Marine Benthic Assemblages Research Articles

Effects of Copper Contamination on Recolonisation of Subtidal Marine Soft Sediments – an Experimental Field Study

Recolonisation patterns of soft-bottom macrofauna on defaunated Cu-spiked sediments were studied in a field experiment at 63 m depth in the Oslofjord, Norway. Field experiments studying colonisation patterns of contaminated sediments are ecologically relevant since settling and metamorphosis of pelagic larvae are generally considered the most critical phases in the development of marine benthic assemblages. Total faunal abundance and the density of the polychaetes Pectinaria koreni and Prionospio cirrifera decreased significantly at sediment Cu-concentrations of 300 mg kg −1. Also the polychaetes Pseudopolydora paucibranchiata, Capitella capitata, Chaetozone setosa, Harmothoe spp., the bivalve Thyasira sarsi and the brittlestar Ophiura affinis were significantly negative correlated to increased sediment Cu-content. Although these species were affected negatively, most of the 116 taxa within the experimental trays showed no response over the entire range of Cu sediment concentrations. Most field studies on recolonisation have been performed in the intertidal or shallow subtidal. These environments are often dominated by robust organisms while animals living in deeper waters are more sensitive to disturbance. The experimental system developed could easily be used at depths between 20 and 100 m to study effects of toxic compounds separately or as mixtures, or colonisation and succession processes of uncontaminated sediments in general.

Time-averaging and postmortem skeletal survival in benthic fossil assemblages: quantitative comparisons among Holocene environments

We used radiocarbon ages on dead Holocene shells of the venerid bivalveChionespp. to investigate how time-averaging and taphonomy in shallow marine benthic assemblages vary with sedimentary and tectonic setting. We compared shells collected from the sediment surface in five depositional environments from two regions of the Gulf of California, Mexico: Bahía Concepción, a young faulted rift basin with high rates of terrigenous and carbonate sedimentation; and Bahía la Choya, an intertidal system along a sediment-starved shelf. Frequency distributions of shell ages in all environments form a hollow curve, with a mode at young ages and a long tail toward older ages. This pattern suggests that shells are added to the taphonomically active zone (TAZ) at roughly constant rates (via continuous shell deaths), and removed from the TAZ at random, either through destruction or by achieving final burial. Shell half-lives (the amount of time to remove half the shells from the TAZ) provide a comparative measure of time-averaging. Time-averaging varies with sedimentary and tectonic setting. The lowest amounts of time-averaging (shell half-lives of 90 to 165 years) occur in Bahía Concepción, where rapid rates of terrigenous sedimentation (on fan-deltas) and carbonate sedimentation (in pocket bays) bury shells rapidly. Time-averaging is higher in the sediment-starved environments of Bahía la Choya (shell half-lives of 285 to 550 years). The highest amounts of time-averaging occur the inner tidal flats of Bahía la Choya (shell half-life of 550 years). Here the conjunction of low sedimentation rates with low rates of shell destruction (due to periodic tidal emergence) permits shells to persist in the TAZ for very long time spans.There is no systematic relationship between a shell's age and its taphonomic condition (taphonomic grade) in any environment, probably because of the complex and random nature of burial-exhumation in the TAZ. Agevariancetends to increase with increasing taphonomic alteration: highly altered shells range in age from young to several thousand years old, while less altered shells are mostly young. The correspondence between time-averaging and the taphonomic condition of entire shell assemblages is also weak, but might be resolved with further study.These results provide quantitative data on time-averaging in benthic assemblages as a function of sedimentary and tectonic setting, and suggest some guidelines for facies appropriate for particular studies. Shallow marine rift basins like Bahía Concepción can potentially contain within-horizon fossil assemblages representing time spans of only a few hundred years—time resolution often beyond reach in paleontology. In contrast, sediment-starved shelf habitats like Bahía la Choya are unlikely to yield assemblages with time resolution finer than several thousands of years.

Relationships between body-size, species abundance and diversity in marine benthic assemblages: facts or artefacts?

Plots of the number of species against body size for a wide range of undisturbed macrobenthic assemblages on the European continental shelf have an approximately lognormal distribution, i.e. there are more species of intermediate size than of either larger or smaller sizes. The total abundance of all macrobenthic species combined also peaks at an intermediate size. The abundance of individual species in relation to their average body size has a pattern similar to that observed for terrestrial guilds of animals; high abundances are found in some intermediate sized species but do not tend to be found in small or large sized species. This does not appear to be a purely statistical consequence of the greater number of intermediate sized species in the assemblage. We show (as these plots imply) that for a given number of individuals there are fewer species of intermediate size but the possibility of sampling artefacts cannot be discounted here. Plots of the number of species per 100 individuals against body-size are J-shaped, the minimum of the curve corresponding to the size of animals for which retention on a 0.5 or 1 mm mesh sieve starts to become inefficient and juveniles will pass through while adults are retained. Thus the real number of species per 100 individuals at these small sizes will tend to be lower than the sample data suggest. At larger body sizes there is a more or less linear increase in the curve (on a log/log scale). This would be an inevitable consequence, in a sample of fixed areal cover (e.g. a 0.1 m 2 grab), of patterns of spatial dispersion scaling with organism size. This problem is of fundamental importance for the sampling of biodiversity and could be examined by scale-stratified sampling and model simulations.

Detection of pollution-induced disturbance in marine benthic assemblages with special reference to nematodes

Detection of pollution-induced disturbance in marine benthic assemblages with special reference to nematodes