Donald C. Rhoads

The Effects of Marine Benthos on Physical Properties of Sediments

The effects of benthic organisms on the physical properties of granular substrata are well documented. The range of effects has been presented in H. B. Moore (1931, 1939), Schwartz (1932), Dapples (1942), D. G. Moore and Scruton (1957), McMaster (1967), Rhoads (1974), Rowe (1974), Powell (1974), Richards and Park (1976), Myers (1977a,b), Self and Jumars (1978), Lee and Swartz (1980), and Carney (1981). These papers relate the effects of benthic species to changes in grain size, sorting, fabric, water content, compaction, shear strength, and bottom stability. Those autecologic parameters that appear to be most highly correlated with physical modifications of sediments include: method of feeding, feeding selectivity, feeding level relative to the sediment—water interface, degree of mobility, organism size and population density, burrowing depth, and, if the organism is a tube dweller, the density, spacing, and length of tubes.

Towards a greater understanding of pattern, scale and process in marine benthic systems: a picture is worth a thousand worms

Historically, advances in our knowledge of benthic community structure and functioning have necessarily relied upon destructive sampling devices (grabs, cores, anchor dredges, etc.) that lose valuable contextual information in the process of sampling. In the last 40 years, instrumentation capable of measuring dynamic events and/or processes within and immediately above the seafloor has been developed that facilitates the collection of ecological information. Of these, both acoustic and optical imaging devices have played a significant role in revealing much about the physiology and behaviour of, and interactions between benthic species, and the sedimentary habitat in which they reside. While a number of reviews have separately considered the methodological and technical aspects of imaging technologies, the collective contribution that imaging has made to benthic ecology has received less attention. In this short review, we attempt to highlight key instances over the last 40 years where either acoustic or optical-based imaging techniques have provided new ecological insights and information about fine-grained sedimentary environments. In so doing, we focus on the ecological advances that have formed the precursor to current research efforts and introduce some of the latest revelations from appropriate and emerging imaging applications.

Macrobenthos and sedimentary facies on the Changjiang delta platform and adjacent continental shelf, East China Sea

Abstract The vertical distribution of macrobenthos and sedimentary structures in near-surface sediments of the East China Sea off the Changjiang has been assessed from analyses of boxcores. The geographic distribution of macrobenthos across the continental shelf reflects sediment type and sedimentation rate. In the rapidly accumulating silty-clays of the submarine delta, sparse populations of small polychaetes predominate. As sediment accumulation rate declines away from the river mouth, denser populations of more deeply burrowing polychaetes, crabs, and ophuroids occur. On the central and outer shelf, sparse populations of surface dwellers relect the sandy, mobile sediment conditions. Based on X-radiographs, five sedimentary facies are identified based on physical and biogenic sedimentary structures in the top 25 cm of sediment. These sedimentary fabrics reflect (1 and 2) high and low rates of accumulation on the submarine delta, in which physical and biogenic structures, respectively, predominate; (3) dynamic sands across the shelf; (4) a muddy, low-depositional environment south of Cheju Island; and (5) a shelf-break regime under the influence of the Kuroshio Current.

Rates of sediment reworking by Yoldia limatula in Buzzards Bay, Massachusetts, and Long Island Sound

ABSTRACT Yoldia limatula is a deposit-feeding pelecypod. In Buzzards Bay, Massachusetts, and Long Island Sound it comprises less than 10 percent (by number) of the total bottom fauna, yet is probably capable of entirely reworking the sediment. Yoldia reworks the sediment by ejecting it several centimeters into the water from its excurrent siphon. Measurements of hourly reworking rates were made under controlled temperature conditions. Extrapolations of yearly reworking rates were determined to be 5-6 liters per square meter per year in Buzzards Bay and 23-51 liters per square meter per year in Long Island Sound. This study suggests that great numbers of organisms may not be required to account for extensive sediment reworking.

SEAFLOOR STABILITY IN CENTRAL LONG ISLAND SOUND: Part I. Temporal Changes In Erodibility of Fine-Grained Sediment

Abstract: This preliminary study documents temporal changes in seafloor erodibility at a 14-meter-deep mud bottom station in central Long Island Sound over a 29-month period. The mean critical rolling and saltation velocities of the bottom were determined in the laboratory with a specially constructed flume containing salt water. A removable 45-cm length of the 10 cm by 10 cm closed flume channel was used as a box core. A removable bottom on the core box allowed scuba divers to obtain relatively undisturbed samples of the seafloor. Samples were returned to the laboratory in thermally insulated containers and critical erosion velocities were determined within a few hours. In 1974 and 1975, minimum annual mean rolling velocities were measured in July (16–19 cm sec -1 at z = 100 cm). Maximum mean rolling velocities were recorded in November 1974 and October 1975 (respectively 28 cm sec -1 and 23 cm sec -1 at z = 100 cm). The observed change in mean critical velocities in 1976 was different from that observed in 1974 and 1975. The minimum mean rolling velocity in 1976 was recorded in October (19 cm sec -1 at z = 100 cm) and the peak threshold velocity was measured in July (28 cm sec -1 at z = 100 cm). We propose tentatively that two opposing biogenic processes influence the observed changes in critical velocities; stabilization by sediment binding and destabilizaton by bioturbation. An estimate of the influence of microbial growth and mucus binding on bottom erodibility was obtained by culturing microorganisms on beds of glass beads of various sizes in the flume. Critical rolling velocities of glass beads increased 25% to 60% after 3–15 days related to mucus production and binding of the beads. Seafloor stabilization by sedentary polychaetes was also studied in the laboratory by comparing mean critical rolling velocities of natural sediment without tube-forming polychaetes with mean rolling velocities of the same sediment after introducing dense aggregations of Heteromastus filiformis. Critical rolling velocities increased by 80% over a period of 11 days.

The Paleoecological and Environmental Significance Of Trace Fossils

Trace fossils have great paleoecologic utility because they are (1) widespread in space and time, (2) found in place, and (3) largely the record of animal behavior and response, making them ideal indicators of environmental conditions.

SEAFLOOR STABILITY IN CENTRAL LONG ISLAND SOUND: Part II. Biological Interactions And Their Potential Importance for Seafloor Erodibility

: Temporal variation in the stability of the seafloor in central Long Island Sound can be qualitatively related to changes in water temperature, sediment surface organic content, sediment surface water content, and numbers of micro-, meio-, and macrofauna. Control of temporal changes in stability appears to result from biological processes which influence sediment stabilization and sediment destabilization. At station NWC, which is below photosynthetic compensation depth, polysaccharides produced by heterotrophic microorganisms bind the sediment. The reworking activities of meio- and macrofauna break up the bound layer of organic-mineral aggregates and cause destabilization of the sediment surface. The stabilization process probably reflects the combined effects of chemical, physical, and biological changes on the formation, stability, and decomposition of polysaccharides in the sediment. We propose that the binding potential of microbially produced polysaccharides is always present. Mucopolysaccharides produced by some meio- and macrobenthos may also be important to the binding process. Although rates of sediment reworking depend on the number of bioturbating macro- and meiofauna and temperature-related seasonal variations in activity, the processes which result in destabilization cannot be adequately explained by these two factors alone. Changes in metabolic activity of all size groups of organisms are related to temperature, but there appears to be a phase lag between temperature, input of organic matter, sediment binding, and bioturbation of the sediment by the benthos. Seasonal changes in biological activities and interactions may contribute to the temporal variations in seafloor stability through their influence on the chemistry of binding as well as the physical characteristics of the sediments. Changes in turbidity levels in the water overlying the bottom resulting from a cycle in seafloor stability may affect the seasonal compensation depth for planktic and benthic plants and probably controls the recruitment success of some groups of benthic larvae.

Growth of Bivalves at Deep-Sea Hydrothermal Vents Along the Galápagos Rift

Direct measurements of shell growth of an unclassified mussel from active hydrothermal vents along the Gal�pagos Rift reveal growth rates of approxmately 1 centimeter per year for mature specimens. The largest mussel collected (with shell length of 18.4 centimeters) was estimated to be 19 � 7 years old at the time of sampling. Recorded growth rates are among the highest documented for deep-sea species.

The Evolution of Infaunal Communities and Sedimentary Fabrics

Mobile infauna are prominent members of modern benthic communities inhabiting granular substrata. In settings below normal wavebase, the benthic fauna is dominated by infaunal deposit-feeders (McCall, 1977; Rhoads et al., 1978), and the substratum is characterized by bioturbated sedimentary fabrics (Moore and Scruton, 1957; Rhoads, 1974). Habitats subjected to wave reworking contain fewer deposit-feeders and retain physical sedimentary fabrics. The increase of biogenic sediment-reworking on an onshore-to-offshore gradient is widespread on modern marine shelves dominated by detrital clastics (Moore and Scruton, 1957; Reineck, 1967; Howard and Reineck, 1972, 1981) or carbonates (Ginsburg and James, 1974; James and Ginsburg, 1979) and has proven useful in reconstructing bathymetric gradients in ancient sedimentary basins (Byers, 1977; Byers and Larson, 1979).

Mechanical properties of the sediment—Water interface

Abstract The hardness of marine sediments in the near-shore waters of Long Island Sound has been measured by penetration tests made both in situ and in the laboratory. Penetration of the sediment—water interface is found to be rate-insensitive and, for a given driving stress, independent of indentor size. Natural sediments in Long Island Sound show no measurable surface cohesion because of the effects of benthic animals, though artificial, azoic sediments prepared in the laboratory do. Hardness of marine mud is found to be sensitive to sand content. Analysis of penetration experiments in terms of punching a rigid-plastic material cannot account for these observations, but a model based on a description of the sediment as a locking material is successful. The observations are applied to defining the bearing capacity of the bottom and its resistance to burrowing by marine animals.