Production Of Microphytobenthos Research Articles

Benthic Heterotrophic Protist Communities of the Southern Baltic Analyzed with the Help of Curated Metabarcoding Studies.

Heterotrophic protists are key components of marine ecosystems. They act as controllers of bacterial and microphytobenthos production and contribute significantly to the carbon flux to higher trophic levels. Still, metabarcoding studies on benthic protist communities are much less frequent than for planktonic organisms. Especially in the Baltic Sea, representing the largest brackish water environment on earth, so far, no extensive metabarcoding studies have been conducted to assess the diversity of benthic protists in this unique and diverse habitat. This study aims to give first insights into the diversity of benthic protist communities in two different regions of the Baltic Sea, Fehmarnbelt, and Oderbank. Using amplicon sequencing of the 18S rDNA V9 region of over 100 individual sediment samples, we were able to show significant differences in the community composition between the two regions and to give insights into the vertical distribution of protists within the sediment (0-20 cm). The results indicate that the differences in community composition in the different regions might be explained by several abiotic factors such as salinity and water depth, but are also influenced by methodological aspects such as differences between DNA and RNA results.

The Ecological Role of Ruppia cirrhosa (Petagna) Grande in a Choked Lagoon

We studied the ecological and functional aspects (such as role in sediment characteristics and plant–animal interactions) of a Ruppia cirrhosa (Petagna) Grande meadow during its vegetative season in the choked Valle Campo lagoon, a sub-basin of the wider Valli di Comacchio, on the Northern Adriatic coast. Sampling campaigns were carried out with a roughly fortnightly frequency in 2017 at two sites, one with the Ruppia meadow and one with bare sediment. Sediment parameters analyzed were microphytobenthic chlorophyll-a, protein, carbohydrate, and lipid content, and total organic matter. The macrobenthos was identified at the lowest possible taxonomic level. Chlorophyll a, as a surrogate of microphytobenthos, showed differences between the two sites, probably mainly related to light intensity; thus, it is expected that the absence of seagrass canopy results in the higher production of microphytobenthos. At both sites, proteins were the dominant class of labile compounds, suggesting that detrital organic matter present at both study sites is of high nutritional quality. The high protein/carbohydrate ratio also suggests the presence of non-aged organic matter. We recorded a total of 18 macroinvertebrate taxa. The Ruppia meadow showed a positive influence on macrofauna abundance, diversity, species richness, and composition of trophic groups. Only the infaunal taxa Capitella capitata and Chironomus salinarius exhibited higher abundance at the bare site. The ecological quality status measured by the M-AMBI index was unsatisfactory everywhere. However, the presence of the Ruppia meadow resulted in index values being consistently higher. The role of this minor seagrass has been proved to be of great importance, improving the nutritional quality of the organic matter in the sediments and, above all, providing new habitats and new niches for a number of benthic macrofauna species.

Assessing biomass and primary production of microphytobenthos in depositional coastal systems using spectral information.

In depositional intertidal coastal systems, primary production is dominated by benthic microalgae (microphytobenthos) inhabiting the mudflats. This benthic productivity is supporting secondary production and supplying important services to humans including food provisioning. Increased frequencies of extreme events in weather (such as heatwaves, storm surges and cloudbursts) are expected to strongly impact the spatiotemporal dynamics of the microphytobenthos and subsequently their contribution to coastal food webs. Within north-western Europe, the years 2018 and 2019 were characterized by record-breaking summer temperatures and accompanying droughts. Field-calibrated satellite data (Sentinel 2) were used to quantify the seasonal dynamics of microphytobenthos biomass and production at an unprecedented spatial and temporal resolution during these years. We demonstrate that the Normalized Difference Vegetation Index (NDVI) should be used with caution in depositional coastal intertidal systems, because it may reflect import of remains of allochthonous pelagic productivity rather than local benthic biomass. We show that the reduction in summer biomass of the benthic microalgae cannot be explained by grazing but was most probably due to the high temperatures. The fivefold increase in salinity from January to September 2018, resulting from reduced river run-off during this exceptionally dry year, cannot have been without consequences for the vitality of the microphytobenthos community and its resistance to wind stress and cloud bursts. Comparison to historical information revealed that primary productivity of microphytobenthos may vary at least fivefold due to variations in environmental conditions. Therefore, ongoing changes in environmental conditions and especially extreme events because of climate change will not only lead to changes in spatiotemporal patterns of benthic primary production but also to changes in biodiversity of life under water and ecosystem services including food supply. Satellite MPB data allows for adequate choices in selecting coastal biodiversity conservation and coastal food supply.

Food sources for camptandriid crabs in an arid mangrove ecosystem of the Persian Gulf: a stable isotope approach

ABSTRACT Crabs of the family Camptandriidae are the most dominant burrowing crabs inhabiting arid mangrove forests of the Persian Gulf. They play important roles in the structuring and functioning of mangrove ecosystems by modulating biogeochemical processes and cycling of nutrients, serving as important ecosystem engineers. We analysed stable carbon (13C) and nitrogen (15N) isotope values of three camptandriid crabs (Opusia indica, Nasima dotilliformis, and Manningis arabicum) and their potential food sources in the Hara Biosphere Reserve, northern Persian Gulf. A Bayesian mixing model was used to estimate the contribution of potential food sources for consumers. The results showed that to some degree, all the four sources selected contributed to the camptandriid diets, but microphytobenthos made the most important contributions to the diet of the consumers. Mangroves do not appear to be a significant source of carbon in the diet of camptandriid crabs in the arid mangrove system of the Persian Gulf. Rather, they offer favourable growing conditions, thus boosting microphytobenthos production and availability for consumers which prefer a high nutritional and palatable source.

Ocean acidification modifies the impact of warming on sediment nitrogen recycling and assimilation by enhancing the benthic microbial loop

Nitrogen that has been recycled in the benthos supports high rates of primary and secondary production in estuaries. However, little is known about the effect of future climate on benthic nitrogen recycling and assimilation. An ex situ core incubation was used to assess the impact of combinations of warming (8°C range) and ocean acidification (OA) (i.e. increased pCO2 and decreased pH) on ammonium (NH4+) and nitrate/nitrite (NOx) fluxes and 15N-nitrate assimilation in shallow unvegetated estuarine sediments. Dissolved inorganic nitrogen (DIN = NH4+ + NOx) fluxes were significantly affected by the interaction of warming and OA, highlighting the importance of considering combined stressor treatments when investigating ecosystem responses to future climates. Warming alone increased DIN efflux from the sediments. At current mean ambient temperatures (23°C) and below (Δ-3°C), OA significantly increased DIN effluxes, but there was little to no effect of OA on DIN fluxes at warmer temperatures (Δ+3°C and Δ+5°C). OA reduced the 15N assimilation/retention of the sediments across all temperatures, suggesting that nitrogen retention in bacterial biomass was reduced, despite OA also increasing primary productivity. As such, under the projected future climate of ~3°C warming and doubling of pCO2 (~1000 µatm), unvegetated estuarine sediments are likely to have a more rapid turnover of DIN driven by greater microphytobenthos production and recycling.

Synoptic Spatio-Temporal Variability of the Photosynthetic Productivity of Microphytobenthos and Phytoplankton in a Tidal Estuary

Tidal estuaries are regarded as highly important ecosystems, mostly due to their high primary productivity and associated role as carbon sinks. In these ecosystems, primary productivity is mainly due to the photosynthetic carbon fixation by phytoplankton and microphytobenthos. The productivity of the two communities has been mostly studied separately, and directly comparable estimates of their carbon fixation rates in the same estuary are relatively scarce. The present study aimed to characterize the spatio-temporal variability of the productivity of phytoplankton and microphytobenthos in a tidal estuary, the Ria de Aveiro (Portugal), and to estimate the annual ecosystem-level budget for carbon fixation by the two communities. Productivity rates were determined based on synoptic in situ measurements of absolute rates of electron transport rate of photosystem II, using PAM fluorometry. Chlorophyll fluorescence indices were accompanied by measurements of salinity, temperature, water turbidity, solar irradiance, and planktonic and benthic microalgal biomass. Measurements were carried out hourly, along four spring-neap tidal cycles distributed along one year, on three sites of the estuary. The most pronounced trends in the spatio-temporal variability of the photophysiology and productivity of the two communities were the following: (i) maximum biomass and productivity were reached later for microphytobenthos (summer-autumn) than for phytoplankton (spring-summer); (ii) the absorption cross-section of PSII was generally higher for phytoplankton; (iii) the two groups showed a similar photoacclimation state, but microphytobenthos appeared as high light-acclimated when compared to phytoplankton. Biomass-specific productivity was on average higher for phytoplankton than for microphytobenthos, averaging 68.0 and 19.1 mg C mg Chl a-1 d-1, respectively. However, areal depth-integrated production rates were generally higher for the microphytobenthos than for the phytoplankton, averaging 264.5 and 140.0 mg C m-2 d-1, respectively. On an annual basis, phytoplankton productivity averaged 49.9 g C m-2 yr-1 while the productivity of microphytobenthos averaged 105.2 g C m-2 yr-1. When upscaling to the whole estuary, annual primary production rates of phytoplankton and microphytobenthos reached 4894.3 and 7534.0 t C yr-1, respectively, representing 39.4% and 60.6% of the combined total of 12428.3 t C yr-1 determined for the two communities in the Ria de Aveiro.

Microbenthic Net Metabolism Along Intertidal Gradients (Cadiz Bay, SW Spain): Spatio-Temporal Patterns and Environmental Factors

The changes in the annual biomass distribution of microphytobenthos (MPB) have been addressed by different studies, although this information is scarce in southern European coastal systems when compared to Northern European estuaries. Moreover, there is little information regarding annual changes in the primary production of MPB related with those in the annual biomass. For this reason, we followed the net production in light, dark respiration rates (determined by oxygen microelectrodes) and MPB abundance (estimated by sediment chlorophyll a) along an intertidal gradient of the inner Cadiz Bay during an annual cycle. Sediment cores were collected seasonally along two transects (5 sampling stations per transect) with contrasting sediment granulometry: muddy (Puerto Real, PR) and muddy-sandy (San Fernando, SF). Our results show that MPB biomass and net production increased seawards reaching maxima in winter. The seasonal maximum of daily solar irradiance and high temperature reached in summer in southern Spain likely inhibits MPB production. Spatial patterns of MPB biomass and net production depend on season and environmental heterogeneity. Environmental variables, analyzed by principal component analyses, were used to explain the variability of MPB metabolism by multiple regression. The principal component axes explained up to 60 % of the microphytobenthic net production and 41 % of the respiration rates in the PR transect while they only explained about 25 % of the same rates in SF. The variability in the environmental variables contributing to the observed patterns implies that no single set of variables can be used to predict net production patterns in the entire system. The different patterns of net production observed between transects highlight the importance of considering the spatial heterogeneity of our system to estimate the contribution of MPB to the inner Cadiz Bay productivity. This production is of major importance since our data showed that microphytobenthic production can contribute up to 49 % of the total benthic production of intertidal sediments of the inner Cadiz Bay.

Spatiotemporal variability in microphytobenthic primary production across bare intertidal flat, saltmarsh, and mangrove forest of Asia and Australia

The ecological role of intertidal microphytobenthos (MPB) is increasingly recognized in coastal production systems. MPB primary production (PP) measured in coastal wetlands of Korea, Cambodia, and Australia confirmed large variability at the global scale. Surprisingly, MPB biomass in mangrove forests almost doubled those measured in nearby bare tidal flats. However, MPB productivity (Pb) in vegetated habitats was significantly reduced (by ~50%) compared to that on bare tidal flats. Extensive measurements of MPB biomass, PP, and Pb across 12 Korean tidal flats revealed large spatiotemporal variations, suggesting complex sediment-MPB coupled dynamics. The key factors included sediment type, tide, bed elevation, irradiation, temperature, and vegetation. Winter MPB blooms and the elevated Pb seem to be unique characteristics of the Korean intertidal flats. The present study provides the baseline data of MPB PPs in mudflat, saltmarsh, and mangrove habitats in the highly productive zones of the Western Indo-Pacific Rim.

Environmental drivers of nematode abundance and genus composition at two spatial scales on an estuarine intertidal flat

Estuarine intertidal flats are important ecosystems characterized by high primary production of microphytobenthos and high secondary production of macro- and meiofauna, especially nematodes. However, the link between both ecosystem components (microphytobenthos and faunal communities) is not fully established yet. In this study, spatial patterns and drivers of nematode density and genus composition were investigated at two different spatial scales (i.e. meso- and microscale), with drivers including sediment granulometry, inundation period and food availability as indicated by various phytopigments. Our study has shown that specific food sources, as represented by different pigments and measures of freshness, are important drivers of nematode genus composition and densities at both scales, especially for the surface layers of the sediments. These food sources mainly comprise microphytobenthos, but also deposited phytodetritus and zooplankton faecal pellets, a resource which had hitherto been largely overlooked in intertidal flats. Tidal level and grain size also had a more pronounced structuring effect in the surface layer of the sediment, while their assumed larger importance at the mesoscale was not outspoken. At both scales, vertical heterogeneity in nematode assemblages was larger than horizontal variability, which has repercussions for future studies into the spatial variability of nematode assemblages of tidal flats.

Effects of key species mud snail Bullacta exarata (Gastropoda) on oxygen and nutrient fluxes at the sediment-water interface in the Huanghe River Delta, China

Since the mud snail Bullacta exarata was introduced for economic aquaculture in the Huanghe River (Yellow River) Delta in 2001, its quick population growth and expanded distribution make it a key-species in the intertidal zone of this area. This significantly contributed to the economic income of the local people, but its potential ecological impact on the benthic ecosystem remains unknown. A mesocosm study was conducted to test whether its bioturbation activities affect the microphytobenthos (MPBs; i.e., sedimentary microbes and unicellular algae) productivity and the nutrient exchange between the sediment-water interface. Our results show that the mud snail significantly impacted the dissolved oxygen (DO) flux across the sediment-water interface on the condition of normal sediment and light treatment, and significantly increased the ammonium efflux during recovery period in the defaunated sediment and dark treatment. The presence of micro- and meiofauna significantly increased the NH4-N flux in dark treatment. Whereas, in light treatment, these small animals had less effects on the DO and NH4-N flux between sediment-water interface. Our results provide better insight into the effect of the mud snail B. exarata on the ecosystem functioning via benthic fluxes.

Water column dissolved silica concentration limits microphytobenthic primary production in intertidal sediments.

Primary production of microphytobenthos (MPB) contributes significantly to the total production in shallow coastal environments. MPB is a diverse community in which diatoms are usually the main microalgal group. Diatoms require N, P, and other nutrients as with other autotrophs, but in addition require silicate to create their outer cell wall. Therefore, dissolved silica (DSi) might be a potential limiting factor for benthic primary production in areas with reduced freshwater input. To test this hypothesis, a microcosm experiment was conducted using intact sediment cores collected from an intertidal mudflat in the Bay of Cádiz and supplied with increasing concentrations of DSi (0, 5, 10, 25, and 45μmol·L-1 ). After 7d of enrichment, we determined chlorophyll a and c (Chl a, c) contents, metabolic rates (Net [Pn ] and Areal Gross [PgA ] Production and Light [RL ] and Dark [RD ] Respiration), as well as fluxes of inorganic nutrients across the sediment-water interface. Chl a and c contents increased significantly with respect to the initial conditions but no differences between treatments were found. Both Pn and PgA showed a saturating-like pattern with silicate concentration, reaching maximum rates at a DSi concentration of 45μmol·L-1 . The addition of DSi also resulted in an increase of DSi and ammonium uptake by the sediment, which was significantly higher in light than in darkness. Our results clearly show that water column DSi concentrations have a direct impact on benthic primary production, also controlling other related processes such as inorganic nutrient fluxes.

Dynamics of phytoplankton productivity and exopolysaccharides (EPS and TEP) pools in the Seine Estuary (France, Normandy) over tidal cycles and over two contrasting seasons

Exopolysaccharides (EPS) play an important role in the carbon flux and may be directly linked to phytoplankton and microphytobenthos production, most notably in estuarine systems. However the temporal and spatial dynamics of estuarine EPS are still not well understood, nor how primary productivity triggers this variability at these different scales.The aim of this study was to investigate the primary productivity of phytoplankton and EPS dynamics in the Seine estuary over a tidal cycle in three different haline zones over two contrasted seasons. The other objectives was to investigate the origin of pools of soluble carbohydrates (S-EPS) and transparent exopolymeric particles (TEP) in phytoplankton, microphytobenthos or other compartments. High frequency measurements of productivity were made in winter and summer 2015. Physical and chemical parameters, biomass and EPS were measured at hourly intervals in sub-surface waters and just above the water sediment-interface.Our results confirmed that high frequency measurements improve the accuracy of primary productivity estimations and associated carbon fluxes in estuaries. The photosynthetic parameters were shown to be strongly controlled by salinity and by the concentrations of suspended particle matter at the smallest temporal and at spatial scales. At these scales, our results showed an inverse relationship between EPS concentrations and biomass and productivity, and a positive relationship with sediment resuspension. Additionally, the distribution of EPS appears to be linked to hydrodynamics with the tide at daily scale and with the winter at seasonal scale. At spatial scale, the maximum turbidity zone played an important role in the distribution of TEP.Our results suggest that, in the Seine estuary, between 9% and 33% of the S-EPS pool in the water column can be attributed to phytoplankton excretion, while only 0.4%–1.6% (up to 6.14% in exceptional conditions) originates from the microphytobenthos compartments. Most EPS was attributed to remobilization of detrital carbon pools in the maximum turbidity zone and in the sediment or allochthonous origin.

Inorganic carbon availability in benthic diatom communities: photosynthesis and migration.

Diatom-dominated microphytobenthos (MPB) is the main primary producer of many intertidal and shallow subtidal environments, being therefore of critical importance to estuarine and coastal food webs. Owing to tidal cycles, intertidal MPB diatoms are subjected to environmental conditions far more variable than the ones experienced by pelagic diatoms (e.g. light, temperature, salinity, desiccation and nutrient availability). Nevertheless, benthic diatoms evolved adaptation mechanisms to these harsh conditions, including the capacity to move within steep physical and chemical gradients, allowing them to perform photosynthesis efficiently. In this contribution, we will review present knowledge on the effects of dissolved inorganic carbon (DIC) availability on photosynthesis and productivity of diatom-dominated MPB. We present evidence of carbon limitation of photosynthesis in benthic diatom mats and highly productive MPB natural communities. Furthermore, we hypothesize that active vertical migration of epipelic motile diatoms could overcome local depletion of DIC in the photic layer, providing the cells alternately with light and inorganic carbon supply. The few available longer-term experiments on the effects of inorganic carbon enrichment on the productivity of diatom-dominated MPB have yielded inconsistent results. Therefore, further studies are needed to properly assess the response of MPB communities to increased CO2 and ocean acidification related to climate change.This article is part of the themed issue 'The peculiar carbon metabolism in diatoms'.

Influence of New Zealand cockles (Austrovenus stutchburyi) on primary productivity in sandflat-seagrass (Zostera muelleri) ecotones

New Zealand cockles (Austrovenus stutchburyi) are ecologically important, intertidal bivalves that have been shown to influence nutrient cycles and the productivity of microphytobenthos on sandflats. Here, we investigated the potential for cockles to impact the productivity of seagrass, Zostera muelleri, and examined interactions between these habitat-defining species where they co-occur. We sampled bivalve densities and sizes, sediment properties, and seagrass shoot densities across the boundaries of two seagrass patches on an intertidal sandflat in northern New Zealand, and measured dissolved oxygen and nutrient fluxes in light and dark benthic incubation chambers in conjunction with a 0–97% gradient in seagrass cover. Although gross primary production (GPP, μmol O2 m−2 h−1) increased predictably with the cover of live seagrass, the density of cockles and sediment properties also contributed directly and indirectly. Seagrass cover was positively correlated with cockle density (ranging from 225 to 1350 individuals per m2), sediment mud percentage (0.5–9.5%), and organic matter content (0.5–2.2%), all of which can affect the efflux of ammonium (readily utilisable inorganic nitrogen) from sediments. Moreover, the cover of green seagrass blades plateaued (never exceeded 70%) in the areas of highest total seagrass cover, adding complexity to cockle-seagrass interactions and contributing to a unimodal cockle-GPP relationship.

Temporal dynamics and spatial heterogeneity of microalgal biomass in recently reclaimed intertidal flats of the Saemangeum area, Korea

Trophodynamics of intertidal mudflats are significantly driven by microphytobenthos (MPB) production but spatial and temporal dynamics of this production source is poorly known. To understand the temporal dynamics and spatial heterogeneity of intertidal MPB, benthic chlorophyll a, phaeopigments, and sediment properties were determined in Gyehwa (sandy) and Gwanghwal (muddy) tidal flats of Saemangeum area over a year at 97 stations. This study set out to: (i) characterize the spatial-temporal patterns in MPB biomass on a year-round basis, (ii) identify the abiotic and biotic factors associated with MPB distributions, (iii) investigate the use of satellite-derived chlorophyll a data and verify with in field measurements, and (iv) determine minimum required sample size for in situ biomass measurement. Concentrations of benthic chlorophyll a and phaeopigments were greater in winter and spring with a high magnitude of variance than in summer and fall at both areas. Benthic chlorophyll a and phaeopigments tended to decrease approaching lower tidal zone, being associated with the corresponding decrease in shore level and/or exposure duration. Compared to available data on macrozoobenthos distribution, the spatial variation of microalgal biomass seems to be attributed to distribution of deposit-feeders. A significant positive correlation (p<0.001) between in situ MPB biomass and satellite-derived normalized difference vegetation index (NDVI) values was observed, but was much weaker in the lower tidal zone. Mirroring algal heterogeneity, the minimum required sample size for in situ biomass measurement were greater in blooming season and sandy bottom, suggesting that sampling design for spatio-temporal mapping of MPB should consider the sampling season and/or abiotic and biotic features of study area. Overall, spatio-temporal dynamics of intertidal MPB seem to be influenced by a combination of abiotic and biotic factors.

The Relationship between Primary Production of Microphytobenthos and Tidal Cycle on the Hwaseong Mudflat, West Coast of Korea

Kwon, B.-O.; Koh, C.-H.; Khim. J. S.; Park, J.; Kang, S-G., and Hwang, J.H., 2014. The relationship between primary production of microphytobenthos and tidal cycle on the Hwaseong mudflat, west coast of Korea.The present study examined variation in sediment temperature and irradiance with respect to microphytobenthos primary production at the Hwaseong mudflat, west coast of Korea, under different tidal cycles. The study was carried out during March and April 2009, which encompassed two spring tides and one neap tide. It was not possible to continuously measure primary production in situ across the tidal cycles; hence, photosynthesis–irradiance (P-I) curves at different temperatures were obtained experimentally in the laboratory. These readings were then used to calculate primary production, by projecting in situ mooring records of irradiance and temperature onto the P-I curves. The sediment temperature and irradiance were measured continuously, and the averages of 10-minute intervals from the mooring records were used. The results show that changes in primary production simulated from the P-I curves were strongly associated with the tidal cycle. During the two spring tides that were recorded, mean daily primary production was 1005 and 1125 mg C m−2 d−1, respectively. However, primary production was lower during the neap tide (898 mg C m−2 d−1). Temperature appeared to influence these differences in mean daily production. In situ mean daily temperatures during exposure were 14°C and 20°C during the first and second spring tides, respectively. In contrast, mean daily total irradiance was noticeably higher in the first spring tide compared with the second spring tide. During the neap tide, the site was exposed in the morning and afternoon, when light intensity and temperature were relatively low. In contrast, during the spring tide, the site was exposed at noon, when light intensity and temperature were high. The current study provided new insights about how changes in environmental parameters during tidal cycles affect microphytobenthic primary production, indicating the complexity of this process and the requirement for further fine-scale research.

Microphytobenthos production potential and contribution to bottom layer oxygen dynamics on the inner Louisiana continental shelf

To investigate the relative importance of microphytobenthos (MPB) oxygen (O2) production on a river-dominated shelf, we made sediment core incubation measurements of MPB O2 production and sediment O2 consumption, and compared these to water-column measures of primary production and respiration during one spring and two summer cruises. Sites were located across a light attenuation (Kd) gradient on the inner Louisiana shelf (<25 m depth) with three sites on the eastern shelf and one site on the western shelf. MPB production rates were highest (range = 0.7-33.4 mmol O2 m-2 d-1) at the western site where light attenuation was lowest (range = 0.13-0.16 m-1). Near the river plumes, where Kd was higher (range = 0.17-0.64 m-1), MPB production was lower (range = 0-2.0 mmol m-2 d-1). MPB production influenced the magnitude and direction of sediment O2 exchanges at the western site and was observed to be a function of light availability. Overall, though, in comparison with integrated bottom layer production and respiration rates, MPB photosynthetic O2 production as a percentage was small (range of means = 0%-2%) at the eastern shelf sites. At the western site, MPB production was larger, and more variable, 39% (SE 34%) of bottom layer O2 production. Though based on a small data set, results suggest distinct differences between eastern and western shelf Kd and MPB contributions to bottom water O2 due to proximity to the freshwater discharges of the Mississippi and Atchafalaya rivers.

Transformation and fate of microphytobenthos carbon in subtropical, intertidal sediments: potential for long-term carbon retention revealed by &amp;lt;sup&amp;gt;13&amp;lt;/sup&amp;gt;C-labeling

Abstract. Microphytobenthos (MPB) are ubiquitous in coastal sediments, but the fate of their production (carbon biomass) is poorly defined. The processing and fate of MPB-derived carbon in subtropical intertidal sediments was investigated through in situ labeling with 13C-bicarbonate. Of the added 13C, 100% was fixed within ~ 4 h, suggesting that MPB productivity was limited by inorganic carbon availability. Although there was rapid transfer of 13C to bacteria (within 12 h), a relatively small fraction of 13C was transferred to heterotrophs (up to 12.5% of total fixed 13C into bacteria and 0.01% into foraminifera). MPB was the major reservoir for 13C throughout the study, suggesting that production of extracellular polymeric substances was limited and/or MPB recycled 13C. This retention of 13C was reflected in remarkably slow estimated turnover times for the MPB community (66–100 d). Over 31 d, ~ 70% of the 13C was lost from sediments. This was primarily via resuspension (~ 55%), enhanced by elevated freshwater flow following rainfall. A further ~ 13% was lost via fluxes of dissolved inorganic carbon during inundation. However, 13C losses via dissolved organic carbon fluxes from inundated sediments (0.5%) and carbon dioxide fluxes from exposed sediments (&lt;0.1%) were minimal. The retention of ~ 30% of the carbon fixed by MPB within one tidal exposure after &gt; 30 d, despite high resuspension, demonstrates the potentially substantial longer term retention of MPB-derived carbon in unvegetated sediments and suggests that MPB may contribute to carbon burial ("blue carbon").

Relevance of macrozoobenthic grazers to understand the dynamic behaviour of sediment erodibility and microphytobenthos resuspension in sunny summer conditions

The quantification of overall microphytobenthos productivity should include the export of biomass from the intertidal zone during high tides, which implies refined estimates and concepts of erosion parameters. For the first time, the export of microphytobenthic cells was assessed over an intertidal mudflat in the Marennes-Oléron Bay, France, during a complete spring/neap tide modulation. In the summer of 2008, resuspension rates of chl-a exported only reached 2.5% of the standing stock of benthic diatoms on each day. Sedimentary factors failed to explain any variation regarding bed and microphytobenthos erodibility. During the early fluff layer erosion phase, there were negative effects of grazing activities exerted by motile infauna (Peringia ulvae) on erosion fluxes of chl-a, while there was a related positive correlation with pheopigment proportion. The erosion process plays an important role in this vegetal–herbivore interaction by reinforcing the decline of the microphytobenthic biomass and provoking a catastrophic shift to mass erosion after a sequence of several days of co-occurring intense grazing by snails and chl-a decline. During mass erosion, the biofilm decline explained the variations of sediment erodibility, with a marked negative correlation between bound extracellular polymeric substance (EPS) proteins and critical threshold for bed erosion, in contrast with the commonly observed positive influence of EPS secretion on bed resistance. The complex nature of the effects of EPS by microphytobenthos must be further investigated to unravel their complex role in bioengineering sediments. The increase of protein proportion in EPS could provide specific properties related to hydrophilic features. Nevertheless, the level of grazing pressure by P. ulvae should be so intense that the top-down control must explain this original finding, since there was a positive correlation of proteins in EPS and snail density that could be related to mucus secretion (as a constitutive part of the EPS pool).

Microphytobenthos and benthic macroalgae determine sediment organic matter composition in shallow photic sediments

Abstract. Microphytobenthos and benthic macroalgae play an important role in system metabolism within shallow coastal bays. However, their independent and interactive influences on sediment organic matter (SOM) are not well understood. We investigated the influence of macroalgae and microphytobenthos on SOM quantity and quality in an experimental mesocosm system using bulk and molecular level (total hydrolyzable amino acids, THAA; phospholipid linked fatty acids, PLFA; pigment) analyses. Our experiment used an incomplete factorial design made up of two factors, each with two levels: (1) light (ambient vs. dark) and (2) macroalgae (presence vs. absence of live macroalgae). Over the course of the 42-day experiment, total organic carbon (TOC) and total nitrogen (TN) increased under ambient light by 173 ± 14 and 141 ± 7%, respectively, compared to in the dark (78 ± 29 and 39 ± 22%). THAA comprised a substantial fraction of SOM (~ 16% of TOC, 35% of TN) and followed TOC and TN accumulation patterns. Mole percent composition of the THAA pool indicated that SOM was composed of more labile organic material (e.g., L-glutamic acid, phenylalanine) under ambient light conditions while SOM in dark treatments was more degraded, with higher proportions of glycine and D-alanine. PLFA content, which represents viable biomass, made up ~ 1% of TOC and contained high levels of algal fatty acids in the light, particularly PLFA derived from diatoms. In the presence of microphytobenthos (i.e., light and macroalgae treatments), SOM lability increased, resulting in the observed increases in bacterial PLFA concentrations. Macroalgae, which were added to half of the light treatments, decreased SOM accumulation compared to light treatments without macroalgae, with TOC and TN increasing by only 130 ± 32 and 94 ± 24%, respectively. This decrease likely resulted from shading by macroalgae, which reduced production of microphytobenthos. The presence of macroalgae decreased SOM lability as well, which resulted in diminished buildup of bacterial biomass. By the final day of the experiment, principal component analysis revealed that sediment composition in treatments with macroalgae was more similar to dark treatments and less similar to light treatments without macroalgae. Overall, microphytobenthos and benthic macroalgae fundamentally altered SOM quality and quantity, which may have notable ecological consequences for shallow-water systems such as increased hypoxia/anoxia, sulfide accumulation, enhanced mineralization and/or stimulated denitrification.