Temporal variability in phytodetritus and megabenthic activity at the seabed in the deep Northeast Atlantic

We report a ten-year study of the abundance and activity of megabenthos on the Porcupine Abyssal Plain, northeast Atlantic, together with observations on the occurrence of phytodetritus at the deep-sea floor (4850 m). Using the Southampton Oceanography Centre time-lapse camera system, ‘Bathysnap’, we have recorded a radical change in the abundance and activity of megabenthos between the two periods of study (1991–1994 and 1997–2000). In 1991–1994, the larger megabenthos occurred at an abundance of c. 71.6/ha and were dominated by large holothurians. In addition, there were very substantial populations of smaller megabenthic ophiuroids (c. 4979/ha). Together, the total megabenthos are estimated to track over some 17 cm²/m²/d (exploiting 100% of the surface of the seabed in c. 2.5 years). In 1997–2000, the larger megabenthos increased to an abundance of c. 204/ha and were joined by exceptional numbers of a small holothurian species (Amperima rosea, 6457/ha) and ophiuroids (principally Ophiocten hastatum, 53,539/ha). The total megabenthos population was tracking at an estimnated rate of c. 247 cm²/m²/d (exploiting 100% of seabed in just 6 weeks). Coincident with these increases in the abundance and activity of the megabenthos, there were apparently no mass depositions of aggregated phytodetritus to the seabed in the summers of 1997–1999. Mass occurrences of phytodetritus had been noted during the summer months of the three years previously studied (1991, 1993 and 1994), with covering between 50 and 96% of the sediment surface. There is a statistically significant (p<0.02) negative correlation between maximum extent of this seabed cover of phytodetritus and seabed tracking by megabenthos. Additional studies [Lampitt et al., Progr. Ocean. 50 (2001)], indicate that there were no substantial changes in surface ocean primary productivity, in export flux, or in the composition of the flux that might otherwise account for the apparent absence of observable concentrations of phytodetritus during the summers of 1997–1999. We postulate that the marked increase in megabenthic tracking activity resulted in the removal (via consumption, disaggregation, burial etc.) of the bulk of the incoming phytodetrital flux during these years. A simple conceptual model, based on the apparent phytodetrital fluxes observed in 1991 and 1993, suggests that the megabenthos tracking rates estimated for 1997–1999 are sufficient to account for near-total removal of this flux. However, we are not able to estimate other processes removing phytodetritus (i.e. other elements of the benthos) that may also have increased between 1991–1994 and 1997–1999. Other independent studies [e.g. Ginger et al., Progr. Ocean. 50 (2001)] of flux constituents support the possibility that just a few species of megabenthos (e.g. A. rosea, and O. hastatum) could well have consumed a major proportion of the incoming flux and so substantially modified the composition of the organic matter available to other components of the benthos.     

Recruitment patterns of Serpula vermicularis L. (Polychaeta, Serpulidae) in Loch Creran, Scotland

This study aimed to contribute to conservation management of reefs of Serpula vermicularis by increasing understanding of the factors influencing larval settlement. The study was carried out in Loch Creran, which supports the most extensive known development of S. vermicularis reefs in the world. Settlement plates were deployed to examine the influence of season, depth, reef density, substrate type and orientation. Monthly deployment of plates revealed settlement of S. vermicularis to occur predominantly from mid-June to mid-October, peaking in late August to early September. Settlement of Pomatoceros spp. peaked much earlier, in late May to early June. Deployment of plates at different depths revealed a marked reduction in S. vermicularis settlement intensity between 6 and 12 m. As this corresponds with the deeper limit of the peripheral fringe of serpulid reefs in the loch, it is suggested that this limit is imposed by a depth-correlated settlement response, rather than reduction in available substrata. Comparisons of various substrata showed a preference by S. vermicularis larvae for a slate over a scallop substrate and no evidence of enhanced recruitment to occupied or unoccupied tubes of S. vermicularis, suggesting that gregarious attraction is unlikely to be a factor causing reef formation. Settlement onto the upper side of a horizontal scallop substrate was found to be insignificant in comparison with the underside or a vertically orientated scallop. Evidence for the role of light in controlling the depth and substrate-orientation preferences of S. vermicularis larvae is discussed. Based on the results of this study, recommendations are made regarding remediation of areas suffering reef damage.     

The morphodynamic modelling of tidal sand waves on the shoreface

An artificial sand wave on the Dutch shoreface of the North Sea has been studied in conditions with relatively strong tidal currents in the range of 0.5 to 1 m/s and sediments in the medium sand size range of 0.2 to 0.5 mm. The sand wave is perpendicular to the tidal current and has a maximum height and length of the order of 5 m and 1 km, respectively. The sand wave is dynamically active and shows migration rates of the order of a few metres per year. A numerical morphodynamic model (DELFT3D model) has been used to simulate the morphological behaviour of the sand wave in the North Sea. This model approach is based on the numerical solution of the three-dimensional shallow water equations in combination with a surface wave propagation model (wind waves) and the advection–diffusion equation for the sediment particles with online bed updating after each time step. The model results show that the sand wave grows in the case of dominant bed-load transport (weak tidal currents; relatively coarse sediment; small roughness height; low waves) and that the sand wave decays in the case of dominant suspended transport (strong currents, relatively fine sediment, large roughness height; storm waves).     

Atmospherically-promoted photosynthetic activity in a well-mixed ecosystem: Significance of wet deposition events of nitrogen compounds

Wet atmospheric deposition of dissolved N, P and Si species is studied in well-mixed coastal ecosystem to evaluate its potential to stimulate photosynthetic activities in nutrient-depleted conditions. Our results show that, during spring, seawater is greatly depleted in major nutrients: Dissolved Inorganic Nitrogen (DIN), Dissolved Inorganic Phosphorus (DIP) and Silicic acid (Si), in parallel with an increase of phytoplanktonic biomass. In spring (March–May) and summer (June–September), wet atmospheric deposition is the predominant source (>60%, relative to riverine contribution) for nitrates and ammonium inputs to this N-limited coastal ecosystem. During winter (October–February), riverine inputs of DIN predominate (>80%) and are annually the most important source of DIP (>90%). This situation allows us to calculate the possibility for a significant contribution to primary production in May 2003, from atmospheric deposition (total input for DIN ≈300 kg km⁻² month⁻¹). Based on usual Redfield ratios and assuming that all of the atmospheric-derived N (AD-N) in rainwater is bioavailable for phytoplankton growth, we can estimate new production due to AD-N of 950 mg C m⁻² month⁻¹, during this period of depletion in the water column. During the same episode (May 2003), photosynthetic activity rate, considered as gross primary production, was estimated to approximately 30 300 mg C m⁻² month⁻¹. Calculation indicates that new photosynthetic activity due to wet atmospheric inputs of nitrogen could be up to 3%.     

The impact of shrimp trawling and associated sediment resuspension in mud dominated, shallow estuaries

To address the relative importance of shrimp trawling on seabed resuspension and bottom characteristics in shallow estuaries, a series of disturbance and monitoring experiments were conducted at a bay bottom mud site (2.5 m depth) in Galveston Bay, Texas in July 1998 and May 1999. Based on pre- and post-trawl sediment profiles of ⁷Be; pore water dissolved oxygen and sulfide concentration; and bulk sediment properties, it was estimated that the trawl rig, including the net, trawl doors, and “tickler chain,” excavate the seabed to a maximum depth of approximately 1.5 cm, with most areas displaying considerably less disturbance. Water column profile data in the turbid plume left by the trawl in these underconsolidated muds (85–90% porosity; <0.25 kPa undrained shear strength) demonstrate that suspended sediment inventories of up to 85–90 mg/cm² are produced immediately behind the trawl net; an order of magnitude higher than pre-trawl inventories and comparable to those observed during a 9–10 m/s wind event at the study site. Plume settling and dispersion caused suspended sediment inventories to return to pre-trawl values about 14 min after trawl passage in two separate experiments, indicating particles re-settle primarily as flocs before they can be widely dispersed by local currents. As a result of the passage of the trawl rig across the seabed, shear strength of the sediment surface showed no significant increase, suggesting that bed armoring is not taking place and the trawled areas will not show an increase in critical shear stress.     

Estimation of Uplift Capacity of Rapidly Loaded Plate Anchors in Soft Clay

Plate anchors are extensively used in civil engineering constructions as they provide an economical alternative to gravity and other embedded anchors. The rate of loading is one of the important factors that affects the magnitude of soil resistance as well as soil suction force. This article outlines the effect of pullout rate on uplift behavior of plate anchors (70 mm diameter) buried in soft saturated clay by varying the pullout rate from 1.4 mm/min to 21.0 mm/min. The variation of breakout force and suction force with embedment depth and rate of pull are presented. A correlation between the rate of increase of undrained strength of clay and anchor capacity with rate of strain has been established. Finally an empirical equation has been proposed that includes the rate of pull in the estimation of breakout capacity of anchors.     

Manifestation of the Arctic Oscillation and El Niño-Southern Oscillation in the tropical stratosphere

Characteristics of the Arctic Oscillation and El Niño-Southern Oscillation effects manifested on interannual scales in the equatorial stratosphere are determined. Wavelet analysis of local phase shifts, coherence, and correlation is used to obtain correlation portraits of the largest factors of climate variability against the background of coherent variations in the equatorial stratospheric wind speed at the 50- and 15-hPa pressure levels. It is shown that the Arctic Oscillation and El Niño-Southern Oscillation signals may reach the tropical stratosphere. The signals are easily identified in a wide range of scales, including quasi-biennial, 3-to 5-year, and 10-to 11-year periods. The results obtained reflect a coherent pattern of the manifestation of these signals at the selected stratospheric levels. It is found that the El Niño-Southern Oscillation effect at periods close to 10–11 years reaches the stratospheric level rather rapidly, in the same or next month, while the effects of the Arctic Oscillation index are delayed by nine months. The estimates obtained show that a phase shift of almost 180° in the Arctic Oscillation index relative to the equatorial stratospheric wind occurred in almost all of the range of interannual periods in 1978 and 1992. For the El Niño-Southern Oscillation, an increase in local correlations on a scale of 3-to 5-year variations was observed in 1980–1990, a 180° phase shift occurred in 1992, and the correlation with stratospheric wind increased in 1992–2004. The estimates obtained are indicative of a change in the atmospheric circulation pattern that took place in the Northern Hemisphere in 1978–1991.     

Geological structure of the Curonian Spit (of the Baltic Sea) and its evolution history (revised)

The origin and evolution history of the Curonian Spit in the Baltic Sea are discussed based on geological and geomorphologic data. Evidence of the correlation between the spit formation stages and the Holocene sea level oscillations is presented. The ledges of moraine basement and marine accumulative features formed a single barrier. Its subsequent transformation occurred under the influence of the wave-related lithodynamic and eolian processes.