David Thistle

Effects of bacterial exopolymer adhesion on the entrainment of sand

Abstract Flow velocity required to erode a bed of acid‐washed sand is increased by intergranular adhesion resulting from growth of the benthic marine bacterium Al‐teromonas atlantica. In general, we find that either pure exopolymer alone or exopolymer generated during in situ growth increases erosion resistance of fine quartz sand. Moreover, the degree of erosion resistance increases in proportion to the concentration of exopolymer‐component uronic acids, which in turn is dependent on relative nitrogen content of peptone‐based growth media. Specifically, we observe that approximately 100 nmol of exopolymer or 1.5 nmol of component uronic acids generated by in situ bacterial growth under nitrogen‐rich conditions per gram of dry sediment can effectively double seawater‐flume flow velocity required for initiation of transport of otherwise noncohesive, 125–177 μm quartz grains. This maximal effect corresponds to an estimated adhesive force that exceeds submerged particle weight by an order of magnitude and ex...

A deep-sea benthic community exposed to strong near-bottom currents on the Scotian Rise (western Atlantic)

Abstract This paper gives an overview of the structure of a benthic community at 4626 m depth on the Nova Scotian continental rise. Here, abundances of polychaetes, bivalves, isopods and tanaids are conspicuously high compared to those reported from comparable depths. Bacterial numbers and ATP concentrations are also high. We suspect that these anomalous abundances result from enhanced food availability caused by the strong near-bottom currents that flow through the area. The polychaete and bivalve faunas have few adults and high species dominance suggesting that currents create large-scale, sediment-transporting disturbances. The composition of the crustacean fauna supports this inference in that it is dominated by forms that can enter the seabed and thereby escape adverse conditions on the sediment surface.

Detecting two-dimensional spatial structure in biological data

SummaryCliff and Ord (1973) made versatile methods available for the direct utilization of location data in the analysis of dispersion patterns, but their monograph has as yet seen little use in the ecological literature. Application of their weighted forms of Gearys c and Morans I indices of spatial autocorrelation to some marine benthos data demonstrates a diversity of population structure not anticipated on the basis of more common measures of pattern. These indices provide objective means to evaluate numerous recent spatial models and hypotheses in geographical ecology and genetics. The procedures are particularly attractive because (1) they efficiently utilize data which are often wasted (i.e., sample coordinates), (2) their application puts few constraints on sampling designs which would otherwise be employed, and (3) they reveal and quantify pattern differences which are not obvious to the untrained eye.

The nematode fauna of a deep-sea site exposed to strong near-bottom currents

Abstract Nematodes are the most abundant metazoans in marine soft-bottom habitats, yet little is known of their role in deep-sea communities. We describe the nematode fauna from 4626 m depth in the North Atlantic (40°24.0′N, 63°07.4′W) at the family and genus level and compare our findings to previous reports on deep-sea nematodes. The results of covariance studies between nematodes and environmental parameters are also given. At the species level, there is no evidence of association between small-scale variation in nematode abundance and variation in abundance of bacteria or several classes of biogenous structures, but there is evidence of an association with pebbles. At the functional-group level, feeding groups show no correlation with environmental variables, nor do groups based on tail morphology. However, four of the tail-morphology groups are significantly disproportionately abundant in either the 0 to 1 or the 1 to 2 cm sediment layer. Individuals of one group may use their long, retractable tails to escape into the sediment to avoid resuspension by noncatastrophic erosive flows, which are common at this hydrodynamically active deep-sea locality.

On the place of origin of deep-sea lsopods

Biogeographic and phylogenetic data on the deep-sea isopod family Ilyarachnidae (Paraselloidea) document its origin and evolutionary radiation in the deep sea. The distribution of eyes among paraselloidean families suggests an in situ evolution for all those families which have primarily deep-sea distributions. Present-day distributions of paraselloidean isopods provide no hints to their ultimate sources in shallow water. These findings contrast to recent suggestions in the literature that the deep-sea isopod fauna has been derived from shallow Antarctic waters.

The deep-sea isopods: A biogeographic and phylogenetic overview

Abstract Isopod crustaceans are among the most numerous and diverse macrofaunal taxa in the deep sea. In contrast to the situation in shallow water, the Asellota encompasses the majority of the deep-sea fauna. Taxa within the Asellota exhibit marked depth zonation. Its most primitive superfamily is limited to fresh water. The rest are marine, where with increasing depth the incidence of more specialized taxa increases. Within its most advanced superfamily, Janiroidea, the most primitive families are primarily restricted to shallow water, while the more specialized families dominate in the deep sea. The most notable exception is the occurrence of these specialized families in shallow water at high latitudes. On the basis of this pattern, it has been suggested that the deep-sea fauna is a result of invasion from shallow water, primarily at high latitudes. The present paper argues that, on the contrary, the specialized deep-sea families evolved in situ, and that their presence at shallow high latitudes is th...

The fauna of the HEBBLE site: patterns in standing stock and sediment-dynamic effects

Abstract At the High Energy Benthic Boundary Layer Experiment (HEBBLE) site at 4820 m depth on the Nova Scotia Rise (40°27′N, 62°20′W), abundances of polychaetes, bivalves, tanaids, and isopods were conspicuously greater than expected for the depth, whereas those of nematodes and harpacticoids, the two dominant meiofaunal taxa, were not. Further, the abundances of polychaetes, bivalves, tanaids, and nematodes were consistent over time and appeared to be unaffected by the periods of strong near-bottom flow that characterized the site. In contrast, isopods and harpacticoid copepods were less abundant in samples taken as a storm abated, apparently because they were eroded during benthic storms. We explore implications of the difference between macrofaunal and meiofaunal abundance for community structure. In addition, because a goal of the HEBBLE project was the parameterization of sediment transport in the deep sea, we summarize ways that the biota affects sediment transport at this high-energy site.

Response of a soft-bottom harpacticoid community to stingray (Dasyatis sabina) disturbance

The recolonization by harpacticoid copepods of patches disturbed by stingrays [Dasyatis sabina (LeSeur)] feeding in a subtidal (2–3 m depth) site off the Florida gulf coast (Lat. 29°54.55′N, Long. 84°31.45′W) was followed. Samples from disturbed patches could no longer be distinguished from control samples in terms of numbers of harpacticoids 29 h after disturbance. No species became significantly disproportionately abundant during the recovery suggesting that small-scale natural disturbances are not an important organizing force in this community.

Response of a harpacticoid copepod to a small-scale natural disturbance☆

Abstract Although several theories have been proposed to explain the manner in which natural disturbances participate in the organization of soft-bottom communities, they have seldom been tested. We examined the competitive-release hypothesis in a study of the response of the harpacticoid copepod Zausodes arenicolus Wilson to the disturbance created by the fecal mounds of the enteropneust Ptychodera bahamensis Spengel in a subtidal site in the northern Gulf of Mexico. Our approach involved field experiments, documenting the response of Zausodes to enteropneust fecal mounds and laboratory preference experiments designed to test the prediction that the reduction of competition in the disturbed patch caused the response. We found no evidence that competitive release caused Zausodess response to the disturbance.

An experimental investigation of enhanced harpacticoid (Copepoda) abundances around isolated seagrass shoots

SummaryAt a site in the Gulf of Mexico (29°54.6′N, 81°31.4′W) off the coast of northern Florida, harpacticoid copepod abundance is significantly enhanced around isolated “plants” (technically short shoots) of the seagrass Syringodium filiforme. Using inanimate mimics of seagrass short shoots, we demonstrate, in the field, that the enhanced abundance does not results from the presence of the plant as a living entity. Our experiments reveal a two-fold increase in bacterial biomass around both short shoots and mimics; the harpacticoids appear to be responding to a local increase in their resources. We suggest that the flow field around a short shoot improves the rate of supply of oxygen and other materials to sedimentary bacteria, thereby driving the effect. Given the ubiquity of structures that have similar flow effects, localized bacterial enhancement may be common and should be considered in studies of the effects of surface structures on soft-bottom community organization.