Chapter 1: provides, I believe, a quasi-complete overview of the existing literature on the hyperbenthos. It thus serves as a general introduction to the work presented in this thesis. Studies are situated in space and time and the main structural characteristics of hyperbenthic communities are summarised. Quite some effort is paid to resolve the overriding semantic disagreement which seems to slow down progress in the field. Sampling methodology and the history of hyperbenthic research are reviewed. The role of hyperbenthic animals in marine food webs is briefly touched upon. Chapter 2: describes the hyperbenthic communities of the Voordelta. Twelve locations were sampled monthly at two depths between August 1988 and July 1989.' Three subareas were covered: the ebb-tidal deltas of the Oosterschelde and the Grevelingen, and the more seaward Banjaard area in between. High densities of a variety of animals were found to be present in the lower metre of the water column. Hyperbenthic community structure was strongly dominated by seasonal fluctuations due to the sequential appearance, high abundance and disappearance of the different species of temporary hyperbenthic species. In winter and early spring, when the community was dominated by its permanent residents, spatial patterns emerged. Averaged over the year these spatial patterns were consistent with the hydrodynamical regime in the system. The geomorphological changes, following the closure of the Grevelingen estuary, in its ebb-tidal delta seem to have created a sheltered area with low current velocities. The hydrodynamical properties of the area are favourable towards the sedimentation of silt and detritus and possibly concentrate fish eggs. Decapod larvae, probably from offshore origin, occur in high densities in the area and macrobenthic larvae find favourable conditions for settlement. This richness and a high primary productivity probably attracts mobile animals (e.g. mysids and fish). In comparison, the equally sheltered ebb-tidal delta of the Oosterschelde is much poorer. Most probably higher current velocities there do not allow sedimentation of silt and, detritus and are not conducive to the entrapment of neutrally buoyant animals. Chapter 3: gives the results of a pilot study on estuarine hyperbenthos conducted in December 1988. 41 stations in the Westerschelde and Oosterschelde estuaries, and in part of the neighbouring shallow coastal area, the Voordelta, were sampled quasi-simultaneously. Winter was chosen as the sampling period to avoid the presence of temporary hyperbenthic species. The dominant components of the hyperbenthic communities were identified. Their spatial distribution, the geographical variation in species composition, density and biomass were described and the observed patterns were related to some environmental variables. Using multivariate analysis all sites sampled could be grouped into 7 geographically coherent clusters corresponding to 7 species assemblages. Mysids dominated the winter hyperbenthos in all communities. Species distributions in the Westerschelde seemed to be primarily determined by a salinity-turbidity-dissolved oxygen gradient. Hyperbenthic animals, mainly the mysid Neomysis integer, reached high densities in the brackish part whereas the more seaward stations were characterized by lower densities but a higher number of species. In the benthic filter-feeder dominated Oosterschelde, the total density of the hyper benthos was very low. The shallow coastal area was characterized by intermediate densities. Chapter 4: describes the hyperbenthos of the Westerschelde, both in space and time. Samples were taken in spring, summer and winter of 1990 at 14 stations along the salinity gradient. Temporal patterns are further described from year cycles of monthly and forthnightly samples taken from April 1990 to December 1991. Mysids dominated the hyperbenthos in each season. Other important species, either permanently (e.g. amphipods and isopods) or temporarily (e.g. fish larvae and decapod larvae) hyperbenthic, belonged to a variety of faunistic groups. Spatial structure was stable through time: the estuary could be divided in the same geographically defined zones in each season. Each zone had a characteristic fauna. Throughout the year, the hyperbenthic community of the mouth region of the estuary was markedly different from that of the upstream brackish area, both in terms of density and species composition. Gradients in salinity, dissolved oxygen and turbidity correlate strongly with the observed variation in community structure. The spatial patterns dominate over the temporal patterns, especially in the brackish part of the estuary. In the marine part, seasonal differences in the communities were more pronounced due to the recruitment, maximal abundance and subsequent disappearance of a series of temporary hyperbenthic species. In the brackish zone seasonal patterns were less obvious. Still, spring was characterized by the presence of postlarval flounder and clupeoids, while other seasonal differences seemed to be mainly due to natural mortality and to migration of permanent hyperbenthic species in and out of the area. In each season, the upstream (brackish) communities were characterized by few species occurring in very high numbers, whereas the downstream (marine) communities were composed of many species but at lower densities. Chapter 5: gives a detailed account on the species of Mysidacea and the one species of Euphausiacea encountered in the preceding chapters. The hyperbenthic mysids and euphausiids of the shallow coastal waters and the Westerschelde were regularly sampled between 1988 and 1991 (see above). These data were supplemented with monthly samples from two salt marshes in the brackish part of the estuary. Eight mysid species and one euphausiid species were identified. The spatial and seasonal distribution patterns of all species were described. The most abundant species in the coastal area was Schistomysis spiritus, followed by Mesopodopsis slabberi, Schistomysis kervillei, and Gastrosaccus spinifer. In the marine part of the Westerschelde the same four species were dominant but densities were generally higher than in the coastal area. Only Siriella armata and the euphausiid Nyctiphanes couchi never entered the estuary. Highest mysid densities, mainly M. slabberi and the estuarine endemic Neomysis integer, were found in the brackish part of the Westerschelde. Praunus flexuosus is a euryhaline species with a preference for the intertidal areas. The estuarine populations of N. integer, M. slabberi, P. flexuosus, and - to a lesser extent - S. spiritus seem to utilise the salt marshes during periods of reproduction. Chapter 6: compares the hyperbenthos of the Westerschelde with that of two other, less polluted European estuaries: the Eems (north of the Westerschelde) and the Gironde (south of the Westerschelde). The three estuaries were sampled at regularly spaced stations covering the entire salinity gradient from marine conditions at the mouth to nearly freshwater conditions upstream within a 15 day period in summer 1991. The diversity of the samples and the distribution of the species along the main estuarine gradients were assessed. Again, hyperbenthic communities were identified using different multivariate statistical techniques. The species composition and the density and biomass of the dominant species of each community were compared among communities. Spatial patterns in density, biomass and diversity of the hyperbenthos were similar in the three estuaries: diversity was highest in the marine zone where density and biomass were lowest. Diversity decreased upstream and was lowest in the brackish part where density and biomass reached maximal values. In the Eems and the Gironde there was a slight increase in diversity towards the freshwater zone. Within each estuary two (Westerschelde) or three (Eems and Gironde) communities could be distinguished and their position along the unidirectional salinity-turbidity temperature gradient was similar: a marine community in the high salinity zone, a brackish water community in the middle reaches and a third community (absent in the Westerschelde) in the stations with the lowest salinities. Qualitative and quantitative differences in the corresponding hyperbenthic communities among estuaries were evident. Some species were restricted to one or two of the estuaries studied, while others, especially the abundant species in the brackish part, were common to all three. Still, these differences were marginal compared to the overriding similarity of the hyperbenthos in the three estuaries. The distribution of single species in the estuaries varied to some extent but the among estuary differences in density and biomass in comparable salinity zones rarely exceeded an order of magnitude. In the Westerschelde, the low salinity hyperbenthic community was completely absent. Upstream of the 10 psu isohaline the dissolved oxygen concentration dropped to a critical threshold value for hyperbenthic life. The populations of a number of species, which in Gironde and Eems reached highest density and biomass in this zone, seem to have (almost) disappeared from the Westerschelde (e.g. Gammarus zaddachi and Palaemon longirostris). Other brackish water species did not occur in their "normal" salinity range and their populations have shifted to higher, atypical salinity zones (e.g. Neomysis integer, Mesopodopsis slabberi, Pomatoschistus microps and Gammarus salinus). The addendum to this chapter concerns an oddity, namely the discovery of a hitherto unreported isopod species in the Gironde estuary. Synidotea laevidorsalis, new for the European fauna, is a nonendemic shallow-water species which was probably introduced into the Gironde from Asia. In the salinity range of 1 to 10 psu of the Gironde the species constitutes an important component of the brackish-water hyperbenthic community. Chapter 7: concerns the population dynamics of the key species of the Westerschelde hyperbenthos: the mysid Neomysis integer. The population was sampled on a fortnightly basis from November 1990 to December 1991. Density, biomass, population structure and brood size were recorded. The Bhattacharya method was applied to the length-frequency data for the detection and separation of cohorts. Growth is described both by a generalised von Bertalanffy function and by a von Bertalanffy function incorporating seasonal oscillations in growth. Secondary production was estimated for each cohort using four approaches. The seasonal pattern in density and biomass showed three peaks: a relatively small, yet distinct, peak in early March and two main peaks in late spring and in summer. Throughout winter, Neomysis density remained low. Three periods of increased reproductive activity and subsequent input of juveniles were found. This suggests that three cohorts were produced per year. The overwintering generation lived from autumn until the following spring. The spring generation was born in early spring and lived for about three months, while the summer generation lived from summer until early winter. The three cohorts showed marked differences in their biology. The overwintering generation showed seasonal growth oscillations, larger brood size, and a larger size at maturity. Individuals belonging to the other two cohorts generally grew faster, produced less young per female, and attained maturity at a smaller size. Within each cohort, both sexes exhibited different growth characteristics: the females generally lived longer, grew faster and consequently became larger than the males. The size-frequency, growth summation and removal summation methods yielded comparable production estimates. The annual production was 0.3 g AFDW m-² yr-1 with an annual P/B ratio of 6. The average cohort P/B was 3. The size-frequency method gave similar results only when applied to the three cohorts and to both sexes separately. The spring cohort accounted for almost half of the annual production. Despite the longer life span of the overwintering generation, it generated only a quarter of the annual production. An independent estimate of production using the mortality rate of the different cohorts, resulted in values comparable to those obtained by the other methods for the overwintering cohort, while the production of the other two cohorts was overestimated. The addendum to this chapter reports on intersexuality in the mysid Neomysis integer collected in the 3 estuaries mentioned above, supplemented with samples from the Elbe (Germany). Individuals which had an irregularly shaped or nearly symmetrically rounded, rather than a typically truncated, telson were also recorded from the four populations studied. A culture experiment with, damaged specimens revealed that every type of abnormal telson morphology found in the field can result from regeneration of damaged parts. It is concluded that both intersexuality and aberrant telson morphology are widespread phenomena among estuarine Neomysis populations. Both abnormalities were found to be rare in the other dominant mysid species in the study area.