Near-bottom Current Velocities Research Articles

Megabenthic Diversity Patterns on a Seamount in the Philippine Sea: Implications for Conservation Planning on the Kyushu-Palau Ridge.

The oligotrophic tropical western Pacific region is characterized by a high density of seamounts, with the Kyushu-Palau Ridge (KPR) being the longest seamount chain here. Effective spatial management plans for seamount ecosystems necessitate an understanding of distribution patterns and key environmental factors influencing benthic communities. However, knowledge regarding deep-sea biodiversity patterns over intricate topography remains limited. In this study, we investigated a seamount with a water depth of 522 m at the summit located in the southern section of KPR. Survey transects were conducted from 522 m to 4059 m. By analyzing video-recorded data obtained by a human-occupied vehicle (HOV) during dives and environmental variables derived from bathymetry, distinct assemblages were identified through noise clustering. α- and β-diversity patterns within the seamount megabenthic community were analyzed across the depth gradient, along with investigation of their environmental drivers. A total of 10,596 megafauna individuals were documented, categorized into 88 morphospecies and statistically separated into six distinct community clusters using noise clustering analysis. Species abundance and richness were highest within the 700-800 m water depth range, declining notably beyond 2100 m, indicating a critical threshold for habitat classification in this region. The β-diversity of megabenthic communities was high (0.836). Although β-diversity patterns along the depth gradient were mostly dominated by differences in species richness, the contribution of species replacement increased with depth, becoming dominant at depths greater than 3000 m. Depth emerged as the primary driver of spatial variation in community structure, while near-bottom current velocity, topographic parameters (bathymetric position index, slope), and substrate type also influenced the formation of microhabitats. The study highlights the depth gradients, thresholds, and other intricate environmental factors shaping the spatial heterogeneity of these communities. It provides valuable insights for the future development of effective survey and conservation strategies for benthic biodiversity on the KPR.

Effects of a retaining wall and an artificial embankment on nearshore littoral habitats and biota in a large Alpine lake

The littoral zones of many Central European lakes are severely altered by lake-side retaining walls. These are suspected to impair littoral biota due to the reflection of incoming wave energy. We conducted a comparative study (armoured shore with a retaining wall and a restored shore with a wedge-shaped cobble embankment vs. a pristine shore) at a wind-exposed shore section of Lake Constance. The retaining wall had a number of significant remote effects on the littoral zone, i.e. increased near-bottom current velocities, higher bed-load transport rates, coarsening of surface sediments, reduction in phytomass density, reduction in total densities and number of taxa of macroinvertebrates as well as a significant decline in the percentages of Limnomysis benedeni Czerniavsky, 1882, Ostracoda, Corbicula fluminea O.F. Muller, 1774, Dreissenapolymorpha (Pallas, 1771) and other Bivalvia. However, the significantly affected zone had a rather narrow width of ~ 3 m. Shore restoration measures are needed to remediate negative effects of retaining walls and re-establish ecological conditions that are comparable to those of pristine shores. We recommend a more site sensitive sampling strategy, e.g. for macroinvertebrates in context with the ecological quality assessment under the European Water Framework Directive.

ДОЛГОВРЕМЕННАЯ ИЗМЕНЧИВОСТЬ ТЕЧЕНИЙ В ЗОНЕ СУБАРКТИЧЕСКОГО ФРОНТА АТЛАНТИЧЕСКОГО ОКЕАНА

Based on time series of near-bottom current velocities and temperatures obtained in the period June, 2016 to July, 2017, at three points in the Atlantic Subarctic Front, along with the use of multi-year (since 1993 up to now) satellite ocean surface sounding data, multi-scale fluctuations of ocean surface and near-bottom flows over the western and eastern flanks of the Reykjanes ridge, as well as near Hatton Rise, on the Rokoll plateau, are studied. Hydrological profiles were carried out from the ocean surface to the bottom with readings every 10 m, when setting and retrieving the buoy stations. Using data from the Bank of hydrological stations (WOD13), SST satellite arrays (Pathfinder), long-term sea level and geostrophic velocities time series (AVISO), and bottom topography (model ETOPO-1), features of longterm cyclical fluctuations of SST, sea level, geostrophic currents on the ocean surface were defined in the sub-polar North Atlantic. It is shown that, in accordance with the large-scale thermohaline structure of the Subarctic front, two branches of the North Atlantic Current are detected on the ocean surface.One is directed from the Hatton towards the Icelandic-Faroese Rise, and the other – alomg the western flank of the Reykjanes Ridge toward Iceland. For the first branch, which is the main continuation of the North Atlantic Current, the average (for 25 years) water drift at a speed of 9.1±0.1 cm/s is determined to the northeast. The second branch, which forms the eastern part of the Subarctic cyclonic gyre, has the average water drift at a speed of 4.0±0.1 cm/s is directed north-northeast, along the western flank of the Reykjanes Ridge. In the intermediate waters of the frontal zone, an average water flow is observed at a speed of 2.7±0.1 cm/s to the north-northeast, along the eastern slope of the Reykjanes ridge.Due to the multy-scale components of the total variability, the average kinetic energy densities(KED) of total currents (109, 45, 97, (±3) erg/cm3, at station points from east to west) are much greater than the mean drift KED. The near-bottom flows on the Reykjanes ridge flanks are opposite to the direction of the North Atlantic Current. Outside the Subarctic gyre, the direction of average transport is maintained from the ocean surface to the bottom. The average (per year) KED of near-bottom currents are 31, 143, 27 (±3 erg/cm3), for three stations from east to west, respectively. In the intermediate waters of the frontal zone, above the eastern slope of the Reykjanes Ridge, there is a powerful reverse (relative to the North Atlantic Current) near-bottom water flow to the south-west, with a high average speed of ~ 15 cm/s. The KED of the currents during the year varies widely from zero to ~ 600 erg/cm3. The overall variability is due to cyclical variations and intermittency (“flashes”) of currents. Perennial cycles, seasonal variations, synoptic fluctuations with periods in the range of 30–300 days, as well as inertial oscillations and semi-diurnal tidal waves are distinguished. The intermittency of oscillations is partly due to changes in low-frequency flows, which can lead to a dopler frequency shift in the cyclic components of the spectrum. The amplitude of temperature fluctuations in the bottom layer for the year was (0.07–0.10) ± 0.01°C by the standard deviation. The seasonal changes of the bottom temperature are not detected. However, a linear trend with a warming of ~ (0.10–0.15) ± 0.01°С per year is noticeable.

Partly standing internal tides in a dendritic submarine canyon observed by an ocean glider

An autonomous ocean glider is used to make the first direct measurements of internal tides within Whittard Canyon, a large, dendritic submarine canyon system that incises the Celtic Sea continental slope and a site of high benthic biodiversity. This is the first time a glider has been used for targeted observations of internal tides in a submarine canyon. Vertical isopycnal displacement observations at different stations fit a one-dimensional model of partly standing semidiurnal internal tides – comprised of a major, incident wave propagating up the canyon limbs and a minor wave reflected back down-canyon by steep, supercritical bathymetry near the canyon heads. The up-canyon internal tide energy flux in the primary study limb decreases from 9.2 to 2.0kWm−1 over 28km (a dissipation rate of 1−2.5×10−7Wkg−1), comparable to elevated energy fluxes and internal tide driven mixing measured in other canyon systems. Within Whittard Canyon, enhanced mixing is inferred from collapsed temperature-salinity curves and weakened dissolved oxygen concentration gradients near the canyon heads. It has previously been hypothesised that internal tides impact benthic fauna through elevated near-bottom current velocities and particle resuspension. In support of this, we infer order 20cms−1 near-bottom current velocities in the canyon and observe high concentrations of suspended particulate matter. The glider observations are also used to estimate a 1°C temperature range and 12μmolkg−1 dissolved oxygen concentration range, experienced twice a day by organisms on the canyon walls, due to the presence of internal tides. This study highlights how a well-designed glider mission, incorporating a series of tide-resolving stations at key locations, can be used to understand internal tide dynamics in a region of complex topography, a sampling strategy that is applicable to continental shelves and slopes worldwide.

Biological valorisation of benthic habitats in the SE Baltic Sea

Biological valorisation offers a biocentric approach for intrinsic valuation of the biodiversity of habitats. This conceptual approach was recently applied in few regional studies within Europe using modern statistical approaches. In this study we aim to test the biological valorisation approach that integrates functional diversity of species, species richness, spatial distinctiveness and the status of the communities. Biological value of communities was predicted using Random forest regression method and several environmental factors, such as sediment types, Secchi depth, minimum near-bottom oxygen concentration, near-bottom current velocity, wave generated orbital near-bottom velocity, topographic features. The assessment showed that most valuable habitats are stony bottom reefs with perennial red algae Furcellaria lumbricalis. The obtained map of biological valuation can provide useful information for ecosystem-based management in the studied area indicating conflict zones and protection areas; it could also be adjusted to sensitivity assessment of benthic habitats.

NEAR-BOTTOM FLOW CHARACTERISTICS OF CURRENTS AT ARBITRARY ANGLE TO 2D RIPPLES

Experimental results for near-bottom current velocity profiles for flows over artificial, definitely 2D ripples made of 1.5 cm high aluminum angle-profile spaced at 10 cm intervals are obtained for the following cases: (i) current alone perpendicular to ripples; (ii) current alone parallel to ripples; (iii) combined orthogonal wave-current flows for current parallel to ripples; and (iv) current alone at an angle of 30° to the ripple axis. The velocity profiles are analyzed by the log-profile method, and show the roughness experienced by the current to increase as the angle between ripple and current direction increases, i.e. demonstrating convincingly the reality of the concept of a direction-dependent roughness for flows over a 2D rippled bottom. Roughness experienced by the velocity component perpendicular to the ripples is, however, found to be independent of the direction of the mainstream flow relative to that of the ripples, and the different roughness experienced by the perpendicular and parallel velocity components gives rise to a turning of the current velocity vector to become increasingly aligned with the ripple crests as the bottom is approached from above. Implications of this feature, in terms of net sediment transport direction in combined wave-current flows in inner-shelf coastal waters, is discussed.

Suspended sediment fluxes and transport processes in the Gulf of Lions submarine canyons. The role of storms and dense water cascading

Contemporary suspended sediment transport was studied in seven submarine canyons of the Gulf of Lions (GoL). Current meters equipped with turbidity sensors were moored 4 m above bottom at 300 m depth in the canyon axis from November 2003 to May 2004. Sediment transport events were monitored and studied in relation to forcing conditions. There was a large flood in early December, during which discharges from all of the coastal rivers increased by more than one order of magnitude. A smaller flood of the Rhone River occurred later in mid-January followed by a persistent high river discharge that lasted until mid-February. There were also several E–SE storm events during the measurement period, two of them causing large swell, one in early December (max H s: 8.4 m), coinciding with the major river flood, and one in late February (max H s: 7 m) during a period of lower river discharge. Most of the variability in down-canyon current speeds was linked to strong downwelling induced by E–SE storms and to cascading of dense shelf water induced by N and NW winds. The intensity and timing of these processes strongly varied spatially. Eastern storms generated higher waves and induced stronger downwelling in the western than in the eastern sector of the GoL. Shelf dense water cascading events were enhanced during eastern storms and they were also more intense in the western sector of the GoL. These events occurred frequently from January to May along the western canyons and from February to April along the eastern canyons. From February to April they occurred simultaneously in all the canyons. Sediment transport was mainly down-canyon and mostly concentrated during storm-induced downwelling and dense water cascading events. During the high river discharge season most of the newly-supplied and resuspended sediment remained stored on the shelf. However, during the late February storm and cascading event, the stored sediment was quickly resuspended and transported, mainly through the westernmost submarine canyon (Cap de Creus), following a flushing pattern. Later, although minor storms and shelf dense water cascading hardly increased suspended sediment concentrations within the canyons, they induced strong near-bottom current velocities, increasing down-canyon sediment fluxes and generating small but significant sediment transport events. Most of the shelf-canyon transfer took place during the late February storm and cascading event through the Cap de Creus canyon where the net suspended sediment flux was between one and two orders of magnitude higher than in the other canyons

Comparison of particulate matter distribution, in relation to hydrography, in the mesotrophic Skagerrak and the oligotrophic northeastern Aegean Sea

The seasonal vertical distribution of particulate matter (PM) was studied in two contrasting areas: (a) the mesotrophic Skagerrak (in the North Sea); and (b) the oligotrophic northeastern Aegean Sea (eastern Mediterranean). Similarities and differences of the PM distribution in the two areas are assessed with respect to the prevailing hydrographic conditions and the PM composition. Hydrographic conditions in both of the areas are characterised by strong density gradients, resulting from the inflow of low-salinity water, i.e. Baltic Sea water and Black Sea water for the Skagerrak and the northeastern Aegean Sea, respectively. Enhanced primary production and particles mainly of biogenic origin characterise the mesotrophic Skagerrak, whereas five-fold lower particle concentrations appeared in the oligotrophic Aegean Sea. The input of riverine particulates was limited in both of the areas. In the Skagerrak, the strong stratification resulted in particle accumulation on/above the pycnocline and the development of pronounced intermediate nepheloid layers (INLs). The pycnocline-related INLs were formed entirely by dinoflagellates. The pycnocline hindered the vertical movement and sinking rates of particles, thus favouring primary production. Particle horizontal advection along the density discontinuities was probably enhanced. This pattern was not observed in the stratified waters of the northeastern Aegean Sea, probably due to the very low particle concentrations and/or the fact that phytoplankton maxima appeared in deeper waters. Pronounced INLs were identified in the Skagerrak below the pycnocline; these are attributed to accumulated or advected dinoflagellate skeletal remains mixed with clay mineral particles. This was revealed only by means of SEM observations. X-ray diffraction analysis could not provide information on the type of phytoplankton present, because dinoflagellates form their skeletons from organic material. Frontal stations in the northeastern Aegean Sea exhibited pronounced vertical movement of particles towards the deeper waters. Benthic nepheloid layers (BNL) were observed in the Skagerrak; these were related to the resuspended fine-grained surface sediments. In the northeastern Aegean Sea, although near-bottom current velocities were sufficient to resuspend surface sediments, resuspension occurred only episodically. The BNLs here are related mostly to near-bottom phytoplankton growth.

Late Quaternary sedimentary processes and ocean circulation changes at the Southeast Greenland margin

A study has been made of late Quaternary depositional processes and bottom current activity on the Southeast Greenland margin, using seismic, sub-bottom profiling and deep-tow side-scan sonar data as well as sediment core information. The seabed data demonstrate the occurrence of strong, southerly bottom currents prevailing on the slope and rise. Well-defined longitudinal bedforms indicate maximum mean near-bottom current velocities of up to at least 1.0 m/s at the depth stratum of Labrador Sea water (800–1500 m). Similarly strong currents occur in Denmark Strait overflow water (DSOW) at the base of the slope, whereas more basinward the maximum DSOW flow speed is lower. Iceberg plow marks were found down to about 700 m water depth. Both at the shelf edge and on the lower slope and rise the seafloor morphology is indicative of downslope sediment transport and mass flow deposition, which is concluded to be a typically glacial feature. After generally more sluggish deep-water circulation during the last glacial maximum, DSOW basin ventilation was re-established shortly before 13.3 ka. On the shelf, in front of the retreating Greenland ice margin, permanent or semi-permanent sea ice conditions prevailed until about 12.5 ka. At that time increased Irminger Current activity had resulted in warming, and East Greenland Current (EGC)-controlled iceberg drift increased. No evidence was found for a return to extreme glacial conditions or a ceasing of DSOW flow during the Younger Dryas. Abundant coarse IRD collected at greater water depth from shallow sub-seabed strata has a provenance mainly in the northern part of East Greenland (68–73°N), which demonstrates the existence of a pre-Holocene EGC system initially extending to beyond the shelf edge.

A new benthic foraminiferal proxy for near-bottom current velocities in the Gulf of Cadiz, northeastern Atlantic Ocean

Recent benthic foraminiferal assemblages were analyzed in the Gulf of Cadiz, northeastern Atlantic, to study the impact of the Mediterranean Outflow Water (MOW) undercurrent on the benthic environment. Foraminiferal counts and the analysis of specimens attached to hard substrates from 26 surface samples reveal a relationship of epibenthic assemblages with sedimentary and hydrodynamic environment. Epibenthic species make up as much as 60% of the living assemblage at proximal sites with high current velocities and 3–18% in distal areas or near the margins of the MOW flow paths at low velocities. These foraminifers inhabit elevated substrates only within the MOW, which evidently provides an ecological niche for opportunistic suspension feeders. They adapt their settling elevation dynamically and occur at greater heights above the ambient sediment surface under stronger currents. Mobility, fixation strength, suspension feeding, and reproduction efficiency emerge as individual capabilities promoting the occupation of elevated substrates by certain epibenthic species. The active microhabitat selection is pursued as basic strategy of these foraminifers to optimize their food acquisition. A better access to food sources stimulates reproduction and leads to a greater contribution of foraminiferal tests to the surface sediments. Elevated epibenthos percentages from the dead assemblage and current velocities prevailing at the sampling sites are closely correlated. A compilation including other data from southern Portugal, Florida Straits, and the English Channel infers an exponential relationship between epibenthic abundances and flow strength implying that endobenthic species prevail even under high current velocities. A linear model provides a significantly better fit for the Gulf of Cadiz data however. This relation is used for a case study in order to estimate near-bottom current strengths for the late Holocene Peak III contourite in core M39008-3. Trends and absolute values of current velocities, inferred from the benthic foraminiferal proxy, are the same scale as estimates obtained from sediment grain-size distribution and hydrodynamic models. Epibenthic foraminifera thus bear a high potential as proxy for palaeocurrent studies that even may overcome objections by predetermined grain-size distributions in deep-sea cores.

Intense mid-slope resuspension of particulate matter in the Faeroe–Shetland Channel: short-term deployment of near-bottom sediment traps

An array of four moorings was deployed on a transect perpendicular to the south-eastern slope of the Faeroe–Shetland Channel to measure near-bottom fluxes during a 12-day period in spring 1999. Each mooring combined current meters and two sediment traps equipped with optical backscatter sensors (OBS) situated at 2 and 30 m above the bottom, at water depths of 470, 700, 800 and 1000 m. During the deployment, near-bottom current velocities at 470, 700 and 800 m water depth increased abruptly to values as high as 55 cm s −1 (within a few hours). The sudden change in the cross-slope component of the current at 470 m was immediately followed by a severe drop in temperature at this depth, and large fluxes were intercepted in the mid-slope traps (700 and 800 m) on the same day, probably associated with the strengthening of the along-slope component. Organic carbon and nitrogen content of the trap samples were, except for the shallowest mooring, much lower than in particulate matter suspended in the water column. During the high flux event, particles possessed lower organic carbon and nitrogen values than during lower flux periods. This indicates that sedimentary material entered the traps. A simple multi-component mixing model was applied to the trap samples to estimate the relative contribution to our total mass fluxes of material from different sources: primary settling from the water surface, rebound material, and the relatively aged sediment. It showed that fluff was the main contributor to the resuspended flux but that sediment proper was resuspended at mid-slope during the strong increase in the current velocities and represented up to 70% of the total mass flux at that time. This study shows that near-bottom resuspension on the slope may be intense and, although the scale is here short-term the phenomena described might be larger in time and space.

Feeding activity of mussels Mytilus edulis related to near-bed currents and phytoplankton biomass

The feeding activity of blue mussels Mytilus edulis was investigated in the field and related to near-bottom current velocities and the phytoplankton biomass in the near-bottom water layers for four days. The body content of Chl-a in mussels and their shell gap size were used as indices of filtration activity. During days 1 and 2 the near-bed current velocities were low (1.2–2.1 cm s −1), and the near-bed phytoplankton biomass was at the same time lower than near the water surface. Between 44 and 69% of the mussels had closed shells and accumulated only small amounts of Chl-a in the body. During day 3 and day 4 the near-bed current velocities increased to 6.5 and 3.9 cm s −1, respectively, and the Chl-a was homogeneously distributed in the water column. Now only 17 and 25% of the mussels had closed valves and they accumulated a larger amount of Chl-a. The actual population filtration rates were estimated from the density of active mussels times the estimated filtration rates (based on the gap size of the mussels and literature values for corresponding filtration rates). Actual population filtration rates ranged from 6.4 to 22.8 m 3 m −2 d −1 on days 1 and 3, respectively, corresponding to 27 and 98% of the estimated population filtration capacity.

Late Glacial to Holocene history of the Mediterranean Outflow. Evidence from benthic foraminiferal assemblages and stable isotopes at the Portuguese margin

Records of benthic foraminiferal assemblage variations and benthic δ 13C along 12 sediment cores from the western Iberian Margin, between 36° and 42°N at water depths from 820 to 3580 m, are used to monitor fluctuations of the Mediterranean Outflow Water (MOW) during the past 30 ka. The chronostratigraphy of the cores is based on planktonic δ 18O records, 14C AMS-dating, and the recognition of Heinrich Events H1 through H4. Increased abundances of suspension feeding benthic foraminifers, denoted as ’Epibenthos Group‘, closely match areas where the recent MOW core layers impinge on the continental slope at 800 and 1300 m water depth, and near-bottom current velocities are enhanced. Elevated ‘Epibenthos Group’ abundances, increased benthic δ 13C, and sedimentological evidence for winnowing and erosion are found in glacial sections up to the earliest Termination I in cores at water depths between 1600 and 2200 m off southern Portugal. The combined evidence reveals enhanced current activity at these depths due to a deep glacial MOW. The MOW advection at the Portuguese margin during the last Glacial was about 700 m deeper than today, conceivably forced by increased MOW density due to higher salinity and colder temperatures of Mediterranean waters. The deep MOW current gradually decreased in strength and shoaled to 1300 m water depth during the Termination and early Holocene. A shallow MOW core layer became active with the onset of Termination I at depths between 600 and 1000 m. Both the shallow and deep MOW current culminated during the Younger Dryas period. The present flow pattern with two MOW core layers centred at 800 and 1300 m water depth was established between 7.5 and 5.5 ka.

Tidal resuspension and deposition of particulate matter in the Oyster Grounds, North Sea

Moored current meters, e uorometers and transmissometers were used in combination with sediment traps (aspect ratio . 4) and shipborne sampling to determine e uxes of deposition and resuspension of total suspended matter (TSM) under tidal action in the 45 m deep Oyster Grounds, North Sea. Here, we present data from the mixed layer below the major thermocline at about 20 m above the bottom (mab) as obtained during a 14-day period of calm weather in July 1994. Around neap tide near-bottom current velocities remained smaller than 0.15 m s 2 1 and TSM was dominated by particles advected from a relatively turbid area to the southeast of the study site. At the onset of spring tide, current speeds increased with maximum values greater than 0.20 m s 2 1 and seabed friction velocities exceeding the threshold value for resuspension. Particles resuspended were strongly enriched with organic carbon compared to the bulk sediment, suggesting that not the bed proper but a e ne-grained e uff fraction was eroded. This resuspended e uff was by far the dominant source for the mass e uxes in the sediment trap (at 3.2 mab), which showed a distinct tidal cycle with highest e uxes directly after low water slack tide and lowest e uxes during maximum ebb current. This pattern was caused by variations in apparent settling velocity of TSM, presumably due to e oc formation during periods in the tidal cycle when current speeds were low and relatively high concentrations of both chlorophyll-a and TSM were found. From a simple model on advection, deposition and resuspension of TSM, we calculated a net accumulation on the sediment of 75 g m 2 2 during the 14-day study period, which is the difference between gross e uxes of deposition and resuspension. Upon deposition, the average retention time of particles until their next resuspension is calculated at 1‐ 2 weeks, which may be sufficient for substantial decomposition of organic matter associated with TSM. This implies that, upon resuspension, particles transported further along the shelf are relatively poor in organic carbon. It is concluded that the Oyster Grounds serve as a mid-shelf temporary depocenter and that mineralization in this and similar areas may play a crucial role in the carbon budget of the North Sea.

Contrasting effects of habitat structure on the recruitment and mortality of an epibiotic macroalga.

Field surveys over 2 years in contiguous beds of the seagrassesZostera capricorni andPosidonia australis showed that the green algaCodium duthieae was consistently more abundant inZ. capricorni than inP. australis. In 1 year, mature plants were also more abundant at the boundary between the seagrass beds than in either bed. Field experiments and programmes of sampling were used to investigate three potential explanations for the unusual distribution of this alga: (1) that the availability of substrata suitable for attachment of the alga differed between the two seagrass beds; (2) that mortality of matureC. duthieae differed between the seagrass beds; and/or (3) that the intensity of recruitment was different in the two seagrass beds.C. duthieae plants were exclusively epibionts of the bivalveAnadara trapezia. Detailed sampling showed that the abundance ofA. trapezia was similar in both seagrass beds and that the distribution of bivalves suitable as substrata forC. duthieae plants was not obviously related to proximity to the boundary between the beds. Two experiments investigated the survival ofC. duthieae plants in each bed. In the first, matureC. duthieae plants transplanted into theP. australis bed suffered similar rates of mortality to plants which were disturbed and moved within theZ. capricorni bed or which were left undisturbed in theZ. capricorni bed. Fewer of the host bivalves were recovered from theZ. capricorni bed, however, indicating that the mechanism of mortality differed between the beds, hosts being more frequently dislodged in theZ. capricorni bed. Removal of the leaves of the seagrasses had consistently greater effects on near-bottom current velocities in theZ. capricorni bed than in theP. australis bed and significantly increased mortality ofC. duthieae in theZ. capricorni bed. Survival of plants was greater in plots of artificial leaves ofP. australis placed in theZ. capricorni bed than in plots of the naturalZ. capricorni leaves or plots where the natural leaves were removed. Most mortality in theZ. capricorni bed was due to dislodgement of the alga and its bivalve substratum. Corresponding manipulations of leaves in theP. australis bed had consistently smaller effects on survivorship of both the alga and its host. Patterns in the recruitment of the alga most clearly reflected the distribution of adults.C. duthieae recruits were 5 times more abundant in theZ. capricorni bed and at the boundary between the two beds than in theP. australis bed. The results demonstrate how habitat structure, provided by the canopy of leaves of the two species of seagrass, can have contrasting effects on the recruitment and mortality of a macroalga. In the case ofC. duthieae, it appears that the differential pattern of recruitment is the primary determinant of the distribution of adult plants.

The history of coastal upwelling off Peru (11°S, ODP Leg 112, Site 680B) over the past 650 000 years

Abstract The coarse sediment fraction (>63 μm) and benthic foraminiferal composition of Quaternary sediment cores from ODP LEG 112 Hole 680B (252.5 m water depth) were sampled from the upwelling area off Peru. High-resolution oxygen isotope stratigraphy based on analyses of the benthic foraminifera Bolivina seminuda allows a detailed reconstruction of the upwelling history since oxygen isotope Stage 16. Variations in the relative abundances of the benthic foraminiferal species Nonionella auris, Bolivina costata , and Bolivina seminuda document changes in fertility, which were found to be enhanced during interglacial Stages 1, 3, 5, 7, 9 and 11. Generally, bioturbated sediments are dominated by the genera Cassidulina, Epistominella, Gyroidina, Trifarina and Valvulineria . These benthic foraminifera are members of the ‘Undercurrent Fauna’, which is commonly associated with sediments that have a high terrigenous detrital component and a low organic carbon content (<3 wt%). These sedimentary deposits are indicative of high near-bottom current velocities of the Peruvian Undercurrent and well oxygenated bottom waters (0.5 to >6 ml/l O 2 ). The entire core interval investigated in Hole 680B appears to have been affected by the Peruvian Undercurrent. Generally, undercurrent-dominated sections were found to correspond with glacial oxygen isotope stages.

Modification of animal habitat by large plants: mechanisms by which seagrasses influence clam growth.

Field experiments withMercenaria mercenaria in a relatively high-energy environment demonstrated that clams on unvegetated sand flats failed to grow during autumn while those within seagrass beds grew substantially. Clam growth rates at the seagrass margin that first receives the faster-flowing, flood-tidal currents were about 25% less than at the opposite edge. In a second experiment, pruning, which reduced average blade length by 50-75%, was shown to enhance near-bottom current velocities and to reduce shell growth ofMercenaria during summer by about 50%. As in the first experiment, clams in the unvegetated sand flats exhibited no net growth. Clam mortality, caused mostly by predatory crabs and whelks, was much higher on sand flats than in seagrass beds and intermediate in clipped seagrass. Although consistent with some previous reports, these growth results are still surprising given that they contradict the generalization that suspension feeders grow faster under more rapid current regimes.Three types of indirect interactions might explain the observed effect of seagrass on growth of buried clams: (1) altering food supply; (2) changing the intensity of biological disturbance on feeding clams; and/or (3) affecting the physical stability of the sediments. Previous research on this question has focused almost exclusively on processes that alter food supply rates. In this study, food concentrations, as indicated by suspended chla, were 30% higher inside than outside one seagrass bed, whereas chla concentrations in two other beds were not different from those on adjacent sand flats. This result is sufficient to show that more intense food depletion was not induced by the reduction in flow velocities under the seagrass canopy. Nevertheless, the possible small difference in food concentrations between vegetated and unvegetated bottom seems insufficient to explain the absence of growth of sand-flat clams, especially given the virtual lack of food limitation among suspension feeders in this system. Two data sets demonstrated that the effects of biological disturbance agents cannot be ignored. An outdoor laboratory experiment showed that even in the absence of physical contact between predator and prey the presence of a whelk reduces the amount of time spent feeding byMercenaria. This result suggests that sand flats, where predation rates are higher, may be sites of lower clam growth than seagrass beds because of greater consumer interference with clam feeding. Furthermore, clam siphons are proportionately larger inside seagrass than on sand flats, implying that siphon nipping may not be as intense inside seagrass. This process, too, would reduce net growth of sand-flat clams. Finally, no explicit test was conducted of the hypothesis that enhanced sediment transport in the absence of flow baffling and root binding by seagrass inhibits net growth of clams on high-energy sand flats. Nevertheless, this is a reasonable explanation for the pattern of enhanced growth of seagrass clams, and could serve to explain the otherwise unexplained pattern of lower clam growth at the edge of the seagrass bed that experiences the faster flood-tidal current velocities. Each broad process, changing fluid dynamics, altering consumer access, and varying sediment stability, represents a mechanism whereby habitat structure, provided by the dominant plant, has an important indirect influence on the functional value of the habitat for resident animals.

Sedimentology of the Kerala mud banks (fluid muds?)

Early workers have used the term “mud bank” to describe the intermittent appearance of calm patches of highly turbid water along the coastal tracts of Kerala, India during the southwest monsoon season. The term “fluid mud” is recommended as a more accurate term. A total of 32 grab and twelve core samples were collected from an area of 60 km 2 off Quilandi, Kozhikode, India to evaluate the sedimentological aspects of mud resuspension in nearshore waters. Information on sediment discharge, rainfall, salinity gradient, suspensate concentration, wave, wind and current patterns were also analysed to understand the fluid mud formation. The topmost unit in the sediment cores is represented by a soft, watery clay with a wedge-shaped geometry. The unit thickens towards the shore and the thin end lies about 9 km offshore. This expanse of fine suspended sediment in water results from interactions of suspended material concentrations, near-bottom current velocity and wave activity in the coastal zone. During the southwest monsoon, strong swells churn up the bottom clay into a thick suspension. The suspended sediment column develops a semicircular periphery in which wave energy is consistently absorbed. Lateral movement of the mud is controlled by the intensity and direction of littoral currents. SEM studies show a weakly disoriented, platy and highly porous fabric in the upper clay zone as opposed to the relatively stable, aligned, supportive framework in the lower clay layer. This suggests an easy dispersion of the upper clay zone with slight mechanical agitation in less saline coastal water. Freshwater discharge through river runoff may be an important factor in maintaining the suspension of mud due to deflocculation of clay particles. The “mud banks” of the Kerala coast differ from mud banks reported from other muddy coasts of the world in that they do not form regular relief-forming features. The transient nature of these “mud banks”, their unpredictable periodicity and the calm and turbid nature of the water column suggest that they are episodes of fluid mud formation.

Long-term measurements of near-bottom currents and suspended sediment concentration on the outer Texas-Louisiana continental shelf

Long-term measurements of near-bottom current velocity, water temperature and transmissivity were collected on the outer Texas-Louisiana shelf, near the shelf-edge, during the spring-summer of 1980 and the winter of 1980–1981. The sediments on this part of the shelf consist primarily of silt and clay and a bottom nepheloid layer is well developed. The winter deployment coincided with the passage of seven northers and Hurricane Jeanne. The spring-summer deployment was relatively quiet in comparison. The observations show that current direction is strongly related to changes in suspended sediment concentration. Flow to the south corresponds to increases in suspended matter concentration, flow to the north corresponds to decreases in suspended sediment concentration, suggesting that suspended sediment concentration is modulated primarily by advective processes. The passage of the northers did not cause an increase in current speed or suspended sediment concentration. However, oscillations that propagate through the area during the winter are important in modulating suspended sediment concentration.

Sedimentary fabric: A tool to delineate a high-velocity zone within a deep western Indian Ocean bottom current

Magnetic particle alignment and silt mean particle size data have been determined for sediment samples taken from the tops of piston cores recovered from the Amirante Passage in the western Indian Ocean. Our results suggest that the deep western boundary current (DWBC) is flowing through the passage fastest in the deeper portions of the passage, while lower velocities are inferred for depths of < 4100 m, possibly indicating the presence of a sharp velocity gradient within the DWBC. Sediment waves occur at the margin of the Amirante Passage where near-bottom current velocities may be below the minimum critical value for sediment resuspension.