The influence of bottom topography on internal seiches in stratified media

Standing internal waves, so-called seiches, are ubiquitous in reservoirs and lakes. Although the stratification in such basin is often continuous, the modeling of seiches has been confined mostly to two-layer models. Such models are unable to give reliable insights into the vertical structure of the seiches, which might be crucial for the understanding of vertical mixing in natural water basins. To obtain this kind of information a two-dimensional computer model has been developed, which takes both continuous stratification and bottom topography into account. The results of this model are presented. The computed seiche modes reveal that (1) several large-scale modes can exist with similar eigenfrequencies, (2) the modes have a tendency to develop narrow jets and (3) only the lowest modes are strongly influenced by the bottom topography.     

A modified profile method for determining the vertical fluxes of NO,NO2, ozone,and HNO3 in the atmospheric surface layer

A modified profile method for determining the vertical deposition (or/and exhalation) fluxes of NO, NO₂, ozone, and HNO₃ in the atmospheric surface layer is presented. This method is based on the generally accepted micrometeorological ideas of the transfer of momentum, sensible heat and matter near the Earth's surface and the chemical reactions among these trace gases. The analysis (aerodynamic profile method) includes a detailed determination of the micrometeorological quantities (such as the friction velocity, the fluxes of sensible and latent heat, the roughness length and the zero plane displacement), and of the height-invariant fluxes of the composed chemically conservative trace gases with group concentrations c ₁=[NO]+[NO₂]+[HNO₃], c ₂=[NO₂]+[O₃]+3/2·[HNO₃], and c ₃=[NO]–[O₃]–1/2·[HNO₃]. The fluxes of the individual species are finally determined by the numerical solution of a system of coupled nonlinear ordinary differential equations for the concentrations of ozone and HNO₃ (decoding method). The parameterization of the fluxes is based on the flux-gradient relationships in the turbulent region of the atmospheric surface layer. The model requires only the vertical profile data of wind velocity, temperature and humidity and concentrations of NO, NO₂, ozone, and HNO₃.The method has been applied to vertical profile data obtained at Jülich (September 1984) and collected in the BIATEX joint field experiment LOVENOX (Halvergate, U.K., September 1989).     

Grid point surface air temperature calculations with a fast turnaround: Combining the results of IMAGE and a GCM

This paper describes a methodology that combines the outputs of (1) the Integrated Model to Assess the Greenhouse Effect (IMAGE Version 1.0) of the Netherlands National Institute of Public Health and Environmental Protection (RIVM) (given a greenhouse gas emission policy, this model can estimate the effects such as global mean surface air temperature change for a wide variety of policies) and (2) ECHAM-1/LSG, the Global Circulation Model (GCM) of the Max-Planck Institute for Meteorology in Hamburg, Germany. The combination enables one to calculate grid point surface air temperature changes for different scenarios with a turnaround time that is much quicker than that for a GCM. The methodology is based upon a geographical pattern of the ratio of grid point temperature change to global mean values during a certain period of the simulation, as calculated by ECHAM-1/LSG for the 1990 Scenarios A and D of the Intergovernmental Panel on Climate Change (IPCC). A procedure, based upon signal-to noise ratios in the outputs, enabled us to estimate where we have confidence in the methodology; this is at about 23% to 83% of the total of 2,048 grid points, depending upon the scenario and the decade in the simulation. It was found that the methodology enabled IMAGE to provide useful estimates of the GCM-predicted grid point temperature changes. These estimates were within 0.5K (0.25K) throughout the 100 years of a given simulation for at least 79% (74%) of the grid points where we are confident in applying the methodology. The temperature ratio pattern from Scenario A enabled IMAGE to provide useful estimates of temperature change within 0.5K (0.25K) in Scenario D for at least 88% (68%) of the grid points where we have confidence; indicating that the methodology is transferable to other scenarios. Tests with the Geophysical Fluid Dynamics Laboratory GCM indicated, however, that a temperature ratio pattern may have to be developed for each GCM. The methodology, using a temperature ratio pattern from the 1990 IPCC Scenario A and involving IMAGE, gave gridded surface air temperature patterns for the 1992 IPCC radiative-forcing Scenarios C and E and the RIVM emission Scenario B; none of these scenarios has been simulated by ECHAM-1/LSG. The simulations reflect the uncertainty range of a future warming.The work reported by the authors was carried out during their stay at the project Forestry and Climate Change of the International Institute for Applied Systems Analysis, Laxenburg, Austria.     

Sub-lethal effects of metal exposure: physiological and behavioural responses of the estuarine bivalve Macoma balthica

Variation in glycogen concentration, condition index (CI) and filtration activity were measured in the bivalve Macoma balthica buried in sediment and experimentally exposed to cadmium (Cd). The stress due to elevated but sub-lethal concentrations (300 ppb Cd) affected the overall fitness of the organism as all parameters monitored responded significantly. Lower concentrations tested (10, 30 and 100 ppb) only induced a significant decrease in filtration activity, which may play a protective role, enabling the organism to slow down its metabolic activity and preserving the integrity of its reserves (reflected by stable CI and glycogen levels). Hence, the various endpoints selected show different thresholds. Our results also demonstrate that under high exposure, small individuals loose proportionally more glycogen per unit of weight than larger ones, thus confirming the higher sensitivity of small individuals to metal contamination. Furthermore, exposure to intermediate concentration (30 ppb) seems to be beneficial to the small individuals as indicated by their high CI values compared to the control. These results showed thus that non-sigmoidal concentration–response relationship and sizes of individuals should be considered in monitoring programmes and risk assessment.     

Distribution and Migrations of two Cerithid Snails on a Sand Flat in Bermuda

Abstract. Two species of cerithid prosobranchs live on a sand flat in Bermuda. At low tide during day light Batillaria minima is found on the surface whereas Cerithium lutosum remains buried in the sediment. This correlates with much higher lethal and coma temperatures, as well as much greater resistance to desiccation, of the former species. However, at other diurnal or tidal stages the two species co-occur in the same layer of sand. Despite greater resistance to physical factors the population of B. minima seems to be declining since in 1980 juveniles of this species were rare on Tuckers Town Flat whereas the population of C. lutosum was thriving.     

In situ hydrocarbon concentrations from pressurized cores in surface sediments, Northern Gulf of Mexico

Two newly developed coring devices, the Multi-Autoclave-Corer and the Dynamic Autoclave Piston Corer were deployed in shallow gas hydrate-bearing sediments in the northern Gulf of Mexico during research cruise SO174 (Oct–Nov 2003). For the first time, they enable the retrieval of near-surface sediment cores under ambient pressure. This enables the determination of in situ methane concentrations and amounts of gas hydrate in sediment depths where bottom water temperature and pressure changes most strongly influence gas/hydrate relationships. At seep sites of GC185 (Bush Hill) and the newly discovered sites at GC415, we determined the volume of low-weight hydrocarbons (C₁ through C₅) from nine pressurized cores via controlled degassing. The resulting in situ methane concentrations vary by two orders of magnitudes between 0.031 and 0.985 mol kg^− 1 pore water below the zone of sulfate depletion. This includes dissolved, free, and hydrate-bound CH₄. Combined with results from conventional cores, this establishes a variability of methane concentrations in close proximity to seep sites of five orders of magnitude. In total four out of nine pressure cores had CH₄ concentrations above equilibrium with gas hydrates. Two of them contain gas hydrate volumes of 15% (GC185) and 18% (GC415) of pore space. The measurements prove that the highest methane concentrations are not necessarily related to the highest advection rates. Brine advection inhibits gas hydrate stability a few centimeters below the sediment surface at the depth of anaerobic oxidation of methane and thus inhibits the storage of enhanced methane volumes. Here, computerized tomography (CT) of the pressure cores detected small amounts of free gas. This finding has major implications for methane distribution, possible consumption, and escape into the bottom water in fluid flow systems related to halokinesis.     

Ammothea hilgendorfi (Böhm, 1879), an Adventitious Pycnogonid new for the Mediterranean Sea

Abstract. The authors demonstrate the probable immigration and stable settling of the pycnogonid Ammothea hilgendorfi (B öhm , 1879) at one station in the lagoon of Venice. The species is widely distributed naturally on both sides of the Pacific Ocean and has probably immigrated on ship hulls through the Suez Canal to the Mediterranean. Two more cases (the pycnogonid Anoplodactylus portus and the amphipod Elasmopus pectenicrus ) of adventitious species in this sea are compared.     

Bottom-up and top-down controls of walleye pollock (Theragra chalcogramma) on the Eastern Bering Sea shelf

Control of walleye pollock (Theragra chalcogramma) recruitment in the Eastern Bering Sea involves complex interactions between bottom-up and top-down processes, although the mechanisms are poorly understood. We used statistical models to test the leading hypotheses linking recruitment variability to biotic and abiotic factors. Consistent with a “cold-pool hypothesis”, recruitment of pollock was significantly stronger if winters preceding the larval (age-0) and juvenile stages (age-1) were mild. However, our results did not support the proposed top-down mechanism (cannibalism) underlying this hypothesis. Several empirical relationships support an “oscillating control hypothesis”. As predicted by it, the effect of ice conditions on survival during the larval and early juvenile stages was modified by the abundance of adult pollock, implying stronger bottom-up control when adult abundance (hence cannibalism) was low. The proposed bottom-up mechanism predicts that the survival of pelagic-feeding walleye pollock (benthic-feeding yellowfin sole), should be higher during years with an early (late) ice retreat, which was confirmed by our analysis. Our results also provide additional evidence for a “larval transport hypothesis”, which states that cannibalism of larval and juvenile pollock is reduced in years when strong northward advection separates juveniles from cannibalistic adults.In addition to testing existing hypotheses, we identified new relationships between spawner-to-recruit survival rates of walleye pollock and several indicators of mixed layer dynamics during the spring and summer. Survival rates and recruitment were significantly reduced when larval or early juvenile stages experienced a delay in the (non-ice-associated) spring bloom as a result of stormy spring conditions, suggesting that the timing of the spring bloom is critical to both first-feeding larvae and age-1 juveniles. Furthermore, a dome-shaped relationship between pollock survival and summer wind mixing at the early juvenile stage is consistent with modeling and laboratory studies showing an increase in survival at low to moderate levels of wind mixing, but a decrease in feeding success at high levels of wind mixing.Top-down controls also regulate recruitment of walleye pollock. At least one-third of the variability in spawner-to-recruit survival could be accounted for by predation mortality at the early juvenile stage (age-1). Predation of juvenile pollock can be attributed largely to cannibalism, which varies with the abundance of adult pollock and with the availability of juveniles to adult predators. A simple index reflecting the spatial overlap between juvenile and adult pollock explained 30–50% of the overall variability in recruitment, similar to the variability explained by the best environmental predictors. Although environmental effects are difficult to separate from the effects of predation, we conclude that bottom-up and top-down processes are equally important in controlling the survival of pollock from spawning to recruitment at age 2. However, the magnitude of top-down control is itself modified by environmental factors that control the availability of juvenile pollock to adults (through impacts on spatial distribution) and the abundance of adult predators (through effects on productivity and carrying capacity).     

Heat flow and quantity of methane deduced from a gas hydrate field in the vicinity of the Dnieper Canyon,northwestern Black Sea

Seismic reflection data document for the first time the existence of a BSR in a limited area west of the Dnieper Canyon in the northwestern Black Sea. Seismic wide-angle data suggest that gas hydrates occupy in average 15±2% of the pore space in a zone of 100 m in thickness. A conservative quantification of the amount of methane associated with this gas hydrate occurrence is about 12±3×10¹¹ m³ (0.6±0.2 Gt of methane carbon). Conductive heat flow deduced from the BSR depth is in the range of 21±6 to 55±15 mW m^–2.