Shallow Mudflats Research Articles

Alpheus perlas, sp. nov., a new infaunal snapping shrimp from the Pacific coast of Panama (Malacostraca: Decapoda: Alpheidae).

A new snapping shrimp, Alpheus perlas sp. nov., is described based on a single complete male specimen collected on a shallow mudflat at Casayeta Island in the Las Perlas Archipelago, Gulf of Panama. The new species belongs to the large A. edwardsii (Audouin, 1821) species group characterised essentially by the presence of two notches on the major chela palm, with the dorsal notch extending posteriorly on the mesial surface. Within the eastern Pacific members of the A. edwardsii group, A. perlas sp. nov. appears to be morphologically closest to A. latus Kim Abele, 1988 and A. burukovskyi Anker Pachelle, 2015. Alpheus perlas sp.nov. does not seem to be specially adapted for digging and may be inquiline of a larger burrowing host, which currently remains unknown.

Geo-Dynamics, Salinity Gradient, and Vegetation Interdependence in Chilika Lagoon, a Tropical Wetland in Eastern India

<p>Chilika a shallow brackish lagoon, India, is shrinking for sediment surplus budget. South Mahanadi deltaic branches <em>i.e.</em> Daya and Bhargavi terminate at the southwest swamps of the Lagoon. The annual average salinity of the lake was depleted from 22.31ppt (1957-58) to 8.5 ppt. (1999-2000) as the mixing process of saline and fresh water was influenced from 1995. Trepidation of conversion of Chilika to a atrophied fresh water lake due to blooming population and their hydrologic interventions like Kolleru lake in (India), Aral Lake (Uzbekistan) was apprehended by 1950’s and was alarming by 1999 when the shallow inlet(s) shifted extreme north. The shallow mud flats of lean salinity were reclaimed further for agriculture. The ecology and biodiversity degraded with substantial pecuniary loss to the lagoon dependents. Anthropogenic interventions like, Hirakud dam (1956), dredging of Sipakuda Inlet (2000), Naraj barrage at delta head (2004), Gobardhanpur barrages (1998) and Gabkund cut with weir (2014) were made to the hydraulic system. The deteriorating health, perturbed biodiversity and declined ecosystem of the lagoon has forced to have a comparative study of the various morphologic changes passed over the Chilika with time. The meteorological, hydrologic and the salinity study of the lagoon area for the period 1990 to 2016 have shown changes. Topographic study using GIS is developed by collecting data from Glovis Classic (Google) and the interpretation is done using ERDAS 9.2 software for various geomorphic features (1984 and 2017) before and after the current anthropogenic interventions and compared with previous studies.</p>

Beneficial use of dredged sediment to enhance salt marsh development by applying a ‘Mud Motor’

We test an innovative approach to beneficially re-use dredged sediment to enhance salt marsh development. A Mud Motor is a dredged sediment disposal in the form of a semi-continuous source of mud in a shallow tidal channel allowing natural processes to disperse the sediment to nearby mudflats and salt marshes. We describe the various steps in the design of a Mud Motor pilot: numerical simulations with a sediment transport model to explore suitable disposal locations, a tracer experiment to measure the transport fate of disposed mud, assessment of the legal requirements, and detailing the planning and technical feasibility. An extensive monitoring and research programme was designed to measure sediment transport rates and the response of intertidal mudflats and salt marshes to an increased sediment load. Measurements include the sediment transport in the tidal channel and on the shallow mudflats, the vertical accretion of intertidal mudflats and salt marsh, and the salt marsh vegetation cover and composition. In the Mud Motor pilot a total of 470,516 m3 of fine grained sediment (D50 of ∼10 μm) was disposed over two winter seasons, with an average of 22 sediment disposals per week of operation. Ship-based measurements revealed a periodic vertical salinity stratification that is inverted compared to a classical estuary and that is working against the asymmetric flood-dominated transport direction. Field measurements on the intertidal mudflats showed that the functioning of the Mud Motor, i.e. the successful increased mud transport toward the salt marsh, is significantly dependent on wind and wave forcing. Accretion measurements showed relatively large changes in surface elevation due to deposition and erosion of layers of watery mud with a thickness of up to 10 cm on a time scale of days. The measurements indicate notably higher sediment dynamics during periods of Mud Motor disposal. The salt marsh demonstrated significant vertical accretion though this has not yet led to horizontal expansion because there was more hydrodynamic stress than foreseen. In carrying out the pilot we learned that the feasibility of a Mud Motor depends on an assessment of additional travel time for the dredger, the effectiveness on salt marsh growth, reduced dredging volumes in a port, and many other practical issues. Our improved understanding on the transport processes in the channel and on the mudflats and salt marsh yields design lessons and guiding principles for future applications of sediment management in salt marsh development that include a Mud Motor approach.

Responses of the infaunal peracarid Monokalliapseudes schubarti (Tanaidacea: Kalliapseudidae) to a bottom trawling disturbance and seasonality in an estuary of southern Brazil

ABSTRACTThe tanaid Monokalliapseudes schubarti is one of the dominant macrobenthic species in the shallow mudflats of southern Brazil. This species plays an important role in local trophic webs as a primary consumer and as a prey. This work investigated (1) the temporal variations in the population densities and structure of M. schubarti in the Patos Lagoon Estuary and (2) the potential impacts of shrimp trawling on the population densities, population structure and vertical stratification of the species in shallow areas of the estuary. The species exhibited significant temporal variability in its total densities and population structure, with a reproductive period observed during late summer and autumn, following an increase in salinity within the estuary. There were significant differences in M. schubarti densities, which appeared to be associated with variations in sediment texture and salinity. No well-defined pattern of trawling impact was identified; however, after high-impact trawling, decreases in the abundance of males and juveniles in the sediment superficial layer were recorded, and ovigerous females disappeared from the sampled area with relatively higher percentages of medium sand. Changes in the densities of different morphological categories observed after trawling were likely due to mortality, resuspension or rapid movement deeper into their tubes.

Response of soft-bottom macrobenthic assemblages to artisanal trawling fisheries in a subtropical estuary

Bottom trawling is one of the most harmful and widespread activities affecting benthic habitats and fauna. In this study, we analyzed the impact of shrimp trawling fisheries on the densities, assemblage structure and vertical stratification of the benthic macrofauna in shallow mudflats of the Patos Lagoon Estuary (Brazil). Experimental trawls of different intensities were performed during three shrimp fishing seasons (2015–2017), comparing macrofauna among before and after trawling, and a control zone. The changes in macrofaunal assemblages were more consistently related to natural variability than to trawling impact, being mostly influenced by sediment structure and salinity variation. The trawling impact was mainly detected in the area with higher percentages of fine sediments, with different macrofaunal responses in each month and stratum. Some non-significant decreases on total densities after trawling were observed, mainly on the superficial stratum, and signs of burial activities. The response to trawling disturbance of each species was different and it highly depended on their natural variability. Only five species showed significant variation to the trawling treatment (Erodona mactroides, Heleobia australis, H. charruana, Heteromastus similis and Laeonereis acuta), with temporally different responses. Decreasing densities were more related to the high impact treatment. In some seasons, decreases on the abundance of Monokalliapseudes schubarti after high impact trawls were up to 60% of the seasonal mean densities. Trawling fisheries may reduce macrobenthic densities but not their vertical stratification. The impact of trawling on key species may compromise the ecosystem function, as benthic macrofauna provide food sources for many aquatic resources. This study highlights trawling impacts in an estuarine nursery area, which should be fully considered since they may be reflected in the trophic webs.

ACOUSTIC DOPPLER VELOCIMETER BACKSCATTER FOR QUANTIFICATION OF SUSPENDED SEDIMENT CONCENTRATION IN SOUTH SAN FRANCISCO BAY

A data set was acquired on a shallow mudflat in south San Francisco Bay that featured simultaneous, co-located optical and acoustic sensors for subsequent estimation of suspended sediment concentrations (SSC). The optical turbidity sensor output was converted to SSC via an empirical relation derived at a nearby site using bottle sample estimates of SSC. The acoustic data was obtained using an acoustic Doppler velocimeter. Backscatter and noise were combined to develop another empirical relation between the optical estimates of SSC and the relative backscatter from the acoustic velocimeter. The optical and acoustic approaches both reproduced similar general trends in the data and have merit. Some seasonal variation in the dataset was evident, with the two methods differing by greater or lesser amounts depending on which portion of the record was examined. It is hypothesized that this is the result of flocculation, affecting the two signals by different degrees, and that the significance or mechanism of the flocculation has some seasonal variability. In the earlier portion of the record (March), there is a clear difference that appears in the acoustic approach between ebb and flood periods, and this is not evident later in the record (May). The acoustic method has promise but it appears that characteristics of flocs that form and break apart may need to be accounted for to improve the power of the method. This may also be true of the optical method: both methods involve assuming that the sediment characteristics (size, size distribution, and shape) are constant

Sound production in two species of eelgrass

There have been numerous anecdotal reports that a species of the aquatic plant known as eelgrass (Zostera marina) makes an audible bubbling sound that can be heard on warm, sunny days in estuarine waters at low tide. This champagne-like fizzing sound results from the expiration of oxygen by the plant during photosynthesis and can be heard as airborne sound by listening closely to the surface of the water. The author recorded this sound in July of 2014 in the shallow mudflats of Puget Sound in Washington. Recordings were made of both airborne and underwater sounds. On the same day in the same general area, the author also recorded similar bubbling from a different, non-native species of eelgrass (Zostera japonica). The sounds of the two species were compared, revealing clear and distinct sound signatures for each. These recordings suggest that sound production in the two species of eelgrass is strongly present and may provide opportunities for further research.

Tide, Wind, and River Forcing of the Surface Currents in the Fraser River Plume

ABSTRACTA long-term record of surface currents from a high-frequency radar system, along with near-surface hydrographic transects, moored current meter records, and satellite imagery, are analyzed to determine the relative importance of river discharge, wind, and tides in driving the surface flow in the Fraser River plume. The observations show a great deal of oceanographic and instrumental variability. However, averaged quantities yielded robust results. The effect of river flow, which determines buoyancy and inertia near the river mouth, was found by taking a long-term average. The resulting flow field was dominated by a jet with two asymmetric gyres; the anticyclonic gyre to the north had flow speeds consistent with geostrophy. The mean flow speed near the river mouth was 14.3 cm s–1, while the flow further afield was 5 cm s–1 or less. Wind stress and surface currents were highly coherent in the subtidal frequency band. Northwesterly winds drive a surface flow to the southeast at speeds of nearly 30 cm s–1. Southeasterly winds drive a surface flow to the northwest at speeds reaching 20 cm s–1; however, there is more spatial variability in speed and direction relative to the northwesterly wind case. A harmonic analysis was used to extract the tidally driven flows. Ellipse parameters for the major tidal constituents varied considerably in both alignment and aspect ratio over the radar domain, in direct contrast to a barotropic model which predicted rectilinear flow along the Strait of Georgia. This is a result of water filling and draining the shallow mud flats north of the Fraser's main channel. The M2 velocities at the surface were also weaker than their barotropic counterparts. However, the shallow water constituent MK3 was enhanced at the surface and nearly as strong as the mean flow, implying that non-linear interactions are important to surface dynamics.

Nearshore movement ecology of a medium-bodied shark, the creek whaler Carcharhinus fitzroyensis

The movement and habitat use patterns of medium-bodied nearshore sharks are poorly understood. However, these species face some of the highest levels of exposure to anthropogenic development. The habitat and space use strategies species exhibit affect their role within communities and how they respond to environmental change. The present study used passive acoustic telemetry to evaluate the residency, space use, and habitat use patterns of the creek whaler Carcharhinus fitzroyensis in a nearshore embayment in Queensland, Australia. Individuals were monitored for approximately 18 months. Half of the monitored population were highly resident to the bay. In contrast, several individuals spent less than 2 weeks in the bay, suggesting that broader movements may occur in a portion of the population. Size had no effect on residency. Activity space size varied between months and time of day but was also not affected by animal size. All C. fitzroyensis spent the majority of time in seagrass habitat (70%) and deep water (>5 m) mud substrate (20%). Shallow mudflat, sandy inshore, and reef habitats were rarely used (7%). Although the sample size of immature individuals was relatively small, results indicated immature and mature C. fitzroyensis shared space and habitats. Overall, C. fitzroyensis used a combination of nearshore movement patterns typically exhibited by small- and large-bodied species. The movement patterns exhibited by C. fitzroyensis suggest that this species has a moderately high degree of seagrass habitat specialisation. Seagrass habitat is typically highly productive and may be an important foraging habitat for this species. Given the consistent use of seagrass habitat, C. fitzroyensis are likely vulnerable to population decline as a result of seagrass habitat loss. Future research should continue to investigate the unique movements of medium-bodied sharks.

Seasonal changes in the abundance and biomass of zooplankton from shallow mudflat river-estuarine system in Persian Gulf

The present study was designed to investigate the seasonal changes in the abundance and biomass of zooplankton and their relationships with water quality parameters in mudflat shallow estuary, Helleh River, Persian Gulf (Iran). The zooplankton consisted of Acartia sp., Euterpina sp., Oithona spp., Oncaea sp., Paracalanus sp., Corycaeus sp., Labidocera sp., Macrosetella sp., Microsetella sp., Temora sp., copepod copepodid, copepod nauplii, Barnacle, Polychaeta, Conchoecia sp. (Ostracoda), Hyperid, Decapoda (zoea and megalopa), Actinula, Echinopluteus, Mollusca, Tintinnopsis sp., Ctenophora (Boliopsis sp.), Protozoa (Discorbis sp.), cumacea (Diastylis sp.), Cnidaria (Siphonophora and Phialidium), Nematoda (Pratylenchus spp.), Urocordata (Oikopleura spp.), fish eggs and fish larvae. The maximum biomass was estimated at estuary mouth in all seasons, the highest values in autumn (97.8-255.6 mg/m3) and the lowest in winter (5.5-68.2 mg/m3). The seasonal abundance (density) of zooplankton was 21237±2419, 45739±6053, 5242±648, and 12905±1867 ind./m3 in summer, autumn, winter and spring, respectively. There was a significant correlation (P<0.01) between zooplankton abundance as well as biomass with salinity, dissolved oxygen and chlorophyll a. Based on PCA (Principal Component Analysis), the most important factors in mudflat shallow river–estuarine system that could describe most changes of biomass and abundance of zooplankton were salinity, chlorophyll a, temperature and pH, respectively.

Radium isotopes as a tracer of sediment‐water column exchange in the North Sea

AbstractSediment‐water column exchange plays an important role in coastal biogeochemistry. We utilize short‐lived radium isotopes (224Ra and 223Ra) to understand and quantify the dominant processes governing sediment‐water column exchange throughout the North Sea. Our comprehensive survey, conducted in September 2011, represents the first of its kind conducted in the North Sea. We find that two main sources regulate surface Ra distributions: minor coastal input from rivers and shallow mudflats and North Sea sediments as the dominant source. Pore waters show 100‐fold larger activities than the water column. North Sea sediment characteristics such as porosity and mean grain size, as well as turbulence at the sediment‐water interface, are the dominant factors contributing to variability of Ra efflux. Ra inventory and mass balance approaches consistently yield high benthic Ra effluxes in the southern North Sea, driven by strong tidal and wind mixing, which in turn cause high sediment irrigation rates. These results exceed incubation‐based Ra flux estimates and the majority of previously reported Ra flux estimates for other regions. Ra‐based estimates of benthic alkalinity fluxes compare well to observed values, and the high rates of Ra efflux imply a potentially significant exchange of other products of sedimentary reactions, including carbon and nutrient species. Passive tracer simulations lend strong support to the Ra source attribution and imply seasonal variation in the surface water Ra distribution depending on stratification conditions.

IMPACTS OF WASHOVER EVENTS ON FATE AND DISTRIBUTION OF MC252 OIL IN A COASTAL SYSTEM

Washover events on coastal headland beaches occur when storm surge from cold fronts, high tides, tropical storms, and hurricanes move across the beach, reworking and depositing sand in the back marshes and shallow mudflat areas. On Fourchon Beach, Louisiana, a 9-mile coastal headland beach impacted by the Deepwater Horizon oil spill, these washover events have intermittently moved oil from the subtidal and intertidal portion of the beach to the supratidal mudflats and marsh areas. In order to determine the impacts of washover events on oil fate, 4 complementary studies are being undertaken. First, washover areas (N=56) have been mapped and sampled for PAHs and n-alkanes in the most heavily impacted area eastern end of Fourchon Beach. Samples indicate the most heavily impacted segments were in areas where a physical breach of the beach developed. Second, characteristics of specific washover events and locations are being studied to establish typical categories of events (i.e., breaches, remobilization of oil on the beach and mobilization of subtidal oil mats) as well as the characteristics of oil mobilized by each category. As an example, presence of MC252 oil was mapped using soil cores in a washover mudflat behind a breach, which developed during TS Isaac in 2011. MC252 oil was present at an average of 6 – 12 cm below the surface. Concentrations of PAHs in this layer ranged from 1.2 to 16 mg/kg consisting primarily of C1-C4 phenanthrenes, C2 and C3 dibenzothiophenes and C1-C2 chrysenes. Third, the chemical signature of crude oil including a range of petrogenic PAHs and n-alkanes relative to poorly biodegradable C30-hopane is being used at these locations to assess the relative weathering of oil at various stages in the washover process. Finally, a laboratory microcosm study is being conducted to understand the biogeochemical controls on oil biodegradation in the new environment (washover mudflat) and how these controls impact its ability to degrade naturally. Anaerobic treatments consistent with the flooded mudflat areas are amended with N and sulfate in various combinations to determine if nutrients or electron acceptors are limiting in the washover environment. Results to date indicate that most PAHs are stable under the flooded conditions with the exception of C1-phenanthrenes. These studies will be discussed in the context of remedial measures during the emergency response phase used on Fourchon Beach to prevent washover impacts.

Habitat characteristics assessmen of the wetlands with breeding Ferruginous Duck Aythya nyroca and Pochard A. ferina in Bulgaria / Ocena habitatnih značilnosti mokrišč z gnezdečimi kostanjevkami Aythya nyroca in sivkami A. ferina v Bolgariji

Abstract The Ferruginous Duck Aythya nyroca and Pochard A. ferina are both regular breeders in Bulgaria. In the 19th and early 20th centuries, the Ferruginous Duck was a widely distributed and abundant breeding species in Bulgaria, while Pochard was first confirmed to breed in the country only in the 1950s. Breeding habitat characteristics of Ferruginous Duck and Pochard were assessed in 2002 during a national census of the former species in 30 and 23 wetlands, respectively, where the species were present. Preferred habitat characteristics were determined based on vegetation cover, water depth, and the presence of mosaic vegetation, bank side vegetation, floating vegetation, shallow or steep banks, shallow mudflats, shallow vegetated areas and woodland along the banks. In 2002, the Ferruginous Duck population in Bulgaria was restricted more to artificial wetlands, like fishponds and micro-reservoirs, while Pochard preferred more natural wetlands, predominantly natural marshes. The altitude of wetlands with Ferruginous Ducks varied markedly, with breeding recorded up to 880 m a.s.l., while Pochard bred at lower altitudes, mostly below 300 m a.s.l. Analysis revealed that Ferruginous Ducks preferred well-vegetated, comparatively shallow wetlands with well-structured mosaic vegetation and a diversity of microhabitats, like shallow mudflats and floating vegetation. The Pochard was found to be more of a generalist in wetland selection, significantly correlated with fewer wetland parameters, and showed preference only for larger, open-water bodies. These results could help explain and give some insight into the reasons for the differences in range size, numbers and distribution of the two species and their respective conservation status in Europe and worldwide. The Pochard, whilst close to the limits of the breeding range in Bulgaria, expands its range in Europe, occupying a variety of wetlands and thus having a favourable status. The Ferruginous Duck is more of a habitat specialist, which limits its distribution and makes it more susceptible to habitat changes. These habitat preferences and the deterioration of the wetland habitats in many parts of its range could probably explain the Ferruginous Duck decreasing population and shrinking breeding range, and thus its current unfavourable conservation status.

Holocene Ostracods from the Borehole Core at Oppama Park, Yokosuka City, Kanagawa Prefecture, Central Japan: Paleoenvironmental Analysis and the Discovery of a Fossil Ostracod with Three-Dimensionally Preserved Soft Parts

Abstract. Forty-seven sediment samples were taken from a borehole core at Oppama Park, Yokosuka City, Kanagawa Prefecture, central Japan. Fifty-eight ostracod species, representing 32 genera, were identified in the 32 samples. Three ostracod biofacies (I–III) were identified using Q-mode cluster analysis. On the basis of ostracod biofacies, the following paleoenvironmental changes were recognized in the study area: land mudflat (no ostracods) → muddy bottom environment of inner bay (biofacies III) → sandy bottom environment of bay near rocky shore (biofacies II) → shallow mudflat (biofacies I). Modern analog technique (MAT) showed that the paleoenvironment of the study area was a warm (maximum bottom temperature in summer around 30°C; minimum bottom temperature in winter around 7°C) and shallow bay, similar to that around Honshu Island today. An ostracod specimen, Parakrithella pseudadonta, with exceptionally well preserved soft parts was discovered in one of the horizons, at a core depth of 20.4–20.5 m. ...

Tidal and wind-driven surface currents in the German Bight: HFR observations versus model simulations

Tidal and wind-driven surface currents in the German Bight between shallow mudflats of the North Frisian islands and the island of Helgoland are studied using coastal high-frequency radar (HFR) observations and hindcasts from a primitive equation numerical model. The setup of the observational system is described, and estimates of expected measurement errors are given. A quantitative comparison of numerical model results and observations is performed. The dominant tidal components are extracted from the two data sources using tidal harmonic analysis and the corresponding tidal ellipses are defined. Results show that the spatial patterns of different tidal ellipse parameters are consistent in the two data sets. Model sensitivity studies with constant and variable salinity and temperature distributions are used to study density-related mechanisms of circulation. Furthermore, the role of the surface wind field in driving the German Bight circulation is investigated using the complex correlation between wind and surface current vectors. The observed change of the respective correlation patterns from the coastal to open ocean is shown to be due to a combination of density effects, the coastline and topography. The overall conclusion is that HFR observations resolve the small-scale and rapidly evolving characteristics of coastal currents well in the studied area and could present an important component for regional operational oceanography when combined with numerical modelling. Some unresolved issues associated with the complex circulation and large instability of circulation in front of the Elbe River Estuary justify further considerations of this area using dedicated surveys and modelling efforts.

Baroclinic tidal flows and inundation processes in Cook Inlet, Alaska: numerical modeling and satellite observations

A wetting and drying (WAD) algorithm is implemented in a baroclinic three-dimensional ocean circu- lation model of Cook Inlet, Alaska, where large tidal ranges (≈10 m) regularly expose extensive mudflats. The model includes tides and wind- and buoyancy-induced flows. In the upper Inlet, the model successfully simulates large amplification of tides and propagation of fast (3∼ 4ms −1 ) tidal bores over shallow mudflats. The simulated return flows during ebb expose large areas (∼100 km 2 )o f the mudflats. Medium-resolution (250- and 500-m) images obtained from the moderate resolution imaging spectro- radiometer(MODIS) instrumentsaboardthe Terraand Aqua satelliteswereused toverify the modelresults byidentifying the location, extent, and temporal changes of the exposed mudflat regions. The results demonstrate the value of operational, medium-resolution remote sensing data in evaluating the WAD model. Sensitivity tests show that WAD produces approximately 20% larger tidal amplitude and 10% slower phase than the corresponding experiment without WAD. In the deep channel of the central Inlet, the confluence of saline water of the lower Inlet with brackish water from rivers and melting ice from land around the upper Inlet produces a salinity front. At the simulated front, strong vertical circulation cells and surface convergence and currents develop, especially during the flood. The char- acteristics resemble those of "rip tides" often observed in this region.

Modelling short-term dynamics of suspended particulate matter in Venice Lagoon, Italy

Sediment stability in the shallow Venice Lagoon was investigated by means of numerical modelling. Results from a hydrodynamic model allowed for the determination of the wave climate and bottom effective parameters so that simulations with a Lagrangian model for suspended particulate matter could be performed. A spring–neap cycle in summer 1998 was chosen as integration period since data for calibration and verification were collected within the European project F-ECTS between summer 1998 and spring 1999. Deposition on shallow mud flats as well as short term erosion during a strong wind event were reproduced and mass balances for two areas computed. A relation of patterns of SPM in the water and in the sediment was found and can be ascribed to the displacement of material during storm events from shallow areas to the bottom of very small channels. Assuming about 10 to 14 storm events during the year comparable to the Bora event during the integration time, estimates for long-term trends of sediment loss on shallow flats by Day et al. [Day, J.W., Rybczyk, J., Scarton, F., Rismondo, A., Are, D., Cecconi, G., 1999. Soil accretionary dynamics, sea-level rise and the survival of wetlands in Venice Lagoon: a field and modelling approach. Estuarine, Coastal and Shelf Science 49, 607–628] are met by the simulation results. Evidently, long-term sediment evolution of the lagoon is therefore not dominated by the average (residual) processes that occur in the lagoon, but by the few peak events that happen randomly over the year.

Microscale distribution of oxygen and nitrate in sediment inhabited by Nereis diversicolor: spatial patterns and estimated reaction rates

Profiles of oxygen and nitrate were measured with microsensors in sediment surrounding burrow structures of the polychaete Nereis diversicolor in a shallow mudflat in Limfjorden, Denmark. Rates and spatial distribution of reactions involving oxygen and nitrate (oxygen consumption, nitrification and nitrate reduction) in the vicinity of burrow structures and surface sediment were calculated and identified from concentration profiles. The burrow walls of the natural N. diversicolor population increased the surface area of the sediment-water interface several-fold and introduced oxic microniches with high metabolic activity into otherwise anoxic sediment. Nitrification and nitrate reduction were spatially separated processes, with nitrification restricted to the 0.9 to 1.5 mm thick oxic zone and nitrate-reduction occurring as a strictly anaerobic process in the nitrate-containing layers next to the oxic zone. At a density of 2560 individuals m-2, burrow walls of N. diversicolor account for 12 to 40% of bulk sediment oxygen uptake and 50 to 77% and 58 to 82% of bulk nitrification and nitrate reduction, respectively. Thus, burrow structures of macro-infauna are potentially a major sediment compartment involved in nitrogen cycling and may promote loss of bioavailable nitrogen from the sediment. [KEYWORDS: NO3-/NO2- biosensor ; Nitrogen cycling ; Sediments ; Bioturbation ; Nereis diversicolor ; Burrow walls]

Effects of suspended, diesel-contaminated sediment on feeding rate in the darter goby, Gobionellus boleosoma (Teleostei: Gobiidae)

The darter goby, Gobionellus boleosoma, a bottom-feeding gobiid fish, was used as a meiofaunal predator in laboratory experiments designed to determine the effect of suspended diesel-contaminated sediment on feeding rate. Estuarine sediments are sinks for hydrophobic contaminants, and contaminant exposure from suspended sediment seems likely in estuarine mudflats. The darter goby is a small estuarine fish that lives in shallow mudflats surrounding Spartina alterniflora marshes, feeding primarily on meio- and small macrofauna. We first showed that darter gobies are effective feeders on suspended meiobenthic copepods and that this feeding was not affected by suspended sediment loads as high as 2000 mg l −1. Harpacticoid copepods then were offered as ad libitum prey to gobies after exposure to either contaminated or non-contaminated sediments via a single turbidity event 24 h prior to feeding. Based on gut content analysis, goby standard length influenced feeding rate as larger gobies ingested more meiobenthic copepods in the 1-h experiment. Diesel-fuel contamination reduced feeding rate by 50–100% in all but the lowest dosage examined; ANCOVA (with length as a co-variate) indicated that feeding rate after exposure to suspended sediments above 200 mg PAH kg dry sediment −1 was significantly reduced relative to uncontaminated controls. No feeding occurred at 687 mg PAH kg dry sediment −1 (the greatest concentration tested). The proportion of non-feeding fishes increased with increasing levels of contamination, and an additional experiment suggested that the source of contamination was associated with direct contact with contaminated sediment rather than exposure to water previously associated with contaminated sediment.