The contribution of atmospheric acid deposition to ocean acidification in the subtropical North Atlantic Ocean

The absorption of anthropogenic CO₂ and atmospheric deposition of acidity can both contribute to the acidification of the global ocean. Rainfall pH measurements and chemical compositions monitored on the island of Bermuda since 1980, and a long-term seawater CO₂ time-series (1983–2005) in the subtropical North Atlantic Ocean near Bermuda were used to evaluate the influence of acidic deposition on the acidification of oligotrophic waters of the North Atlantic Ocean and coastal waters of the coral reef ecosystem of Bermuda. Since the early 1980's, the average annual wet deposition of acidity at Bermuda was 15 ± 14 mmol m^− 2 year^− 1, while surface seawater pH decreased by 0.0017 ± 0.0001 pH units each year. The gradual acidification of subtropical gyre waters was primarily due to uptake of anthropogenic CO₂. We estimate that direct atmospheric acid deposition contributed 2% to the acidification of surface waters in the subtropical North Atlantic Ocean, although this value likely represents an upper limit. Acidifying deposition had negligible influence on seawater CO₂ chemistry of the Bermuda coral reef, with no evident impact on hard coral calcification.     

Structural controls on the formation of BSR over a diapiric anticline from a dense MCS survey offshore southwestern Taiwan

A dense seismic reflection survey with up to 250-m line-spacing has been conducted in a 15 × 15 km wide area offshore southwestern Taiwan where Bottom Simulating Reflector is highly concentrated and geochemical signals for the presence of gas hydrate are strong. A complex interplay between north–south trending thrust faults and northwest–southeast oblique ramps exists in this region, leading to the formation of 3 plunging anticlines arranged in a relay pattern. Landward in the slope basin, a north–south trending diapiric fold, accompanied by bright reflections and numerous diffractions on the seismic profiles, extends across the entire survey area. This fold is bounded to the west by a minor east-verging back-thrust and assumes a symmetric shape, except at the northern and southern edges of this area, where it actively overrides the anticlines along a west-verging thrust, forming a duplex structure. A clear BSR is observed along 67% of the acquired profiles. The BSR is almost continuous in the slope basin but poorly imaged near the crest of the anticlines. Local geothermal gradient values estimated from BSR sub-bottom depths are low along the western limb and crest of the anticlines ranging from 40 to 50 °C/km, increase toward 50–60 °C/km in the slope basin and 55–65 °C/km along the diapiric fold, and reach maximum values of 70 °C/km at the southern tip of the Good Weather Ridge. Furthermore, the local dips of BSR and sedimentary strata that crosscut the BSR at intersections of any 2 seismic profiles have been computed. The stratigraphic dips indicated a dominant east–west shortening in the study area, but strata near the crest of the plunging anticlines generally strike to southwest almost perpendicular to the direction of plate convergence. The intensity of the estimated bedding-guided fluid and gas flux into the hydrate stability zone is weaker than 2 in the slope basin and the south-central half of the diapiric fold, increases to 7 in the northern half of the diapiric fold and plunging anticlines, and reaches a maximum of 16 at the western frontal thrust system. Rapid sedimentation, active tectonics and fluid migration paths with significant dissolved gas content impact on the mechanism for BSR formation and gas hydrate accumulation. As we begin to integrate the results from these studies, we are able to outline the regional variations, and discuss the importance of structural controls in the mechanisms leading to the gas hydrate emplacements.     

An unsteady wave driver for narrowbanded waves: modeling nearshore circulation driven by wave groups

In this paper, we derive an unsteady refraction–diffraction model for narrowbanded water waves for use in computing coupled wave–current motion in the nearshore. The end result is a variable coefficient, nonlinear Schrödinger-type wave driver (describing the envelope of narrow-banded incident waves) coupled to forced nonlinear shallow water equations (describing steady or unsteady mean flows driven by the short-wave field). Comparisons with experimental data show that good accuracy can be obtained for cases of nonbreaking wave transformation. Numerical simulations show that the interaction of wave groups with longshore topographic nonuniformities generates strong edge wave resonances, providing a generating mechanism for low-order edge waves.     

Impact of urban stormwater runoff on estuarine environmental quality

Urban stormwater runoff could contribute to the deterioration of water quality of a receiving water body. In this research, field studies and laboratory experiments were conducted to assess the microbial contamination resulting from urban stormwater runoff into the Lake Pontchartrain estuary. Fecal coliform, Escherichia coli and enterococci were used as indicator organisms. The specific objectives of the research were to examine the distribution of the indicator organisms in different environmental elements (water column, suspended particles and sediment) and to further investigate the mechanisms related to their fate. Results of the research indicated satisfactory water quality at the study sites during dry weather periods. However, a significant increase was observed in the concentrations of the indicator organisms in the water columns and sediment at specific study sites following a given stormwater event. Three to seven days were needed for the elevated indicator organisms to return back to their background levels in the water column and sediment, respectively. The mechanism of sedimentation contributed to a reduction in the microbial concentration in the water column, as the indicator organisms were found to attach to the suspended particles in the stormwater. The percentage of fecal coliform, E. coli and enterococci attached to the suspended particles was found to be within the range of 9.8–27.5%, 21.8–30.4%, and 8.4–11.5% of the total indicator organisms in the stormwater loaded into the estuary, respectively. About 75–80% of the total indicator organisms remained free-floating for some distance in the water column before dying off.     

Plastic buckling of ring-stiffened conical shells under external hydrostatic pressure

The paper describes experimental tests carried out on three ring-stiffened cones that were tested to destruction under external hydrostatic pressure. The cones were carefully machined from EN1A Steel. All three cones failed by plastic non-symmetric bifurcation buckling in a mode commonly known as general instability. In this mode the entire ring-shell combination buckles bodily.The paper also provides a design chart using the results obtained from these three vessels, together with the results of six other vessels obtained from other tests. The design chart allows the possibility of obtaining a plastic knock down factor, so that the theoretical buckling pressures, based on elastic theory, can be divided by the plastic knockdown factor, to give the predicted buckling pressure. This method can also be used for the design of full-scale vessels.     

Regulation of Tilapia metallothionein gene expression by heavy metal ions

Tilapia is a common fish species inhabiting inland waters and estuarine regions in Hong Kong and Southeast Asia, and useful for bio-monitoring of metal pollution. Metallothionein (MT) gene expression in fish tissues has been useful to sub-lethal risk assessment as biomarker of exposure to metal ions in fishes inhabiting metal contaminated area. To investigate metal inductions of Tilapia MT gene expression in vivo, Tilapias were injected with different concentrations of heavy metals and tissues were then removed for quantitative PCR assay using mimic PCR methods. All of the metal ions tested (Cu(2+), Cd(2+), Hg(2+), Ni(2+), Pb(2+) and Zn(2+)) were able to induce hepatic MT mRNA levels. Renal MT mRNA levels of Cd(2+) and Zn(2+) treated fish was not induced with significant fold induction, however MT mRNA levels in gills were sensitive to the administrations of these metal ions. These data indicated that Tilapia MT mRNA levels in gills and liver are sensitive biomarker of exposure to various metal ions.     

Biological Responses to Seasonally Varying Fluxes of Organic Matter to the Ocean Floor: A Review

Deep-sea benthic ecosystems are sustained largely by organic matter settling from the euphotic zone. These fluxes usually have a more or less well-defined seasonal component, often with two peaks, one in spring/early summer, the other later in the year. Long time-series datasets suggest that inter-annual variability in the intensity, timing and composition of flux maxima is normal. The settling material may form a deposit of “phytodetritus” on the deep-seafloor. These deposits, which are most common in temperate and high latitude regions, particularly the North Atlantic, evoke a response by the benthic biota. Much of our knowledge of these responses comes from a few time-series programmes, which suggest that the nature of the response varies in different oceanographic settings. In particular, there are contrasts between seasonal processes in oligotrophic, central oceanic areas and those along eutrophic continental margins. In the former, it is mainly “small organisms” (bacteria and protozoans) that respond to pulsed inputs. Initial responses are biochemical (e.g. secretion of bacterial exoenzymes) and any biomass increases are time lagged. Increased metabolic activity of small organisms probably leads to seasonal fluctuations in sediment community oxygen consumption, reported mainly in the North Pacific. Metazoan meiofauna are generally less responsive than protozoans (foraminifera), although seasonal increases in abundance and body size have been reported. Measurable population responses by macrofauna and megafauna are less common and confined largely to continental margins. In addition, seasonally synchronised reproduction and larval settlement occur in some larger animals, again mainly in continental margin settings. Although seasonal benthic responses to pulsed food inputs are apparently widespread on the ocean floor, they are not ubiquitous. Most deep-sea species are not seasonal breeders and there are probably large areas, particularly at abyssal depths, where biological process rates are fairly uniform over time. As with other aspects of deep-sea ecology, temporal processes cannot be encapsulated by a single paradigm. Further long time-series studies are needed to understand better the nature and extent of seasonality in deep-sea benthic ecosystems. This revised version was published online in August 2006 with corrections to the Cover Date.