Recently developed techniques for estimating bacterial biomass and productivity indicate that bacterial biomass in the sea is related to phytoplankton concentration and that bacteria utilise 10 to 50 % of carbon fixed by photosynthesis. Evidence is presented to suggest that numbers of free bacteria are controlled by nanoplankton~c heterotrophic flagellates which are ubiquitous in the marine water column. The flagellates in turn are preyed upon by microzooplankton. Heterotrophic flagellates and microzooplankton cover the same size range as the phytoplankton, thus providing the means for returning some energy from the 'microbial loop' to the conventional planktonic food chain.

Species abundance distributions (SADs) follow one of ecologyÕs oldest and most universal laws -every community shows a hollow curve or hyperbolic shape on a histogram with many rare species and just a few common species. Here, we review theoretical, empirical and statistical developments in the study of SADs. Several key points emerge. (i) Literally dozens of models have been proposed to explain the hollow curve. Unfortunately, very few models are ever rejected, primarily because few theories make any predictions beyond the hollow-curve SAD itself. (ii) Interesting work has been performed both empirically and theoretically, which goes beyond the hollow-curve prediction to provide a rich variety of information about how SADs behave. These include the study of SADs along environmental gradients and theories that integrate SADs with other biodiversity patterns. Central to this body of work is an effort to move beyond treating the SAD in isolation and to integrate the SAD into its ecological context to enable making many predictions. (iii) Moving forward will entail understanding how sampling and scale affect SADs and developing statistical tools for describing and comparing SADs. We are optimistic that SADs can provide significant insights into basic and applied ecological science.

Eutrophication is one of the most severe and widespread forms of disturbance affecting coastal marine systems. Whilst there are general models of effects on benthos, such as the PearsonRosenberg (P-R) model, the models are descriptive rather than predictive. Here we first review the process of increased organic matter production and the ensuing sedimentation to the seafloor. It is shown that there is no simple relationship between nutrient inputs and the vertical flux of particulate organic matter (POM). In particular, episodic hydrographic events are thought to be the key factor leading to high rates of sedimentation and accompanying hypoxia. We extend an earlier review of effects of hypoxia to include organisms living in the water column. In general, fishes are more sensitive to hypoxia than crustaceans and echinoderms, which in turn are more sensitive than annelids, whilst molluscs are the least sensitive. Growth is affected at oxygen concentrations between 6.0 and 4.5 mg O 2 l -1

Summary 1.One of the general characteristics of ecological communities is that the number of species accumulates with increasing area sampled. However, it is important to distinguish between the species-area relationship and species accumulation curves. The species-area relationship is concerned with the number of species in areas of different size irrespective of the identity of the species within the areas, whereas the species accumulation curve is concerned with accumulation rates of new species over the sampled area and depends on species identity. 2. We derive an exact analytical expression for the expectance and variance of the speciesaccumulation curve in all random subsets of samples from a given area. The analytical species accumulation curve may be approximated by a semilog curve. Both the exact accumulation curve and its semilog approximation are independent of the underlying species abundance distributions, but are influenced strongly by the distribution of species among the samples and the spatial relationship of the samples that are randomized. 3. To estimate species richness in larger areas than that sampled we take account of the spatial relationship between samples by dividing the sampled area into subareas. First a species-accumulation curve is obtained for randomized samples of all the single subareas. Then the species-accumulation curve for all combinations of two subareas is calculated and the procedure is repeated for all subareas. From these curves a new total species (T-S) curve is obtained from the terminal point of the subarea plots. The T-S curve can then be extrapolated to estimate the probable total number of species in the area studied. 4. Data from the Norwegian continental shelf show that extrapolation of the traditional species-accumulation curve gave a large underestimate of total species richness for the whole shelf compared with that predicted by the T-S curve. Application of nonparametric methods also gave large underestimates compared with actual data obtained from more extensive sampling than the data analysed here. Although marine soft sediments sampled in Hong Kong were not as variable as those from the Norwegian shelf, nevertheless here the new method also gave higher estimates of total richness than the traditional species-accumulation approaches. 5. Our data show that both the species-accumulation curve and the accompanying T-S curve apply to large heterogeneous areas varying in depth and sediment properties as well as a relatively small homogeneous area with small variation in depth and sediment properties.

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Reductions in number of species and diversity and increased dominance of opportunistic species occurred late in the sequence of response to oil as a stress factor (within 500 to 1000 m of discharge sources). However, multivariate analyses, (classification analysis using the Bray-Curtis dissimilarity index) and ordination (multi-dimensional scaling) clearly distinguished site groupings related to oil activities at distances of up to 2 to 3 km from the Ekofisk pollution source and up to 1.5 km from the Eldfisk source. The first recorded changes in benthic communities in response to oil were increased abundance patterns of some species and changes in the presence and absence patterns of rare species, with species being mostly present in one site group and mostly absent in another site group. Only under severe pollution did the opportunistic species, which have often been suggested as universal indicators of pollution, dominate. The major site groupings could still be distinguished after aggregation to higher taxa (families and even phyla) when using multivariate analyses. If this finding proves to be a general one then great savings in time and effort, with little or no loss of precision, wdl be possible in environmental monitoring.

Summary 1.We examined data on soft-sediment macrobenthos (organisms retained on a 1-mm sieve) from a transect of c. 1960 km along the Norwegian continental shelf (56-71°N), covering a range of water depths (65-434 m) and varying sediment properties. 2. A total of 809 species was recorded from 101 sites. Of these, 36% were restricted to one or two sites, and 29% were represented by one or two individuals. No species spanned the entire transect. Polychaetes were the dominant taxonomic group, followed by crustaceans, molluscs and echinoderms. 3. Alpha diversity (sample species richness) was highly variable (35-148 species) but showed no evidence of a relationship to latitude or other environmental variables. 4. Beta diversity was measured as Whittaker's β W , the number of shared species, complementarity (biotic distinctness) and Bray-Curtis similarity, and there was no evidence of a latitudinal trend on the shelf. Beta diversity increased with the level of environmental variability, and was highest in the southern-central area, followed by the most northern area. Change in environmental variables had a stronger effect on beta diversity than spatial distance between sites. 5. Gamma diversity was computed by pooling samples over large areas. There was no convincing evidence of a latitudinal cline in gamma diversity, but gamma diversity increased with the level of environmental heterogeneity. Mean alpha diversity and gamma diversity were not significantly correlated. Whereas mean complementarity and mean Bray-Curtis similarity were related to gamma diversity, β W was not.

Marine soft sediments comprise one of the largest and oldest habitats in the world, yet remarkably little is known about patterns of species richness. Here I present a short review of patterns of species richness and possible factors that influence such patterns. Species richness in general is remarkably high in both shallow coastal areas and the deep sea. However, there are clear differences -the deep-sea has higher number of species for a given number of individuals than the coast. This can be explained by the larger amounts of primary production that reach coastal compared with deep-sea sediments, leading to higher numbers of individuals per unit area. Species density (the number of species per unit area) is also higher in the deep-sea than in coastal areas, but it is not obvious why this is so. Most studies of the broad patterns of species richness have used samples taken at small scales only. The problem with such analyses is that unless a large number of samples are taken, the true underlying pattern (or lack of it) may be wrongly interpreted. Recent studies have analysed species richness at larger scales. In general there seems to be a cline of increasing species richness from the Arctic to the tropics, but this is not the case in the southern hemisphere, where Antarctic species richness is high. However, it is not known whether high species richness in the Antarctic occurs at all spatial scales. To what extent these patterns are determined by evolutionary factors remains to be determined by the application of molecular methods. The available evidence suggests that environmental factors such as productivity, temperature, and sediment grain-size diversity play dominant roles in determining patterns of regional-scale species richness and patterns in species turnover, and it is probably the regional scale that primarily determines local species richness.