MMHg production and export from intertidal sediments to the water column of a tidal lagoon (Arcachon Bay,France)

Hg cycling in biologically productive coastal areas is of special importance given the potential for bioaccumulation of monomethylmercury (MMHg) into aquatic organisms. Field experiments were performed during three different seasons in Arcachon Bay, a mesotidal lagoon (SW France), to assess the variability of the water column concentrations, sediment–water exchanges and potential formation and degradation of MMHg. The objectives were to evaluate the contribution of intertidal mudflats to MMHg production and the various pathways of Hg species export. Dissolved and bulk concentrations of Hg species in the water column downstream of tidal flats were measured throughout several tidal cycles. The Hg benthic fluxes at the sediment–water interface were determined by means of benthic chambers for three different stations. Hg methylation and demethylation potentials were determined in surficial sediments and the water column using isotopic tracers. The tidal surveys demonstrated that benthic remobilization of Hg occurs primarily in association with sediment erosion and advection during ebb tide. However, elevated dissolved Hg concentrations observed at low tide were found to be caused by a combination of pore-waters seeping, benthic fluxes and methylation in the water column. Benthic fluxes were more intense during late winter conditions (median MMHg and inorganic Hg (IHg) fluxes: 64 and 179 pmol m^?2 h^?1, respectively) and subsequently decreased in spring (median 0.7 and ?5 pmol m^?2 h^?1, respectively) and fall (median ?0.4 and ?1.3 pmol m^?2 h^?1, respectively). The trends in methylation and demethylation potentials were at the opposite of the fluxes, two times lower during winter than for spring or fall conditions. In this tidal environment, MMHg production in surface sediments and its subsequent release is estimated to be the major source of MMHg to the water column during winter and spring time. However, during the more productive summer period, the Hg methylation extent in the water column may be very significant and equivalent to the sediment contribution.     

Rare but large bivalves alter benthic respiration and nutrient recycling in riverine sediments

Bioturbation studies have generally analyzed small and abundant organisms while the contribution to the benthic metabolism by rare, large macrofauna has received little attention. We hypothesize that large, sporadic bivalves may represent a hot spot for benthic processes due to a combination of direct and indirect effects as their metabolic and bioturbation activities. Intact riverine sediments with and without individuals of the bivalve Sinanodonta woodiana were collected in a reach with transparent water, where the occurrence of the mollusk was clearly visible. The bivalve metabolism and its effects on sedimentary fluxes of dissolved gas and nutrients were measured via laboratory incubations of intact cores under controlled conditions. S. woodiana contributed significantly to O₂ and TCO₂ benthic fluxes through its respiration and to \({\text{NH}}_{4}^{ + }\), SRP and SiO₂ regeneration via its excretion. The bivalve significantly stimulated also microbial denitrification and determined a large efflux of CH₄, likely due a combination of bioturbation and biodeposition activities or to anaerobic metabolism within the mollusk gut. This study demonstrates that a few, large individuals of this bivalve produce significant effects on aerobic and anaerobic benthic metabolism and nutrient mobilization. Random sediment sampling in turbid waters seldom catches these important effects due to low densities of large fauna.     

The effects of bioturbation on particle redistribution in Mediterranean coastal sediment. Preliminary results

In order to quantify bioturbation processes in a coastal Mediterranean ecosystem, experiments were performed to determine sediment mixing rates resulting from macrobenthos activity. Particle flux was measured in situ for 22 days using luminophores, which are colored sediment particles with sizes ranging from 10 to 200 µm.In sediment depths from 0–5 cm, particle mixing was intensive due to high macrobenthos abundance. A small quantity of luminophores was transported down to a depth of 14 ± 2 cm, where the macrofauna was represented principally by Polychetes. In a control experimental structure — without benthic fauna — no transfer of luminophores into the sediment was recorded.Sediment particle mixing measured in the ecosystem studied is intensive, and is the result of high macrobenthos activity. Different mixing modes occur with scales and rates depending on the organisms present. The luminophore profile resulting from bioturbation processes is explained by an intensive bioadvection sediment mixing added to a biodiffusive mixing with an order of magnitude of 10^–6 cm² s^–1. Tracer accumulations between 1 and 2 ± 1 cm and between 4 and 5 ± 1 cm are attributed to bioadvection activity of two or more distinct populations. Studies over a larger time scale have been undertaken to monitor developments in the observed subsurface maxima.     

Benthic transfer and speciation of mercury in wetland sediments downstream from a sewage outfall

In order to evaluate the role of hypoxic conditions of overlying water in the benthic flux and speciation of Hg, we analyzed sediment cores from hypoxic or oxic sites downstream from a sewage outfall in the Damyang Riverine Wetland, Korea. Each core was analyzed for total Hg (THg), monomethylmercury (MMHg), and elemental Hg (Hg⁰) from sediment, and for THg and MMHg from pore water. Hypoxic conditions of the overlying water near the sewage outfall were associated with a peak production of Hg⁰, but the lowest production of MMHg, in the upper 2 cm sediments. The benthic fluxes of THg and MMHg were estimated at 130-2109 ng m⁻² day⁻¹ and −12 to 260 ng m⁻² day⁻¹, respectively. The order of MMHg flux from sediment to overlying water at each site did not follow the order of MMHg concentration in sediment, but was highest in hypoxic water conditions. The results suggest that maintaining oxic conditions in wetland water is important for decreasing the transfer of MMHg from sediment into overlying water.     

Biogeochemical cycling of methylmercury at Barn Island Salt Marsh,Stonington, CT,USA

Monomethylmercury (MMHg) is toxic, and is the primary form of Hg thatbioaccumulates in the food web. An understanding of its distribution,production, and transport is needed. Prior investigations indicate thatmethylation is mediated by sulfate-reducing bacteria, yet limited in highsulfate environments. High rates of microbial respiration and strong oxygengradients are found in salt marshes. It is hypothesized that significant in situ methylation takes place in the redox transition zone of sulfate rich( 28 mM) salt marsh sediment. Results from a water column surveyof Barn Island Salt Marsh in October 1996 showed that ca. 61pmol m^-2 d^-1 of dissolved MMHg were discharged toadjacent coastal waters, while 16 pmol m^-2 d^-1 ofparticulate MMHg were entrained in the marsh, implying an in situsource. In-sediment MMHg production rates were determined by²⁰³Hg radiotracer studies. At the surface, methylation rates variedover both long (i.e., 100's m; 11–1120 pmol m^-2 d^-1) andshort (i.e., 10 cm; 11–108 pmol m^-2 d^-1) spatial scales. Methylation rate profiles from both low and high MMHg production sitesexhibited an exponential decrease below the redox transition zone. Porewater was collected with multi-chambered in situ dialysis (30 kDa)samplers [Peepers] and analyzed for MMHg. Temporal differences in porewater MMHg accumulation (i.e., May > September > November)were found. Results from May showed a significant gradient at thesediment water interface. The transport out of the sediments estimated byFick's Law (ca. 390 pmol MMHg m-2 d^-1) suggeststhat MMHg entered the marsh water by diffusion. This workdemonstrates the potential for elevated in situ Hg methylation in highsulfate environments.     

Influence of the mode of macrofauna-mediated bioturbation on the vertical distribution of living benthic foraminifera: First insight from axial tomodensitometry

We investigated the influence of bioturbation by macrofauna on the vertical distribution of living (stained) benthic foraminifera in marine intertidal sediments. We investigated the links between macrofaunal bioturbation and foraminiferal distribution, by sampling from stations situated on a gradient of perturbation by oyster-farming, which has a major effect on benthic faunal assemblages. Sediment cores were collected on the French Atlantic coast, from three intertidal stations: an oyster farm, an area without oysters but affected by oyster biodeposits, and a control station. Axial tomodensitometry (CT-scan) was used for three-dimensional visualization and two-dimensional analysis of the cores. Biogenic structure volumes were quantified and compared between cores. We collected the macrofauna, living foraminifera, shells and gravel from the cores after scanning, to validate image analysis. We did not investigate differences in the biogenic structure volume between cores. However, biogenic structure volume is not necessarily proportional to the extent of bioturbation in a core, given that many biodiffusive activities cannot be detected on CT-scans. Biodiffusors and larger gallery-diffusors were abundant in macrofaunal assemblage at the control station. By contrast, macrofaunal assemblages consisted principally of downward-conveyors at the two stations affected by oyster farming. At the control station, the vertical distribution of biogenic structures mainly built by the biodiffusor Scorbicularia plana and the large gallery-diffusor Hediste diversicolor was significantly correlated with the vertical profiles of living foraminifera in the sediment, whereas vertical distributions of foraminifera and downward-conveyors were not correlated at the station affected by oyster farming. This relationship was probably responsible for the collection of foraminifera in deep sediment layers (> 6 cm below the sediment surface) at the control station. As previously suggested for other species, oxygen diffusion may occur via the burrows built by S. plana and H. diversicolor, potentially increasing oxygen penetration and providing a favorable microhabitat for foraminifera in terms of oxygen levels. By contrast, the absence of living foraminifera below 6 cm at the stations affected by oyster farming was probably associated with a lack of biodiffusor and large gallery-diffusor bioturbation. Our findings suggest that the effect of macrofaunal bioturbation on the vertical distribution of foraminiferal assemblages in sediments depends on the effects of the macrofauna on bioirrigation and sediment oxidation, as deduced by Eh values, rather than on the biogenic structure volume produced by macrofauna. The loss of bioturbator functional diversity due to oyster farming may thus indirectly affect infaunal communities by suppressing favorable microhabitats produced by bioturbation.     

生物扰动对沉积物中污染物环境行为的影响研究进展

生物扰动由于显著改变沉积物结构和性质,进而影响沉积物中污染物的环境行为。综述生物扰动对沉积物中氮、磷、重金属和疏水性有机污染物环境行为的影响。生物扰动促进这些污染物从沉积物向水体释放。生物扰动还对不同的污染物产生其它不同的影响。对于氮,生物扰动还影响其硝化与反硝化作用;对于磷,生物扰动不仅改变其化学形态,还提高有机磷降解。对于重金属,生物扰动还能改变其在沉积物中的分布及化学形态。对于疏水性有机污染物,生物扰动主要增强生物富集和代谢,以及提高生物降解。     

Empirical Evidence Reveals Seasonally Dependent Reduction in Nitrification in Coastal Sediments Subjected to Near Future Ocean Acidification

Research so far has provided little evidence that benthic biogeochemical cycling is affected by ocean acidification under realistic climate change scenarios. We measured nutrient exchange and sediment community oxygen consumption (SCOC) rates to estimate nitrification in natural coastal permeable and fine sandy sediments under pre-phytoplankton bloom and bloom conditions. Ocean acidification, as mimicked in the laboratory by a realistic pH decrease of 0.3, significantly reduced SCOC on average by 60% and benthic nitrification rates on average by 94% in both sediment types in February (pre-bloom period), but not in April (bloom period). No changes in macrofauna functional community (density, structural and functional diversity) were observed between ambient and acidified conditions, suggesting that changes in benthic biogeochemical cycling were predominantly mediated by changes in the activity of the microbial community during the short-term incubations (14 days), rather than by changes in engineering effects of bioturbating and bio-irrigating macrofauna. As benthic nitrification makes up the gross of ocean nitrification, a slowdown of this nitrogen cycling pathway in both permeable and fine sediments in winter, could therefore have global impacts on coupled nitrification-denitrification and hence eventually on pelagic nutrient availability.     

Benthic fauna bio-irrigation effects on nutrient regeneration in fish farm sediments

Impacts of organic enrichment and a modified benthic fauna community (caused by fish farming) on benthic mineralization rates and nutrient cycling were studied in sediments at one Danish and one Cypriote fish farm. Sediment organic matter concentration and macrofauna community composition were manipulated in microcosms and changes in total benthic metabolism (oxygen consumption, TCO₂ production), anaerobic metabolism (sulfate reduction rates), nutrient fluxes and sediment parameters were followed for a period of 3 weeks. Mineralization rates were found to be highly correlated with irrigation velocities and largest fauna effects were found in the Danish sediments with the large and active irrigating climax species (Nereis diversicolor and Macoma balthica). Eastern Mediterranean climax species (Glycera rouxii and Naineris laevigata) also stimulated mineralization rates but to a smaller extent due to lower irrigation, whereas the opportunistic species (Capitella in Danish sediment and Hermodice carunculata in Cypriote sediment) showed less effect on mineralization. Ammonium and phosphate release increased with increasing irrigation velocities, but much less in Cyprus indicating higher nutrient retention at the ultra-oligotrophic location compared to eutrophic Danish site. Irrigation velocities, and thus mineralization rates, increased by organic matter loading, indicating larger fauna-induced oxidation in enriched environments. The result implies that a change in fauna structure in fish farm sediment towards smaller opportunistic polychaete species with lower irrigation will result in slower mineralization rates and potentially increase accumulation of organic waste products.     

A comparison of two nitrification inhibitors used to measure nitrification rates in estuarine sediments

Abstract: Nitrification rates were measured using intact sediment cores from South San Francisco Bay and two different nitrification inhibitors: acetylene and methyl fluoride. Sediment oxygen consumption and ammonium and nitrate fluxes were also measured in these cores. Four experiments were conducted in the spring, and one in the fall of 1993. There was no significant difference in nitrification rates measured using the two inhibitors, which suggests that methyl fluoride can be used as an effective inhibitor of nitrification. Nitrification was positively correlated with sediment oxygen consumption and numbers of macrofauna. This suggests that bioturbation by macrofauna is an important control of nitrification rates. Irrigation by the tube-dwelling polychaete, Asychis elongata , which dominates the benthic biomass at this location, appears particularly important. Ammonium fluxes out of the sediment were greatest about one week after the spring bloom, while nitrification peaked about one month later.     

Rapid reworking of subtidal sediments by burrowing spatangoid urchins

The mixing and displacement of sediment by benthic macrofauna (bioturbation) has major biogeochemical implications, and can control rates of organic matter degradation and carbon burial. Large, abundant, mobile macrofauna often dominate sediment bioturbation, and heart urchins of the genus Echinocardium are regarded as key sediment bioturbators in marine systems throughout the world. To better understand the bioturbation potential and functional role of Echinocardium, we developed a mathematical model and parameterized it with field data from six locations in northern New Zealand in order to estimate bioturbation rates in these places. Although urchin sizes and densities were measured in consecutive years at all six locations, we obtained a third model parameter, urchin movement rate, from one time and place only (Site OB5). Because confidence in model output was greatest at OB5, and since OB5 had the highest sediment reworking rate of all sites, our model yielded a good upper bound estimate for the bioturbation potential of Echinocardium in the areas examined. The volume of sediment displaced by Echinocardium populations reached 20,000 cm³ m⁻² d⁻¹ at OB5, suggesting that surface sediment is reworked about every 3 days at sites where Echinocardium is abundant. Experimental work with a fluorescent tracer at OB5 suggested limited downward particle movement as a result of Echinocardium bioturbation, though vertical profiles of chlorophyll a and organic matter content indicated well mixed sediment. The loss of Echinocardium because of broad-scale anthropogenic disturbance to the seabed could have major consequences on marine ecosystem functioning.     

Bioturbation: impact on the marine nitrogen cycle

Sediments play a key role in the marine nitrogen cycle and can act either as a source or a sink of biologically available (fixed) nitrogen. This cycling is driven by a number of microbial remineralization reactions, many of which occur across the oxic/anoxic interface near the sediment surface. The presence and activity of large burrowing macrofauna (bioturbators) in the sediment can significantly affect these microbial processes by altering the physicochemical properties of the sediment. For example, the building and irrigation of burrows by bioturbators introduces fresh oxygenated water into deeper sediment layers and allows the exchange of solutes between the sediment and water column. Burrows can effectively extend the oxic/anoxic interface into deeper sediment layers, thus providing a unique environment for nitrogen-cycling microbial communities. Recent studies have shown that the abundance and diversity of micro-organisms can be far greater in burrow wall sediment than in the surrounding surface or subsurface sediment; meanwhile, bioturbated sediment supports higher rates of coupled nitrification-denitrification reactions and increased fluxes of ammonium to the water column. In the present paper we discuss the potential for bioturbation to significantly affect marine nitrogen cycling, as well as the molecular techniques used to study microbial nitrogen cycling communities and directions for future study.     

Influence of macrofaunal assemblages and environmental heterogeneity on microphytobenthic production in experimental systems

Despite the complexity of natural systems, heterogeneity caused by the fragmentation of habitats has seldom been considered when investigating ecosystem processes. Empirical approaches that have included the influence of heterogeneity tend to be biased towards terrestrial habitats; yet marine systems offer opportunities by virtue of their relative ease of manipulation, rapid response times and the well-understood effects of macrofauna on sediment processes. Here, the influence of heterogeneity on microphytobenthic production in synthetic estuarine assemblages is examined. Heterogeneity was created by enriching patches of sediment with detrital algae (Enteromorpha intestinalis) to provide a source of allochthonous organic matter. A gradient of species density for four numerically dominant intertidal macrofauna (Hediste diversicolor, Hydrobia ulvae, Corophium volutator, Macoma balthica) was constructed, and microphytobenthic biomass at the sediment surface was measured. Statistical analysis using generalized least squares regression indicated that heterogeneity within our system was a significant driving factor that interacted with macrofaunal density and species identity. Microphytobenthic biomass was highest in enriched patches, suggesting that nutrients were obtained locally from the sediment-water interface and not from the water column. Our findings demonstrate that organic enrichment can cause the development of heterogeneity which influences infaunal bioturbation and consequent nutrient generation, a driver of microphytobenthic production.     

The microbial landscape in bioturbated mangrove sediment: A resource for promoting nature-based solutions for mangroves

Globally, soils and sediments are affected by the bioturbation activities of benthic species. The consequences of these activities are particularly impactful in intertidal sediment, which is generally anoxic and nutrient-poor. Mangrove intertidal sediments are of particular interest because, as the most productive forests and one of the most important stores of blue carbon, they provide global-scale ecosystem services. The mangrove sediment microbiome is fundamental for ecosystem functioning, influencing the efficiency of nutrient cycling and the abundance and distribution of key biological elements. Redox reactions in bioturbated sediment can be extremely complex, with one reaction creating a cascade effect on the succession of respiration pathways. This facilitates the overlap of different respiratory metabolisms important in the element cycles of the mangrove sediment, including carbon, nitrogen, sulphur and iron cycles, among others. Considering that all ecological functions and services provided by mangrove environments involve microorganisms, this work reviews the microbial roles in nutrient cycling in relation to bioturbation by animals and plants, the main mangrove ecosystem engineers. We highlight the diversity of bioturbating organisms and explore the diversity, dynamics and functions of the sediment microbiome, considering both the impacts of bioturbation. Finally, we review the growing evidence that bioturbation, through altering the sediment microbiome and environment, determining a ‘halo effect’, can ameliorate conditions for plant growth, highlighting the potential of the mangrove microbiome as a nature-based solution to sustain mangrove development and support the role of this ecosystem to deliver essential ecological services.     

The effect of macrofauna,meiofauna and microfauna on the degradation of Spartina maritima detritus from a salt marsh area

Decomposition of salt marsh plants results from physical, chemical and biological processes including abiotic and biotic fragmentation, microbial decay and chemical transformation. According to literature data, only a few species have the ability to feed directly on living plant material, so fungi and bacteria seem to be the principal competitors for the organic substrates. Nevertheless, by consuming bacteria, protists and fungi associated to the detritus, macrofauna and meiofauna recycle the incorporated nutrients. Moreover, this nutrient regeneration may be seen as an effective factor in maintaining and stimulating bacterial production. In fact, it is well known that many detritus feeding species have very low assimilation efficiencies. The objective of the present study was to compare the nutrient mass balance of carbon; nitrogen and phosphorus in Spartina maritima covered areas and bare bottom sediment, with and without contribution of macrofauna, meiofauna and microbial populations. Nutrients mass balance was studied taking into account the initial and final nutrient concentrations in the sediment, water and plant material. Faunal activity was measured as a function of remineralised carbon, nitrogen and phosphorus. The experimental set-up included sixteen sub-experiments, which varied with respect to type of fauna, plant biomass and oxic status. Each sub-experiment was performed in small glass containers (3 L) containing about 900 g wwt sediment and 2.5 L estuarine water. Plant material, cut from intact plants, sediment cores and estuarine water were brought from the southern arm of the Mondego estuary (Portugal). The results showed that although the bacterial activity was responsible for the Spartina maritima degradation, the presence of meiofauna and macrofauna significantly enhanced the process. Moreover, the presence of Spartina maritima positively affected the mineralisation of the sediment carbon and nitrogen, especially when the three faunal components were present, and denitrification rates were highest in the presence of the macrofauna and meiofauna. The present study suggests that macrofauna and meiofauna have an important role on the ecosystem nutrient flux and that fauna might function as a sink for excess nutrients, that otherwise could be exported to the coastal waters.     

Bioturbation in a declining oxygen environment, in situ observations from Wormcam

Bioturbation, the displacement and mixing of sediment particles by fauna or flora, facilitates life supporting processes by increasing the quality of marine sediments. In the marine environment bioturbation is primarily mediated by infaunal organisms, which are susceptible to perturbations in their surrounding environment due to their sedentary life history traits. Of particular concern is hypoxia, dissolved oxygen (DO) concentrations ≤2.8 mg l(-1), a prevalent and persistent problem that affects both pelagic and benthic fauna. A benthic observing system (Wormcam) consisting of a buoy, telemetering electronics, sediment profile camera, and water quality datasonde was developed and deployed in the Rappahannock River, VA, USA, in an area known to experience seasonal hypoxia from early spring to late fall. Wormcam transmitted a time series of in situ images and water quality data, to a website via wireless internet modem, for 5 months spanning normoxic and hypoxic periods. Hypoxia was found to significantly reduce bioturbation through reductions in burrow lengths, burrow production, and burrowing depth. Although infaunal activity was greatly reduced during hypoxic and near anoxic conditions, some individuals remained active. Low concentrations of DO in the water column limited bioturbation by infaunal burrowers and likely reduced redox cycling between aerobic and anaerobic states. This study emphasizes the importance of in situ observations for understanding how components of an ecosystem respond to hypoxia.     

Soil ecological interactions: comparisons between tropical and subalpine forests

Soil fauna can influence soil processes through interactions with the microbial community. Due to the complexity of the functional roles of fauna and their effects on microbes, little consensus has been reached on the extent to which soil fauna can regulate microbial activities. We quantified soil microbial biomass and maximum growth rates in control and fauna-excluded treatments in dry and wet tropical forests and north- and south-facing subalpine forests to test whether soil fauna effects on microbes are different in tropical and subalpine forests. Exclusion of fauna was established by physically removing the soil macrofauna and applying naphthalene. The effect of naphthalene application on the biomass of microbes that mineralize salicylate was quantified using the substrate induced growth response method. We found that: (1) the exclusion of soil fauna resulted in a higher total microbial biomass and lower maximum growth rate in the subalpine forests, (2) soil fauna exclusion did not affect the microbial biomass and growth rate in the tropical forests, and (3) the microbial biomass of salicylate mineralizers was significantly enhanced in the fauna-exclusion treatment in the tropical wet and the south-facing subalpine forests. We conclude that non-target effects of naphthalene on the microbial community alone cannot explain the large differences in total microbial biomass found between control and fauna-excluded treatments in the subalpine forests. Soil fauna have relatively larger effects on the microbial activities in the subalpine forests than in tropical dry and wet forests.     

Effects of Deposit-Feeding Macrofauna on Benthic Bacteria,Viruses, and Protozoa in a Silty Freshwater Sediment

In microcosm experiments, we simultaneously tested the effects of increased numbers of deposit-feeding macrofauna (chironomids, oligochaetes and cladocerans) on the standing stock, activities and interactions of heterotrophic bacteria, viruses, and bacterivorous protozoa (heterotrophic nanoflagellates and ciliates) in the aerobic layer of a silty littoral freshwater sediment. On average, bacterial secondary production was stimulated between 11 and 29% by all macrofaunal groups compared to control experiments without macrofauna addition. Bacterial standing stock increased significantly by 8 and 13% in case of chironomids and cladocerans, respectively. Oligochaetes and chironomids produced significant negative effects on viral abundance while the results with cladocerans were inconsistent. The addition of oligochaetes and chironomids resulted in a significant decrease by on average 68 and 32% of viral decay rates, respectively, used as a measure of viral production. The calculated contribution of virus-induced lysis to benthic bacterial mortality was low, with 2.8 to 11.8% of bacterial secondary production, and decreased by 39 to 81% after the addition of macrofauna compared to the control. The abundances of heterotrophic nanoflagellates were significantly reduced by 20% by all tested macrofauna groups, while ciliates showed inconsistent results. The importance of heterotrophic nanoflagellate grazing on benthic bacteria was very low (<1% of bacterial secondary production) and was further reduced by elevated numbers of macrofauna. Thus, the selected deposit feeding macrofauna groups seem to have several direct and indirect and partly antagonistic effects on the benthic bacterial compartment through the enhancement of bacterial production and the reduction of virus-induced cell lysis and protozoan grazing.     

The effects of chemical pollution on the bioturbation potential of estuarine intertidal mudflats

Bioturbation by benthic infauna has important implications for the fate of contaminants as well as for changes to the sediment structure, chemistry and transport characteristics. There is an extensive literature dealing with the influence of sedimentary variables on the structure and function of infaunal marine and estuarine organisms but less is known of the converse, the influence of biota on sedimentary structure. Although some work has been carried out regarding spatial and temporal patterns of bioturbation, little attention has been given to the effects of pollution. The paper gives a framework of animal sediment relationships in an intertidal environment and discusses the general role of macrofauna in structuring and modifying sedimentary features. A brief outline of the various techniques used for quantifying the degree of bioturbation is given and some of these techniques have then been used to demonstrate the effect of a petrochemical discharge on the bioturbation potential of intertidal communities in the Humber estuary, eastern England. These studies indicate an increase in bioturbation with increasing distance from the source of pollution, not only because of differences in abundance, animal size and depth of activity but also because of the difference in species composition between the communities. As a means of interpreting the responses, the species present have been broadly classified in terms of their feeding strategy and sediment modification potential. The paper concludes by discussing the potential impact, in terms of effect on sediment transport, of selectively removing the different guilds (by pollution). Received: 8 February 1999 / Received in revised form: 10 May 1999 / Accepted: 14 May 1999     

Impact of microphytobenthos and macroinfauna on temporal variation of benthic metabolism in shallow coastal sediments

The impact of microphytobenthos and different abundances of macrofauna (Nereis diversicolor) on temporal variation of benthic metabolism was investigated in laboratory microcosms. Measurements primarily included diurnal fluxes of O₂ and CO₂ as well as sediment profiles of Chlorophyll a and extracellular polymeric substances (EPS). Net and gross primary production (2-5 and 4-7 mmol CO₂ m^{− 2} h^{− 1}, respectively) were relatively stable in both defaunated and faunated sediment throughout a 12 h light period. The CO₂ release from sediments immediately after onset of darkness ranged from 1.5 to 3.5 mmol CO₂ m^{− 2} h^{− 1} followed by a consistent decrease during the next 12 h in the dark. The decrease was more conspicuous in faunated (about 50%) than defaunated (9%) sediment. Total carbon oxidation was in both cases fuelled primarily by microphytobenthic biomass, while EPS only contributed by 1-4%. Diurnal measurements of Nereis diversicolor ventilation activity showed a significant decrease in the dark that corresponds well to the observed decrease in total metabolic activity. It is concluded that changes in solute exchange associated with animals and burrows (e.g. microbial respiration) is a major controlling factor for total sediment metabolism. In general, the faunal impact was evident as about 50% enhanced CO₂ release in the dark, while net primary production was reduced by 30-50%. The turnover time of produced organic carbon is therefore considerably shorter in the presence than absence of macrofauna. Thus, the daily average exchange of CO₂ was almost balanced in bioturbated sediment with a 43% share of carbon oxidation mediated by direct faunal respiration. Defaunated sediment was net autotrophic with daily primary production exceeding microbial carbon oxidation by 40%. The present study clearly demonstrates that knowledge on interactions between microphytobenthos and macrofauna is essential for understanding carbon dynamics in shallow sediments.