DISTRIBUTION OF THE NEW INVADER MNEMIOPSIS SP AND THE RESIDENT AURELIA-AURITA AND PLEUROBRACHIA-PILEUS POPULATIONS IN THE BLACK-SEA IN THE YEARS 1991-1993

Since the outburst of Mnemiopsis sp. in 1988-1990 in the Black Sea, two international cruises in June 1991 and in July 1992, and a subsequent survey in August 1993, have determined the distributions of both the invading Mnemiopsis sp. and the resident Aurelia aurita and Pleurobrachia pileus. Aurelia aurita and Pleurobrachia pileus displayed the lowest and the highest population densities of 3-14 individuals m(-2) and 172-523 individuals m(-2), respectively. The abundances of Mnemiopsis sp. varied from 12-45 individuals m(-2). The lower ends of the ranges were found in June 1991, whilst the highest abundances occurred in July 1992 and August 1993. Despite the differences in abundance, the average biomass of all three species was about 200 g m(-2) wet weight in July 1992 and August 1993, though in June 1991 biomass values were approximately half those found in succeeding years. The abundances and biomass of Mnemiopsis sp. were more than three times higher in the eastern than in the western Black Sea during 1992 and 1993. The biomass of Mnemiopsis sp. and Aurelia aurita decreased in 1992 and 1993 compared to its value in the 1980s, but the biomass of the deep-dwelling Pleurobrachia pileus doubled between 1990/1991 and 1993

DISTRIBUTION OF ANCHOVY EGGS AND LARVAE (ENGRAULIS-ENCRASICOLUS CUV) IN THE BLACK-SEA IN 1991-1992

Two international surveys of anchovy eggs and larvae were carried out in the Black Sea at the beginning of the spawning season in June 1991 and during the main spawning period in July 1992. Horizontal tows demonstrated that the bulk of anchovy eggs acid larvae were distributed in the upper 3-m layer, but in downwelling areas eggs and larvae were found down to 70 m depth. In contrast to earlier studies, vertical hauls obtained during the present investigation contained higher egg numbers in the southern and particularly south-eastern Black Sea than in the north-western region, which is known as the main spawning area of anchovy. Long-term sampling by the Institute of Biology of the Southern Seas, Sevastopol, shows that, in the northern Black Sea, the number of anchovy eggs and larvae found since the mid-1980s is lower than in the early 1960s. The sudden decline of anchovy ichthyoplankton in 1989, coinciding with the outburst of the recently introduced Mnemiopsis sp. (Ctenophora), supports the hypothesis that this gelatinous zooplankton species has played a role in diminishing the Black Sea anchovy fisheries, although the drastic changes in the Black Sea ecosystem (due to pollution, eutrophication, and heavy fishing) have also had an effect

Meiobenthos of the Oxic/Anoxic Interface in the Southwestern Region of the Black Sea: Abundance and Taxonomic Composition

The Black Sea contains the World’s largest body of anoxic water. Based on new and published data, we describe trends among selected protozoan and metazoan meiofaunal taxa at water depths of 120–240 m in the northwestern part of the Black Sea near the submarine Dnieper Canyon. This transect spans the transition between increasingly hypoxic but non-sulfidic bottom water and the deeper anoxic/sulfidic zone, the boundary between these two domains being located at approximately 150–180 m depth. This transition zone supports a rich rose-Bengal-stained fauna. Among the protozoans, gromiids are common only at 120 and 130 m. All other groups exhibit more or less distinct abundance maxima near the base of the hypoxic zone. Foraminifera peak sharply at ?160 m while ciliates are most abundant at 120, 160–190, and 240 m, where they are possibly associated with concentrations of bacterial cells. The three most abundant metazoan taxa also exhibit maxima in the hypoxic zone, the nematodes and polychaetes at 160 m, and the harpacticoid copepods at 150 m. Most of the polychaetes belong to two species, Protodrilus sp. and Vigtorniella zaikai, the larvae of which are widely distributed in severely hypoxic water just above the anoxic/sulfidic zone of the Black Sea. Both protozoans and metazoans are usually concentrated in the 0–1 cm layer of the sediment, except at the shallowest (120–130 m) site where deeper layers may yield a substantial proportion of the assemblage. The concentration of nematodes in the 3–5 cm layer at 120 m is particularly notable. Our data suggest that some benthic species can tolerate anoxic/sulfidic conditions in the Black Sea. An important caveat is that anoxia or severe hypoxia may lead to the corpses of nonindigenous organisms being preserved in our samples. However, we argue that the morphological integrity of specimens, the high population densities (associated with high bacterial concentrations in the case of ciliates), the presence of taxa often found in hypoxic settings, and the presence of all life stages (including gravid females) among nematodes and harpacticoids, suggests that at least some of the organisms are indigenous. Further comparative studies of shallow- and deep-water meiobenthic communities in the Black Sea are necessary in order to establish which species are characteristic and indicative of hypoxic/anoxic conditions

Evidence of Diel Vertical Migration in Mnemiopsis leidyi

The vertical distribution and migration of plankton organisms may have a large impact on their horizontal dispersal and distribution, and consequently on trophic interactions. In this study we used video-net profiling to describe the fine scale vertical distribution of Mnemiopsis leidyi in the Kattegat and Baltic Proper. Potential diel vertical migration was also investigated by frequent filming during a 24-hour cycle at two contrasting locations with respect to salinity stratification. The video profiles revealed a pronounced diel vertical migration at one of the locations. However, only the small and medium size classes migrated, on average 0.85 m h(-1), corresponding to a total migration distance of 10 m during 12 h. Larger individuals (with well developed lobes, approx. >27 mm) stay on average in the same depth interval at all times. Biophysical data suggest that migrating individuals likely responded to light, and avoided irradiance levels higher than approx. 10 mu mol quanta m(-2) s(-1). We suggest that strong stratification caused by low surface salinity seemed to prohibit vertical migration