David K. Young

Monsoons boost biological productivity in Arabian Sea

Monsoons over the Arabian Sea—the oceanic basin that separates the Arabian peninsula from the Indian subcontinent—follow seasonal cycles, reversing directions twice a year, in summer and winter. Rather than spreading across the expanse of the sea, the southwest (summer) monsoon is often concentrated into a jet over the central Arabian Sea. Evidence suggests that variations in wind stress force substantial upwelling in the ocean to the west of the jet, and weaker upwelling or even downwelling to the east. This upwelling provides nutrients to the euphotic zone and enhances biological productivity.

Physical processes affecting availability of dissolved silicate for diatom production in the Arabian Sea

A passive tracer to represent dissolved silicate concentrations, with biologically realistic uptake kinetics, is successfully incorporated into a three-dimensional, eddy-resolving, ocean circulation model of the Indian Ocean. Hypotheses are tested to evaluate physical processes which potentially affect the availability of silicate for diatom production in the Arabian Sea. An alternative mechanism is offered to the idea that open ocean upwelling is primarily responsible for the high, vertical nutrient flux and consequent large-scale phytoplankton bloom in the northwestern Arabian Sea during the southwest monsoon. Model results show that dissolved silicate in surface waters available for uptake by diatoms is primarily influenced by the intensity of nearshore upwelling from southwest monsoonal wind forcing and by the offshore advective transport of surface waters. The upwelling, which in the model occurs within 200±50 km of the coast, appears to be a result of a combination of coastal upwelling, Ekman pumping, and divergence of the coastal flow as it turns offshore. Localized intensifications of silicate concentrations appear to be hydrodynamically driven and geographically correlated to coastal topographic features. The absence of diatoms in sediments of the eastern Arabian Basin is consistent with modeled distributional patterns of dissolved silicate resulting from limited westward advection of upwelled coastal waters from the western continental margin of India and rapid uptake of available silicate by diatoms. Concentrations of modeled silicate become sufficiently low to become unavailable for diatom production in the eastern Arabian Sea, a region between 61°E and 70°E at 8°N on the south, with the east and west boundaries converging on the north at ∼67°E, 20°N.

Photographs of deep-sea Lebensspuren: A comparison of sedimentary provinces in the Venezuela Basin, Caribbean Sea

Abstract Twenty-five types of Lebensspuren were identified and quantified from sites representative of the three major sedimentary provinces in the Venezuela Basin, Caribbean Sea. Only seven types were unique to any one site. Most of the Lebensspuren have been found elsewhere in the deep sea and some have identities as ichnogenera. Densities of Lebensspuren and megafauna were inversely correlated, suggesting that the relative abundances of functional groups may be more important in determining densities of Lebensspuren than overall megafaunal densities. Diversities of Lebensspuren and megafauna were positively correlated. Lebensspuren attributable to sedentary and mobile animal activities were identified and compared among sites. Percent organic carbon content of sediment and the ratio of sedentary to mobile types were positively correlated, suggesting that mobile deposit-feeding activity predominates when organic carbon is low. Mean densities of total Lebensspuren and the ratio of sedentary to mobile features were positively correlated, suggesting that mobile deposit-feeding activity tends to “smooth out” seafloor microrelief while sedentary deposit-feeding helps maintain and accentuate it. In the absence of hydrodynamic influences at the deep-sea floor, as the case with the provinces studied here, densities of Lebensspuren are maintained in a steady state by rates of destruction and production of these features by various sediment-reworking activities of animals.

Effects of biological activity by abyssal benthic macroinvertebrates on a sedimentary structure in the Venezuela Basin

Abstract Macrobenthic standing stock estimates from the Venezuela Basin were in agreement with those calculated from other ocean basins. Biomass and density values were significantly higher at a site characterized by hemipelagic sedimentation compared to two sites characterized by carbonate and turbidite sedimentation. The higher standing stock is probably related to the higher input of organic matter to that site. Vertical distribution of fauna suggests that sediment was well mixed to 6–8 cm depth at the carbonate and turbidite sites and to 10–12 cm depth at the hemipelagic site. Infrequent mixing probably extends to 30 cm at the turbidite site, 18 cm at the hemipelagic site and 10 cm at the carbonate site. Mixing rates were predicted to be the highest at the hemipelagic site where benthic standing stocks were highest. These mixing depths and rates were in general agreement with those determined by radiochemical methods and by visual and X-ray photographic observations. Gradients of decreased porosity and increased density were probably the result of biological activity (burrowing and tube dwelling) as opposed to effects of overburden pressure. Sediment shear strength was controlled by biological activity coupled with biologically mediated chemical bonding at the redox discontinuity layer. Destruction of sand-sized foraminiferan tests by faunal ingestion of sediments contributes to a reduction in mean grain size at the sediment surface.

The interaction of southwest monsoon upwelling, advection and primary production in the northwest Arabian Sea

Abstract A biological model comprising phytoplankton, Zooplankton, detritus, and nitrogen pools has been coupled to a 3-layer reduced-gravity ocean circulation model of the Arabian Sea through the vertical mixing and horizontal transport fields. Coupled biophysical interactions during the southwest monsoon have been investigated using experiments with progressively more physical forcing: (1) initial-value biological calculations with no forcing; (2) upwelling simulations for the southwest monsoon that incorporate vertical forcing only; and (3) coupled physical-biological model experiments that use both upwelling and horizontal advection, and are forced by monthly varying climatological wind stresses and density profiles for the Arabian Sea. The initial-value and upwelling experiments indicate the roles of grazing pressure and remineralization of detritus when physical forcing is present. The coupled model experiments demonstrate the importance of coastal upwelling and offshore advection in determining patterns of total nitrogen in surface waters, both near the coast and offshore. Offshore jets are significant as sources of both transported nutrients and biomass in open waters far from coastal upwelling areas.

Abyssal benthos of the Venezuela Basin, Caribbean Sea: standing stock considerations

Abstract Standing stocks of four size classes of benthic organisms were compared among three sedimentary provinces in the Venezuela Basin. Total biomass was dominated by microbiota and filter-feeding glass sponges that were abundant in both macrofaunal and megafaunal size classes. The remaingng biomass was divided among predominantly deposit-feeding meiofauna, macrofauna and megafauna. The concentration of biomass in smaller sized classes in the Venezuela Basin was in general agreement with the results of other deep-sea investigations. By contrast, biomass in shallow-water benthic assemblages is concentrated in the larger sized classes. We postulate that these differences in the distribution of biomass among benthic size classes results, in part, from differences in the quality and quantity of organic matter available to shallow-water and deep-sea communities. Most of the labile organic matter that reaches the deep-sea bottom in the Venezuela Basin is consumed at or near the sediment-water interface by surface deposit-feeding and filter-feeding benthos. Transformation of refractory organic matter into utilizable substrate by bacteria in the upper 10–20 cm of sediment provides a source of food for subsurface deposit feeders. Biological mixing may also play a role in the transfer of recently deposited organic matter to subsurface deposit feeders.

Effects of hydrodynamic and biological processes on sediment geoacoustic properties in Long Island Sound, U.S.A.

Abstract Concurrent acoustical, physical, and biological properties were measured from replicate core liner and box-core samples collected by SCUBA divers from each of two locations in Long Island Sound, 27–28 August, 1980. A very low diversity pioneer assemblage dominated by the filter-feeding bivalve Mulinia lateralis was found at the shallower (10 m water depth) FOAM site. Nearby sediments were dominated by surface-dwelling tubiculous polychaetes and amphipods. Sediment laminations produced by storm-induced erosional and depositional events were preserved at the FOAM site because sediment mixing by macrofauna was uncommon below the upper few centimeters of sediment. Horizontal patchiness of macrofauna and preservation of sediment laminations resulted in vertical and horizontal variability of sediment physical and acoustical properties. Spatial and temporal changes of dominant species at the FOAM site resulted in considerable large scale (10–100 m) variability of physical and acoustical properties of surficial sediment. A low-diversity equilibrium assemblage dominated by surface deposit-feeding bivalves and deeper-dwelling errant and tubiculous polychaetes was found at the deeper (16 m water depth) NWC site. Intense bioturbation by surface deposit-feeding bivalves precluded preservation of primary laminations created by storm-induced erosional and depositional events. Bioturbation was responsible for spatial and temporal large-scale homogeneity of physical and acoustical properties of NWC sediments. Deeper-dwelling polychaetes mixed sediment to depths of 15 cm, creating random variability of fine-scale physical and acoustical structure by production of burrows, tubes and feeding voids, and by mixing shell remains throughout the upper 15 cm. Physical and acoustical properties of many coastal marine sediments are controlled by the interaction of biological and hydrodynamic processes. Study of relationships between these two processes and resultant sedimentary properties should lead to improvement of predictive geoacoustic models for coastal environments, especially where high levels of biological activity are expected.

Geoacoustic Models and Bioturbation

Abstract Two types of geoacoustic models are used to describe relationships between physical and acoustic properties of unconsolidated marine sediments: simple predictive, and theoretical models. Simple predictive models use the apparent correlation between physical properties of sediments (usually porosity, grain size or density) and acoustic properties of sediments to predict velocity and attenuation of compressional and shear waves. Theoretical models use elastic properties of sediments (rigidity, compressibility, etc.) to calculate acoustic properties. It has been shown that bioturbation by benthic animals profoundly affects the physical properties of marine sediments. Activities including burrowing, ingestion/digestion/defecation, tube building, biodeposition, cementation and metabolic activities modify porosity, grain size, density, fabric, rigidity and compressibility of sediments. We hypothesize that bioturbation by benthic animals alters acoustic properties of unconsolidated marine sediments. An example is provided for predicted effects of bioturbation on selected acoustic and elastic properties of a silty mud sediment. Knowledge of effects of bioturbation on unconsolidated marine sediments may increase our understanding of the relationship between predicted and measured acoustic properties.

Variability in geoacoustic and related properties of surface sediments from the Venezuela Basin, Caribbean Sea

Abstract The spatial variability of sediment geoacoustic and related properties was determined from replicate 0.25 m 2 USNEL box-core samples collected from three sedimentary provinces in the Venezuela Basin. The maximum variation in sediment physical, chemical and geoacoustic properties occurred among distant sites representing pelagic, turbidite or hemipelagic sedimentary regimes rather than within sites or within core depth gradients. Variability among sites was a function of differences in calcium carbonate dissolution among a water-depth gradient of 1550 m (3500–5050 m) and differences in the relative contribution of particles from pelagic and terrigenous sources. The least amount of variation was evident among subcores obtained from the same box core or site, suggesting that a single subcore was adequate to describe sediment physical and geoacoustic properties over large areas of the sea floor. The considerable amount of downcore variability in sediment properties was a function of gradients in biological activity, biologically mediated chemical changes and the presence of ponded terrigenous sediments deposited by intermittent turbidite flows interspersed between sediments of pelagic origin. For sediments with greater than 35% calcium carbonate, porosity is a good predictor of compressional wave velocity; whereas mean grain size is a good predictor of compressional wave attenuation. Porosity—mean grain-size relationships had little predictive value in carbonate sediments. The differences among these and previously published empirical relationships show that particle-size distribution of larger carbonate material and composition of the sediment matrix must be considered when predicting sediment geoacoustic properties. Sediment shear strength did not show any correlation with other sediment physical or acoustic properties and is primarily controlled by biological and chemical processes.

Effects of waste disposal on benthic faunal succession on the abyssal seafloor

Strategies to predict, and thus limit, potentially detrimental environmental impacts of abyssal disposal of wastes are severely limited by our lack of knowledge of deep-sea processes and lack of opportunity to directly study waste disposal in abyssal environments. Probable successional sequences following burial of benthic faunas by sewage sludge and dredged material on the abyssal seafloor are drawn by analogy with well-known processes in shallow-marine water. Scales of change and recovery of abyssal benthic faunas from episodic deposition of waste material are examined by extrapolation from what is currently known about turbidite sedimentary provinces, in particular, the Venezuela Abyssal Plain and the Great Meteor East area of the Madeira Abyssal Plain. Results suggest that initial benthic faunal recolonization would take place within years following episodic depositions of waste on the abyssal seafloor. Anoxic conditions or chemical inhibitory effects may delay initial benthic recolonization for hundreds of years. Establishment of equilibrium benthic faunal assemblages probably takes hundreds to potentially thousands of years. Potentially detrimental effects dictate that the surface areas of individual waste deposits should be minimized and the deposits should be isolated by capping with nontoxic materials or chemical barriers.