Laboratory observation of boundary layer flow under spilling breakers in surf zone using particle image velocimetry

A boundary layer flow under spilling breakers in a laboratory surf zone with a smooth bottom is investigated using a high resolution particle image velocimetry (PIV) technique. By cross-correlating the images, oscillatory velocity profiles within a viscous boundary layer of O(1) mm in thickness are resolved over ten points. Using PIV measurements taken for an earlier study and the present study, flow properties in the wave bottom boundary layer (WBBL) over the laboratory surf zone are obtained, including the mean velocities, turbulence intensity, Reynolds stresses, and intermittency of coherent events. The data are then used to estimate the boundary layer thickness, phase variation, and bottom shear stress. It is found that while the time averaged mass transport inside the WBBL is onshore in the outer surf zone, it changes to offshore in the inner surf zone. The zero Eulerian mass transport occurs at h/h_b ≈ 0.92 in the outer surf zone. The maximum overshoot of the streamwise velocity and boundary layer thickness are not constant across the surf zone. The bottom shear stress is mainly contributed by the viscous stress through mean velocity gradient while the Reynolds stress is small and negligible. The turbulence level is higher in the inner surf zone than that in the outer surf zone, although only a slight increase of turbulent intensity is observed inside the WBBL from the outer surf zone to the inner surf zone. The variation of phase inside and outside the WBBL was examined through the spatial velocity distribution. It is found the phase lead is not constant and its value is significantly smaller than previous thought. By analyzing instantaneous velocity and vorticity fields, a remarkable number of intermittent turbulent eddies are observed to penetrate into the WBBL in the inner surf zone. The size of the observed large eddies is about 0.11 to 0.16 times the local water depth. Its energy spectra follow the − 5/3 slope in the inertial subrange and decay exponentially in the dissipation subrange.     

Turbulence-induced steady streaming in an oscillating boundary layer: On the reliability of turbulence closure models

The accuracy of several closure models of the Reynolds-Averaged Navier–Stokes Equations in predicting the characteristics of an oscillating turbulent wall boundary layer is analyzed. The analysis involves four low Reynolds number k − ε models and a k − ω model and it is carried out by comparing the model results both with experimental data and with data obtained by a Direct Numerical Simulation (DNS) of the Navier–Stokes equations. The boundary layer is generated by a spatially constant time-oscillating pressure gradient given by the sum of two harmonic components characterized by angular frequencies Ω^∗ and 2Ω^∗ respectively, which generates a steady streaming because of the asymmetry of turbulence intensity during the cycle. Thus the results are relevant to the boundary layer at the bottom of nonlinear sea waves. The attention is therefore focused on the accuracy of the models in reproducing the period averaged profiles of the hydrodynamic characteristics of the steady streaming. The instantaneous quantities, such as time development of the wall shear stress, profiles of the streamwise velocity, Reynolds stresses and turbulent kinetic energy are also considered and analyzed. The results shows that a model can be judged better or worse than other models depending on the specific flow characteristic under investigation. However, an approach has been adopted which allowed to rank the models according to their accuracy in predicting the values of the hydrodynamic quantities involved in the present study.     

Numerical study of turbulence and wave damping induced by vegetation canopies

Vegetation canopies control mean and turbulent flow structure as well as surface wave processes in coastal regions. A non-hydrostatic RANS model based on NHWAVE (Ma et al., 2012) is developed to study turbulent mixing, surface wave attenuation and nearshore circulation induced by vegetation. A nonlinear k − ϵ model accounting for vegetation-induced turbulence production is implemented to study turbulent flow within the vegetation field. The model is calibrated and validated using experimental data from vegetated open channel flow, as well as nonbreaking and breaking random wave propagation in vegetation fields. It is found that the drag-related coefficients in the k − ϵ model C_fk and C_fϵ can greatly affect turbulent flow structure, but seldom change the wave attenuation rate. The bulk drag coefficient C_D is the major parameter controlling surface wave damping by vegetation canopies. Using the empirical formula of Mendez and Losada (2004), the present model provides accurate predictions of vegetation-induced wave energy dissipation. Wave propagation through a finite patch of vegetation in the surf zone is investigated as well. It is found that the presence of a finite patch of vegetation may generate strong pressure-driven nearshore currents, with an onshore mean flow in the unvegetated zone and an offshore return flow in the vegetated zone.     

Particulate suspended matter concentrations in the Bahía Blanca Estuary,Argentina: Implication for the development of phytoplankton blooms

The inner zone of the Bahía Blanca Estuary is shallow, nutrient-rich and turbid. Tidal energy and water turbulence strongly affect the water column resulting in a well-mixed structure and high concentrations of suspended sediment. The phytoplankton community is mostly dominated by diatoms and the annual pattern has been characterized by a recurrent winter-early spring bloom. Here, we investigated to what extent the temporal variations of suspended particulate matter (SPM) regulate the phytoplankton blooms in the head of the estuary by light-limitation. Sampling was done on a fortnightly basis (weekly during the blooming season) at a fixed station in the inner zone of the estuary from January 2007 to February 2008. SPM concentrations and light extinction coefficients (k) in the water column were significantly correlated and showed relatively lower values during the phytoplankton maximal biomass levels. During winter, SPM and k reached values of 23.6 mg l⁻¹ and 0.17 m⁻¹ which were significantly lower than the annual means of 77.6 mg l⁻¹ and 2.94 m⁻¹, respectively. The particulate organic matter (POM) concentration was significantly correlated with the calculated phytoplankton biomass although the contribution of the latter to the total POM was rather low. Both, POM and biomass, had maximal values during winter (21.8 mg l⁻¹ and 393.5 μg C l⁻¹) and mid summer (24.3 mg l⁻¹ and 407.0 μg C l⁻¹), with cell densities up to 8 × 10⁶ cells l⁻¹ and chlorophyll a up to 24.6 μg l⁻¹. Our results suggest that the decrease of SPM concentrations in the water column with a concomitant increase in the penetration of solar radiation seems to be one of the main causes for the development of the phytoplankton winter bloom in the Bahía Blanca Estuary.     

Turbidity limits gas exchange in a large macrotidal estuary

In estuaries, the gas transfer velocity (k) is driven by a combination of two major physical drivers, wind and water current. The k values for CO₂ in the macrotidal Gironde Estuary were obtained from 159 simultaneous pCO₂ and floating chamber flux measurements. Values of k increased with wind speed and were significantly greater when water currents and wind were in opposing directions. At low wind speeds (<1 m s⁻¹), k increased with water current velocities (0–1.5 m s⁻¹) following an exponential trend. The latter was a good proxy for the Y-intercept in a generic equation for k versus wind speed in estuaries. We also found that, in this turbid estuary, k was significantly lower at high turbidity. The presence of suspended material in great concentrations (TSS > 0.2 g L⁻¹) had a significant role in attenuating turbulence and therefore gas exchange. This result has important consequences for modeling water oxygenation in estuarine turbidity maxima. For seven low turbidity estuaries previously described in the literature, the slope of the linear regression between k and wind speed correlates very well with the estuary surface area due to a fetch effect. In the Gironde Estuary, this slope follows the same trend at low turbidity (TSS < 0.2 g L⁻¹), but is on average significantly lower than in other large estuaries and decreases linearly with the TSS concentration. A new generic equation for estuaries is proposed that gives k as a function of water current velocity, wind speed, estuarine surface area and TSS concentration.     

Effect of bed roughness on turbulent boundary layer and net sediment transport under asymmetric waves

This paper presents an investigation of the roughness effects in the turbulent boundary layer for asymmetric waves by using the baseline (BSL) k–ω model. This model is validated by a set of the experimental data with different wave non-linearity index, N_i (namely, N_i = 0.67, N_i = 0.60 and N_i = 0.58). It is further used to simulate asymmetric wave velocity flows with several values of the roughness parameter (a_m/k_s) which increase gradually, namely from a_m/k_s = 35 to a_m/k_s = 963. The effect of the roughness tends to increase the turbulent kinetic energy and to decrease the mean velocity distribution in the inner boundary layer, whereas in the outer boundary layer, the roughness alters the turbulent kinetic energy and the mean velocity distribution is relatively unaffected. A new simple calculation method of bottom shear stress based on incorporating velocity and acceleration terms is proposed and applied into the calculation of the rate of bed-load transport induced by asymmetric waves. And further, the effect of bed roughness on the bottom shear stress and bed-load sediment transport under asymmetric waves is examined with the turbulent model, the newly proposed method, and the existing calculation method. It is found that the higher roughness elements increase the magnitude of bottom shear stress along a wave cycle and consequently, the potential net sediment transport rate. Moreover, the wave non-linearity also shows a big impact on the bottom shear stress and the net sediment transport.     

The cycling and oxidation pathways of organic carbon in a shallow estuary along the Texas Gulf Coast

The cycling and oxidation pathways of organic carbon were investigated at a single shallow water estuarine site in Trinity Bay, Texas, the uppermost lobe of Galveston Bay, during November 2000. Radio-isotopes were used to estimate sediment mixing and accumulation rates, and benthic chamber and pore water measurements were used to determine sediment-water exchange fluxes of oxygen, nutrients and metals, and infer carbon oxidation rates. Using ⁷Be and ²³⁴Th_XS, the sediment-mixing coefficient (D_b) was 4.3 ± 1.8 cm² y⁻¹, a value that lies at the lower limit for marine environments, indicating that mixing was not important in these sediments at this time. Sediment accumulation rates (S_a), estimated using ¹³⁷Cs and ²¹⁰Pb_XS, were 0.16 ± 0.02 g cm⁻² y⁻¹. The supply rate of organic carbon to the sediment-water interface was 30 ± 3.9 mmol C m⁻² d⁻¹, of which ∼10% or 2.9 ± 0.44 mmol C m⁻² d⁻¹was lost from the system through burial below the 1-cm thick surface mixed layer. Measured fluxes of O₂ were 26 ± 3.8 mmol m⁻² d⁻¹ and equated to a carbon oxidation rate of 20 ± 3.3 mmol C m⁻² d⁻¹, which is an upper limit due to the potential for oxidation of additional reduced species. Using organic carbon gradients in the surface mixed layer, carbon oxidation was estimated at 2.6 ± 1.1 mmol C m⁻² d⁻¹. Independent estimates made using pore water concentration gradients of ammonium and C:N stoichiometry, equaled 2.8 ± 0.46 mmol C m⁻² d⁻¹. The flux of DOC out of the sediments (DOC_efflux) was 5.6 ± 1.3 mmol C m⁻² d⁻¹. In general, while mass balance was achieved indicating the sediments were at steady state during this time, changes in environmental conditions within the bay and the surrounding area, mean this conclusion might not always hold. These results show that the majority of carbon oxidation occurred at the sediment-water interface, via O₂ reduction. This likely results from the high frequency of sediment resuspension events combined with the shallow sediment mixing zone, leaving anaerobic oxidants responsible for only ∼10–15% of the carbon oxidized in these sediments.     

Dynamics of surf-zone turbulence in a strong plunging breaker

The characteristics of turbulence created by a plunging breaker on a 1 on 35 plane slope have been studied experimentally in a two-dimensional wave tank. The experiments involved detailed measurements of fluid velocities below trough level and water surface elevations in the surf zone using a fibre-optic laser-Doppler anemometer and a capacitance wave gage. The dynamical role of turbulence is examined making use of the transport equation for turbulent kinetic energy (the k-equation). The results show that turbulence under a plunging breaker is dominated by large-scale motions and has certain unique features that are associated with its wave condition. It was found that the nature of turbulence transport in the inner surf zone depends on a particular wave condition and it is not similar for different types of breakers. Turbulent kinetic energy is transported landward under a plunging breaker and dissipated within one wave cycle. This is different from spilling breakers where turbulent kinetic energy is transported seaward and the dissipation rate is much slower. The analysis of the k-equation shows that advective and diffusive transport of turbulence play a major role in the distribution of turbulence under a plunging breaker, while production and dissipation are not in local equilibrium but are of the same order of magnitude. Based on certain approximate analytical approaches and experimental measurements it is shown that turbulence production and viscous dissipation below trough level amount to only a small portion of the wave energy loss caused by wave breaking. It is suggested that the onshore sediment transport produced by swell waves may be tied in a direct way to the unique characteristics of turbulent flows in these waves.     

Surface layer mixing during the SAGE ocean fertilization experiment

Vessel-based observations of the oceanic surface layer during the 14-day 2004 SAGE ocean fertilization experiment were conducted using ADCP, CTD and temperature microstructure in a frame of reference moving with a patch of injected SF₆ tracer. During the experiment the mixed layer depth z_mld ranged between 50 and 80 m, with several re-stratifying events that brought z_mld up to less than 40 m. These re-stratifying events were not directly attributable to local surface-down development of stratification and were more likely associated with horizontal variation in density structure. Comparison between the CTD and a one-dimensional model confirmed that the SAGE experiment was governed by 3-d processes. A new method for estimating z_mld was developed that incorporates a component that is proportional to density gradient. This highlighted the need for well-conditioned near-surface data which are not always available from vessel-based survey CTD profiles. A centred-displacement scale, L_c, equivalent to the Thorpe lengthscale, reached a maximum of 20 m, with the eddy-centroid located at around 40 m depth. Temperature gradient microstructure-derived estimates of the vertical turbulent eddy diffusivity of scalar (temperature) material yielded bin-averaged values around 10⁻³ m² s⁻¹ in the pycnocline rising to over 10⁻² m² s⁻¹ higher in the surface layer. This suggests transport rates of nitrate and silicate at the base of the surface layer generate mixed layer increases of the order of 38 and 13 mmol/m²/day, respectively, during SAGE. However, the variability in measured vertical transport processes highlights the importance of transient events like wind mixing and horizontal intrusions.     

Effects of upwelling,tides and biological processes on the inorganic carbon system of a coastal lagoon in Baja California

The role of coastal lagoons and estuaries as sources or sinks of inorganic carbon in upwelling areas has not been fully understood. During the months of May–July, 2005, we studied the dissolved inorganic carbon system in a coastal lagoon of northwestern Mexico during the strongest period of upwelling events. Along the bay, different scenarios were observed for the distributions of pH, dissolved inorganic carbon (DIC) and apparent oxygen utilization (AOU) as a result of different combinations of upwelling intensity and tidal amplitude. DIC concentrations in the outer part of the bay were controlled by mixing processes. At the inner part of the bay DIC was as low as 1800 μmol kg⁻¹, most likely due to high water residence times and seagrass CO₂ uptake. It is estimated that 85% of San Quintín Bay, at the oceanic end, acted as a source of CO₂ to the atmosphere due to the inflow of CO₂-rich upwelled waters from the neighboring ocean with high positive fluxes higher than 30 mmol C m⁻² d⁻¹. In contrast, there was a net uptake of CO₂ and HCO₃⁻ by the seagrass bed Zostera marina in the inner part of the bay, so the pCO₂ in this zone was below the equilibrium value and slightly negative CO₂ fluxes of −6 mmol C m⁻² d⁻¹. Our positive NEP and ΔDIC values indicate that Bahía San Quintín was a net autotrophic system during the upwelling season during 2005.     

Tiller dynamic and production on a SW Atlantic Spartina alterniflora marsh

We used non-destructive methods to study the bi-monthly changes in standing stock, turnover, and net aerial primary productivity (NAPP) of Spartina alterniflora in the Bahía Blanca Estuary, Argentina, from 2005 to 2007. Tillers were tagged and counted bimonthly and a weight:height relationship developed for the live and dead stems in a regularly flooded zone (low marsh, LM) and an irregularly flooded one (high marsh, HM). The annual tiller natality in year one compared to year two decreased from 440 ± 68 to 220 ± 58 new individuals m^–2 yr^–1 in the HM and from 500 ± 103 to 280 ± 97 new individuals m⁻² yr⁻¹ in the LM (μ ± 1 SE). Tiller mortality averaged 670 ± 70 individuals m⁻² yr⁻¹.     

The influence of prolonged mouth closure on selected components of the hyperbenthos in the littoral zone of the temporarily open/closed Kasouga Estuary,South Africa

The influence of prolonged mouth closure on the population dynamics of the caridian shrimp, Palaemon peringueyi and the estuarine isopod, Exosphaeroma hylocoetes, in the littoral zone of temporarily open/closed Kasouga Estuary located on the south-eastern coastline of southern Africa was assessed monthly over the period October 2007 to September 2008. Prolonged mouth closure of the estuary contributed to hypersaline conditions (psu > 35) prevailing throughout the estuary for the last four months of the study. The high salinities coincided with a decrease in the areal extent (up to 80%) of the submerged macrophytes, mainly Ruppia maritima, within the littoral zone of the estuary. Total abundance and biomass values of the shrimp and isopod over the period of investigation ranged from 0 to 14.6 ind m⁻², from 0 to 13.3 mg dwt m⁻², from 12 to 1540 ind m⁻² and from 0.1 to 2.16 mg dwt m⁻², respectively. Maximum values of both the shrimp and isopod were recorded in the upper reaches of the estuary in close association with R. maritima. Over the course of the investigation, both the abundance and biomass values of the shrimp decreased significantly (P < 0.05 in both cases) which could be related to reduced habitat availability, R. maritima, that acts as a refuge against fish predation. Additionally, the decrease in abundance and biomass values could be attributed to reduced recruitment opportunities for the shrimp and the cessation of reproduction in the estuarine isopod. The establishment of a link to the marine environment following an overtopping event in September 2008 contributed to a decrease in salinity within the system although no recruitment of either the isopod or shrimp was recorded.     

Habitat use by larval fishes in a temperate South African surf zone

Larval fishes were sampled in the Kwaaihoek surf zone on the south east coast of South Africa. On six occasions between February and May 2002, larval fishes were collected in two habitat types identified in the inner surf zone using a modified beach-seine net. The surf zone habitats were classified as either sheltered trough areas or adjacent exposed surf areas. Temperature, depth and current measurements were taken at all sites. Trough habitats consisted of a depression in surf topography characterised by reduced current velocities and greater average depth than adjacent surf areas. In total, 325 larval fishes were collected. Of these, 229 were collected in trough and 96 in surf habitats. At least 22 families and 37 species were represented in the catch. Dominant families were the Mugilidae, Sparidae, Atherinidae, and Engraulidae. Dominant species included Liza tricuspidens and Liza richardsonii (Mugilidae), Rhabdosargus holubi and Sarpa salpa (Sparidae), Atherina breviceps (Atherinidae) and Engraulis japonicus (Engraulide). Mean CPUE of postflexion larvae of estuary-dependent species was significantly greater in trough areas. The proportion of postflexion larval fishes in trough habitat was significantly greater than that of preflexion stages, a result that was not apparent in surf habitat sampled. CPUE of postflexion larvae of estuary-dependent fishes was negatively correlated with current magnitude and positively correlated with habitat depth. Mean body length of larval fishes was significantly greater in trough than in surf habitats. These results provide evidence that the CPUE of postflexion larvae of estuary-dependent fishes is higher in trough habitat in the surf zone and this may be indicative of active habitat selection for areas of reduced current velocity/wave action. The implications of this behaviour for estuarine recruitment processes are discussed.     

Swash zone sediment fluxes: Field observations

This paper describes newly obtained, high-frequency observations of beach face morphological change over numerous tidal cycles on a macrotidal sandy beach made using a large array of ultrasonic altimeters. These measurements enable the net cross-shore sediment fluxes associated with many thousands of individual swash events to be quantified. It is revealed that regardless of the direction of net morphological change on a tidal time scale, measured net fluxes per event are essentially normally distributed, with nearly equal numbers of onshore and offshore-directed events. The majority of swash events cause net cross-shore sediment fluxes smaller than ± 50 kg m^− 1 and the mean sediment flux per swash event is only O(± 1 kg m^− 1) leading to limited overall morphological change. However, much larger events which deposit or remove hundreds of kilograms of sand per meter width of beach occur at irregular intervals throughout the course of a tide. It was found that swash–swash interactions tend to increase the transport potential of a swash event and the majority of the swash events that cause these larger values of sediment flux include one or more interactions. The majority of the larger sediment fluxes were therefore measured in the lower swash zone, close to the surf/swash boundary where swash–swash interactions are most common. Despite the existence of individual swash events that can cause fluxes of sediment that are comparable to those observed on a tidal time scale, frequent reversals in transport direction act to limit net transport such that the beach face volume remains in a state of dynamic equilibrium and does not rapidly erode or accrete.     

Response of Nereis diversicolor population (Polychaeta,Nereididae) to the pollution impact – Authie and Seine estuaries (France)

A survey within the French National Programme of Ecotoxicology was carried out in 2002, 2003 and 2004 to study the response of Nereis diversicolor populations (Polychaeta, Nereididae) to the impact of pollution in the Authie estuary (non-contaminated site) and in the Seine estuary (contaminated site). In the period studied, the density varied from 672 ind. m⁻² to 3584 ind. m⁻² in the Authie estuary and from 80 ind. m⁻² to 920 ind. m⁻² in the Seine estuary. Biomass varied from 3.94 g m⁻² (dry weight) in February 2004 to 38.0 g m⁻² in August 2003 in the Authie estuary and from 3.4 g m⁻² in February 2002 to 0.6 g m⁻² in February 2004 in the Seine estuary. Density and biomass of the populations of N. diversicolor were consistently lower in the Seine estuary than in the Authie estuary. Size frequency histograms permit the analysis of the cohorts as well as the elaboration of the growth curves. For the individuals from the Authie estuary, the relation between dry weight (DW) and length L3 (prostomium, peristomium and chaetiger 1) was DW = 4.2205 L3^2.9832. For those from the Seine estuary, the relation between dry weight and L3 was DW = 0.4697e^1.7209L3. The individuals of N. diversicolor should belong to eight cohorts in Authie estuary (two cohorts each year) instead of six cohorts for those from the Seine estuary. These differences can be attributed to the effect of pollution on the population of N. diversicolor.     

Particulate organic carbon fluxes to the ocean interior and factors controlling the biological pump: A synthesis of global sediment trap programs since 1983

Particulate organic carbon (POC) is vertically transported to the oceanic interior by aggregates and their ballasts, mainly CaCO₃ and biogenic opal, with a smaller role for lithogenic aerosols through the mesopelagic zone. Diel migrating zooplankton communities effect vertical transport and remineralization of POC in the upper layers of the ocean. Below 1.5 km, the presence of zooplankton is reduced and thus the aggregates travel mainly by gravitational transport. We normalized the fluxes of POC, CaCO₃, and biogenic opal from data published on samples collected at 134 globally distributed, bottom-tethered, time-series sediment trap (TS-trap) stations to annual mole fluxes at the mesopelagic/bathypelagic boundary (m/b) at 2 km and defined them as F_m/bC_org, F_m/bC_inorg, and F_m/bSi_bio. Using this global data set, we investigated (1) the geographic contrasts of POC export at m/b and (2) the supply rate of ∑CO₂ to the world mesopelagic water column. F_m/bC_org varies from 25 (Pacific Warm Pool) to 605 (divergent Arabian Sea) mmolC m⁻² yr⁻¹; F_m/bC_inorg varies from >8 (high latitude Polar Oceans) or 15 (Pacific Warm Pool) to 459 (divergent Arabian Sea) mmolC m⁻² yr⁻¹; and F_m/bSi_bio, the most spatially/temporally variable flux, ranges from 6 (North Atlantic Drift) to 1118 (Pacific Subarctic Gyre) mmolSi m⁻² yr⁻¹. The oceanic region exhibiting the highest POC flux over a significantly large region is the area of the North Pacific Boreal Gyres where the average F_m/bC_org = 213, F_m/bC_inorg = 126, and F_m/bSi_bio = 578 mmol m⁻² yr⁻¹. F_m/bC_org and F_m/bC_inorg are particularly high in large upwelling margins, including the divergent Arabian Sea and off Cape Verde. One of the data sets showing the lowest flux over a significant region/basin is F_m/bC_org = 39, F_m/bC_inorg = 69, and F_m/bSi_bio = 22 mmol m⁻² yr⁻¹ in the North Pacific subtropical/tropical gyres; Pan-Atlantic average fluxes are similar except F_m/bSi_bio fluxes are even lower. Where C_org/C_inorg and Si_bio/C_inorg are <1 defines the “Carbonate Ocean”, and where these ratios are ?1 defines the “Silica Ocean”. The Carbonate Ocean occupies about 80% of the present world pelagic ocean between the two major oceanographic fronts, the North Pacific Polar Front and the Antarctic Polar Front, and the Silica Ocean is found on the polar sides of these fronts. The total global annual fluxes of F_m/bC_org, F_m/bC_inorg, and F_m/bSi_bio at m/b calculated by parameterizations of the export flux data from 134 stations are surprisingly similar; 36.2, 33.8, and 34.6 teramol yr⁻¹ (120, 112, and 114 mmol m⁻² yr⁻¹), respectively, resulting in a near uniform binary ratio between the above three elements of about one. The global ternary % ratios estimated from 152 TS-trap samples of the three elements are 35:32:33. From our global F_m/bC_org and a published model estimate of the global export production, we estimate the regeneration rate of CO₂ through the mesopelagic zone by the biological pump is 441 teramolC yr⁻¹. Based on our global F_m/bC_inorg and recently estimated global primary production of PIC, 36-86 teramolC yr⁻¹ of PIC is assumed to be dissolved within the upper 2 km of the water column.     

Persistently declining oxygen levels in the interior waters of the eastern subarctic Pacific

Fifty years of measurements at Ocean Station Papa (OSP, 50°N, 145°W) show trends in the interior waters of the subarctic Pacific that are both impacted by short term (few years to bi-decadal) atmospheric or ocean circulation oscillations and by persistent climate trends. Between 1956 and 2006, waters below the ocean mixed layer to a depth of at least 1000 m have been warming and losing oxygen. On density surfaces found in the depth range 100-400 m (σ_θ = 26.3-27.0), the ocean is warming at 0.005-0.012 °C y⁻¹, whereas oxygen is declining at 0.39-0.70 μmol kg⁻¹ y⁻¹ or at an integrated rate of 123 mmol m⁻² y⁻¹ (decrease of 22% over 50 years). During this time, the hypoxic boundary (defined as 60 μmol O₂ kg⁻¹) has shoaled from ∼400 to 300 m. In the Alaska Gyre, the 26.2 isopycnal occasionally ventilates, whereas at OSP 26.0σ_θ has not been seen at the ocean surface since 1971 as the upper ocean continues to stratify. To interpret the 50 year record at OSP, the isopycnal transport of oxygenated waters within the interior of the subarctic Pacific is assessed by using a slightly modified “NO” parameter [Broecker, W., 1974. “NO” a conservative water-mass tracer. Earth and Planetary Science Letters 23, 100-107]. The highest nitrate-oxygen signature in interior waters of the North Pacific is found in the Bering Sea Gyre, Western Subarctic Gyre and East Kamchatka Current region as a consequence of winter mixing to the ∼26.6 isopycnal. By mixing with low NO waters found in the subtropics and Okhotsk Sea, this signature is diluted as waters flow eastward across the Pacific. Evidence of low NO waters flowing north from California is seen along the coasts of British Columbia and SE Alaska. Oxygen in the subsurface waters of the Alaskan Gyre was supplied ∼60% by subarctic and 40% by subtropical waters during WOCE surveys, whereas such estimates are shown to periodically vary by 20% at OSP. Other features discernable in the OSP data include periods of increased ventilation of deeper isopycnals on an ∼18 year cycle and strong, short term (few month) variability caused by passing mesoscale eddies. The potential impacts of declining oxygen on coastal ecosystems are discussed.     

Organic carbon fluxes through the mesozooplankton and their variability at different time-scales in the Gullmarsfjord,Sweden

Study of rate process responses and carbon transfer rates in the plankton is a well developed field, but attention has focused on seasonal or longer scales and knowledge on variability patterns and modulating mechanisms at shorter scales is very limited. Here we investigated variability in pelagic carbon transfer rates on daily and seasonal scales in the Gullmar Fjord, Sweden. Hierarchical sampling was carried out in five campaigns in different seasons each of which consisted of 8–12 d with sampling every other day. Algal ingestion, copepod egg and pellet production and vertical flux measured as pigments and pellets varied significantly between days as well as between seasons. Algal ingestion was highest in autumn (weight-specific rate: 0.3–1 d⁻¹) and at the start of the spring bloom (0.3–1.5 d⁻¹), while egg production was highest in summer–autumn (0.1–0.5 d⁻¹). Vertical flux peaked in summer with 4–6 mg pigment m⁻² d⁻¹ and up to 150 mg pellet Cm⁻² d⁻¹. Consonant responses between species were observed indicating a common food source for the mesozooplankton. Changes in trophic functions occurred on scales of days and the coupling between pelagic processes and vertical flux was strong. There were, however, both positive and negative responses which could not be explained by simple predator–prey interactions. The food quality in terms of prey type and biochemical composition are suggested as crucial factors for the variability both within the pelagic environment as well as for the vertical flux out of the upper layers in the ocean.     

In vitro simulation of oxic/suboxic diagenesis in an estuarine fluid mud subjected to redox oscillations

Estuarine turbidity maxima (ETMs) are sites of intense mineralisation of land-derived particulate organic matter (OM), which occurs under oxic/suboxic oscillating conditions owing to repetitive sedimentation and resuspension cycles at tidal and neap-spring time scales. To investigate the biogeochemical processes involved in OM mineralisation in ETMs, an experimental set up was developed to simulate in vitro oxic/anoxic oscillations in turbid waters and to follow the short timescale changes in oxygen, carbon, nitrogen, and manganese concentration and speciation. We present here the results of a 27-day experiment (three oxic periods and two anoxic periods) with an estuarine fluid mud from the Gironde estuary. Time courses of chemical species throughout the experiment evidenced the occurrence of four distinct characteristic periods with very different properties. Steady oxic conditions were characterised by oxygen consumption rates between 10 and 40 μmol L⁻¹ h⁻¹, dissolved inorganic carbon (DIC) production of 9–12 μmol L⁻¹ h⁻¹, very low NH₄⁺ and Mn²⁺ concentrations, and constant NO₃⁻ production rates (0.4 - 0.7 μmol L⁻¹ h⁻¹) due to coupled ammonification and nitrification. The beginning of anoxic periods (24 h following oxic to anoxic switches) showed DIC production rates of 2.5–8.6 μmol L⁻¹ h⁻¹ and very fast NO₃⁻ consumption (5.6–6.3 μmol L⁻¹ h⁻¹) and NH₄⁺ production (1.4–1.5 μmol L⁻¹ h⁻¹). The latter rates were positively correlated to NO₃⁻ concentration and were apparently caused by the predominance of denitrification and dissimilatory nitrate reduction to ammonia. Steady anoxic periods were characterised by constant and low NO₃⁻ concentrations and DIC and NH₄⁺ productions of less than 1.3 and 0.1 μmol L⁻¹ h⁻¹, respectively. Mn²⁺ and CH₄ were produced at constant rates (respectively 0.3 and 0.015 μmol L⁻¹ h⁻¹) throughout the whole anoxic periods and in the presence of nitrate. Finally, reoxidation periods (24–36 h following anoxic to oxic switches) showed rapid NH₄⁺ and Mn²⁺ decreases to zero (1.6 and 0.8–2 μmol L⁻¹ h⁻¹, respectively) and very fast NO₃⁻ production (3 μmol L⁻¹ h⁻¹). This NO₃⁻ production, together with marked transient peaks of dissolved organic carbon a few hours after anoxic to oxic switches, suggested that particulate OM mineralisation was enhanced during these transient reoxidation periods. An analysis based on C and N mass balance suggested that redox oscillation on short time scales (day to week) enhanced OM mineralisation relative to both steady oxic and steady anoxic conditions, making ETMs efficient biogeochemical reactors for the mineralisation of refractory terrestrial OM at the land-sea interface.