Microphytobenthic Biomass Research Articles

Biofilm polymers: relationship between carbohydrate biopolymers from estuarine mudflats and unialgal cultures of benthic diatoms

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 38:169-180 (2005) - doi:10.3354/ame038169 Biofilm polymers: relationship between carbohydrate biopolymers from estuarine mudflats and unialgal cultures of benthic diatoms B. J. Bellinger1,*, A. S. Abdullahi1, M. R. Gretz1, G. J. C. Underwood2 1Department of Biological Sciences, Michigan Technological University, 1400 Townsend Drive, Houghton, Michigan 49931, USA2Department of Biological Sciences, University of Essex, Wivenhoe Park, Colchester CO4 3SQ, UK *Email: bjbellin@mtu.edu ABSTRACT: Benthic microalgae (microphytobenthos) are the dominant group of primary producers in many marine intertidal and subtidal habitats. Estuarine mudflat diatoms are thought to be major contributors of extracellular polymeric substances (EPS), which are important for sediment stabilization and in benthic food chains. Biofilms from 6 sites in the Colne estuary, UK, were fractionated to isolate biopolymers (colloidal, colloidal EPS [cEPS], low molecular weight [LMW] carbohydrates, hot water [HW] and hot bicarbonate [HB] soluble) and the same techniques were applied to diatoms cultured from these sediments. At sites dominated by benthic diatoms, colloidal carbohydrate concentration and chlorophyll a were closely related. With increasing biomass, the proportion of cEPS within the colloidal fraction decreased from 60 to 20%. Carbohydrate analysis revealed significant differences in monosaccharide and uronic acid composition of different carbohydrate fractions. Principal component analysis (PCA) of monosaccharide composition of HB polymers from both field and culture samples grouped closely along fucose and rhamnose vectors and formed 2 distinct clusters. HW and LMW fractions grouped along the glucose vector and cEPS polymers along the galactose and arabinose vectors. These data indicate that the simple relationship between colloidal carbohydrate concentration and microphytobenthic biomass in biofilms masks a high degree of potential complexity within the sediment carbohydrate pool and in the different proportions of polymeric and nonpolymeric material between different biofilms. Comparing monosaccharide composition of extracts generated using the same protocol, natural assemblages showed close relationships with unialgal cultures, confirming the important role of diatom-derived polymers in mudflat ecology. KEY WORDS: Diatoms · Biofilms · Microphytobenthos · EPS · Monosaccharide distribution · Uronic acids · Biopolymers · Fractionation Full article in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 38, No. 2. Online publication date: February 09, 2005 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 2005 Inter-Research.

Response of microphytobenthic biomass to experimental nutrient enrichment and grazer exclusion at different land-derived nitrogen loads

Effects of eutrophication on the relative importance of nutrients and macroherbivores as controls of microphytobenthic standing crop were examined in estuaries with different nitrogen load- ing rates: Sage Lot Pond (14 kg ha -1 yr -1 ), Green Pond (178 kg ha -1 yr -1 ), and Childs River (601 kg ha -1 yr -1 ). We selected 5 sites with similar salinity ranges on shallow-water, sandy substrates per estuary. In year-round experiments, we fertilized sediments with nitrogen + phosphorus to examine nutrient limitation. We conducted exclusion experiments to determine the significance of macroherbivores as controls of microphytobenthic biomass and examined possible interactions between nutrients and grazing in cages fertilized with nitrogen + phosphorus. Cages fertilized with nitrogen only were also included to determine if nitrogen availability was limiting. Nitrogen + phosphorus addition increased sediment chlorophyll a( chl a) content (herein used as a proxy for biomass) by a similar magnitude across estuaries. Grazer exclusion also increased chl a, but to a different extent across estuaries: the magnitude of the response increased with increasing nitrogen loading rates. We found no interac- tions between nutrients and grazing. Strong chl a increases in response to nitrogen only addition indicated N limitation in Sage Lot Pond and Green Pond. In the highly eutrophic Childs River estuary we found virtually no response to nitrogen-only additions, suggesting the possibility of phos- phorus limitation in this estuary.

Microphytobenthos, their Extracellular Polymeric Substances, and the Morphogenesis of Intertidal Sediments

Intertidal sediments are important areas that separate the land from the sea and form natural coastal defenses. They are known as highly productive ecosystems, fueling the coastal food web. It is also conceived that microphytobenthos contribute to the stability of intertidal sediments by increasing the erosion threshold and that they are major players in coastal morphodynamics. Depending on the sedimentary composition of intertidal flats, different types of microphytobenthos colonize the sediment surface. Fine sand sediment is often colonized by cyanobacteria, prokaryotic algae, which form dense and rigid microbial mats. Mudflats on the other hand are characterized by the development of thin biofilms of epipelic diatoms. Both groups of phototrophic microorganisms excrete extracellular polymeric substances (EPS), but they do so in different ways and for different reasons. Two operationally defined fractions, water- and EDTA-extractable EPS, have been obtained from intertidal diatom biofilms and from cultures. They differ in composition and their production seems to be under different metabolic control. Water-extractable EPS are considered to be closely associated with the diatoms and are rich in neutral sugars, notably glucose. These EPS show a dynamic relationship with the microphytobenthic biomass. EDTA-extractable EPS are tightly bound to the sediment, probably through bridging by divalent ions. This material is rich in uronic acids and other acid sugars and is weakly related to chlorophyll. These EPS have been conceived to be a major factor in the structuring and diagenesis of coastal sediments and essential for increasing the sediment erosion threshold. However, this relationship is now questioned.

Enzymatically hydrolyzable protein and carbohydrate sedimentary pools as indicators of the trophic state of detritus sink systems: A case study in a Mediterranean coastal lagoon

In order to classify the trophic state of detritus sink systems, instead of the conventional indicators based on inorganic nutrient availability and algal biomass and productivity in the water column, we used new biochemical descriptors based on the amount of sedimentary organic carbon (C) and nitrogen (N) potentially available to heterotrophs. We investigated spatial and temporal changes in microphytobenthic biomass, organic matter biochemical composition, and enzymatically hydrolyzable protein and carbohydrate pools along a north-south transect in the Marsala lagoon (Mediterranean Sea, Italy) at three stations characterized by different hydrodynamic conditions and organic matter content in the sediment. In the Marsala lagoon water currents decreased from north to south and this pattern was reflected by organic matter distribution and composition. Sediment organic matter concentrations were among the highest reported in the literature and, in the central area where large meadows of the seagrassPosidonia oceanica were present, display a strong dominance of highly refractory carbohydrates. The protein to carbohydrate ratio was always < 1, indicating the dominance of aged organic detritus. Microphytobenthic biomass displayed an increasing pattern southward, and its contribution to the biopolymeric C pools ranged from negligible in the central sector of the lagoon to 50% in its northern part, indicating that sources of sediment organic C also changed along the hydrodynamic gradient. The percentage contribution of the enzymatically hydrolyzable fraction of proteins and carbohydrates was inversely related to total protein and total carbohydrate concentrations, respectively, suggesting that bioavailability of organic C and N increased with decreasing organic matter content in the sediment and with increasing hydrodynamic regime. Microphytobenthic contribution to biopolymeric C (as a proxy of autotrophic organic C) and the ratio of the enzymatically digestible fraction to biopolymeric C (as an indicator of organic matter liability) were significantly correlated, suggesting that chlorophylla sediment content might be used as an indicator of food promptly available to consumers. The present study also highlighted that the ratio of labile (i.e., enzymatically digestible) versus biopolymeric organic C in the sediments tends to decrease with increasing organic matter content, due to the increase of the refractory fraction of organic C.

Carbohydrate production in relation to microphytobenthic biofilm development: an integrated approach in a tidal mesocosm.

Experiments were performed to evaluate short-term changes in sediment extracellular carbohydrates for a multispecific assemblage of benthic diatoms in relation to physiological status, endogenous migratory rhythms, and environmental conditions. For this purpose, a mesocosm was used, which simulated both tidal and dark: light alternating cycles under controlled conditions. Scanning electronic microscopy in combination with picture analyses indicated that natural diatom migration patterns were reproduced in the mesocosm. Two EPS fractions were operationally separated in colloidal carbohydrate measurements: alcohol-soluble EPS (termed "soluble EPS") and alcohol-insoluble EPS (termed "bound EPS"). Microphytobenthic biomass followed a logistic-type curve and converged toward a maximal value termed the "biotic capacity of the local environment." Both EPS fractions showed oscillations with production during photosynthetic periods and sharp decreases during night immersion periods. Productions of both EPS fractions increased with Chl a production during light periods suggesting a light dependence in relation to migratory patterns. The decreases in both EPS fractions, which occurred during night immersion periods suggest that carbohydrate hydrolysis and/or washaway affected both EPS fractions similarly in benthic environments. Our results confirm the theory according to which the two distinct fractions are under different metabolic controls. No change in soluble EPS release was obtained during the transition from logarithmic to stationary phase. On the other hand, a metabolism modification of microalgae, probably related to ammonium depletion, occurred when cells entered the stationary phase, since there was a high enhancement in bound EPS production. Mesocosm results can serve as a system of reference useful to characterize biofilm development in field investigations and to revisit the effective implication of each EPS fraction in sediment stability.

Impacts of longline mussel farming on oxygen and nitrogen dynamics and biological communities of coastal sediments

Benthic communities and benthic mineralization were studied in two shallow coastal regions of New Zealand: Tasman Bay, a possible future site for mussel farm development, and Beatrix Bay, which already hosts several longline mussel farms. In Tasman Bay, microphytobenthic (MPB) production added significantly to the total primary production of the bay. The activity of benthic microalgae had a pronounced effect on oxic conditions, solute exchange and denitrification rates. Benthic mineralization, quantified as the dark oxygen uptake, was in the range of 675±11 μmol m −2 h −1. Denitrification rates were high and fueled entirely by nitrate produced by the nitrifying community within the sediment. Competition for inorganic nitrogen between benthic microalgae and nitrifiers/denitrifiers resulted in diel variation in nitrogen cycling and reduced the inorganic nitrogen efflux and denitrification activity in the light. Calculated in electron equivalents, denitrification accounted for 11–20% of the total carbon mineralization—one of the highest numbers reported for coastal sediments. Reduced sediments, containing low MPB biomass and few subsurface macroinvertebrate species, were observed below a mussel farm in Beatrix Bay, presumably due to the intensified sedimentation of organic matter. Oxygen consumption increased in the organic-rich sediments, and ammonium effluxes were up to 14 times higher than those of unaffected sediments 250 m away from the farm. Denitrification rates below the farm were low as the coupled nitrification–denitrification was inhibited by the presence of sulfide. The dissimilative reduction of nitrate to ammonium (DRNA) was, however, stimulated in the reduced sediment. The enhanced benthic mineralization was associated with sulfidic sediments and a lower nitrogen removal rate due to impeded benthic photosynthesis and denitrification activity. The described local conditions associated with mussel farming should be taken into account when new areas are considered for development.

Seasonal and interannual patterns of intertidal microphytobenthos in combination with laboratory and areal production estimates

From April 1994 to December 1997, we studied the microphytobenthic assemblages in sur- face (0 to 0.5 cm) and subsurface (0.5 to 2 cm) sediments at spring low tide along a transect of 5 stations in an estuarine sandflat of the Seto Inland Sea, Japan. At the innermost sampling station, microphyto- benthos biomass (chl a) was also investigated in a vertical profile to 10 cm depth from December 1994 to April 1996. The chl a contents at the 2 uppermost layers were well correlated with each other, with a mean decrease of 34% from the surface to subsurface layer. Chl a tended to decrease rapidly through the vertical profile and was reduced to 3.2 ± 1.4% SD in the 9 to 10 cm layer. There was a progressive de- crease in the chl a content every year in fall and the occurrence of major peaks in early spring and/or summer. This was accompanied by a significant increase in microphytobenthos biomass from 1994 to 1995 and from 1995 to both 1996 and 1997. The microphytobenthos biomass in surface sediments (mean of 5 stations) ranged between 27.7 (October 1994) and 120 mg chl a m -2 (July 1997), or between 3.9 (November 1994) and 20.3 µg chl a g -1 dry wt (July 1996). Annual mean (1995 to 1997) biomass was 72.3 ± 27.1 mg chl a m -2 and 11.0 ± 4.3 µg chl a g -1 dry wt. These values rank in the mid-upper range of microphytobenthic biomass for intertidal sediments. In addition to the field investigations, we conducted laboratory experiments on a dominant diatom species, Navicula sp. The photosynthetic rate of Navi- cula sp. was saturated at a light intensity of 165 µE m -2 s -1 at 21°C. No photoinibition was found at higher light intensities up to 400 µE m -2 s -1 . The relationship between temperature and photosynthetic rate was positive and linear within a temperature range between 10 and 35°C at 55 µE m -2 s -1 . Areal 'potential' primary production of microphytobenthos was between 0.32 (December 1994) and 3.0 g C m -2 d -1 (July 1997), with an annual mean of 1.2 g C m -2 d -1 . Uni- (summer) or bi-modal (spring and summer) peaks of microphytobenthos biomass and primary production highlighted a marked interannual variability. Marked seasonal patterns were also recognizable, with primary production of microphytobenthos significantly higher both in spring and summer than in winter and fall.

Site-specific features influence sediment stability of intertidal flats

Abstract. The factors that influence the sediment stability and the transport of estuarine mudflats are not yet fully understood but knowledge of them is essential in coastal engineering applications and pollution ecology studies. The suggestion that variation in predictive models of sediment stability might be due to site-specific characteristics is investigated using data from four estuarine mudflats (Eden Estuary, Scotland, the Biezelingsche Ham, Zandkreek, and Molenplaat mudflats in The Netherlands). These estuaries differ in their environmental conditions, macrofaunal species composition and local features (e.g. Enteromorpha mats, migratory biofilms). Stable and unstable sediments were compared, and mean chlorophyll-a concentrations and granulometry of the sediments were significantly different between the two groups. Step-wise multiple linear regressions were applied to the sediment stability data of all sites to establish the influences on erosion threshold of microphytobenthic biomass, water content, granulometry, organic carbon content and the abundance of dominant macrofaunal species. The stability of each site was influenced by different factors. Sediment stability of the Eden Estuary was affected by the Enteromorpha bloom; Biezelingsche Ham was influenced by the highly migratory nature of the diatom biofilms and the abundance of Corophium volutator; the polychaete worm Arenicola marina had a net negative effect on sediment stability of the Zandkreek; and the Molenplaat was influenced by microphytobenthic biomass. This research highlights the need for site-specific calibration of models and suggests that a universal proxy parameter for sediment stability is unlikely to be obtained. Keywords: sediment stability; erosion threshold; cohesive strength meter; microphytobenthos; Enteromorpha spp.

Properties of the dynamics of intertidal microphytobenthic biomass

The short-term variability of intertidal microphytobenthic biomass was characterized by analysing a 30-day time series encompassing two spring–neap tide cycles. Chlorophyll-a concentration was measured every day at the beginning and at the end of every diurnal exposure periods. Microalgal biomass exhibited predictible net increases during diurnal exposures due to a positive production-loss balance. In addition, our data series shows that after an environmental disturbance, even though biomass decreased strongly, it converged quickly back to its initial steady-state oscillation pattern. This study supports some fundamental properties of the dynamics of microphytobenthic biomass, as previously predicted in our mathematical model of primary production.

Biodiversity and trophic structure of nematode assemblages in seagrass systems: evidence for a coupling with changes in food availability

Nematode assemblage composition, trophic structure and biodiversity were followed over an annual cycle in a Posidonia oceanica bed of the NW Mediterranean to test the response to temporally changing food availability (sediment organic matter, bacterial and microphytobenthic biomass). The sediment–water interface of the seagrass meadow was characterised by high particulate organic matter concentrations. Also, seagrass sediments had high organic matter content, chloropigment concentrations and bacterial biomass. All organic matter components (i.e. carbohydrates, proteins and lipids) changed temporally, with higher concentrations in winter and spring and lower concentrations in summer; however, overall, large amounts of organic compounds were potentially available for (seagrass) benthic consumers throughout the year. Nematode assemblage in the P. oceanica bed maintained a high genus number (88 genera), and a trophic structure tightly coupled with the composition of the potential food sources. In agreement with the relevance of microphytobenthos, epigrowth-feeder nematodes (2A) represented the dominant trophic guild. The biomass of predator nematodes (2B) was significantly correlated with the biomass of other nematodes. Non-selective deposit feeders (1B) displayed a close relationship with the concentration of proteins, carbohydrates and biopolymeric carbon (i.e. labile organic detritus). Conversely to what was expected, epigrowth feeders were not correlated with chloropigment concentrations, but showed a significant relationship with the number of dividing bacteria, possibly suggesting a trophic plasticity of the 2A feeding guild. The coupling between temporal changes in food sources and temporal variability of the nematode trophic structure was highlighted by a CANOCO analysis, which allowed us to identify and associate, at each sampling time, nematode genera and their feeding habits, with specific environmental variables and food indicators. Furthermore, species diversity (H′) and evenness (J) calculated on nematodes identified to the genus level displayed evident temporal changes, also reflected by the index of trophic diversity (ITD). Both structural and functional diversity were coupled with high concentration and highly heterogeneous composition of the food sources (including organic detritus, microphytobenthic algae and bacteria) potentially available to nematodes. These data suggest that temporal changes in quantity and quality of food sources do not only influence nematode dynamics and trophic composition, but also influence nematode structural and functional diversity.

Self-organized criticality in intertidal microphytobenthos patch patterns

A 2D distribution of microphytobenthic biomass is determined from a superficial sediment sampling on an exposed sandy shore. The distribution undergoes a very structured pattern, displaying a few dense patches over a wide range of low density patches. We perform a statistical analysis of this 2D patterns using statistical tools developed in the field of fractal theory. It is shown that the patch pattern belongs to fractal and multifractal structures, and exhibits specific power-law in the probability space, involving the appearance of a self-organized critical state. To our knowledge, this is the first experimental study of self-organized criticality in benthic ecology. Some theoretical consequences are outlined, and their practical applications to improve our understanding of intertidal ecosystems structures and functions are discussed.

Environmental and behavioural factors affecting activity in the intertidal gastropod Hydrobia ulvae

Laboratory microcosms were used to investigate the mud snail Hydrobia ulvae (Pennant) bioturbation activities and behavioural changes in response to snail density, algal food, sediment moisture content, light regime and water cover conditions. Density-dependent kinetics of bioturbated muddy areas were described by von Bertalanffy equations, which provided reliable estimates of mud surface covering rates by snail tracks (m 2 h −1 snail −1). Snails need a wet habitat to be active either covered by seawater or by moving in fluid layers for low-tide conditions. Light and microphytobenthic biomass, which are less potent to affect snail activity, are positively interrelated to increase covering rates in the tested chl a concentrations within the range of 1–15 μg g −1. Experimental results suggested us the relevance of microphytobenthos migration processes in affecting crawling activities of H. ulvae that appeared to adjust their foraging efforts in response to benthic algal biomass. Behavioural processes of H. ulvae, in terms of floating, crawling, burrowing and inactive snails, were described using a Markov model. Finally, an empirical model based on von Bertalanffy equations was proposed to describe kinetics of sediment covering by snail tracks under the influences of snail density, sediment moisture content, chl a concentrations and the four combinations of presence/absence of light and seawater. This model should provide a base for further development of a hydrosedimentary model to simulate the effects of H. ulvae bioturbation activities on the resuspension of the intertidal cohesive sediment–water interface for various in situ conditions.

Seasonal changes in selected physico-chemical and biological variables in the temporarily open/closed Kasouga estuary, Eastern Cape, South Africa

Seasonal changes in selected physico-chemical (temperature, salinity and turbidity) and biological (chlorophyll-a (chl-a), microphytobenthos and microheterotrophs) variables were measured at 7–9 day intervals for a period of 14 months in the temporarily open/closed Kasouga estuary. In addition, zooplankton biomass was determined monthly over the same period. During the course of the study, 13 overtopping and two breaching events were documented. Freshwater inflow into the estuary resulted in a decrease in salinity coupled with increases in total suspended solids (TSS), particulate organic matter (POM) and turbidity. Chlorophyll-a ranged between 0.29 and 8.01mg chl-a m−3 and microphytobenthic biomass between 3.87 and 209.09mg chl-a m−2. Generally, the highest chl-a concentrations were recorded during the wet season (spring to early summer). In contrast the microphytobenthic biomass attained highest levels during periods of little or no freshwater influx into the estuary. The influx of freshwater into the estuary resulted in a change in the size structure of the phytoplankton community from one dominated by nano- (2–20μm) and picophytoplankton (<2.0μm) to a community dominated by microphytoplankton (>20μm). Densities of microheterotrophs (<200μm) (range 1760 to 13750cells l-1) demonstrated an inverse relationship to total phytoplankton with maximum densities (>5000cells l-1) recorded during winter. Total zooplankton biomass ranged between 15.91mg Dwt m−3 and 106.93mg Dwt m−3 and demonstrated a similar pattern to chl-a with maximum biomass recorded during the wet season. These data indicate that riverine input into the temporarily open closed Kasouga estuary is important for estuarine structure and function.

Factors affecting microphytobenthic biomass, species composition and production in the Colne Estuary (UK)

Biomass, species composition and primary production of the microphytobenthos (MPB) were investigated along environmental gradients in the Colne Estuary between August 1996 and March 1998. Mean monthly sediment chlorophyll a (chl a) concentrations varied between 6 and 241 mg m -2 . There was no seasonal pattern in chl a distribution. Low chl a concentrations were asso- ciated with the estuary mouth, which was characterised by exposed sandy sediments and low water column nutrient concentrations. Position on the shore affected biomass, with greater chl a concentra- tions on the high shore. Chl a concentrations followed the same pattern at both high and low shore positions. There was a significant relationship between concentrations of sediment colloidal carbo- hydrate and chl a. Epipelic diatoms dominated the MPB, with cluster analysis identifying 10 distinct species assemblages. Navicula spp. were an important component of all assemblages. Peaks in chl a biomass occurred when a single species made up >65% of the cell numbers. The distribution of the diatom assemblages was related to salinity, temperature and dissolved inorganic nitrogen (DIN) con- centrations. Net production of oxygen occurred in sediments under illumination at all sites on almost all occasions. Oxygen uptake by sediment in the dark was positively correlated with temperature at the head of the estuary, but not at any other site. Mean annual chl a concentrations correlated posi- tively with total annual dark oxygen uptake. Annual primary production was estimated by 4 models using chl a concentrations and a model based on gross O2 fluxes across the sediment-water interface. Production was estimated to be in the range of 25 to 1199 gC m -2 yr -1 , depending on site and model. There was greater production on the high shore than on the low shore, due to higher chl a concen- trations and longer emersion periods. Total production of MPB in the estuary was estimated to be between 117 and 852 tons C yr -1 . Assimilation by the MPB provided a temporary sink for DIN, equiv- alent to 12.8% of the annual 1996 load to the Colne Estuary. N assimilation rates by the MPB were in the same order of magnitude as denitrification, indicating that assimilation by the MPB is an impor- tant process affecting the flux of DIN through this mesotidal estuary.

Dynamic behaviour of benthic microalgal biomass in intertidal mudflats

The dynamic behaviour of microphytobenthic biomass in a European intertidal mudflat has been analysed by comparing field and laboratory measurements. In an experimental mesocosm, where the effects of grazing by deposit-feeders and resuspension by tides had been significantly decreased, the benthic microalgal biomass followed a logistic-type growth curve, and thus converged towards a maximum value at which production is theoretically equal to zero. In the field, the kinetics of microalgal biomass—measured during 14 consecutive days at the beginning and at the end of every daytime exposures—exhibited a different pattern with biomass increases during daytime exposures and biomass decreases during the other periods, thus describing a series of oscillations. It is suggested that in the field, the effect of grazing and resuspension prevents biomass from holding steady at its maximum level. Moreover, it has been found a significant negative relationship which states that the higher the biomass level at the beginning of daytime exposures, the lower the net production during that daytime exposure. In other words, it turned out that in the field, the biomass also tends to converge towards a “field maximum” where net production is equal to zero. Taken together, these observations allow to hypothesize that the high productivity of the microphytobenthic community in intertidal mudflats is due to the tight coupling between physical and biological processes.

Small-Scale Distribution of Bacteria, Enzymatic Activities, and Organic Matter in Coastal Sediments.

The small-scale distribution of several structural (bacterial abundance, phytopigment, total and soluble protein, and carbohydrate content) and functional ecological variables (enzymatic activities, frequency of dividing cells) was investigated in coastal sediments during a spring bloom. For bacterial abundance, enzymatic activity, and organic matter determinations, samples were collected at 5-m depth from a sediment surface delimited by a 42 x 42-cm frame, divided into 49 squares. In order to test the influence of the bottom microtopography on the investigated variables, the size of this frame was defined to cover the distance between two subsequent ripples. As indicated by Fisher's index, benthic bacteria, enzymatic activities, proteins, carbohydrates, and their soluble fractions exhibited an aggregate distribution. Sampling size (i.e., sample unit of 36 cm2) was appropriate for all variables, except for chlorophyll a and frequency of dividing cells that displayed a contagious distribution. To estimate the reliability of the current sampling strategy, we compared the mean values from three randomly selected sample units with the average value of the entire sediment surface (i.e., 49 samples). For all variables reported in this study, the use of three replicates was representative of the mean values of the sampled area with a confidence limit within +/-20%. Bacterial population sizes did not correlate with their potential food sources (e.g., phytopigments, proteins, and carbohydrates), or with enzymatic activities, suggesting the presence of possible time lags between organic inputs and microbial response. Chlorophyll a during the spring bloom displayed much higher concentrations than phaeopigments and correlated with carbohydrates. Chlorophyll a distribution was autocorrelated and displayed a large patch size (1,134 cm2). Phaeopigments and proteins accumulated in the central depression of the ripple-mark structure, apparently depending upon a passive accumulation due to the reduced current action. In contrast, microphytobenthic biomass and bacterial numbers were highest in the two opposite ripples, suggesting that different driving forces operate selectively on the living components.

Microscale analysis of chlorophyll-a in cohesive, intertidal sediments: the implications of microphytobenthos distribution

Microphytobenthos are significant primary producers in many coastal systems. It is therefore important to quantify their biomass and productivity. Chlorophyll-a is often used as an index for microphytobenthic biomass. However, complications arise as most studies of sediment properties have been on a millimetre scale, whilst chemical and biological gradients in the surface layers of sediment occur over a microscale. The development of a new technique, the Cryolander (Wiltshire et al., 1997; Wiltshire, 2000), now allows microscale analysis of the sediment surface. Areas of high and low diatom biomass were compared using two coring techniques of different vertical resolution; the Cryolander method, with a vertical resolution of 0·2 mm and plastic core tubes (coarse coring), with a vertical resolution of 5 mm. Results indicated that, except at extreme biomass levels, coarse coring does not detect statistically significant differences in chlorophyll-a between obviously diverse sample sites. This may lead to misinterpretation of seasonal and spatial data when coarse coring is used. Furthermore microscale sectioning allows distinctions to be made between chlorophyll-a measured in the photic zone (photosynthetically active biomass (PAB)) and chlorophyll-a measured below the photic zone (photosynthetically inactive biomass (PIB)), allowing accurate determination of biomass specific primary production.

Seasonal Dynamics of Microphytobenthos Biomass and Photosynthetic Characteristics in the Northern German Wadden Sea, Obtained by the Photosynthetic Light Dispensation System

Microphytobenthic biomass and primary production were studied at two sampling sites in the northern German Wadden Sea from October 1996 to September 1997. Net oxygen production of the algae in the first millimetre of sediment, which was suspended in filtered seawater, was measured in the laboratory. The ‘photosynthetic light dispensation system’ in combination with a Clark type electrode was tested for its applicability for sediment slurries. Advantages and limitations of the system compared with other methods of primary production measurement are discussed. Primary production (PBmax) was between 0·01 and 2·66mgCmgChla−1h−1. Photosynthetic parameters such as the initial slope of the P-E curve (αB) and the light saturation parameter (Ek) reached a maximum of 0·034μgCμgChla−1h−1(μEm−2s−1)−1and 141μEm−2s−1, respectively. Daily production, calculated on the basis of daily irradiance, normally ranged from 0·27 to 448·9mgCm−2d−1. Chlorophyll a content varied between 2·21 and 38·34mgm−2. Both sampling sites showed large variation in chlorophyll a concentration during the seasons studied, which might be caused by changing environmental conditions and patchy distribution. Variation in photosynthetic parameters might also be due to varying physiological status of the algae and changing environmental conditions. A maximum in spring and autumn biomass was found at ‘Hedwigenkoog’. On the contrary, no clear seasonal trend was found in chlorophyll a at ‘Büsum’, but photosynthetic parameters showed an increase during summer. Only at this station were daily production and photosynthetic parameters significantly correlated with physical parameters.

The different primary producers in a small African tropical reservoir during a drought: temporal changes and interactions

1. The biomass of the different primary producers, and their relative contributions to the total, were measured quarterly from July 1997 to September 1998 in a tropical reservoir (Brobo, Côte‐d'Ivoire). The study period was marked by an exceptional decrease in the water level as a result of a long drought with only one significant rainfall in June 1998. 2. In July 1997, at the beginning of the usual rainy season, the level of the lake was normal and the ecosystem was in a moderately clear water state dominated by littoral macrophytes, mostly Potamogeton octandrus (more than 55% of the 6.4 tons of carbon of the total primary biomass). The microphytobenthos (19%) and the phytoplankton (25%) were secondary contributors, whereas the periphyton on macrophytes (0.5%) and the epixylon (periphyton on dead flooded trees, 0.3%) were negligible. 3. As a result of the steady decline in water level due to a lack of rain, the macrophytes were progressively stranded on the shores and had disappeared by March 1998. From this time, microalgae (microphytobenthos and phytoplankton) became dominant. Their combined biomass increased slowly from 2.8 tons of carbon in July 1997 to 3.7 tons in September 1998. The microphytobenthic biomass contributed 78% to the total in March, 55% in June and 60% in September, while other contributions were mainly due to the phytoplankton. The epixylon remained negligible throughout the study (≤0.5% of the whole primary biomass). 4. The distribution and temporal changes in the biomass during each survey were mainly linked to hydrology and to interactions between primary producers. 5. The effect of water level changes on free (planktonic) and fixed primary producers (periphytic microalgae and rooted macrophytes) is discussed.

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

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.