ALBERT

Notes: Abstract The day/night vertical distributions of mesozooplankton and micronekton biomass and that of a large number of copepod species to a depth of 1 000 m are described and contrasted in detail from two eastern tropical Pacific stations, the DOME station, in a region of continuous upwelling and the BIOSTAT station, in a non-upwelling area. The effects of various biological parameters such as the zones of primary production and chlorophyll maxima plus physical parameters, such as temperature, salinity and oxygen concentrations, on the distributions of the species and mesozooplankton and micronekton biomass are examined. The thermocline depth appeared to have the greatest influence of all the physical factors on the vertical distribution of the copepod species. The vertical distribution of many species was truncated in the region of the oxygen minimum layer with very few species found below this layer. The vertical distribution of copepod molts in the top 1 000 m showed that the molts probably originated at the depths at which they were found and that the molts from the euphotic zone did not reach the deep water. Copepod species association at different depths showed that few species had common depth distributions during both day and night. The highest degree of association occurred at night in the region of the thermocline. At depths below the thermocline, the degree of similarity in vertical distributions was greatly reduced. The biomass of mesozooplankton at these two stations is compared to other regions of the tropical oceans described in the literature and shown to be among the highest values recorded. The oxygen minimum layer and the thermocline had the greatest impact on species vertical distribution. The biomass of mesoplankton was about twice as high and the number of copepods between three and four times as high at the DOME site as compared to the BIOSTAT site, and evidence suggested that this was influenced by the higher primary production in this region. The difference in micronekton biomass between sites was less, with DOME having about 1.2 to 2.2 the biomass. This suggests that the influence of the higher level of primary productivity at the DOME was evidence of the higher trophic levels.

Notes: Horizontal samples taken with the BIONESS at various depths on 13 stations within 115 km of the coast of Peru showed nine species of fish larvae present with the most common of these being, in order of abundance, Leuroglossus stilbius, Sardinops sagax, Diogenichthys laternatus, Merluccius gayi and Engraulis ringens. All the species except S. sagax and E. ringens showed a diurnal migration to the upper 30 m of water at night. The two above species were present in the top 30 m at all times. The size of the larvae of all species did not vary with time, depth or sample location. The numbers of larvae increased with zooplankton m−3 up to 1000 copepods m−3 above which no further increase in larvae occurred. A significant linear relationship was found between the total number of larvae m−2 and biomass and zooplankton m−2, but not between individual species of larvae and zooplankton biomass. Significant correlations were found between E. ringens and seven species of copepods found in the upper 50 m of water.Polymodal analysis showed that the larval populations of all species, with the possible exception of S. sagax, were made up of more than one sub-population. From this analysis the length of the spawning season forE. ringens was estimated at 54–68 days.A significant correlation was found between the numbers of E. ringens larvae m−2 and the maximum concentration of chlorophyll α, as measured with Batfish, in the area of the BIONESS stations. This showed the largest numbers of larvae in areas of the highest chlorophyll concentrations.

Notes: Abstract Between March 23 and April 4, 1981, samples were taken in the eastern tropical Pacific. The day/night vertical distribution of euphausiid species and biomass are described and contrasted in detail on two eastern tropical Pacific stations, the DOME station, in a region of continuous upwelling and the BIOSTAT station, in a nonupwelling area. The effects of various biological parameters, such as temperature, salinity and oxygen concentrations on the distributions of the species are examined. The numbers of euphausiids m-2 on both stations were highest during the day, indicating that avoidance of the sampler was not a problem. During the day the largest concentration of adult euphausiids was between 300 and 350 m whereas the juveniles were concentrated between 170 and 80 m on both stations. Very few individuals were found within the oxygen minimum layer, but low concentrations of some species were found below the oxygen minimum down to 1 000 m. At night the euphausiid concentration migrated upward into the mixed layer (20 to 30 m) at BIOSTAT and to the base of the mixed layer at the DOME. Significant differences in the night depths of the species were found on both stations. The oxygen minimum layer appeared to act as a barrier to the vertical distribution of all species. Only two species were found in water with an oxygen concentration of 〈0.1 ml O2 l-1. Twentyone species of euphausiids were found on the two stations but the adult population was dominated by only two or three species on both stations. The reproductive state of the species suggested that some species reproduced earlier on the DOME than on BIOSTAT. Analysis of the depth distribution by cluster analysis showed that the most abundant species occupied different depths during the night and day at BIOSTAT but the two most abundant species were concentrated at the same depth at the DOME station although portions of each species population occupied different pelagic zones.

Notes: Summary Vertical distributions of various species and stages of zooplankton at different times of the day were determined by stratified sampling with the BIONESS in northeastern Baffin Bay during early August. The water column was divided into an upper subarctic zone (〉0° C, salinity 〈 32‰), a lower subarctic zone (〈 0° C, salinity 32‰ to 34‰) and a deep zone of Atlantic water (〉0° C, salinity 34‰). The upper subarctic zone was dominated by two species of pteropod molluscs; the lower subarctic water was dominated by the copepods, Calanus finmarchicus, C. glacialis and C. hyperboreus whose copepodite stages showed depth distributions that were different from one another, with the copepodite stage 5 and adult females generally shallower than the younger stages. All stages of all Calanus species were in the zone of primary production (10 to 50 m) while the copepods Pseudocalanus, Metridia and Oithona were generally found below this zone. Only C. finmarchicus and C. glacialis showed evidence of diurnal migration, migrating to the surface waters when the sun was at its lowest position on the horizon (i.e. at 0100 h).

Notes: Summary Primary productivity in Lancaster Sound was estimated from chlorophyll and light irradiance profiles and light saturation measurments. Mesozooplankton concentrations were measured using an in situ zooplankton counter mounted on a Batfish and with the BIONESS. Microzooplankton concentrations were estimated from samples taken with the BIONESS. The horizontal and vertical distribution of the two groups of zooplankton are described and related to the distribution of chlorophyll primary production and physical features of the Sound. A westward intrusion of Baffin Bay water along the north shore of the sound which mixed with Arctic water flowing easterly resulted in a complex physical environment in the sound. The admixing of these water masses resulted in a highly variable biological structure. The thermocline was the main physical feature that affected the depth of the chlorophyll layer and the levels of primary production, with a shallow thermocline resulting in higher primary production. The admixing of Baffin Bay water with Arctic water at different areas of the sound combined with the different times of appearance of open water resulted in the populations of Calanus hyperboreus and C. glacialis showing different age structures, the youngest being in the Arctic water at the western end of the Sound. Microzooplankton populations were concentrated at various depths depending on the species. Oithona juveniles preferred the upper 10 m when the thermocline was near the surface. Most macrozooplankton were found below the thermocline and the primary production maxium. The largest concentrations of microzooplankton were found at the western end of the Sound in the Arctic water believed to be still in the biological spring.

Notes: Summary The myctophid, Benthosema glaciale, was found to occur in the region of Davis Strait, but not in northern Baffin Bay. The factors that limited its northern range were believed to be the cold temperature and the long periods of day light in Baffin Bay. During the day B. glaciale was found at a depth of between 300 and 900 m with the population concentrating at about 500 m. At night about 46% of the population migrated to the upper 65 m to feed on copepods, primarily Calanus finmarchicus stage V and C. hyperboreus stage IV for which they showed a high degree of selectivity. The weight of stomach contents of fish with 100% full stomachs was equal to about 3% of their body weight. Feeding occurred mainly at night in the upper layers above the coldest layer of water with a temperature of about 0.5 °C. The biomass of zooplankton and myctophids m-2 were similar to those for these groups on the Nova Scotia slope during June. The fish fed on the same size range of prey in Davis Strait as on the Nova Scotia slope even though the zooplankton species composition was different in the two regions.