Benthic Boundary Layer Research Articles

Biomixing in Stagnant Water Above Population of Blue Mussels (Mytilus edulis)

Dense beds of filter-feeding mussels can exert a considerable grazing impact on phytoplankton in many marine areas depending on downmixing promoted by current, wave- and wind action. But downmixing may also be promoted by biomixing caused by the action of the strong exhalent jets of water from the mussels. Here we study the strength of biomixing exerted by large actively filtering blue mussels Mytilus edulis in stagnant water. Vertical concentration profiles of added algal cells (Rhodomonas salina) were measured (as chl a) over a 70 cm high and stagnant water column in an aquarium above a population of 48 ind.m-2 of mussels of shell length 69.5 ± 2.3 mm. Due to the intense agitation (biomixing) generated by exhalant jets of the actively feeding mussels the profiles remained nearly uniform over the full water column while decreasing exponentially with time, reaching a level of about 40% of the initial level after 120 min, which implied a population filtration rate of about 0.3 m3.h-1.m-2 in agreement with prior clearance measurements. Comparing to numerical solutions of a one-dimensional diffusion model, varying the eddy diffusivity, a value of D = 550 × 10-6 m2.s-1 was estimated. This high strength of biomixing far exceeds those of previous similar studies on the filter-feeding polychaete Nereis diversicolor (0.3 × 10-6 m2.s-1) and the ascidian Ciona intestinalis (150 × 10-6 m2.s-1) and suggests that biomixing in moderate benthic boundary layer flows past mussel beds may contribute to the downmixing of phytoplankton.

Sustained in situ measurements of dissolved oxygen, methane and water transport processes in the benthic boundary layer at MC118, northern Gulf of Mexico

Within months of the BP Macondo Wellhead blowout, elevated methane concentrations within the water column revealed a significant retention of light hydrocarbons in deep waters plus corresponding dissolved oxygen (DO) deficits. However, chemical plume tracking efforts were hindered by a lack of in situ monitoring capabilities. Here, we describe results from in situ time-series, lander-based investigations of physical and biogeochemical processes controlling dissolved oxygen, and methane at Mississippi Canyon lease block 118 (~18km from the oil spill) conducted shortly after the blowout through April 2012. Multiple sensor arrays plus open-cylinder flux chambers (“chimneys”) deployed from a benthic lander collected oxygen, methane, pressure, and current speed and direction data within one meter of the seafloor. The ROVARD lander system was deployed for an initial 21-day test experiment (9/13/2010–10/04/2010) at 882m depth before a longer 160-day deployment (10/24/2011–4/01/2012) at 884m depth. Temporal variability in current directions and velocities and water temperatures revealed strong influences of bathymetrically steered currents and overlying along-shelf flows on local and regional water transport processes. DO concentrations and temperature were inversely correlated as a result of water mass mixing processes. Flux chamber measurements during the 160-day deployment revealed total oxygen utilization (TOU) averaging 11.6mmol/m2day. Chimney DO concentrations measured during the 21-day deployment exhibited quasi-daily variations apparently resulting from an interaction between near inertial waves and the steep topography of an elevated scarp immediately adjacent to the 21-day deployment site that modulated currents at the top of the chimney. Variability in dissolved methane concentrations suggested significant temporal variability in gas release from nearby hydrocarbon seeps and/or delivery by local water transport processes. Free-vehicle (lander) monitoring over time scales of months to years utilizing in situ sensors can provide an understanding of processes controlling water transport, respiration and the fate and impacts of accidental and natural gas and oil releases.

Microfossils, a Key to Unravel Cold-Water Carbonate Mound Evolution through Time: Evidence from the Eastern Alboran Sea.

Cold-water coral (CWC) ecosystems occur worldwide and play a major role in the ocean's carbonate budget and atmospheric CO2 balance since the Danian (~65 m.y. ago). However their temporal and spatial evolution against climatic and oceanographic variability is still unclear. For the first time, we combine the main macrofaunal components of a sediment core from a CWC mound of the Melilla Mounds Field in the Eastern Alboran Sea with the associated microfauna and we highlight the importance of foraminifera and ostracods as indicators of CWC mound evolution in the paleorecord. Abundances of macrofauna along the core reveal alternating periods dominated by distinct CWC taxa (mostly Lophelia pertusa, Madrepora oculata) that correspond to major shifts in foraminiferal and ostracod assemblages. The period dominated by M. oculata coincides with a period characterized by increased export of refractory organic matter to the seafloor and rather unstable oceanographic conditions at the benthic boundary layer with periodically decreased water energy and oxygenation, variable bottom water temperature/density and increased sediment flow. The microfaunal and geochemical data strongly suggest that M. oculata and in particular Dendrophylliidae show a higher tolerance to environmental changes than L. pertusa. Finally, we show evidence for sustained CWC growth during the Alleröd-Younger-Dryas in the Eastern Alboran Sea and that this period corresponds to stable benthic conditions with cold/dense and well oxygenated bottom waters, high fluxes of labile organic matter and relatively strong bottom currents

The Influence of Bed Roughness on Turbulence: Cabras Lagoon, Sardinia, Italy

Estimates of bed roughness used for predictions of sediment transport are usually derived either from simple scalars of the physical roughness (i.e., ripple height or grain size) or from the hydrodynamic roughness length (Zo) based upon velocity gradient estimates in the benthic boundary layer. Neither parameter accounts for irregular bed features. This study re-evaluates the relation between hydrodynamic roughness and physical bed roughness using high-resolution seabed scanning in the inlet of a shallow lagoon. The statistically-robust relationship, based on a 1D statistical analysis of the seabed elevation at different locations of the Cabras lagoon. Sardinia, has been obtained between Zo and the topographical bed roughness Ks by defining Ks = 2*STD + skin friction, with STD the standard deviation of the seabed elevation variations. This correlation between Ks and Zo demonstrates that the roughness length is directly influenced by irregular bed features, and that the Reynolds number accounts for the total drag of the bed: the data points collapse on the Law of the Wall curves with a fitting factor x = 0.5. Further testing must be done in other locations and in the fully-rough domain in order to test how widely those new parameters can be applied.

First insights into genus level diversity and biogeography of deep sea benthopelagic calanoid copepods in the South Atlantic and Southern Ocean

Calanoid copepods constitute the most numerous organisms not only in the pelagic realm, but also in the benthic boundary layer, which gives them an important role in the turnover of organic matter in the benthopelagic habitat. During seven expeditions to the South Atlantic and Southern Ocean, the diversity and biogeography of deep-sea benthopelagic calanoid copepods were studied. The communities of calanoids living in the vicinity of the seabed were characterized by high diversity comparable to many pelagic habitats, but low abundance of individuals. Members of the taxon Clausocalanoidea dominated the communities, and within this taxon most individuals belonged to detritivore calanoids characterized by sensory setae on the second maxillae or aetideid copepods. 73% of all genera classified as obligate or predominantly benthopelagic copepods detected during these expeditions were new to science and a vast number of genera and species have been described since then. Comparing the communities of calanoid genera between different regions, the assemblages in the Southern Ocean differed significantly from the Southeast and Southwest Atlantic. A latitudinal diversity gradient could be observed, with highest numbers of genera in the Southwest Atlantic and low numbers at stations in the Southern Ocean. Reviewing the literature, endemism for benthopelagic calanoids appeared to be low on a latitudinal range caused by connectivity in benthopelagic habitats through spreading water masses. However, considering the habitats structuring the water column vertically, a high number of genera are endemic in the benthopelagial and specialized to living within the vicinity of the seabed.

A field experiment on trophic relations within the benthic boundary layer (BBL) over an oligotrophic continental shelf

The benthic boundary layer (BBL) macrofauna consists of epibenthic, hyperbenthic and zooplanktonic organisms with different degrees of mobility and bottom dependence. The top-level predators within this habitat include demersal fish species. The BBL species are considered to be the most readily available prey for these fish making them an attractive source of food and therefore key taxa in marine food webs. The present study is a field experiment in the oligotrophic continental shelf of Heraklion Bay (Cretan Sea, eastern Mediterranean) investigating the ways in which demersal fish utilize BBL sources as food. Species diversity patterns derived from samples collected by using a modified hyperbenthic sledge as well as different benthic, hyperbenthic and pelagic samplers are described. These patterns are compared with those derived from stomach content analysis of demersal fish species inhabiting the same area. Results reveal that BBL macrofaunal communities which comprise most of the available prey communities are largely exploited by demersal fish species. Furthermore, the application of different samplers provides complementary information on prey selectivity of macrofaunal organisms by demersal fish species, highlighting further the trophic interactions within the BBL habitat.

Seasonal variation in concentration, size, and settling velocity of muddy marine flocs in the benthic boundary layer

AbstractSuspended Particulate Matter (SPM) concentration profiles of the lowest 2 m of the water column and particle size distribution at 2 m above the bed were measured in a coastal turbidity maximum area (southern North Sea) during more than 700 days between 2006 and 2013. The long‐term data series of SPM concentration, floc size, and settling velocity have been ensemble averaged according to tidal range, alongshore residual flow direction, and season, in order to investigate the seasonal SPM dynamics and its relation with physical and biological processes. The data show that the SPM is more concentrated in the near‐bed layer in summer, whereas in winter, the SPM is better mixed throughout the water column. The decrease of the SPM concentration in the water column during summer is compensated by a higher near‐bed concentration indicating that a significant part of the SPM remains in the area during summer rather than being advected out of it. The opposite seasonality between near‐bed layer and water column has to our knowledge not yet been presented in literature. Physical effects such as wave heights, wind climate, or storms have a weak correlation with the observed seasonality. The argument to favor microbial activity as main driver of the seasonality lies in the observed variations in floc size and settling velocity. On average, the flocs are larger and thus settling velocities higher in summer than winter.

Benthic fluxes on the Oregon shelf

Benthic chamber incubations were performed in the mid-shelf region on the Oregon shelf in June and August 2009 to measure fluxes of oxygen, nutrients, and iron and their effect on water column chemistry. Chamber oxygen and nitrate fluxes were into the sediments while silicate, iron and ammonium fluxes were out of the sediments. Benthic fluxes were similar between the two months, except that dissolved iron fluxes were higher at some sites in August. Bottom waters were consistently hypoxic (43–64 μM O2) and had ammonium concentrations from 0 to 2.6 μM in the mid-shelf region. Given measured ammonium fluxes (0.2–1.4 mmol m−2 d−1), we used a simple stoichiometric model for a 10 m bottom boundary layer to calculate that benthic fluxes only contributed ∼16–41% of the bottom water ammonia. Benthic oxygen fluxes (−4.3 to −12.5 mmol O2 m−2 d−1) were responsible for ∼38–51% of oxygen drawdown in the benthic boundary layer. In both cases, the remainder may be attributed to water column respiration. Benthic iron and nitrate fluxes have opposite effects on productivity. Iron fluxes (0–71 μmol m−2 d−1, average: 5 μmol m−2 d−1) increased bottom water concentrations while nitrate was lost (−1.2 to −2.9 mmol m−2 d−1NO3−) due to denitrification. By supplying iron and consuming nitrogen, benthic diagenetic processes reinforce an iron-replete, nitrate-limited coastal ecosystem.

Water column oxygen dynamics within the coastal gradient in the northeastern Gulf of Mexico inner shelf

This study quantified the coastal gradients of salinity, temperature, nutrients, chlorophyll and oxygen along a transect from 5 to 18m depth in the northeastern Gulf of Mexico (NEGOM) through measurements of water column profiles over 4.5-years and recordings of probes mounted at 0.5m above the seafloor over 2-years. The main goal was to determine the influences of the coastal gradients on water column oxygen concentration distributions. Apalachicola River discharge affected the study region as reflected in low-salinity surface layers. Decreasing rates of oxygen production and consumption from the shallowest station to the deepest station were attributed to measured coastal gradients of nutrients and the decreasing influence of seafloor processes with increasing distance from shore. Water column photosynthetic oxygen production at a realistic irradiance of 200μEm−2s−1 ranged from 0.5 to 4.5mmolO2m−3h−1, and dark consumption rates from 0.2 to 2.6mmolO2m−3h−1. Based on July irradiance profiles and associated production and consumption rates, the water column in July 2009 produced 0.9 (1SD=0.2)gCm−2d−1 with rates remaining relatively constant over the length of this coastal transect. Benthic boundary layer production reaching 26–43% of the surface layer production underlines the role of the benthic boundary layer for production in the inner shelf. Oxygen in the benthic boundary layer never decreased below 73% air saturation, and hypoxic zones, as reported from shelf areas west of the study region, did not occur. Oxygen production exceeded consumption suggesting a net autotrophic system.

Vertical convergence of resuspended sediment and subducted phytoplankton to a persistent detached layer over the southern shelf of Monterey Bay, California

AbstractWork done by the present authors and collaborators in the first year of a 2‐year field study established the frequent presence of layers of suspended particulate matter detached from the seafloor over the southern shelf of Monterey Bay, California. In this contribution, we document similar findings over a 1‐month period in Fall 2012 and investigate physical processes leading to vertical convergence of particles to the observed layers, both from resuspended sediment originating below the layer depth and from phytoplankton originating above the layer depth. Physical and optical vertical structure was measured by an autonomous vertical profiler, thermistor chain, and acoustic Doppler current profiler, and optical sensors fixed to a bottom‐mounted frame measured beam attenuation and particle size distribution within the benthic boundary layer (BBL). These data support a conceptual model for layer formation in which (1) bottom material was mobilized into the BBL by semidiurnal internal tidal currents; (2) brief “updraft” events regularly injected particles into the interior water column during the downslope phase of the semidiurnal internal tide; and (3) particles converged at the detached layer due to a measured, but previously unreported, long time scale mean vertical convergence in flow. Subduction and vertical mixing of phytoplankton to the subeuphotic layer were also observed and are presented in two case studies in connection with horizontal convergences of surface water masses.

Distribution and sources of suspended particulate matter in the Kara Sea

Abstract. The distribution of suspended particulate matter (SPM) concentration in the Kara Sea was analysed based on ship and satellite data during the lowest ice cover periods in the Arctic (2007 and 2011). The distance of the river runoff with terrigenous SPM was recognized up to at least 76–76.5° N on the basis of isotope data and SPM concentrations and composition. The complex studies of SPM in the water column and in the benthic boundary layer provided new information about the supply and transformation of particles in the marine ecosystem.

Near seafloor bioluminescence, macrozooplankton and macroparticles at the Mid-Atlantic Ridge

The benthic boundary layer is a region often perceived to be high in faunal abundance and biomass. In this study, we investigated the distribution of near seafloor bioluminescent zooplankton (BL), macrozooplankton (>1cm) and macroparticles (>430μm) at the Northern Mid-Atlantic Ridge at ca. 2500m depth.At sites south of 52°N, the Charlie Gibbs Fracture Zone, BL density increased weakly towards the seafloor. This trend was driven by small bioluminescent crustaceans, comprising ca. 90% of the total BL density. Macroparticle density was coherent with BL density, exhibiting a small increase towards the seafloor. Appendicularians (animals as well as occupied and discarded houses) were the most abundant macrozooplankton, and the only group to show a significant increase in density towards the seafloor. The absence of pronounced increases in BL and macroparticle density, and no increase in macrozooplankton (except appendicularians), towards the seafloor do not support the conventional view of high concentrations of particulate organic matter and zooplankton biomass in the benthic boundary layer relative to overlying waters.

Radium isotope distributions during the US GEOTRACES North Atlantic cruises

Radium isotopes are produced in sediments via the decay of thorium isotopes and are generally soluble in seawater. As such, isotopes of the radium quartet (224Ra, 223Ra, 228Ra, 226Ra) have been used as tracers of ocean boundary inputs and mixing processes. We measured radium isotopes on the US GEOTRACES North Atlantic cruises in 2010 and 2011, which crossed a number of key ocean boundary features including the Mediterranean outflow, the Mauritanian upwelling and oxygen minimum zones (OMZ) along western Africa, a hydrothermal plume over the mid-Atlantic Ridge (MAR), and the broad continental margin of the north-eastern United States. Radium isotope features at these locations are discussed in the context of their potential to quantify fluxes of trace elements and isotopes (TEIs) from common sources. Notable observations include: (1) a Mediterranean outflow spreading rate of 0.52–0.60cm/s derived from 228Ra, (2) evidence of substantial sediment–water interaction in the benthic boundary layer along the OMZ, (3) decoupling between 223Ra and the other Ra isotope sources over the MAR, and (4) significant continental inputs (e.g. submarine groundwater discharge—SGD) in the western Atlantic. Lastly, 228Ra inventories have remained constant over the past 30years, which suggests that SGD is steady-state for the North Atlantic on decadal time scales.

Advances in Field Methods for the Study of Benthic Boundary Layer (BBL) Biodiversity

The marine sediment-water interface is one of the most clearly defined ecological boundaries and the most extensive habitat on the planet occupying 75% of the Earth’s surface. The ecological processes which appear to link the domain below the sediment-water interface with that above are likely to have global importance. However, the study of the different ecosystem structure above and below the sediment-water interface usually requires different hypotheses and methodological approaches leading often to scientific isolation of the two domains. Consequently, the role of biodiversity related to sediment-water interface is underestimated, especially in studies of benthicpelagic coupling related to energy fluxes and also in holistic approaches of the marine ecosystem. At the interface between the sediment and the overlying water there is a benthic boundary layer (BBL) which is characterized by strong gradients of flow as well as high concentrations of dissolved and particulate matter [1]. This dynamic zone can be extended from a few centimetres to tens of metres, depending both temporally and spatially on various physical and biogeochemical processes whereby organisms on either side of the sediment-water interface interact. Hyperbenthos constitutes the dominant BBL faunal component and includes all swimming bottom-dependent organisms (mainly crustaceans) which perform, with varying amplitude, intensity and regularity, seasonal or daily vertical migrations into the water column [2]. The BBL also supports epibenthic and several infaunal species emerging into the water column as well as zooplanktonic species usually derived from downward extensions [3]. Most of the BBL species are considered to be key taxa in marine food webs [4]. They exploit a large diversity of food resources, mobilizing organic and detritus particles and associated microbes or preying on meiobenthos and plankton, while constituting an important food source for demersal fish, epibenthic crustaceans and cephalopods. For the study of BBL macrofauna, specially designed samplers, generally known as hyperbenthic sledges, have been used over the last 40 years. Nevertheless, there are still practical technical difficulties in sampling efficiently the lowest layer of the water column above the seabed where most of these organisms are concentrated.

Iron-binding ligands and humic substances in the San Francisco Bay estuary and estuarine-influenced shelf regions of coastal California

Dissolved iron (dFe) and organic dFe-binding ligands were determined in San Francisco Bay, California by competitive ligand exchange adsorptive cathodic stripping voltammetry (CLE-ACSV) along a salinity gradient from the freshwater endmember of the Sacramento River (salinity <2) to the mouth of the estuary (salinity >26). A range of dFe-binding ligand classes was simultaneously determined using multiple analytical window analysis, involving titrations with multiple concentrations of the added ligand, salicylaldoxime. The highest dFe and ligand concentrations were determined in the low salinity end of the estuary, with dFe equal to 131.5nmolL−1 and strong ligand (log KFeL,Fe′cond≥12.0) concentrations equal to 139.5nmolL−1. The weakest ligands (log KFeL,Fe′cond<10.0) were always in excess of dFe in low salinity waters, but were rapidly flocculated within the estuary and were not detected at salinities greater than 7. The strongest ligands (log KFeL,Fe′cond>11.0) were tightly coupled to dFe throughout the estuary, with average excess ligand concentrations ([L]–[dFe]) equal to 0.5nmolL−1. Humic-like substances analyzed via both CLE-ACSV and proton nuclear magnetic resonance in several samples were found to be a significant portion of the dFe-binding ligand pool in San Francisco Bay, with concentrations ranging from 559.5μgL−1 to 67.5μgL−1 in the lowest and highest salinity samples, respectively. DFe-binding ligands and humic-like substances were also found in benthic boundary layer samples taken from the shelf near the mouths of San Francisco Bay and Eel River, suggesting estuaries are an important source of dFe-binding ligands to California coastal shelf waters.

N2O production in the eastern South Atlantic: Analysis of N2O stable isotopic and concentration data

AbstractThe stable isotopic composition of dissolved nitrous oxide (N2O) is a tracer for the production, transport, and consumption of this greenhouse gas in the ocean. Here we present dissolved N2O concentration and isotope data from the South Atlantic Ocean, spanning from the western side of the mid‐Atlantic Ridge to the upwelling zone off the southern African coast. In the eastern South Atlantic, shallow N2O production by nitrifier denitrification contributed a flux of isotopically depleted N2O to the atmosphere. Along the African coast, N2O fluxes to the atmosphere of up to 46 µmol/m2/d were calculated using satellite‐derived QuikSCAT wind speed data, while fluxes at the offshore stations averaged 0.04 µmol/m2/d. Comparison of the isotopic composition of the deeper N2O in the South Atlantic (800 m to 1000 m) to measurements made in other regions suggests that water advected from one or more of the major oxygen deficient zones contributed N2O to the mesopelagic South Atlantic via the Southern Ocean. This deeper N2O was isotopically and isotopomerically enriched (δ15Nbulk− N2O = 8.7 ± 0.1‰,δ18O − N2O = 46.5 ± 0.2‰, and Site Preference = 18.7 ± 0.6‰) relative to the shallow N2O source, indicating that N2O consumption by denitrification influenced its isotopic composition. The N2O concentration maximum was observed between 200 m and 400 m and reached 49 nM near the Angolan coast. The depths of the N2O concentration maximum coincided with those of sedimentary particle resuspension along the coast. The isotopic composition of this N2O (δ15Nbulk− N2O = 5.8 ± 0.1‰,δ18O − N2O = 39.7 ± 0.1‰, and Site Preference = 9.8 ± 1.0‰) was consistent with production by diffusion‐limited nitrate (NO3−) reduction to nitrite (NO2−), followed by NO2−reduction to N2O by denitrification and/or nitrifier denitrification, with additional N2O production by NH2OH decomposition during NH3oxidation. The sediment surface, benthic boundary layer, or particles resuspended from the sediments are likely to have provided the physical and chemical conditions necessary to produce this N2O.

Natural 222Rn and 220Rn indicate the impact of the Water–Sediment Regulation Scheme (WSRS) on submarine groundwater discharge in the Yellow River estuary, China

Submarine groundwater discharge (SGD) in estuaries brings important influences to coastal ecosystems. In this study, we observed significant SGD in the Yellow River estuary, including a fresh component, during the Water–Sediment Regulation Scheme (WSRS) period. We used the 222Rn and 220Rn isotope pair to locate sites of significant SGD within the study area. Three apparent SGD locations were found during a non-WSRS period, one of which became much more pronounced, according to the remarkably elevated radon levels, during the WSRS. Increased river discharge (from 245m3s−1 to 3560m3s−1) and the elevated river water level (from 11m to 13m) during the WSRS led to a higher hydraulic head, enhancing groundwater discharge in the estuary. Our results suggest that high river discharge (>3000m3s−1) might be necessary for elevated fresh submarine groundwater discharging (FSGD). Vertical profiles of salinity, DO and turbidity anomalies along the benthic boundary layer also indicated significant FSGD in the estuary during the WSRS. Nutrient concentrations had positive correlations with 222Rn during a 24-h observation, which indicates that SGD is a dominant nutrient pathway in this area.

Laboratory simulation of the influence of geothermal heating on the interior ocean

This study, using laboratory experiments and scaling analysis, evaluates the influence of geothermal heating on global oceanic circulation. Upon a well-developed large-scale convective flow, an additional heat flux perturbation delta F/F is employed. The increments of flow and thermal properties, including eddy diffusivity K-T, flow velocity V and bottom temperature T-b, are found to be independent of the applied heat flux F. Together with the scaling analysis of convective flow at different configurations, where the flow is thermally driven in the relatively low or extremely high turbulent thermal convections or the horizontal convection, the variances of flow properties, delta K-T/K-T KT and delta V/V, are found to be close to 0.5% and 0.75% at delta F/F=2%. This means that the small heat flux perturbation plays a negligible role in the global convective flow. However, delta T-b/Delta T is found to be 1.5% at delta F/F=2%, which would have a significant effect in the local region. The results might provide a clue to understanding the influence of geothermal heating on global oceanic circulation. It is expected that geothermal heating will contribute less than 1% in turbulent mixing and volume flux to global oceanic circulation, so its influence can be negligible in this situation. However, when it comes to the local environment, the influence of geothermal heating cannot be ignored. For example, temperature increases of about 0.5 degrees C with geothermal heating would have a significant effect on the physical environments within the benthic boundary layer.

Short-term and seasonal variability of oxygen fluxes at the sediment–water interface in a riverine lake

In situ measurements of sediment-water oxygen fluxes conducted in a riverine lake during different seasons were analyzed with the aim of quantifying the combined effects of hydrodynamic forcing and seasonal changes in temperature on sediment oxygen uptake rate. Oxygen fluxes measured using the eddy correlation (EC) technique varied widely between −6.4 and −84 mmol m−2 day−1, while variations observed on hourly time scales were of comparable magnitude to seasonal variations. Oxygen fluxes were most strongly correlated to current speed in the benthic boundary layer and water depth, which both co-varied with discharge, temperature, and oxygen concentration. A direct correlation of measured fluxes with temperature and corresponding seasonal flux variations could not be observed. To explore the potential effect of temperature on oxygen fluxes, we applied a simplified analytical model, which couples the effect of hydrodynamic forcing with a temperature-dependent oxygen consumption rate within the sediment. The results suggest that the flux is a non-linear function of both variables and both can have comparable effects on the magnitude of the oxygen fluxes. The model confirms our observation that short-term variations of oxygen fluxes in response to hydrodynamic forcing can mask longer-term seasonal variations driven by temperature. The model further indicates that the magnitude and form of the temperature dependence of oxygen uptake and mineralization rates in freshwater sediments obtained from laboratory incubations can be strongly affected by flow conditions during incubations. We conclude that predictions of oxygen uptake and mineralization rates under changing climatic conditions should also take potential changes of flow conditions into account.

Field evidence of the viscous sublayer in a tidally forced developing boundary layer

AbstractField observations of boundary layer development within a tidally forced estuary revealed evidence of an observable viscous sublayer. Evidence is provided by several independent measures of the flow field, including hydrodynamic smoothness, an immobile bed, and characteristic velocity, constant stress, and higher‐order moment structures. This investigation reports what may be the second comprehensive observation of the viscous sublayer in a marine environment, and what could be the first observation of a momentum balance that includes the viscous sublayer within a shallow estuarine environment. Hydrodynamic observations were made in a straight channel within the Great Bay Estuary of New Hampshire over a flat sandy mud with low water depth of 1.5 m at the sampling location. Beyond quantifying the role of the benthic boundary layer in nutrient dynamics, these observations are useful to provide insight into very near boundary stress estimates leading to incipient motion in estuarine and coastal environments.