Impact of fiddler crabs (Uca spp.) on rates and pathways of benthic mineralization in deposited mangrove shrimp pond waste

The impact of fiddler crabs (Uca spp.) on benthic mineralization rates and pathways in deposits of shrimp pond waste (SPW) with planted mangrove trees (Rhizophora apiculata) were determined in the Ranong mangrove forest, Thailand. Sediment metabolism, measured as CO₂ flux, increased by 2- to 3-fold when either fiddler crabs or mangrove trees were present compared to control plots. Sulfate reduction rates (SRR) were always high and partitioning of various electron acceptors to total carbon oxidation revealed that sulfate reduction contributed by >90%, with iron reduction being important only near the sediment-water interface. However, significant iron reduction appeared down to 7 cm when bioturbation and plant roots were combined in the easily oxidizable mangrove sediment (MS), indicating that infaunal activity and plant roots were able to alter the substratum. Microprofiles around individual Uca burrows showed 46% lower SRRs in a 15-mm-thick oxidized layer around the burrows compared to the surrounding sediment. The burrow wall environment appears to be a zone of intense reoxidation of reduced compounds as indicated by low pools of reduced sulfide compounds and a high Fe(III) content. Despite the decreased SRRs near the burrow, and the introduction of Fe(III) to deeper sediment layers, fiddler crabs and mangrove trees have only limited impact on rates and partitioning of anaerobic carbon mineralization in the SPW. This lack of response was attributed to the relative small volume of sediment affected by crab activities.     

Influences of Infaunal Burrows on the Community Structure and Activity of Ammonia-Oxidizing Bacteria in Intertidal Sediments

Influences of infaunal burrows constructed by the polychaete (Tylorrhynchus heterochaetus) on O₂ concentrations and community structures and abundances of ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) in intertidal sediments were analyzed by the combined use of a 16S rRNA gene-based molecular approach and microelectrodes. The microelectrode measurements performed in an experimental system developed in an aquarium showed direct evidence of O₂ transport down to a depth of 350 mm of the sediment through a burrow. The 16S rRNA gene-cloning analysis revealed that the betaproteobacterial AOB communities in the sediment surface and the burrow walls were dominated by Nitrosomonas sp. strain Nm143-like sequences, and most of the clones in Nitrospira-like NOB clone libraries of the sediment surface and the burrow walls were related to the Nitrospira marina lineage. Furthermore, we investigated vertical distributions of AOB and NOB in the infaunal burrow walls and the bulk sediments by real-time quantitative PCR (Q-PCR) assay. The AOB and Nitrospira-like NOB-specific 16S rRNA gene copy numbers in the burrow walls were comparable with those in the sediment surfaces. These numbers in the burrow wall at a depth of 50 to 55 mm from the surface were, however, higher than those in the bulk sediment at the same depth. The microelectrode measurements showed higher NH₄⁺ consumption activity at the burrow wall than those at the surrounding sediment. This result was consistent with the results of microcosm experiments showing that the consumption rates of NH₄⁺ and total inorganic nitrogen increased with increasing infaunal density in the sediment. These results clearly demonstrated that the infaunal burrows stimulated O₂ transport into the sediment in which otherwise reducing conditions prevailed, resulting in development of high NH₄⁺ consumption capacity. Consequently, the infaunal burrow became an important site for NH₄⁺ consumption in the intertidal sediment.     

The impact of polychaete (Nereis virens Sars) burrows on nitrification and nitrate reduction in estuarine sediments

The influence of Nereis virens Sars burrows on nitrification, denitrification and total nitrate reduction was assessed in poor (0.7% organic matter) and rich (2.0% organic matter) sediment from the estuary, Norsminde Fjord. The experiments were performed as assays of potential activity, since natural conditions proved impossible to simulate in the unpredictable burrow environment. The measurements were made in two microprofiles, extending 15 mm into the sediment from the surface and from the burrow wall lining. Both sediment types showed higher potential nitrification in the wall linings than in the surface sediment. This was positively correlated with the content of silt + clay particles and organic matter (i.e. the mucous lining of burrow walls). An elevated nitrate reduction activity was evident in the oxic layer of surface sediment. No such activity pattern was observed in the burrow walls. Denitrification accounted for 27–53 % of the total nitrate reduction. An empirical relationship between the ratio of predicted oxygen penetration into surface and wall sediment and the contribution of nereid burrows to bulk actual nitrification and nitrate reduction is presented. The burrow contribution to bulk nitrification was, in contrast to bulk nitrate reduction, very sensitive to variable oxygen penetrations. Thus, possible short-time changes in nitrate exchange across the wall lining will apparently be regulated by changes in nitrification activity rather than nitrate reduction activity.     

Sulfate reduction and sediment metabolism in Tomales Bay,California

Sulfate reduction rates (SRR) in subtidal sediments of Tomales Bay, California, were variable by sediment type, season and depth. Higher rates were measured in near-surface muds during summer (up to 45 nmol cm^-3 h^-1), with lower rates in sandy sediments, in winter and deeper in the sediment. Calculations of annual, average SRR throughout the upper 20 cm of muddy subtidal sediments (about 30 mmol S m^-2 d^-1) were much larger than previously reported net estimates of SRR derived from both benthic alkalinity flux measurements and bay wide, budget stoichiometry (3.5 and 2.6 mmol m^-2 d^-1, respectively), indicating that most reduced sulfur in these upper, well-mixed sediments is re-oxidized. A portion of the net alkalinity flux across the sediment surface may be derived from sulfate reduction in deeper sediments, estimated from sulfate depletion profiles at 1.5 mmol m^-2 d^-1. A small net flux of CO₂ measured in benthic chambers despite a large SRR suggests that sediment sinks for CO₂ must also exist (e.g., benthic microalgae).     

Influences of infaunal burrows on the community structure and activity of ammonia-oxidizing bacteria in intertidal sediments

Influences of infaunal burrows constructed by the polychaete (Tylorrhynchus heterochaetus) on O(2) concentrations and community structures and abundances of ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) in intertidal sediments were analyzed by the combined use of a 16S rRNA gene-based molecular approach and microelectrodes. The microelectrode measurements performed in an experimental system developed in an aquarium showed direct evidence of O(2) transport down to a depth of 350 mm of the sediment through a burrow. The 16S rRNA gene-cloning analysis revealed that the betaproteobacterial AOB communities in the sediment surface and the burrow walls were dominated by Nitrosomonas sp. strain Nm143-like sequences, and most of the clones in Nitrospira-like NOB clone libraries of the sediment surface and the burrow walls were related to the Nitrospira marina lineage. Furthermore, we investigated vertical distributions of AOB and NOB in the infaunal burrow walls and the bulk sediments by real-time quantitative PCR (Q-PCR) assay. The AOB and Nitrospira-like NOB-specific 16S rRNA gene copy numbers in the burrow walls were comparable with those in the sediment surfaces. These numbers in the burrow wall at a depth of 50 to 55 mm from the surface were, however, higher than those in the bulk sediment at the same depth. The microelectrode measurements showed higher NH(4)(+) consumption activity at the burrow wall than those at the surrounding sediment. This result was consistent with the results of microcosm experiments showing that the consumption rates of NH(4)(+) and total inorganic nitrogen increased with increasing infaunal density in the sediment. These results clearly demonstrated that the infaunal burrows stimulated O(2) transport into the sediment in which otherwise reducing conditions prevailed, resulting in development of high NH(4)(+) consumption capacity. Consequently, the infaunal burrow became an important site for NH(4)(+) consumption in the intertidal sediment.     

Continuously measured changes in redox potential influenced by oxygen penetrating from burrows of Callianassa subterranea

A continuously recording multiple redox sensor was used to measure oxygenation effects in otherwise anoxic sediments. The sensor reacted within three minutes (increase > 50 mV) to oxygen penetrating into the sediment. Different patterns of oxidation/reduction were observed along burrows of sediment-dwelling infauna. Examples of an increase of more than 600 mV lasting 50 hours and short, 1 to 4 hours long, oxidation pulses of 280 mV increase are given. Redox patterns similar to the ones observed along natural burrows were generated using artificial burrows. Oxygen penetrated up to 1 mm by diffusion from burrow walls into the adjacent sediment, whereas elevated Eh values were measured within 3 mm of the burrow. The oxygenation effect of a burrowing shrimp, Callianassa subterranea, was studied in a 3-month experiment with ten sensors and five animals. For one individual we recorded 108 events with Eh increasing by more than 300 mV. It was estimated that below every square meter of sediment surface 0.7 m². burrow surface are subjected to oxygenation at least once per day by the population of Callianassa in the southern North Sea.     

The effects of temperature and salinity on ventilation behavior of two species of ghost shrimp (Thalassinidea) from the northern Gulf of Mexico: a laboratory study

Thalassinidean shrimp are among the most important bioturbators in coastal ecosystems. The species Lepidophthalmus louisianensis and Callichirus islagrande are found in dense aggregations (up to 400 burrows m⁻²) along sandy and muddy shores of the northern Gulf of Mexico. These shrimp actively ventilate their burrows to provide oxygen and eliminate wastes. In doing so, they expel nutrient-rich burrow water to the overlying water column, potentially altering nutrient cycling and benthic primary productivity. To develop a mechanistic understanding of the role of burrowing shrimp in nutrient processes, we must first examine how changes in environmental conditions alter the frequency, strength, and duration of ventilation. Field measurements of burrow temperature and salinity suggest that the burrow serves as a buffer from the highly variable conditions found in these estuarine, intertidal habitats. Temperatures at sediment depths >30 cm were generally warmer in winter and cooler in summer than at the sediment surface. Burrow salinities, measured at low tide, were consistently higher than adjacent open water. We used these measurements to parameterize laboratory studies of burrow ventilation in artificial burrows made of plastic tubing and in more natural sediment mesocosms, and studies of oxygen consumption in small glass containers. Rates of oxygen consumption and burrow ventilation by L. louisianensis were lower than those of C. islagrande, perhaps reflecting a lower overall activity rate in the former species which resides in less permeable sediments. Generally, increased temperature had a significant positive effect on oxygen consumption for both species. Salinity had no effect on oxygen consumption by L. louisianensis, reflecting the ability of this species to exist in a wide range of salinities. In contrast, oxygen consumption rates of C. islagrande, which is less tolerant of low salinity, were significantly higher at 35‰ than at 20‰. Ventilation rates were highly variable, and shrimp in artificial burrows tended to have consistently higher ventilation rates than those in sediment mesocosms. There is a trend toward more frequent ventilation at 30 °C for both species. Salinity had no effect on ventilation for either species. Our results suggest that thalassinideans exhibit highly variable and species-specific ventilation patterns that are more likely to be affected by temperature than salinity. Increased ventilation at higher temperatures seems to coincide with increased oxygen consumption at these temperatures, although a similar finding was not made for salinity treatments.     

Burrows of the Semi-Terrestrial Crab Ucides cordatus Enhance CO2 Release in a North Brazilian Mangrove Forest

Ucides cordatus is an abundant mangrove crab in Brazil constructing burrows of up to 2 m depth. Sediment around burrows may oxidize during low tides. This increase in sediment-air contact area may enhance carbon degradation processes. We hypothesized that 1) the sediment CO₂ efflux rate is greater with burrows than without and 2) the reduction potential in radial profiles in the sediment surrounding the burrows decreases gradually, until approximating non-bioturbated conditions. Sampling was conducted during the North Brazilian wet season at neap tides. CO₂ efflux rates of inhabited burrows and plain sediment were measured with a CO₂/H₂O gas analyzer connected to a respiration chamber. Sediment redox potential, pH and temperature were measured in the sediment surrounding the burrows at horizontal distances of 2, 5, 8 and 15 cm at four sediment depths (1, 10, 30 and 50 cm) and rH values were calculated. Sediment cores (50 cm length) were taken to measure the same parameters for plain sediment. CO₂ efflux rates of plain sediment and individual crab burrows with entrance diameters of 7 cm were 0.7–1.3 µmol m⁻² s⁻¹ and 0.2–0.4 µmol burrows⁻¹ s⁻¹, respectively. CO₂ released from a Rhizophora mangle dominated forest with an average of 1.7 U. cordatus burrows⁻¹ m⁻² yielded 1.0–1.7 µmol m⁻² s⁻¹, depending on the month and burrow entrance diameter. Laboratory experiments revealed that 20–60% of the CO₂ released by burrows originated from crab respiration. Temporal changes in the reduction potential in the sediment surrounding the burrows did not influence the CO₂ release from burrows. More oxidized conditions of plain sediment over time may explain the increase in CO₂ release until the end of the wet season. CO₂ released by U. cordatus and their burrows may be a significant pathway of CO₂ export from mangrove sediments and should be considered in mangrove carbon budget estimates.     

Organic matter diagenesis at the oxic/anoxic interface in coastal marine sediments,with emphasis on the role of burrowing animals

The present paper reviews the current knowledge on diagenetic carbon transformations at the oxic/anoxic interface in coastal marine sediments. Oxygen microelectrodes have revealed that most coastal sediments are covered only by a thin oxic surface layer. The penetration depth of oxygen into sediments is controlled by the balance between downward transport and consumption processes. Consumption of oxygen is directly or indirectly caused by respiration of benthic organisms. Aerobic organisms have the enzymatic capacity for complete oxidation of organic carbon. Anaerobic decay occurs stepwise, involving several types of bacteria. Large organic molecules are first fermented into small moieties. These are then oxidized completely by anaerobic respirers using a sequence of electron acceptors: Mn⁴⁺, NO₃ ^-, Fe³⁺, SO₄ ^2- and CO₂. The quantitative role of each electron acceptor depends on the sediment type and water depth. Since most of the sediment oxygen uptake is due to reoxidation of reduced metabolites, aerobic respiration is of limited importance. It has been suggested that sediments contain three major organic fractions: (1) fresh material that is oxidized regardless of oxygen conditions; (2) oxygen sensitive material that is only degraded in the presence of oxygen; and (3) totally refractory organic matter. Processes occurring at the oxic/anoxic boundaries are controlled by a number of factors. The most important are: (1) temperature, (2) organic supply, (3) light, (4) water currents, and (5) bioturbation. The role of bioturbation is important because the infauna creates a three-dimensional mosaic of oxic/anoxic interfaces in sediments. The volume of oxic burrow walls may be several times the volume of oxic surface sediment. The infauna increases the capacity, but not the overall organic matter decay in sediments, thus decreasing the pool of reactive organic matter. The increase in decay capacity is partly caused by injection of oxygen into the sediment, and thereby enhancing the decay of old, oxygen sensitive organic matter several fold. Finally, some future research directions to improve our understanding of diagenetic processes at the oxic/anoxic interface are suggested.     

Microscale oxygen distribution in various invertebrate burrow walls

Profiles of dissolved oxygen were measured in pore waters of unburrowed sediment and the burrow walls of seven invertebrate dwellings. Burrows studied include those of Corophium volutator, Heteromastus filiformis, Arenicola marina, Saccoglossus bromophenolosus, Clymenella sp., Hemigrapsus oregonensis and Cirriformia luxuriosa all from mudflats in Willapa Bay, Washington. These animals comprise a range of burrow architectures ranging from simple, unlined burrows to more complex, mucous lined burrows. Oxygen penetrated unburrowed sediment between depths of 0.4–2.6 mm, whereas oxygen penetrated the burrow walls from 0.3 mm to 2.3 mm. Three groups of burrows are recognized based on the oxygen diffusive properties relative to the unburrowed sediment including those that: (1) slightly impeded oxygen penetration, (2) clearly inhibited oxygen penetration, and (3) enhanced oxygen penetration. Differences in the diffusive properties of the burrow wall are related to the burrow microstructure and presumably the microbial communities living within the burrow microenvironment. The results of this study suggest that burrow shape and burrow‐wall architecture may play an important role in controlling the diffusion of oxygen, and possibly of other dissolved gases (i.e. CO₂, H₂S). The results further demonstrate that simplified assumptions (i.e. that bioturbation uniformly enhances oxygen diffusion into suboxic and anoxic sediments), while requisite for numerical modelling, are not necessarily representative of field data.     

Denitrification and nitrogen fixation dynamics in the area surrounding an individual ghost shrimp (Neotrypaea californiensis) burrow system

Bioturbated sediments are thought of as areas of increased denitrification or fixed-nitrogen (N) loss; however, recent studies have suggested that not all N may be lost from these environments, with some N returning to the system via microbial dinitrogen (N(2)) fixation. We investigated denitrification and N(2) fixation in an intertidal lagoon (Catalina Harbor, CA), an environment characterized by bioturbation by thalassinidean shrimp (Neotrypaea californiensis). Field studies were combined with detailed measurements of denitrification and N(2) fixation surrounding a single ghost shrimp burrow system in a narrow aquarium (15 cm by 20 cm by 5 cm). Simultaneous measurements of both activities were performed on samples taken within a 1.5-cm grid for a two-dimensional illustration of their intensity and distribution. These findings were then compared with rate measurements performed on bulk environmental sediment samples collected from the lagoon. Results for the aquarium indicated that both denitrification and N(2) fixation have a patchy distribution surrounding the burrow, with no clear correlation to each other, sediment depth, or distance from the burrow. Field denitrification rates were, on average, lower in a bioturbated region than in a seemingly nonbioturbated region; however, replicates showed very high variability. A comparison of denitrification field results with previously reported N(2) fixation rates from the same lagoon showed that in the nonbioturbated region, depth-integrated (10 cm) denitrification rates were higher than integrated N(2) fixation rates (～9 to 50 times). In contrast, in the bioturbated sediments, depending on the year and bioturbation intensity, some (～6.2%) to all of the N lost via denitrification might be accounted for via N(2) fixation.     

Oxygen measurements in the burrows of freshwater insects

1. Thin-tipped micro-electrodes were used to measure oxygen concentrations in the burrows of two common aquatic insects, the mayfly Hexagenia limbata and the alderfly Sialis velata . Both species maintain their surroundings oxygenated by drawing water from above the sediment surface into their tubes. 2. The temporal pattern of oxygen in the burrows differed between the species. The constant high oxygen concentration (>75% of air saturation) measured in the tubes of the mayfly suggest that this animal pumps water almost continuously, which is consistent with its high oxygen requirements. In contrast, oxygen concentration in burrows of the alderfly fluctuated widely over time, suggesting that this animal irrigates only irregularly, probably because it can tolerate short periods of low oxygen concentration in its burrow. 3. The interval between pumping episodes by the alderfly decreased with increasing temperature, a result of increased oxygen consumption by the animal and by sediment at high temperature. 4. Based on the tube dimensions, oxygen penetration depth and animal density in lakes, we estimate that Hexagenia could create an oxic micro-environment equivalent to 3–35% of the volume of the surface oxidized sediment layer created by molecular diffusion. The mosaic of oxic micro-environments created by the burrowing and irrigation of freshwater animals could influence chemical and biological processes in sediments, the fluxes of materials between the sediment and the overlying water column, and the exposure of benthic animals to sedimentary contaminants.     

Thermophilic anaerobes in Arctic marine sediments induced to mineralize complex organic matter at high temperature

Marine sediments harbour diverse populations of dormant thermophilic bacterial spores that become active in sediment incubation experiments at much higher than in situ temperature. This response was investigated in the presence of natural complex organic matter in sediments of two Arctic fjords, as well as with the addition of freeze‐dried Spirulina or individual high‐molecular‐weight polysaccharides. During 50°C incubation experiments, Arctic thermophiles catalysed extensive mineralization of the organic matter via extracellular enzymatic hydrolysis, fermentation and sulfate reduction. This high temperature‐induced food chain mirrors sediment microbial processes occurring at cold in situ temperatures (near 0°C), yet it is catalysed by a completely different set of microorganisms. Using sulfate reduction rates (SRR) as a proxy for organic matter mineralization showed that differences in organic matter reactivity determined the extent of the thermophilic response. Fjord sediments with higher in situ SRR also supported higher SRR at 50°C. Amendment with Spirulina significantly increased volatile fatty acids production and SRR relative to unamended sediment in 50°C incubations. Spirulina amendment also revealed temporally distinct sulfate reduction phases, consistent with 16S rRNA clone library detection of multiple thermophilic Desulfotomaculum spp. enriched at 50°C. Incubations with four different fluorescently labelled polysaccharides at 4°C and 50°C showed that the thermophilic population in Arctic sediments produce a different suite of polymer‐hydrolysing enzymes than those used in situ by the cold‐adapted microbial community. Over time, dormant marine microorganisms like these are buried in marine sediments and might eventually encounter warmer conditions that favour their activation. Distinct enzymatic capacities for organic polymer degradation could allow specific heterotrophic populations like these to play a role in sustaining microbial metabolism in the deep, warm, marine biosphere.     

Rates and controls of anaerobic microbial respiration across spatial and temporal gradients in saltmarsh sediments

This study was undertaken to determine the rates and controls ofanaerobic respiration reactions coupled to organic matter mineralization as afunction of space and time along a transect from a bioturbated creekbank to themidmarsh in Georgia saltmarsh sediments. Sulfate reduction rates (SRR) weremeasured at 3 sites during 5 sampling periods throughout the growth season. Thesites differed according to hydrologic regime and the abundance of dominantplants and macrofauna. SRR and pore water / solid phase geochemistry showedevidence of enhanced sediment oxidation at sites exposed to intensebioturbation. Iron(III) reduction rates (FeRR) were directly determined insaltmarsh sediments for the first time, and in agreement with measured SRR,higher rates were observed at the bioturbated, unvegetated creekbank (BUC) andbioturbated, vegetated levee (BVL) sites in comparison to a vegetated mid-marsh(MM) site. An unexpected result was the fact that SRR varied nearly as muchbetween sites (2–3 x) as it did with temperature or season (3–4 x).The BVL site, vegetated by the tall form of Spartinaalterniflora, always exhibited the highest SRR and carbon oxidationrates (> 4000 nmol cm^–3 d^–1) with high activity levels extending deep ( 50 cm)into the sediment, while the MM site, dominated by the short form ofSpartina, always exhibited the lowest SRR which werelocalized to the top 15 cm of sediment. SRR and FeRR at BUC wereintermediate between those measured at the BVL and MM. Acetate was the mostabundant microbial fermentation product (concentrations up to > 1mM) in marsh porewaters, and its distribution reflectedrespirationactivity. Chemical exchange, caused by bioturbation, appeared to be the primarycontrol explaining trends in rates of sulfate and Fe(III) reduction withmacrophytes and carbon source acting as secondary controls.     

Unusual nonmarine burrows from the Middle Jurassic of Scotland

The Kilmaluag Formation of the Great Estuarine Group (Middle Jurassic) of Scotland represents deposition of mixed carbonate and clastic sediments in a low‐salinity coastal lagoon to floodplain lake setting. Large, unusual trace fossils occur at two horizons within the formation. One type consists of platelike structures about 50 cm in diameter, which are found on wave‐rippled sandstone. These structures, strikingly similar to burrows produced by modern mudskippers, are assigned to fish that shallowly burrowed into the lagoon‐shore sediment.

The second type of burrows, found in brecciated, dolomitic limestones, are pipelike, about 4 to 7 cm in diameter and as much as 50 cm deep. One example has a chamber at the base of the pipe. Although most features of these structures appear similar to modern lungfish burrows, the chamber is most similar to structures produced by modern crayfish. The animal probably burrowed into the moist, mudflat sediment to escape desiccation during seasonal aridity.     

Strategies of burrowing in soft muddy sediments by diverse polychaetes

Muddy sediments are elastic solids through which morphologically diverse animals extend burrows by fracture. Muddy sediments inhabited by burrowing infauna vary considerably in mechanical properties, however, and at high enough porosities, muds can be fluidized. In this study, we examined burrowing behaviors and mechanisms of burrow extension for three morphologically diverse polychaete species inhabiting soft muddy sediments. Worms burrowed in gelatin, a transparent analog for muddy sediments, and in natural sediments in a novel viewing box enabling visualization of behaviors and sediment responses. Individuals of Scalibregma inflatum and Sternaspis scutata can extend burrows by fracture, but both also extended burrows by plastic deformation and by combinations of fracture and plastic deformation. Mechanical responses of sediments corresponded to different burrowing behaviors in Scalibregma; direct peristalsis was used to extend burrows by fracture or a combination of plastic deformation and fracture, whereas a retrograde expansive peristaltic wave extended burrows by plastic deformation. Burrowing speeds differed between behaviors and sediment mechanical responses, with slower burrowing associated with plastic deformation. Sternaspis exhibited less variability in behavior and burrowing speed but did extend burrows by different mechanisms consistent with observations of Scalibregma. Individuals of Ophelina acuminata did not extend burrows by fracture; rather individuals plastically deformed sediments similarly to individuals of the related Armandia brevis. Our results extend the range of natural sediments in which burrowing by fracture has been observed, but the dependence of burrow extension mechanism on species, burrowing behavior, and burrowing speed highlights the need for better understanding of mechanical responses of sediments to burrowers.     

Burrow morphology and dynamics of mudshrimp in Asian soft shores

Mudshrimps are important soft shore bioturbators but research on the ecology of tropical species has received less attention when compared with their temperate counterparts. The mudshrimp Austinogebia edulis is common on Asian soft shores and lives in burrows for its entire adult life. Epoxy resin casting of A. edulis burrows showed that they were approximately Y-shaped, with an upper U-part and the lower central shaft part. The burrows had two openings extending to the surface; the mean distance between the two openings was 11.0 cm in Hong Kong and 26.4 cm in Taiwan. Openings of the burrows had small chimneys. The tunnels of the burrows were circular, narrow and with a smooth surface (tunnel diameter corresponded to shrimp carapace width). Each burrow was inhabited by a single shrimp and burrows were inter-connected during the mating and reproductive season. Each burrow had four to 12 spherical chambers, which were free of detritus. The chambers were thought to be used for suspension feeding, current generation and as turning points. The depth of burrows was up to 1.1 m. Multivariate analysis on various burrow parameters showed that burrows collected on a mud flat in Taiwan were deeper, had a wider distance between the openings and a larger volume than burrows collected from a sandy shore in Hong Kong, suggesting that burrow architecture is variable between shore types. Burrow architecture, however, did not vary between tidal levels, seasons and shrimp density on the shores in Hong Kong, indicating that the burrows were quite stable within the substratum and were not affected by environmental and biological factors.     

Depth of artificial Burrowing Owl burrows affects thermal suitability and occupancy

Many organizations have installed artificial burrows to help bolster local Burrowing Owl (Athene cunicularia) populations. However, occupancy probability and reproductive success in artificial burrows varies within and among burrow installations. We evaluated the possibility that depth below ground might explain differences in occupancy probability and reproductive success by affecting the temperature of artificial burrows. We measured burrow temperatures from March to July 2010 in 27 artificial burrows in southern California that were buried 15–76 cm below the surface (measured between the surface and the top of the burrow chamber). Burrow depth was one of several characteristics that affected burrow temperature. Burrow temperature decreased by 0.03°C per cm of soil on top of the burrow. The percentage of time that artificial burrows provided a thermal refuge from above‐ground temperature decreased with burrow depth and ranged between 50% and 58% among burrows. The percentage of time that burrow temperature was optimal for incubating females also decreased with burrow depth and ranged between 27% and 100% among burrows. However, the percentage of time that burrow temperature was optimal for unattended eggs increased with burrow depth and ranged between 11% and 95% among burrows. We found no effect of burrow depth on reproductive success across 21 nesting attempts. However, occupancy probability had a non‐linear relationship with burrow depth. The shallowest burrows (15 cm) had a moderate probability of being occupied (0.46), burrows between 28 and 40 cm had the highest probability of being occupied (>0.80), and burrows >53 cm had the lowest probability of being occupied (<0.43). Burrowing Owls may prefer burrows at moderate depths because these burrows provide a thermal refuge from above‐ground temperatures, and are often cool enough to allow females to leave eggs unattended before the onset of full‐time incubation, but not too cool for incubating females that spend most of their time in the burrow during incubation. Our results suggest that depth is an important consideration when installing artificial burrows for Burrowing Owls. However, additional study is needed to determine the possible effects of burrow depth on reproductive success and on possible tradeoffs between the effects of burrow depth on optimal temperature and other factors, such as minimizing the risk of nest predation.     

A modern analogue for the trace fossil Gyrolithes: burrows of the thalassinidean shrimp Axianassa australis

The tidal flats at Praia do Araça, Brazil have muddy siliciclastic sediments on the surface and a layer of heavily packed shells down to 30–40 cm depth. The most obvious element of the infauna is the thalassinidean shrimp Axianassa australis. Several animals were captured with a yabby pump. Burrow openings were characterized by a low mound (1-2 cm high and 10–30 cm in diameter at the base) with one or two simple holes nearby (20-70 cm away). Counts along two transects showed a mean density of Axianassa burrow openings of 1.4 m⁻² (range: 0–7), mounds ranged in density from 0 to 3 m⁻² (mean 1.25). Three nearly complete (and several incomplete) resin casts showed a unique burrow shape, with spiral shafts leading to wide horizontal galleries from which several evenly proportioned corkscrew-shaped spirals branched off, leading to further horizontal galleries at greater sediment depths. Burrows had up to 15 such spirals and a total length of over 8 m. The total burrow depth was between 106 and 130 cm. The role of the spirals and the similarity of Axianassa burrows to the trace fossil Gyrolithes are discussed.     

Pathways and Microbiology of Thiosulfate Transformations and Sulfate Reduction in a Marine Sediment (Kattegat, Denmark)

Reductive and oxidative pathways of the sulfur cycle were studied in a marine sediment by parallel radiotracer experiments with ³⁵SO₄^2-, H₂³⁵S, and ³⁵S₂O₃^2- injected into undisturbed sediment cores. The distributions of viable populations of sulfate- and thiosulfate-reducing bacteria and of thiosulfate-disproportionating bacteria were concurrently determined. Sulfate reduction occurred both in the reducing sediment layers and in oxidized and even oxic surface layers. The population density of sulfate-reducing bacteria was >10⁶ cm^-3 in the oxic layer, high enough that it could possibly account for the measured rates of sulfate reduction. The bacterial numbers counted in the reducing sediment layers were 100-fold lower. The dominant sulfate reducers growing on acetate or H₂ were gas-vacuolated motile rods which were previously undescribed. The products of sulfide oxidation, which took place in both oxidized and reduced sediment layers, were 65 to 85% S₂O₃^2- and 35 to 15% SO₄^2-. Thiosulfate was concurrently oxidized to sulfate, reduced to sulfide, and disproportionated to sulfate and sulfide. There was a gradual shift from predominance of oxidation toward predominance of reduction with depth in the sediment. Disproportionation was the most important pathway overall. Thiosulfate disproportionation occurred only as cometabolism in the marine acetate-utilizing sulfate-reducing bacteria, which could not conserve energy for growth from this process alone. Oxidative and reductive cycling of sulfur thus occurred in all sediment layers with an intermediate “thiosulfate shunt” as an important mechanism regulating the electron flow.