition was studied with a combination of optical (Confocal Laser Scanning Microscopy, PAM fluorometry) and chemical methods (EPS extraction, HPLC, TOC determination). The development of algal-bacterial biofilms was followed from bare surfaces to mature biofilms in two water reservoirs on Dutch filtration dunes. Biofilms developed under the influence of grazing, light limitation or undisturbed by either of these two factors. Biofilms appeared similar at the beginning of the colonization period at the three sites and laser microscopy observations revealed the predominance of bacteria and capsular EPS (extracellular polymeric substances) in young biofilms. After 3 weeks development, the biofilms had a higher number of taxa; undisturbed biofilms presented the highest biomass, the longest developmental period and showed a significant correlation between organic carbon, chlorophyll and EPS production, indicating a close coupling between autotrophic carbon production and EPS. In light-limited biofilms, this coupling was weaker (although the organic carbon content was similar to that of the undisturbed biofilms) and a lower algal biomass was reached. Light-limited biofilms were mostly composed of diatoms, which are more efficient in low irradiances than green microalgae. Biofilms grazed by the snail Potamopyrgus antipodarum presented the lowest biomass level, but the highest proportion of EPS. Grazing seemed to favour the predominance of EPS-rich algae, as well as firmly attached diatoms. Although filamentous cyanobacteria were found in mature biofilms at the three locations, they were more abundant in the grazed biofilms. The differences in carbon uptake with respect to its allocation indicated that external factors influencing biofilm development affect the cycling and transport of carbon in biofilms and hence influence the effect of biofilm metabolism on the overlying water quality. [K

Item does not contain fulltext, We investigated the effects of copper on the structure and physiology of freshwater biofilm microbial communities. For this purpose, biofilms that were grown during 4 weeks in a shallow, slightly polluted ditch were exposed, in aquaria in our laboratory, to a range of copper concentrations (0, 1, 3, and 10 muM). Denaturing gradient gel electrophoresis (DGGE) revealed changes in the bacterial community in all aquaria. The extent of change was related to the concentration of copper applied, indicating that copper directly or indirectly caused the effects. Concomitantly with these changes in structure, changes in the metabolic potential of the heterotrophic bacterial community were apparent from changes in substrate use profiles as assessed on Biolog plates. The structure of the phototrophic community also changed during the experiment, as observed by microscopic analysis in combination with DGGE analysis of eukaryotic microorganisms and cyanobacteria. However, the extent of community change, as observed by DGGE, was not significantly greater in the copper treatments than in the control. Yet microscopic analysis showed a development toward a greater proportion of cyanobacteria in the treatments with the highest copper concentrations. Furthermore, copper did affect the physiology of the phototrophic community, as evidenced by the fact that a decrease in photosynthetic capacity was detected in the treatment with the highest copper concentration. Therefore, we conclude that copper affected the physiology of the biofilm and had an effect on the structure of the communities composing this biofilm.

Biofilms in surface waters are characterized by their spatial structure and the heterogeneous distribution of the microorganisms that interact in a complex and dynamic way. Therefore, the assessment of phototrophic and heterotrophic biofilms' metabolic activity should be done without disturbing their structure. Several optical methods that allow the study of undisturbed living biofilms have been developed and are claimed to have high potential in the analysis of biofilms. However, natural biofilms can develop into thick packages of cells that may limit light penetration into the biofilm matrix, hindering the use of optical methods. In this paper we evaluate the use of non-destructive optical and destructive methods for the study of natural algal-bacterial biofilms. Pulse amplitude modulation (PAM) fluorimetry (with both single and multiple wavelength excitation) and confocal laser scanning microscopy (CLSM) are used as optical methods and compared to chemical extraction of plant pigments and exopolysaccharides. We demonstrate that the attenuation of the light intensity by the biofilm matrix represents a limitation to optical methods that is difficult to overcome in mature natural biofilms; but nevertheless, optical methods are very reliable for the study of thin or young biofilms. Apart from the biofilm thickness, the degree of compactness should also be taken into account. The density of some natural biofilms could be a limitation of CLSM especially if high molecular weight probes are used for specific biofilm components. In conclusion, a combination of both approaches still appears to be necessary in order to follow the complete developmental period of biofilms. [KEYWORDS: lms ; Microalgae ; PAM fluorimetry ; CLSM ; Chlorophyll ; Lectins ; Bacteria], Biofilms in surface waters are characterized by their spatial structure and the heterogeneous distribution of the microorganisms that interact in a complex and dynamic way. Therefore, the assessment of phototrophic and heterotrophic biofilms' metabolic activity should be done without disturbing their structure. Several optical methods that allow the study of undisturbed living biofilms have been developed and are claimed to have high potential in the analysis of biofilms. However, natural biofilms can develop into thick packages of cells that may limit light penetration into the biofilm matrix, hindering the use of optical methods. In this paper we evaluate the use of non-destructive optical and destructive methods for the study of natural algal-bacterial biofilms. Pulse amplitude modulation (PAM) fluorimetry (with both single and multiple wavelength excitation) and confocal laser scanning microscopy (CLSM) are used as optical methods and compared to chemical extraction of plant pigments and exopolysaccharides. We demonstrate that the attenuation of the light intensity by the biofilm matrix represents a limitation to optical methods that is difficult to overcome in mature natural biofilms; but nevertheless, optical methods are very reliable for the study of thin or young biofilms. Apart from the biofilm thickness, the degree of compactness should also be taken into account. The density of some natural biofilms could be a limitation of CLSM especially if high molecular weight probes are used for specific biofilm components. In conclusion, a combination of both approaches still appears to be necessary in order to follow the complete developmental period of biofilms. [KEYWORDS: lms ; Microalgae ; PAM fluorimetry ; CLSM ; Chlorophyll ; Lectins ; Bacteria]

The effects of copper (Cu) on photosynthetic riverine biofilms were studied in artificial stream channels. Direct effects on the composition and functioning of the biofilms were investigated using plant pigments, community-level physiological profiles (CLPP), and pulse-amplitude-modulated (PAM) fluorescence. Copper caused a significant reduction of microalgal biomass and induced a shift in the population from diatoms to cyanobacteria. However, a decrease in biomass indicated that the replacement of species was not totally effective to counteract the toxic effects of Cu. A direct effect of Cu could also be shown in the bacterial community, and, furthermore, changes in the CLPP could be related to the Cu treatment. Copper-exposed biofilms lost the capacity to use between 11 and 15% of the substrates, but many of the remaining capacities became more robust, indicating an increased Cu tolerance due to the exposure. The change in the biofilm microbial composition points to the indirect effects of Cu on biofilms due to the close interdependence between biofilm autotrophic and heterotrophic compartments. Grazing by snails, which appeared to be an important factor structuring biofilms without any Cu addition, had a very minor effect on Cu-exposed biofilms. Although grazing changed the bacterial composition, its effects were not detected either on the algal community or on the biofilm community tolerance to Cu [KEYWORDS: Copper Biofilms Grazing Bacteria Photosynthesis], The effects of copper (Cu) on photosynthetic riverine biofilms were studied in artificial stream channels. Direct effects on the composition and functioning of the biofilms were investigated using plant pigments, community-level physiological profiles (CLPP), and pulse-amplitude-modulated (PAM) fluorescence. Copper caused a significant reduction of microalgal biomass and induced a shift in the population from diatoms to cyanobacteria. However, a decrease in biomass indicated that the replacement of species was not totally effective to counteract the toxic effects of Cu. A direct effect of Cu could also be shown in the bacterial community, and, furthermore, changes in the CLPP could be related to the Cu treatment. Copper-exposed biofilms lost the capacity to use between 11 and 15% of the substrates, but many of the remaining capacities became more robust, indicating an increased Cu tolerance due to the exposure. The change in the biofilm microbial composition points to the indirect effects of Cu on biofilms due to the close interdependence between biofilm autotrophic and heterotrophic compartments. Grazing by snails, which appeared to be an important factor structuring biofilms without any Cu addition, had a very minor effect on Cu-exposed biofilms. Although grazing changed the bacterial composition, its effects were not detected either on the algal community or on the biofilm community tolerance to Cu [KEYWORDS: Copper Biofilms Grazing Bacteria Photosynthesis]

Photosynthetic biofilms are capable of immobilizing important concentrations of metals, therefore reducing bioavailability to organisms. But also metal pollution is believed to produce changes in the microalgal species composition of biofilms. We investigated the changes undergone by natural photosynthetic biofilms from the River Meuse, The Netherlands, under chronic copper (Cu) exposure. The suspended particles in the river water had only a minor effect on reduction of sorption and toxicity of Cu to algae. Biofilms accumulated Cu proportionally to the added concentration, also at the highest concentration used (9 muM Cu). The physiognomy of the biofilms was affected through the growth of the chain-forming diatom Melosira varians, changing from long filaments to short tufts, although species composition was not affected by the Cu exposure. The Cu decreased phosphate uptake and algal biomass measured as chl a, which degraded exponentially in time. Photosynthetic activity was always less sensitive than algal biomass; the photon yield decreased linearly in time. The protective and insulating role of the biofilm, supported by ongoing autotrophic activity, was indicated as essential in resisting metal toxicity. We discuss the hypothesis that the toxic effects of Cu progress almost independently of the species composition, counteracting ongoing growth, and conclude that autotrophic biofilms act as vertical heterogeneous units. Effective feedback mechanisms and density dependence explain several discrepancies observed earlier. [KEYWORDS: biofilms; copper; diatoms; fluorescence; River Meuse], Photosynthetic biofilms are capable of immobilizing important concentrations of metals, therefore reducing bioavailability to organisms. But also metal pollution is believed to produce changes in the microalgal species composition of biofilms. We investigated the changes undergone by natural photosynthetic biofilms from the River Meuse, The Netherlands, under chronic copper (Cu) exposure. The suspended particles in the river water had only a minor effect on reduction of sorption and toxicity of Cu to algae. Biofilms accumulated Cu proportionally to the added concentration, also at the highest concentration used (9 muM Cu). The physiognomy of the biofilms was affected through the growth of the chain-forming diatom Melosira varians, changing from long filaments to short tufts, although species composition was not affected by the Cu exposure. The Cu decreased phosphate uptake and algal biomass measured as chl a, which degraded exponentially in time. Photosynthetic activity was always less sensitive than algal biomass; the photon yield decreased linearly in time. The protective and insulating role of the biofilm, supported by ongoing autotrophic activity, was indicated as essential in resisting metal toxicity. We discuss the hypothesis that the toxic effects of Cu progress almost independently of the species composition, counteracting ongoing growth, and conclude that autotrophic biofilms act as vertical heterogeneous units. Effective feedback mechanisms and density dependence explain several discrepancies observed earlier. [KEYWORDS: biofilms; copper; diatoms; fluorescence; River Meuse]

At two intertidal sites (one sandy and one silty, in the Scheldt estuary, The Netherlands), the fate of microphytobenthos was studied through an in situ C-13 pulse- chase experiment. Label was added at the beginning of low tide, and uptake of C-13 by algae was linear during the whole period of tidal exposure (about 27 mg m(-2) h(-1) in the top millimeter at both sites). The C-13 fixed by microphytobenthos was rapidly displaced toward deeper sediment layers (down to 6 cm), in particular at the dynamic, sandy site. The residence times of microphytobenthos with respect to external losses (resuspension and respiration) were about 2.4 and 5.6 d at the sandy and silly stations, respectively. The transfer of carbon from microphytobenthos to benthic consumers was estimated from the appearance of C-13 in bacterial biomarkers, handpicked nematodes, and macrofauna. The incorporation of C-13 into bacterial biomass was quantified by carbon isotope analysis of polar lipid derived fatty acids specific for bacteria. The bacterial polar lipid-derived fatty acids (i14:0, i15:0, a15:0, i16:0, and 18:1 omega 7c) showed rapid, significant transfer from benthic algae to bacteria with maximum labeling after 1 d. Nematodes became enriched after 1 h, and C-13 assimilation increased until day 3. Microphytobenthos carbon entered all heterotrophic components in proportion to heterotrophic biomass distribution (bacteria > macrofauna > meiofauna). Our results indicate a central role for microphytobenthos in moderating carbon flow in coastal sediments. [KEYWORDS: Water marine habitats; microbenthic communities; westerschelde estuary; microbial biomass; epipelic diatoms; ecological role; organic-matter; grazing rates; chlorophyll-a; secret garden], At two intertidal sites (one sandy and one silty, in the Scheldt estuary, The Netherlands), the fate of microphytobenthos was studied through an in situ C-13 pulse- chase experiment. Label was added at the beginning of low tide, and uptake of C-13 by algae was linear during the whole period of tidal exposure (about 27 mg m(-2) h(-1) in the top millimeter at both sites). The C-13 fixed by microphytobenthos was rapidly displaced toward deeper sediment layers (down to 6 cm), in particular at the dynamic, sandy site. The residence times of microphytobenthos with respect to external losses (resuspension and respiration) were about 2.4 and 5.6 d at the sandy and silly stations, respectively. The transfer of carbon from microphytobenthos to benthic consumers was estimated from the appearance of C-13 in bacterial biomarkers, handpicked nematodes, and macrofauna. The incorporation of C-13 into bacterial biomass was quantified by carbon isotope analysis of polar lipid derived fatty acids specific for bacteria. The bacterial polar lipid-derived fatty acids (i14:0, i15:0, a15:0, i16:0, and 18:1 omega 7c) showed rapid, significant transfer from benthic algae to bacteria with maximum labeling after 1 d. Nematodes became enriched after 1 h, and C-13 assimilation increased until day 3. Microphytobenthos carbon entered all heterotrophic components in proportion to heterotrophic biomass distribution (bacteria > macrofauna > meiofauna). Our results indicate a central role for microphytobenthos in moderating carbon flow in coastal sediments. [KEYWORDS: Water marine habitats; microbenthic communities; westerschelde estuary; microbial biomass; epipelic diatoms; ecological role; organic-matter; grazing rates; chlorophyll-a; secret garden]

Microphytobenthos primary production was measured using C fixation and compared with production estimates based on measurements on intact cores using variable chlorophyll fluorescence with a PAM (pulse-amplitude-modulated) fluorometer at a number of different sites on intertidal flats in the Westerschelde and Oosterschelde estuaries (The Netherlands). We observed a linear relationship between the minimal fluorescence, F-0, and the chlorophyll a content of the sediments, but the relationship was quite noisy, which was partly due to a vertical scale mismatch between the depth of sampling (1 mm) and the measuring depth the of PAM fluorometer (100 to 200 mum) On most sites, C fixation was a linear function of photosystem II electron transport (ETR, obtained from PAM measurements), although for Stn B, a relatively silty site on the Molenplaat, an intertidal flat in the Westerschelde, the agreement between C fixation and ETR was often non-linear at irradiances saturating C fixation. This was sometimes observed for some of the other stations as well. The differences at high irradiance might be related to vertical migration of epipelic diatoms, optical properties of the sediment or alternative electron accepters. A conversion factor was calculated, allowing estimation of C fixation from ETR measurements. This conversion factor was not different for the different stations and no clear seasonal influence was observed, although the conversion factor could vary within hours. The seasonally averaged conversion factor for all stations and each site was used to calculate areal rates of production and compared with primary production estimates obtained from the C fixation measurements. It appeared that the PAM-based estimates gave a good prediction of the (potential) C fixation, with the exception of a number of dates for Molenplaat Stn B. This demonstrates that the variable fluorescence technique can become, after further calibration with C-14 for Other shallow sediments, a very u, Microphytobenthos primary production was measured using C fixation and compared with production estimates based on measurements on intact cores using variable chlorophyll fluorescence with a PAM (pulse-amplitude-modulated) fluorometer at a number of different sites on intertidal flats in the Westerschelde and Oosterschelde estuaries (The Netherlands). We observed a linear relationship between the minimal fluorescence, F-0, and the chlorophyll a content of the sediments, but the relationship was quite noisy, which was partly due to a vertical scale mismatch between the depth of sampling (1 mm) and the measuring depth the of PAM fluorometer (100 to 200 mum) On most sites, C fixation was a linear function of photosystem II electron transport (ETR, obtained from PAM measurements), although for Stn B, a relatively silty site on the Molenplaat, an intertidal flat in the Westerschelde, the agreement between C fixation and ETR was often non-linear at irradiances saturating C fixation. This was sometimes observed for some of the other stations as well. The differences at high irradiance might be related to vertical migration of epipelic diatoms, optical properties of the sediment or alternative electron accepters. A conversion factor was calculated, allowing estimation of C fixation from ETR measurements. This conversion factor was not different for the different stations and no clear seasonal influence was observed, although the conversion factor could vary within hours. The seasonally averaged conversion factor for all stations and each site was used to calculate areal rates of production and compared with primary production estimates obtained from the C fixation measurements. It appeared that the PAM-based estimates gave a good prediction of the (potential) C fixation, with the exception of a number of dates for Molenplaat Stn B. This demonstrates that the variable fluorescence technique can become, after further calibration with C-14 for Other shallow sediments, a very u

The microbenthic communities of a tidal flat in the Westerschelde estuary were studied at 4 stations in late spring and early autumn 1996. Additional information on the diatom component of these communities was obtained from a one-year survey of these organisms. Total biomass of pigmented (PIG) protists greatly exceeded that of non-pigmented (NPIG) protists in late spring, especially at the more silty stations. However, in autumn, the ratio of PIG/NPIG protists was much lower and is, The microbenthic communities of a tidal flat in the Westerschelde estuary were studied at 4 stations in late spring and early autumn 1996. Additional information on the diatom component of these communities was obtained from a one-year survey of these organisms. Total biomass of pigmented (PIG) protists greatly exceeded that of non-pigmented (NPIG) protists in late spring, especially at the more silty stations. However, in autumn, the ratio of PIG/NPIG protists was much lower and is

Microphytobenthic primary production and biomass were studied in an intertidal flat located in the Westerschelde (SW Netherlands) with a vertical resolution of 1 mm. Short-term variability of primary production and photosynthetic characteristics were compared during low tide in 2 types of sediment: sandy and muddy sands. The changes observed were compared with abiotic factors and the biomass abundance, which was dominated by benthic diatoms. C-14 uptake values obtained from incubations in a photosynthetron were used for the construction of P-I (photosynthesis-irradianee) curves. Annual averages indicate that both sediments were equally productive (34.5 +/- 23.6 mg C m(-2) h(-1) and 41.1 +/- 11.6 mg C m(-2) h(-1) for the sands and muddy sands respectively), but production rates were highly variable on monthly time scales and were regulated by different mechanisms. Light and temperature played an important role in determining the production rates, especially in the muddy sediments, where changes in I-k (light saturation) were correlated with temperature. I-k showed seasonal changes, suggesting that algae adapted to the seasonal light conditions but there was not a significant correlation between the I-k and PAR (photosynthetic available radiation) at any of the stations. Vertical migration of the algae, as followed by spectroradiometric measurements, probably accounted for a general absence of photoinhibition. In the sandy sediments, production appeared to be limited by the low biomass of algae, due to resuspension and export. The fact that gross oxygen production rates measured on intact cores using microelectrodes were not lower than potential production obtained from 14C fixation suggests that short-term limitation of production due to nutrients and/or carbon is not frequent in the microphytobenthos of the Westerschelde. [KEYWORDS: microphytobenthos; photosynthetic characteristics; primary production; tidal flats Benthic microalgal production; ems-dollard estuary, Microphytobenthic primary production and biomass were studied in an intertidal flat located in the Westerschelde (SW Netherlands) with a vertical resolution of 1 mm. Short-term variability of primary production and photosynthetic characteristics were compared during low tide in 2 types of sediment: sandy and muddy sands. The changes observed were compared with abiotic factors and the biomass abundance, which was dominated by benthic diatoms. C-14 uptake values obtained from incubations in a photosynthetron were used for the construction of P-I (photosynthesis-irradianee) curves. Annual averages indicate that both sediments were equally productive (34.5 +/- 23.6 mg C m(-2) h(-1) and 41.1 +/- 11.6 mg C m(-2) h(-1) for the sands and muddy sands respectively), but production rates were highly variable on monthly time scales and were regulated by different mechanisms. Light and temperature played an important role in determining the production rates, especially in the muddy sediments, where changes in I-k (light saturation) were correlated with temperature. I-k showed seasonal changes, suggesting that algae adapted to the seasonal light conditions but there was not a significant correlation between the I-k and PAR (photosynthetic available radiation) at any of the stations. Vertical migration of the algae, as followed by spectroradiometric measurements, probably accounted for a general absence of photoinhibition. In the sandy sediments, production appeared to be limited by the low biomass of algae, due to resuspension and export. The fact that gross oxygen production rates measured on intact cores using microelectrodes were not lower than potential production obtained from 14C fixation suggests that short-term limitation of production due to nutrients and/or carbon is not frequent in the microphytobenthos of the Westerschelde. [KEYWORDS: microphytobenthos; photosynthetic characteristics; primary production; tidal flats Benthic microalgal production; ems-dollard estuary

A pulse amplitude modulated fluorometer (PAM) was used to investigate photosynthetic activity of microphytobenthos on an intertidal mudflat. Spectral irradiance measurements indicate that 75% of the signal detectable by the PAM originates in the upper 150 mu m of the sediment. From the photosynthetic electron transport rate (ETR) measurements, it was concluded that the PAM could be used to observe changes in photosynthetic parameters during the day or the season. Photoacclimation to lower irradiance was indicated by changes in the maximum ETR and the saturating photon irradiance parameter I-k. When cores were exposed to a high photon irradiance for several hours, vertical migration could be followed using reflectance spectra. The data also showed that the benthic algae did not seem to experience photoinhibition or CO2 limitation. To explain this, it is hypothesised that there is a continuous vertical migration in the top layer of the sediment, where algae can avoid photoinhibition due to prolonged periods of high irradiance and lack of CO2 by migrating downwards while others migrate upwards. [KEYWORDS: microphytobenthos; chlorophyll fluorescence photosynthesis; vertical migration; C-limitation Estuary sw netherlands; photosystem-ii; marine-phytoplankton; light; photoinhibition; sediments; stabilization; respiration;communities; limitations], A pulse amplitude modulated fluorometer (PAM) was used to investigate photosynthetic activity of microphytobenthos on an intertidal mudflat. Spectral irradiance measurements indicate that 75% of the signal detectable by the PAM originates in the upper 150 mu m of the sediment. From the photosynthetic electron transport rate (ETR) measurements, it was concluded that the PAM could be used to observe changes in photosynthetic parameters during the day or the season. Photoacclimation to lower irradiance was indicated by changes in the maximum ETR and the saturating photon irradiance parameter I-k. When cores were exposed to a high photon irradiance for several hours, vertical migration could be followed using reflectance spectra. The data also showed that the benthic algae did not seem to experience photoinhibition or CO2 limitation. To explain this, it is hypothesised that there is a continuous vertical migration in the top layer of the sediment, where algae can avoid photoinhibition due to prolonged periods of high irradiance and lack of CO2 by migrating downwards while others migrate upwards. [KEYWORDS: microphytobenthos; chlorophyll fluorescence photosynthesis; vertical migration; C-limitation Estuary sw netherlands; photosystem-ii; marine-phytoplankton; light; photoinhibition; sediments; stabilization; respiration;communities; limitations]

Microphytobenthos biomass and community composition vi ere studied by the use of pigment biomarkers during one year at a tidal flat located in the Westerschelde (SW Netherlands). Benthic microphytes appeared to be an important carbon source in the Westerschelde, especially in the central part of the flat with a mean biomass as high as 65 mg Chi a m(-2) in the superficial 1 mm of sediment. Diatoms (fucoxanthin, Chl c) dominated the population during both spring and autumn blooms. In summer microphytobenthic biomass decreased, and diatoms coexisted with Cyanobacteria (zeaxanthin) and Euglenophyceae (Chl b, zeaxanthin and lutein). The shift in the community composition may possibly be linked with the decrease in silicon concentration in the overlying water. The degradation and recycling of microphytobenthic biomass was assessed by the study of degraded chlorophyll pigments, and pigments in deeper sediment layers. The fate of microphytobenthos appears to include rapid bacterial degradation in spring, episodic grazing by benthic animals in surface layers of sediments in the centre of the flat, and export by resuspension in sediments more exposed to currents. [KEYWORDS: microphytobenthos; plant pigments; tidal flats; Westerschelde Performance liquid-chromatography; dutch wadden sea; marine-phytoplankton; coastal sediments; chlorophyll-a; water; algae; atlantic; diatoms; light], Microphytobenthos biomass and community composition vi ere studied by the use of pigment biomarkers during one year at a tidal flat located in the Westerschelde (SW Netherlands). Benthic microphytes appeared to be an important carbon source in the Westerschelde, especially in the central part of the flat with a mean biomass as high as 65 mg Chi a m(-2) in the superficial 1 mm of sediment. Diatoms (fucoxanthin, Chl c) dominated the population during both spring and autumn blooms. In summer microphytobenthic biomass decreased, and diatoms coexisted with Cyanobacteria (zeaxanthin) and Euglenophyceae (Chl b, zeaxanthin and lutein). The shift in the community composition may possibly be linked with the decrease in silicon concentration in the overlying water. The degradation and recycling of microphytobenthic biomass was assessed by the study of degraded chlorophyll pigments, and pigments in deeper sediment layers. The fate of microphytobenthos appears to include rapid bacterial degradation in spring, episodic grazing by benthic animals in surface layers of sediments in the centre of the flat, and export by resuspension in sediments more exposed to currents. [KEYWORDS: microphytobenthos; plant pigments; tidal flats; Westerschelde Performance liquid-chromatography; dutch wadden sea; marine-phytoplankton; coastal sediments; chlorophyll-a; water; algae; atlantic; diatoms; light]