Vertical distribution and nocturnal migration of Nyctiphanes couchi (Crustacea: Euphausiacea) in relation to the summer thermocline in the Celtic Sea

Vertical distributions and nocturnal migrations of the developmental stages of Nyctiphanes couchi (Bell) in relation to the summer thermocline in the Celtic Sea, 25 to 26 August 1982, have been investigated using the Longhurst-Hardy Plankton Recorder (LHPR). The vertical distributions of the metanauplii and adult females suggest that N. couchi liberates its young within the euphotic zone as mature metanauplii which, in a matter of hours, moult into the first feeding stage (Calyptopis I). The ascent migration by adult females took a maximum of 3 h (17.10 to 20.05 hrs) and had an amplitude of 50m (54 to 4 m) from below to above the thermocline. A 7C° thermocline occurred between 20 to 30 m in these profiles. The nocturnal migrations by the females were for the purpose of breeding as well as feeding within the euphotic zone and were not influenced by the presence of the thermocline. The majority of the calyptopes and furciliae remained above the thermocline over the sampling period. The post-larval males and females migrated; their vertical distributions showed a pattern similar to those of the adult females. The larger the developmental stage, the deeper was the mode of its vertical distribution. The zooplankton dry weight in the profiles ranged from 3.74 to 6.91 g per haul (=1.85 to 3.45 g C m^-2, 0 to 100 m). The euphausiids represented 35% of total zooplankton dry weight and their migrations removed a large percentage of the total zooplankton biomass from the euphotic zone for 18 h d^-1. Such a large displacement of biomass would have a major impact on the biological interactions within the ecosystem.     

Functional feeding morphology of the euphausiid Nyctiphanes australis

The structure of the feeding basket, mandible and stomach armature of the krill Nyctiphanes australis (G. O. Sars, 1883) was examined by scanning electron microscope with the aim of predicting its diet. N. australis were collected during February 1986 and October 1986 to February 1987 in Otago Harbour, New Zealand (45° 50 S; 170° 37 E). Predictions based on the functional morphology were tested by examining stomach contents with SEM. Intersetule distances of the feeding basket (1 to 7.5 m) are finer than in other krill species, suggesting that N. australis can efficiently collect nanoplankton-sized particles (2 to 20 m). The mandibular edge index (0.74) matched the edge index of Meganyctiphanes norvegica. This indicates, in contrast to the fine feeding-basket setulation, that N. australis has a mandible resembling that of predominantly carnivorous krill species. The ratio of mandibular palp length to mandible width is 3.2±0.2, or relatively longer than the elongate palp thought to be indicative of herbivorous habit in Euphausia superba. The fine structure of the molar shows specialized surfaces which differ from those of other krill species. The internal armature of the stomach is heavily spinose, as is common in krill of herbivorous habit. Layers of various-shaped spines at differing densities were observed. The functional morphology suggests that N. australis is an opportunistic omnivore. The stomach contents tentatively support this prediction, containing fragments of phytoplankton and detrital material. However, stomach contents were generally amorphous, making identification of dietary components in the field difficult.     

Recherches sur la situation trophique d'un groupe d'organismes pélagiques (Euphausiacea). VI. Conclusions sur le rôle des euphausiacés dans les circuits trophiques de l'océan Pacifique intertropical

The role of euphausiids in the food webs of the Intertropical Pacific Ocean is defined through analysis of their nutrition, vertical distributions and migrations, and their utilization by pelagic predators. It is suggested that the abundance of the group, the extensive vertical migrations of many species and the fact that feeding takes place mainly in subsurface layers, result in a leading role of euphausiids in energy transfer between different bathymetric levels. For night-time feeding predators, they represent a noticeable food source only in the 0 to 300 m water layer, as 97% of the euphausiid biomass concentrates in this layer at night. In the daytime, only the smaller specimens (chiefly genus Stylocheiron), accounting for 10 to 15% of the whole biomass of the group, remain available for epipelagic (0 to 400 m) predators, larger individuals dwelling deeper. Euphausiids account for 8 to 10% of the food ingested by micronektonic fishes, but the species are not the same for different categories of fishes. Migrating fishes caught by pelagic trawls, more or less connected with the deep scattering layer, feed on migrating species in subsurface layers at night as well as in deeper layers during the daytime, and on non-migrating species inhabiting shallower and intermediate layers. On the other hand, fishes which comprise the prey of large long-line tunas, which are not caught by trawls because they are fast swimmers, feed almost solely on species which remain above 400 m in the daytime. These results suggest a certain degree of independance between the trophic webs which concern, on the one hand, epipelagic ichthyofauna (including tuna), and, on the other hand, migrating and deep-living faunas. Migrating populations are able to feed at night upon subsurface organisms, a part of this resource being then transmitted during the day to the deep-living fauna; but the epipelagic ichthyofauna, with a feeding activity restricted to light hours, has few possibilities to benefit from the migrating or deepliving biomass. Therefore, energy transfers seem to be intense only from subsurface (0 to 400 m) to deeper layers. From a more general point of view, these investigations suggest that, in the pelagic system, vertical distributions and migrations, and feeding rhythms, are the main factors determining the structure of the food webs.     

Direct and indirect trophic interactions of <Emphasis Type="Italic">Aurelia </Emphasis>sp. (Scyphozoa) in a stratified marine environment (Mljet Lakes,Adriatic Sea)

The pattern of diel vertical migration and the trophic interactions of moon jelly (Aurelia sp.) were investigated in the sea lakes of Mljet Island (Adriatic Sea) where this scyphomedusa is present throughout the year. Water column characteristics, plankton and in situ behaviour of Aurelia were followed over several 24-h cycles (6–8 times during each cycle) from the surface to the bottom (44 m). Aurelia exhibited a consistent pattern of diel vertical migration. Most of the time Aurelia were located at the bottom of the thermocline layer at temperatures lower than 19°C. Aurelia migrated towards the surface at dusk when the majority was found within the thermocline or just above it. During the night the medusae sank into the deepest layers below 25 m. The main medusa food items inferred from stomach contents were small adult copepods like Oithona nana and Paracalanus parvus and copepodites of small calanoids and cyclopids. In addition, in situ feeding experiments indicated high clearance rates for nauplii and naked ciliates and clear response of bacterial populations pointing to indirect cascade effects of Aurelia on microbial in addition to classical food web.     

Diel vertical migration and feeding patterns of Mysis mixta (Crustacea,Mysidacea) in the Baltic Sea

The extent of the nocturnal vertical migration of Mysis mixta Lilljeborg varied between early July and late October (of 1985 and 1986) in a coastal area of the Baltic Sea. Migration was more restricted in early July and late October. Seasonal changes in surface light levels and transparency were sufficient to explain the observed differences. Mysids avoided light levels above 10^-4 lux throughout the study period. Smaller juveniles migrated higher up than larger juveniles and adults. A two-layered distribution with part of the population close to the bottom was observed at night. Zooplankton were more abundant in water layers above the main concentration of mysids. M. mixta fed on phytoplankton, detritus, copepods, cladocerans, rotifers and tintinnids. Diel changes in gut fluorescence indicated a higher intake of phytoplankton at night, but levels were low compared to primarily herbivorous zooplankton. Comparisons of stomach contents of mysids caught at the bottom in the evening and in the water column at night showed a higher ingestion of zooplankton at night and of detritus during the day. Mysids caught at the bottom at night had an intermediate diet. Copepods and cladocerans constituted between 90 and 100% of ingested material by weight in all mysid groups.     

Life-history parameters and vertical distribution of Maurolicus muelleri in Masfjorden in summer

Population characteristics, individual life-history variables, feeding and vertical distribution of the mesopelagic fish Müller's pearlside Maurolicus muelleri collected in 1990 in Masfjorden, western Norway, are reported as well as environmental variables from the fjord. Minimum size at maturity was far smaller than reported from previous investigations in the same region. Fecundity was size-dependent and total egg numbers were higher than reported from other investigations of M. muelleri world wide, while the number of maturing eggs was far lower than observed in the same region earlier. Food concentration in the fjord was an order of magnitude lower than previous early summer observations, and several factors indicated that feeding opportunities may have been low for a long period. Daily feeding rate (g prey g^-1 fish) decreased with increasing fish size. These observations fit well with a model of maximizing fitness by means of a flexible size at maturity. Minimum age at maturity seems to be achieved at the expense of fecundity. M. muelleri was concentrated in a 20 to 30 m deep sound scattering layer (SSL). The SSL stayed close to the surface during the night and at 100 to 180 m during the daytime. The vertical position of the SSL varied instantaneously with changes in surface light intensity, remaining at 10^-3 to 10^-4 mol m^-1 s^-1 at the top of the SSL. Stomach fullness was highest during the night; feeding intensity seems to have been peaked at dusk. Cladocerans were the main prey ranked by number, copepods by biomass. Intake of large copepods increased with fish size.     

Ontogenetic changes in the vertical distribution of giant scallop larvae,Placopecten magellanicus,in 9-m deep mesocosms as a function of light,food, and temperature stratification

To understand how thermal stratification and food abundance affects the vertical distribution of giant scallop larvae Placopecten magellanicus (Gmelin), a mesocosm study was conducted in January and February 1992. The position of larvae was followed over 55 d in replicated 9-m deep tanks in relation to a sharp thermocline and the presence or absence of phytoplankton. Growth and vertical position of larvae were monitored in separate treatments which included phytoplankton added above the thermocline, below the thermocline, throughout the mesocosm, or absent from the mesocosm. Changes in the vertical position of larvae over time were quantified with a new, profiling, video-optical instrument capable of semi-automatically identifying, counting and sizing larvae. The strong diurnal migration of scallop larvae resulted in aggregations at two interfaces: the air/water interface during the night, and at the thermocline during the day. At times, the concentration of larvae within cm of the surface was > 100 times that in the remaining water column. The formation of bioconvective cells of swimming larvae at the air/water interface allowed larval aggregations to persist throughout the period of darkness. Regardless of the distribution of food, larvae remained above the thermocline during most of the experiment. Therefore, only in those treatments where food was also present above the thermocline was larval growth relatively high. Larger larvae penetrated the thermocline only after reaching a shell length of about 200 m; thus larval size, rather than chronological age, was more important in describing their vertical distribution. The rapid increase in kinematic viscosity with decreasing water temperature at the thermocline may retard the movement of larvae and contribute to aggregation at this interface. The influence of larval size on their vertical distribution, and the resulting potential for horizontal transport to settlement sites, points to the importance of persistent hydrographic features as critical factors contributing to settlement variance in scallops.     

Omnivorous feeding behavior of the Antarctic krill Euphausia superba

Feeding experiments were conducted at Palmer Station from December 1985 to February 1986 to examine the potential role of copepod prey as an alternative food source for Euphausia superba. Copepod concentration, copepod size, phytoplankton concentration, the duration of krill starvation and the volume of experimental vessels were altered to determine effects on ingestion and clearance rates. Krill allowed to feed on phytoplankton and copepods in 50-litre tubs showed greatly increased feeding rates relative to animals feeding in the much smaller volumes of water traditionally used for krill-feeding studies. Clearance rates on copepods remained constant over the range of concentrations offered, but clearance rates on phytoplankton increased linearly with phytoplankton concentration. Feeding rates increased when larger copepods were offered and when krill were starved for two weeks prior to experiments. Clearance rates of krill feeding on copepods were higher than, but not correlated with, their clearance rates on phytoplankton in the same vessel. E. superba may have a distinct mechanism for capturing copepods, perhaps through mechanoreception. Although our observed clearance rate of 1055 ml krill^-1 h^-1 indicates that krill can feed very efficiently on copepod prey, such feeding would meet less than 10% of their minimum metabolic requirements at the typical copepod concentrations reported for Antarctic waters. However, substantial energy could be gained if krill fed on the patches of high copepod concentrations occasionally reported during the austral summer, or if krill and copepods were concentrated beneath the sea ice during the winter or spring months. Our results, indicating efficient feeding on zooplankton and higher clearance rates on phytoplankton than previously believed, represent a step towards balancing the energy budget of E. superba in Antarctic waters.     

Feeding patterns of Lampanyctus pusillus (Pisces: Myctophidae) throughout its ontogenetic development

The trophic ecology of the lanternfish Lampanyctus pusillus was investigated using individuals captured off the Balearic Islands (39°N, 2°E) (western Mediterranean) in December 2009. Based on gut content analyses, the trophic niche breadth, diet composition and selectivity were determined for the entire life cycle of L. pusillus. The larval stages fed actively near the surface during the day, with a feeding incidence (FI) of approximately 71 %. In contrast, the adults fed at night, both in near-surface depths and in the 400 m deep scattering layer, with a higher FI (83 %). Diet analysis revealed a shift in the prey choice throughout ontogenetic development, from preflexion individuals, which selected nauplii and small oncaeids, to postflexion larvae, which consumed a variety of calanoids, mainly Clausocalanus spp., to the adults, which preyed on large organisms, exhibiting positive selectivity for Pleuromamma spp. and euphausiids. These results show that the vertical distribution of larvae and adults is partly conditioned by their respective feeding habits, with larvae feeding on small zooplankton in the upper layer and adults preferring to consume larger taxa that perform nycthemeral migrations.     

Settlement of larvae of the giant scallop,Placopecten magellanicus,in 9-m deep mesocosms as a function of temperature stratification,depth, food,and substratum

In January and February 1992 an experiment was conducted in a 10.5-m deep tank (diameter: 3.7 m, volume: 117 m³) to examine the effects of food distribution with respect to a stable thermocline, depth, and substratum type on the settlement and metamorphosis of larvae of the giant scallop, Placopecten magellanicus (Gmelin). Polyethylene tube bags (diameter: 0.60 m) were used to enclose 9-m deep columns of seawater which were then used as treatment replicates. A sharp thermocline (i.e. 7 to 11°C gradient) was created between a depth of 4.0 and 5.0 m. At the beginning of the experiment, one million 6-d old larvae were added to the surface of each tube. Two or three replicate tubes of each of four feeding treatments were established: (1) food (Isochrysis galbana) added to the top 1 m of the water column (top-fed, n=3); (2) food added to the bottom 1 m of the water column (bottom-fed, n=3); (3) food added throughout the water column (mixed, n=3); and (4) no food added (unfed, n=2). Settlement collectors were placed in two replicate tubes of each treatment at depths of 0.1, 4, 5, and 9 m and contained two different substrata, Polysiphonia lanosa (a red filamentous alga) and aquarium filter-wool as an algal mimic. Spat settlement in the different feeding treatments was a function of larval growth rate. Most spat were collected in the mixed tubes. Fewer individuals were collected in the top-fed treatment and fewer still in the bottom-fed treatment; minimal numbers of spat were found in the unfed tubes. Filter-wool collected more spat than P. lanosa, but this was evident only in the 4-m deep collectors in the mixed tubes. Most spat were found in the 0.1-m or 4-m deep collectors; generally few were located below the thermocline in collectors at 5 or 9 m. We suggest that, in areas of intense stable stratification, spat collection of the giant scallop may be enhanced by the placement of collectors with appropriate substratum material at or above the zone of stratification, rather than near the bottom. Furthermore, we propose that natural settlement may be increased in areas where a stratification layer intersects with the sea floor or where the layer is disrupted by turbulent mixing.Contribution to the programs of OPEN (Ocean Production Enhancement Network, one of the 15 Networks of Centres of Excellence supported by the Government of Canada) and GIROQ (Group Interuniversitaire de Recherches Océanographiques du Québec)     

Trophic strategies of euphausiids in a low-latitude ecosystem

Vertical distribution, diet, and morphology of adults were examined in 27 species of euphausiids occurring in the upper 1000 m in the eastern Gulf of Mexico. Vertical distribution patterns were similar to those found in the central ocean gyres and oceanic equatorial waters of the Atlantic, Indian and Pacific Oceans. Most species migrated vertically from their daytime depths of 300 to 600 m to the upper 300 m at night. Exceptions were the non-migrating species of Stylocheiron, which remained in the epipelagic zone day and night, and Nematobrachion boopis, which remained in the mesopelagic zone. Based on gut-contents analysis, the Gulf euphausiids were largely zooplanktivorous, with cyclopoid and calanoid copepods being the most common items in stomachs. ostracods were especially common in the stomachs of Thysanopoda spp. and phytoplankton in the guts of Euphausia spp. Nearly every species' diet contained a considerable amount of olive-colored debris, which may have been marine snow generated in the epipelagic zone. Cluster analysis grouped the euphausiids into nine diet guilds. Euphausiids with a generalized morphology (i.e., spherical eyes, uniform thoracic appendages) tended to group together and demonstrated little variety in stomach contents among species. Euphausiids with a specialized morphology (i.e., bilobed eyes, elongate thoracic appendages) showed considerable variety in stomach contents among species, and several species had diets that were highly specific. Many of the species that had similar gut contents fed on prey of different sizes, as indicated by the width of the calanoid copepod mandibles found in stomachs. Principal-components analysis of seven morphological characters yielded species groups that were similar, but not identical, to those generated by cluster analysis of stomach contents data. We inferred from this that morphological characters partly determine diet, but that behavior is also important. Using the 20 most abundant species and 3 niche parameters, we attempted to identify the degree of separation among euphausiids based on the level of overlap in vertical distribution and diet composition, and on differences in mean prey size. Overlap of <60% in vertical distribution or diet composition was considered to indicate distinction of that parameter. Of 190 total species pairs, only 4 pairs did not demonstrate niche separation in at least one of these categories. We found that differences in these niche parameters were greatest among species with a specialized morphology and least among species that were morphologically generalized.     

Vertical distribution,and seasonal and diurnal migration of Calanus helgolandicus in the Celtic Sea

The vertical distribution and migration (seasonal, diel and ontogenetic) of Calanus helgolandicus are described from the shallow (100 m) shelf-seas to the south-west of the British Isles. In 1978 and 1979, the overwintering population of C. helgolandicus consisted primarily of Stage V copepodites and adults. By late winter/early spring the copepodites had moulted to adult females (>90%), which matured and bred the first cohorts of the year, prior to onset of the spring phytoplankton bloom in April/May. C. helgolandicus reached a peak of numerical abundance in August of 20x10³ copepodites m^-2 (over the depth range sampled -0 to 70 m), which was 200 times the population in winter. The seasonal peak of abundance occurred 4 mo after the peak of the bloom of phytoplankton in spring. The yearly development of the copepod was not always out of phase with the diatom bloom, as seen when the data from 1978 was placed in the context of a longer time-series collected at 10 m over 22 yr (1960–1981, inclusive). Large vertical migrations were observed in the younger copepodites (CI and II) in May from below to above the thermocline. In the remainder of the year, the CI and CII stages behaved differently and were located above the thermocline within the euphotic zone. The largest vertical displacements of biomass were seen in the summer months due to the migrations of the CV stages and adults, which had developed from the spring cohorts. It was contended that the seasonal and vertical migrations of C. helgolandicus are part of a more complex pattern of inherent behavior than has been reported previously and that, however difficult this is to discern in the natural populations, it always expresses itself.     

The feeding habits of three species of lanternfishes (family myctophidae) off oregon,USA

The common myctophids Stenobrachius leucopsarus, Diaphus theta and Tarletonbeania crenularis were found to feed primarily on the euphausiid Euphausia pacifica, the copepods Metridia lucens and Calanus spp., and the amphipod Parathemisto pacifica. The diets of these species of fishes were diverse and overlapped broadly, suggesting that they are feeding generalists. Most stomachs contained either all copepods or all euphausiids. Euphausiids were the most important prey on the basis of biomass. They comprised over one-half the weight of the stomach contents in over 40% of individuals of each of the three fishes. Stomach fullness and state of digestion of stomach contents differed over the diel period, but not enough to indicate feeding only during the nighttime. Average stomach fullness was greatest during the morning and night and well-digested material, which predominated in most stomachs, was most prevalent in the morning and afternoon. Either some feeding occurs throughout the day or digestion rates are slow.     

Ontogenetic diel vertical migration of the planktonic copepodCalanus sinicus in the Inland Sea of Japan

The ontogenetic diel vertical migration of the planktonic copepodCalanus sinicus was investigated in the Inland Sea of Japan in summer 1988, when the water was thermally stratified with a thermocline of ca 5 °C between 35 and 45 m. Stage-specific differences in the diel vertical migration behavior ofC. sinicus were found. Eggs were spawned primarily within the surface-waters between midnight and dawn by ascending females, and sank gradually to deeper waters until they hatched into nauplii. Non-feeding nauplius stages (NI and II) were distributed throughout the water column, but the first feeding stage (NIII) performed an ontogenetic upward migration. NIV to VI and copepodite (C) stages I to III continuously aggregated in the phytoplankton-rich euphotic layer. However, the depth of the median CI to III populations descended as stage progressed. The onset of prominent diel vertical migration took place in CIV, and the amplitude of vertical migration increased with age, being maximal in adult females (CVI). Adult males (CVI), however, remained in the layer below 20 m, and did not migrate dielly. The ecological significance of ontogenetic diel vertical migration is discussed.     

Observations on the diurnal vertical migrations of an oceanic animal community

Diurnal changes in abundance caused by vertical migrations have been examined in populations of copepods, ostracods, euphausiids, amphipods, decapods, chaetognaths, siphonophores and fish. The animals were taken in a series of hauls made over a 24 h period with an opening-closing midwater trawl system (RMT 1+8), consisting of a net of 1 m² mouth area combined in the same frame as one of 8 m² mouth area. The samples were taken at 250 m depth in a position 30°N; 23°W on 7/8 April 1972. The specific composition of the community and the numbers of individuals changed continuously with time. The numbers of fish, decapods and chaetognaths increased at night, but those of copepods, ostracods and euphausiids decreased. More species of fish, decapods and copepods were present by night than by day, whereas the numbers of species per haul for other groups remained fairly constant. The relative abundances of groups caught by the RMT 1 have been analysed, but similar treatment of the RMT 8 samples was impossible as only 3 groups were taken from this net. Non-migrants were a minority in every group except chaetognaths. Migrant species have been put into one of 6 transitory categories according to their patterns of abundance and hence migrations. Within each category, migratory behaviour varied both inter- and intraspecifically. The patterns of abundance of many species were smooth and continuous, suggesting slow migratory cycles of small amplitude. Conversely, extensive migrants had discontinuous patterns and presumably more rapid movements. Few migrants had a steady numerical plateau between their upward and downward migrations, and most apparently moved up or down continuously. The presence of migratory species in the sampled layer depended upon the time of day or night. It is concluded that, in a vertical series of hauls, the depths of occurrence of migrants will vary with the sampling time. Further-more, a vertical series will show a species minimum migration range but not necessarily its maximum. Individuals of some species were out of phase with the migrations of their main populations. There is evidence that the distributions and migrations of some species of decapods, euphausiids, copepods and fish could be related to the distribution of underwater light. Three pairs of congeneric copepod species were both spatially and temporally segregated for at least part of their diurnal cycles. Such an orderly arrangement could provide a means of reducing competition between species. Some species, however, overtook others on their migrations and the pattern of underwater light cannot, therefore, regulate the distribution of all species in the same way.     

Feeding ecology of the lanternfish Benthosema pterotum from the Indian Ocean

The feeding ecology of lanternfish Benthosema pterotum (Alcock) from the north Arabian Sea, Mozambique and the Bay of Bengal was studied. Samples were collected on cruises carried out by R.V. Dr. Fridtjof Nansen during the period 1978 to 1983. A wide variety of zooplankton organisms were identified in the diet of B. pterotum with crustanceans dominating the diet. Copepods constituted ca. 40 to 90% of the diet. Dry weight analyses of the stomach contents from the Gulf of Oman in February 1983 showed copepods to be 35 to 55% in weight (average in samples). Ontogenetic differences were observed in the diet. Prey size increased as the fish length increased, but the largest fish did not exclude the smaller prey organisms from their diet. Regional variation in diet was also shown in B. pterotum. The degree of filling and the state of digestion of stomach contents revealed that this species feeds intensively at night in the epipelagic layer. All copepods indentified were epipelagic species, providing additional evidence of diurnal pattern in the feeding chronology of B. pterotum. Identification of copepods from the Gulf of Oman in February 1983, revealed that herbivorous species dominated in biomass. Quantitative analyses show that B. pterotum probably have a daily food intake of ca. 4.5% of the body weight.     

Photosynthetic characteristics of nanoplankton and picoplankton from the surface mixed layer

Nanoplankton and picoplankton primary production has been studied at two oceanic stations in the Porcupine Sea-bight and at one shelf station in the Celtic Sea. At both sites, low wind conditions in June and July 1985 resulted in greatly reduced vertical turbulent mixing and a secondary, temporary thermocline developed in what is usually a well-mixed surface layer; as a result, there was physical separation of the phytoplankton within two zones of the surface mixed layer. The photosynthetic characteristics of three size fractions (>5 m, <5 to >1 m and <1 to >0.2 m) of phytoplankton populations from the two zones have been measured. Phytoplankton was more abundant at the oceanic stations and chlorophyll a values were between 1.3 and 2.2 mg chlorophyll a m^-3, compared with 0.3 to 0.6 mg chlorophyll a m^-3 at the shelf station; at both stations, numbers of cyanobacteria were slightly higher in the lower zone of the surface mixed layer. There was no effect of the temporary thermocline on the vertical profiles of primary production and most phtosynthesis occurred in the surface 10 m. Photosynthetic parameters of the three size fractions of phytoplankton have been determined; there was considerable day-to-day variation in the measured photosynthetic parameters. Assimilation number (P _m ^B ) of all >5 m phytoplankton was lower for the deeper than for the surface populations, but there was little change in initial slope (a ^B ). The small oceanic nanoplankton (<5 to >1 m) showed changes similar to the >5 m phytoplankton, but the same size fraction from the shelf station showed changes that were more like those shown by the picoplankton (<1 m) viz, little change in P _m ^B but an increase in a ^B with depth. Values of a ^B were generally greater for the picoplankton fraction than for the larger phytoplankton, but values of adaptation parameter (I _k )(=P _m ^B /) were not always less. There was little evidence to support the hypothesis that these populations of picoplankton were significantly more adapted to low light conditions than the larger phytoplankton cells. When photosynthetic parameters of the picoplankton were normalised to cell number (P _m ^C /a ^C ) rather than chlorophyll a, P _m ^C was comparable to other published data for picoplankton, but a ^C was much lower. The maximum doubling time of the picoplankton at saturating irradiance is calculated to be ca. 8.5 h for the oceanic population and ca. 6.2 h for the shelf population.     

Microbial activities as a function of water depth in the Ligurian Sea: an autoradiographic study

Fourteen species of sergestid shrimps were collected in the Sargasso Sea between the surface and 1500 m near Bermuda on 4 cruises. The vertical distribution and feeding activity of the most abundant species are discussed in relation to interspecific competition and the adaptive significance of vertical migration. Each species lives within a narrow depth range and exhibits a diel vertical migration. Sergestes splendens migrated as much as 825 m, while S. japonicus migrated less than 100 m. Neither the seasonal nor permanent thermocline influenced the migration range. The only species which occurred together both day and night were S. pectinatus with S. vigilax and S. pectinatus with S. sargassi. Morphological differences in the third maxillipeds of these species suggest differences in feeding. Although most species eat a variety of organisms, the foreguts of S. grandis, S. corniculum, and S. splendens contained euphausiids more often than those of other species, and S. grandis and S. robustus fed more frequently on fishes. In contrast, S. japonicus appears to feed on detritus. Food was found in the foreguts of most species less frequently during the day than night, but no species fed only at night. S. sargassi and S. pectinatus fed equally day and night.     

Vertical habitat-partitioning by copepodites and adults of subtropical oceanic copepods

Vertical distributions of the abundant larger copepods, both adults and late copepodites, were observed day and night in the upper 500 m of the North Pacific central gyre in early November, 1971. Densities of the copepodites usually equalled or exceeded those of the adults. Copepod species with maximum densities at or above 100 m (Calanus spp., Nannocalanus minor f. major, Undinula darwini, and Euchaeta rimana) usually had no ontogenetic or diel migration. Neocalanus spp. and Haloptilus longicornis exhibited ontogenetic but not diel migrations. Nannocalanus minor f. minor, Aetideus acutus, Euchaeta media, Scolecithrix spp. and Pleuromamma spp., had both ontogenetic and diel migrations. Adults and copepodites of E. media and Pleuromamma spp. usually had their night modes at the same depth, but the daytime modes were at progressively deeper depths for progressively older stages. Daytime modes for adults and copepodites of A. acutus and Scolecithrix bradyi were at the same depth, but the nighttime modes were at shallower depths for progressively older stages. Night modes of all these migrators were usually in the mixed layer (75 m), where primary production rates were maximal. Congeners usually had similar migratory behavior, but competition probably has been a significant determinant of vertical distribution, since congeners, particularly sibling species, consistently had different depths of maximum occurrence during both day and night.     

Diel vertical migration and feeding of copepods at an oceanic site near South Georgia

Twelve Longhurst Hardy Plankton Recorder (LHPR) profiles were taken over a 16 h period in January 1990, in order to study feeding of four copepod species at an Antarctic oceanic site near South Georgia. Vertical distributions of their life stages, as well as those of dominant competitors and predators, are described in relation to the feeding cycles of Calanoides acutus CV, Calanus simillimus CV, Calanus propinquus CV and Rhincalanus gigas CIII, CV and CVI. Comparisons with vertical ring-net catches, which were used for concomitant gutevacuation experiments, demonstrated the suitability of the LHPR for these fine-scale studies. Planktonic predators, with the exception of the diel migrant Themisto gaudichaudii, resided deeper than the herbivores. During the day and around midnight, when feeding rates were low, species and stages reached their maximum vertical separation. At these times, new generation copepodites of the four species lived progressively deeper and the overwintered generation (i.e., R. gigas Stages CIV, CV, CVI) were progressively shallower. During the afternoon or evening (depending on species), all stages older than CII, as well as Euphausia frigida and T. gaudichaudii, migrated upwards, to amass in the surface mixed layer. Feeding was restricted to darkness, although R. gigas commenced several hours before dusk. In detail their migration and feeding differed widely, with combinations of unimodal and apparent bimodal cycles. As a whole, the results suggest that (1) feeding could occur during sinking as well as during upward migrations, (2) upward migrations were not always associated with feeding increases, and (3) individuals appeared to descend after filling their guts.