In situ organism-sediment interactions: Bioturbation and biogeochemistry in a highly depositional estuary.
ABSTRACT: Organic matter (OM) production and degradation is important in coastal estuaries, and OM fate is strongly influenced by the coupled interactions of bioturbation and biogeochemistry. From April to September 2013 sediment cores and a benthic observing system, Wormcam, were used to investigate the in situ relationship of biogeochemistry and macrofauna bioturbation in Cape Lookout Bight North Carolina. Wormcam imagery provided a vivid depiction of macrofauna functioning in an environment not previously observed, and affirmed the importance of fine-scale temporal observations of the benthic environment in situ. Observation of macrofauna presence and bioturbation during the summer contradicted previous studies that found this area to be azoic during methane activity and sulfide build-up. Sulfate concentrations decreased while sulfide and dissolved inorganic carbon concentrations increased during the summer. This coincided with changes in the depth and rates of bioturbation. Summer burrow depths (~0.8 cm) and rates (~0.4 cm h-1) were significantly less than spring burrow depths (~3.0 cm) and rates (~1.0 cm h-1). While sulfate reduction and OM degradation increased with temperature at a microscopic level, macroscopic OM degradation was reduced. As a result, reduced conditions dominated and a thin aerobic sediment layer, a few millimeters in thickness, was visible at the sediment surface. Decreases in macrofauna burrow depth and rates diminishes the area of influence of bioturbators, limiting bioturbation and subsequently the important ecosystem functions these organisms provide.
Project description:In marine environments, macrofauna living in or on the sediment surface may alter the structure, diversity and function of benthic microbial communities. In particular, microbial nitrogen (N)-cycling processes may be enhanced by the activity of large bioturbating organisms. Here, we study the effect of the burrowing mud shrimp Upogebia deltaura upon temporal variation in the abundance of genes representing key N-cycling functional guilds. The abundance of bacterial genes representing different N-cycling guilds displayed different temporal patterns in burrow sediments in comparison with surface sediments, suggesting that the burrow provides a unique environment where bacterial gene abundances are influenced directly by macrofaunal activity. In contrast, the abundances of archaeal ammonia oxidizers varied temporally but were not affected by bioturbation, indicating differential responses between bacterial and archaeal ammonia oxidizers to environmental physicochemical controls. This study highlights the importance of bioturbation as a control over the temporal variation in nitrogen-cycling microbial community dynamics within coastal sediments.
Project description:Carbon mineralization processes and their dependence on environmental conditions (e.g. through macrobenthic bioturbation) have been widely studied in temperate coastal sediments, but almost nothing is known about these processes in subtropical coastal sediments. This study investigated pathways of organic carbon mineralization and associated effects of macrobenthic bioturbation in winter and summer (September 2012 and February 2014) at the SE Brazilian coast. Iron reduction (FeR) was responsible for 73-81% of total microbial carbon mineralization in September 2012 and 32-61% in February 2014. Similar high rates of FeR have only been documented a few times in coastal sediments and can be sustained by the presence of large bioturbators. Denitrification accounted for 5-27% of total microbial carbon mineralization while no SO4(2-) reduction was detected in any season. Redox profiles suggested that conditions were less reduced in February 2014 than in September 2012, probably associated with low reactivity of the organic matter, higher rates of aerobic respiration and bioirrigation by the higher density of small-macrofauna. Bioturbation by small macrofauna may maintain the sediment oxidized in summer, while large-sized species stimulate the reoxidation of reduced compounds throughout the year. Therefore, bioturbation seems to have an important role modulating the pathways of carbon mineralization in the area.
Project description:Seasonal hypoxia on the Louisiana continental shelf (LCS) has grown to over 22,000 km2 with limited information available on how low oxygen effects the benthos. Benthic macrofaunal colonization and sediment biogeochemical parameters were characterized at twelve stations in waters 10 - 50 m deep along four transects spanning 320 km across the LCS hypoxic zone in the early fall of 2010 when bottom waters typically return to oxic conditions. Chemical data and sediment profile imaging (SPI) support three primary mechanistic pathways of organic matter degradation on the LCS: (i) metal oxide cycling in depositional muds, (ii) infauna-driven bioturbation delivering oxygen below the sediment-water interface, and (iii) sulfate reduction in sediments where iron oxide availability is limited. The transect nearest the Mississippi River delta had the highest concentrations of porewater and solid phase Mn and Fe with SPI images of recently deposited reddish, mixed muddy sediments suggestive of metal cycling. The deepest stations had high oxidized iron concentrations and rust colored sediments with faunal colonization that suggests sediments are oxidized via bioturbation. Many nearshore and central LCS stations had more black sediments, more disturbed clay layers, lower amounts of oxidized iron, and higher sulfate reduction rates than the deepest stations. Sediment mixing coefficients, DB , determined from chlorophyll-a concentration profiles varied between 33 and 183 cm-2 y-1. DB values were highest at the deepest stations where sediments were colonized. DB were not determined at two nearshore stations where chlorophyll-a concentrations were highly variable in surficial sediments, and on the eastern shelf where sedimentation is high. This study provides a regional view of benthic faunal colonization and sediment biogeochemistry on the LCS, describes regions with potentially different pathways of organic matter degradation, and demonstrates the importance of both bioturbation and physical mixing in processing the large amounts of organic matter in river-dominated continental shelf systems.
Project description: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.
Project description:One of the most common approaches for investigating the ecology of spatially complex environments is to examine a single biotic assemblage present, such as macroinvertebrates. Underlying this approach are assumptions that sampled and unsampled taxa respond similarly to environmental gradients and exhibit congruence across different sites. These assumptions were tested for five benthic groups of various sizes (archaea, bacteria, microbial eukaryotes/protists, meiofauna and macrofauna) in Plymouth Sound, a harbour with many different pollution sources. Sediments varied in granulometry, hydrocarbon and trace metal concentrations. Following variable reduction, canonical correspondence analysis did not identify any associations between sediment characteristics and assemblage composition of archaea or macrofauna. In contrast, variation in bacteria was associated with granulometry, trace metal variations and bioturbation (e.g. community bioturbation potential). Protists varied with granulometry, hydrocarbon and trace metal predictors. Meiofaunal variation was associated with hydrocarbon and bioturbation predictors. Taxon turnover between sites varied with only three out of 10 group pairs showing congruence (meiofauna-protists, meiofauna-macrofauna and protists-macrofauna). While our results support using eukaryotic taxa as proxies for others, the lack of congruence suggests caution should be applied to inferring wider indicator or functional interpretations from studies of a single biotic assemblage.
Project description:Ocean acidification (OA), caused by the dissolution of increasing concentrations of atmospheric carbon dioxide (CO2) in seawater, is projected to cause significant changes to marine ecology and biogeochemistry. Potential impacts on the microbially driven cycling of nitrogen are of particular concern. Specifically, under seawater pH levels approximating future OA scenarios, rates of ammonia oxidation (the rate-limiting first step of the nitrification pathway) have been shown to dramatically decrease in seawater, but not in underlying sediments. However, no prior study has considered the interactive effects of microbial ammonia oxidation and macrofaunal bioturbation activity, which can enhance nitrogen transformation rates. Using experimental mesocosms, we investigated the responses to OA of ammonia oxidizing microorganisms inhabiting surface sediments and sediments within burrow walls of the mud shrimp Upogebia deltaura. Seawater was acidified to one of four target pH values (pHT 7.90, 7.70, 7.35 and 6.80) in comparison with a control (pHT 8.10). At pHT 8.10, ammonia oxidation rates in burrow wall sediments were, on average, fivefold greater than in surface sediments. However, at all acidified pH values (pH ? 7.90), ammonia oxidation rates in burrow sediments were significantly inhibited (by 79-97%; p < 0.01), whereas rates in surface sediments were unaffected. Both bacterial and archaeal abundances increased significantly as pHT declined; by contrast, relative abundances of bacterial and archaeal ammonia oxidation (amoA) genes did not vary. This research suggests that OA could cause substantial reductions in total benthic ammonia oxidation rates in coastal bioturbated sediments, leading to corresponding changes in coupled nitrogen cycling between the benthic and pelagic realms.
Project description:Understanding spatial and temporal patterns of bioirrigation induced by benthic fauna ventilation is critical given its significance on benthic nutrient exchange and biogeochemistry in coastal ecosystems. The quantification of this process challenges marine scientists because faunal activities and behaviors are concealed in an opaque sediment matrix. Here, we use a hybrid medical imaging technique, positron emission tomography and computed tomography (PET/CT) to provide a qualitative visual and fully quantitative description of bioirrigation in 4D (space and time). As a study case, we present images of porewater advection induced by the well-studied lugworm (Arenicola marina). Our results show that PET/CT allows more comprehensive studies on ventilation and bioirrigation than possible using techniques traditionally applied in marine ecology. We provide a dynamic three-dimensional description of bioirrigation by the lugworm at very high temporal and spatial resolution. Results obtained with the PET/CT are in agreement with literature data on lugworm ventilation and bioirrigation. Major advantages of PET/CT over methods commonly used are its non-invasive and non-destructive approach and its capacity to provide information that otherwise would require multiple methods. Furthermore, PET/CT scan is versatile as it can be used for a variety of benthic macrofauna species and sediment types and it provides information on burrow morphology or animal behavior. The lack of accessibility to the expensive equipment is its major drawback which can only be overcome through collaboration among several institutions.
Project description:Coastal ocean acidification research is dominated by laboratory-based studies that cannot necessarily predict real-world ecosystem response given its complexity. We enriched coastal sediments with increasing quantities of organic matter in the field to identify the effects of eutrophication-induced acidification on benthic structure and function, and assess whether biogenic calcium carbonate (CaCO3) would alter the response. Along the eutrophication gradient we observed declines in macrofauna biodiversity and impaired benthic net primary productivity and sediment nutrient cycling. CaCO3 addition did not alter the macrofauna community response, but significantly dampened negative effects on function (e.g. net autotrophy occurred at higher levels of organic matter enrichment in +CaCO3 treatments than -CaCO3 (1400 vs 950 g dw m-2)). By identifying the links between eutrophication, sediment biogeochemistry and benthic ecosystem structure and function in situ, our study represents a crucial step forward in understanding the ecological effects of coastal acidification and the role of biogenic CaCO3 in moderating responses.
Project description:Methane and nitrous oxide are potent greenhouse gases (GHGs) that contribute to climate change. Coastal sediments are important GHG producers, but the contribution of macrofauna (benthic invertebrates larger than 1 mm) inhabiting them is currently unknown. Through a combination of trace gas, isotope, and molecular analyses, we studied the direct and indirect contribution of two macrofaunal groups, polychaetes and bivalves, to methane and nitrous oxide fluxes from coastal sediments. Our results indicate that macrofauna increases benthic methane efflux by a factor of up to eight, potentially accounting for an estimated 9.5% of total emissions from the Baltic Sea. Polychaetes indirectly enhance methane efflux through bioturbation, while bivalves have a direct effect on methane release. Bivalves host archaeal methanogenic symbionts carrying out preferentially hydrogenotrophic methanogenesis, as suggested by analysis of methane isotopes. Low temperatures (8?°C) also stimulate production of nitrous oxide, which is consumed by benthic denitrifying bacteria before it reaches the water column. We show that macrofauna contributes to GHG production and that the extent is dependent on lineage. Thus, macrofauna may play an important, but overlooked role in regulating GHG production and exchange in coastal sediment ecosystems.
Project description:Denitrification is a critical process that can alleviate the effects of excessive nitrogen availability in aquatic ecosystems subject to eutrophication. An important part of denitrification occurs in benthic systems where bioturbation by meiofauna (invertebrates <1 mm) and its effect on element cycling are still not well understood. Here we study the quantitative impact of meiofauna populations of different abundance and diversity, in the presence and absence of macrofauna, on nitrate reduction, carbon mineralization and methane fluxes. In sediments with abundant and diverse meiofauna, denitrification is double that in sediments with low meiofauna, suggesting that meiofauna bioturbation has a stimulating effect on nitrifying and denitrifying bacteria. However, high meiofauna densities in the presence of bivalves do not stimulate denitrification, while dissimilatory nitrate reduction to ammonium rate and methane efflux are significantly enhanced. We demonstrate that the ecological interactions between meio-, macrofauna and bacteria are important in regulating nitrogen cycling in soft-sediment ecosystems.