Chaetognaths and ctenophores in the holoplankton of the Bristol Channel

The geographical distributions, seasonal variations in numerical abundance and biomass (mg C m^-3) of the predators of the holoplankton of the Bristol Channel, between November 1973 and February 1975, are described. The predator numbers and biomass were dominated by the chaetognath Sagitta elegans Verrill. This species represented 96% of the holoplankton carnivore biomass in the outer, seaward region of the Channel and 60% in the inner region; the remainder being ctenophores. The maximum numerical abundance of S. elegans occurred in September at 129 individuals m^-3 (18 mg C m^-3). Juveniles (<5 mm) reached maximum numbers of 55 individuals m^-3 during June, August and September, demonstrating the reproductive activity of the population. The peak numbers were probably the result of the development of two major generations over the 90 d period from mid-June to mid-September. The tentaculate ctenophores were represented by Pleurobrachia pileus (O. F. Müller). The highest abundance was 81 individuals m^-3 (3.0 mg C m^-3) at a single site in July in the South Central Channel. However, June was the only month when the ctenophores dominated the carnivore biomass in all regions of the Channel; thereafter, S. elegans was more abundant. Reproduction of the ctenophore occurred from April to September, with juveniles reaching maximum abundance in June at 12 individuals m^-3. The estimated food demand of the population in May for the outer region of the Channel was approximately 31% of the daily production of copepods. When the population reached its peak abundance in June, the estimated food requirement outstripped the daily production of copepods and a decline in both the prey and predator standing stocks was observed. Similar estimations were derived for the inner region of the Channel. S. elegans increased from a standing stock of 0.038 mg C m^-3 in March to 6.35 mg C m^-3 in September. Estimates of the copepod production compared with the derived demand of the chaetognath population showed that the decline in the copepods in the late summer was the result of feeding by this predator. The holoplankton carnivore population was approximately 66% of the copepod standing stock for the 10 mo period November 1973 to September 1974 in the outer region of the Channel and 45% of that in the inner region. The carnivores formed the greater part of the total holoplankton biomass from September through the winter months to February, suggesting a predator-dominated community.     

Zooplankton biomass in the ice-covered Weddell Sea,Antarctica

Zooplankton was sampled by a Rectangular Midwater Trawl (RMT 1 + 8) in Weddell Sea surface waters (0 to 300 m) between 66 and 78°S during austral summer (February – March 1983). Sixty-nine taxa including different developmental stages were considered and divided into 16 size classes between <1 and >39.5 mm length. Biomass was determined by taxon and size class for three different meso- and macroplankton communities in the oceanic region, on the northeastern shelf and on the southern shelf of the Weddell Sea. The highest biomass of 11.2 mg DW m^–3 (3.4 g DW m^–2) was found in the northeastern shelf community (70 to 74°S), where juvenile and adultEuphausia crystallorophias accounted for 3.7 mg DW m^–3 (1.1 g DW m^–2). Although not quantitatively sampled, early copepodite stages (CI to CIII) ofCalanoides acutus andCalanus propinquus ranked second with 2.7 mg DW m^–3 (0.8 g DW m^–2). Biomass in the northeastern shelf community was concentrated in the size ranges 1 to 4 mm and 19.5 to 39.5 mm. The oceanic community of the central Weddell Sea was dominated by copepods smaller than 5 mm, which made up half of the total oceanic biomass. The tunicateSalpa thompsoni (7.0 to 8.5 mm) was the dominant single species with 1.6 mg DW m^–3 (0.5 g DW m^–2). Euphausiids, mainly juvenile and adult krillEuphausia superba, comprised 1.2 mg DW m^–3 (0.4 g DW m^–2). Total standing stock in the oceanic community was 9.4 mg DWm^–3 (2.8 g DW m^–2). Lowest biomass values were found in the southern shelf community (south of 75°S) with 4.0 mg DW m^–3 (1.2 g DW m^–2), concentrated in the 1 to 4 mm and 14.5 to 34.5 mm size classes. Abundant species were the pteropodLimacina helicina (1 to 2 mm; 0.7 mg DW m^–3; 0.2 g DW m^–2) andE. crystallorophias (24.5 to 39.5 mm; 0.9 mg DW m^–3; 0.3 g DW m^–2). The data reveal that it is essential to distinguish among subsystems in the Southern Ocean. This leads to a better understanding of the structure and function of those pelagic food webs which represent alternatives to the paradigmatic krill-centered system.     

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.     

Present-day genetic composition suggests contrasting demographic histories of two dominant chaetognaths of the North-East Atlantic, <Emphasis Type="Italic">Sagitta elegans</Emphasis> and <Emphasis Type="Italic">S. setosa</Emphasis>

Sagitta elegans and S. setosa are the two dominant chaetognaths in the North-East (NE) Atlantic. They are closely related and have a similar ecology and life history, but differ in distributional ranges. Sagitta setosa is a typical neritic species occurring exclusively above shelf regions, whereas S. elegans is a more oceanic species with a widespread distribution. We hypothesised that neritic species, because of smaller and more fragmented populations, would have been more vulnerable to population bottlenecks resulting from range contractions during Pleistocene glaciations than oceanic species. To test this hypothesis we compared mitochondrial Cytochrome Oxidase II DNA sequences of S. elegans and S. setosa from sampling locations across the NE Atlantic. Both species displayed very high levels of genetic diversity with unique haplotypes for every sequenced individual and an approximately three times higher level of nucleotide diversity in S. elegans (0.061) compared to S. setosa (0.021). Sagitta setosa mitochondrial DNA (mtDNA) haplotypes produced a star-like phylogeny and a uni-modal mismatch distribution indicative of a bottleneck followed by population expansion. In contrast, S. elegans had a deeper mtDNA phylogeny and a multi-modal mismatch distribution as would be expected from a more stable population. Neutrality tests indicated that assumptions of the standard neutral model were violated for both species and results from the McDonald-Kreitman test suggested that selection played a role in the evolution of their mitochondrial DNA. Congruent with these results, both species had much smaller effective population sizes estimated from genetic data when compared to census population sizes estimated from abundance data, with a factor of ~10⁸–10⁹ difference. Assuming that selective effects are comparable for the two species, we conclude that the difference in genetic signature can only be explained by contrasting demographic histories. Our data are consistent with the hypothesis that in the NE Atlantic, the neritic S. setosa has been more severely affected by population bottlenecks resulting from Pleistocene range shifts than the more oceanic S. elegans.     

On the life history of Centropages typicus: Responses to a fall diatom bloom in the New York Bight

Laboratory studies and field collections show that egg production by Centropages typicus (Krøyer) in New York shelf waters in autumn 1984 responded to both food and temperature. Rates of egg production were high (43 to 76 eggs female^-1 d^-1) in October, early in the fall diatom bloom. Later, although food concentrations remained high and female size actually increased, egg production declined, presumably in response to seasonally decreasing temperatures. Carnivorous diets did not support egg production. Development time for autumn-hatched C. typicus was 33 d at 15°C, a rate that gives a Q₁₀ of 2.21 when compared with the spring development rate of 49 d at 10°C. We could find no evidence of physiological adjustments being made by this copepod for overwintering. Development was not arrested at any subadult stage and resting eggs were not produced. Trends in body size of copepodid stage V, however, suggest that an overwintering strategy may be invoked by this copepod in Junuary or February.     

Spatial and temporal variations in the population size structure of three lanternfishes (Myctophidae) off Oregon,USA

Size-frequency distributions were determined for 3 common lantern-fishes (Stenobrachius leucopsarus, Diaphus theta, and Tarletonbeania crenularis) off Oregon in the summer. The fishes were caught mainly in sound-scattering layers by a large pelagic trawl with 5 opening-closing nets. Changes in depth distribution and diel vertical migration with growth were evident for all 3 species. The size of S. leucopsarus increased markedly with depth both at 0 to 90 m at night and 250 to 500 m during the day. Larger D. theta were also found deeper during the day (between 250 and 450 m), but neither D. theta nor T. crenularis demonstrated size segregation in the upper 90 m at night. Large D. theta and small T. crenularis did not appear to migrate into surface waters at night. Age-Group O (15 to 20 mm) S. leucopsarus were most abundant in deep water (400 to 480 m) in the daytime and did not migrate into near-surface waters at night. Age-Group I (30 to 40 mm) S. leucopsarus were common at about 300 m by day and within the upper 30 m at night. Age-Group II–III (50 to 60 mm) apparently followed the evening ascent of Age-Group I fish and most resided at 75 to 90 m at night, beneath Age-Group I fish. Age-Group III+fish (70 to 80 mm) were associated with Age-Group O at 400 to 480 m by day and usually did not migrate above 200 m at night. The size structure of S. leucopsarus differed among the nets of a single tow at one depth, or between two tows that fished the same depths on successive nights, indicating horizontal patchiness in age structure. D. theta demonstrated low within-tow variability in size composition which indicated a spatially more uniform age structure on a scale of kilometers. The size structures of these 3 lanternfishes were different in the same area and the same season during two different years, suggesting variable survival of year classes or horizontal patchiness of age composition in the area sampled.     

Aspects of the ecology and growth of an intertidal juvenile population of Concholepas concholepas (Mollusca: Gastropoda: Muricidae) at Las Cruces,Chile

The habitat, density and growth rate for an intertidal population of Concholepas concholepas (Bruguière, 1789) were studied at Las Cruces, Chile (Lat. 33°30′S; Long. 71°38′W) during 1977–1978. The growth rate (3.67 mm month^-1) was determined in a newly settled group, whose average length was 11.3 mm, and whose age was estimated at 3 months. The densities found ranged between 1.1 and 107.3 “locos” m^-2. Based on these results, the time of settlement of C. concholepas was calculated; capsule deposition, maximum maturity and recruitment reported by other authors for different localities are discussed.     

Reproduction and distribution of the invasive European fanworm Sabella spallanzanii (Polychaeta: Sabellidae) in Port Phillip Bay, Victoria, Australia

 The European fanworm Sabella spallanzanii (Gmelin, 1791) was recently introduced to Port Phillip Bay and is now a conspicuous component of most benthic communities. Reproduction of the worm was investigated in a population at Queenscliff over a 2 yr period (October 1995 to October 1997) using gonadal histology. The worms are dioecious (sex ratio 1:1, n=250), and attained sexual maturity at ∼50 mm body length. Reproductive periodicity followed a distinct annual cycle, and spawning proceeded through an extended autumn/winter period. Spawning was broadly synchronous between sexes, and coincided with falling seawater temperatures and shorter day-lengths. The females were highly fecund, and >50 000 eggs were probably shed from large females (>300 mm body length) during the annual spawning period. Breeding cycles of S. spallanzanii in Port Phillip Bay are ∼6 mo out of phase with endemic populations located at similar latitudes in the northern hemisphere. The spread of S. spallanzanii within Port Phillip Bay has been monitored by divers on an annual basis since 1994. The most recent dive survey (1998) indicates that S. spallanzanii has extended its range through out the entire 2000 km² embayment, and has invaded most subtidal habitats. Quantitative estimates of S. spallanzanii abundances were highest on pier pylons (12.5 individuals m⁻², 0.5 to 7 m depths). On sediments, estimates were highest at shallow sites (0.3 m⁻², 7 m depth), but numbers declined significantly with depth (0.1 m⁻², 17 to 22 m depth). Mean worm lengths and biomass were, by contrast, significantly higher at intermediate depths (12 to 17 m) than in shallower (7 m) or deeper (22 m) locations. S. spallanzanii demonstrates a clear preference for growth in sheltered, nutrient-enriched waters, so it may not spread from Port Phillip Bay into the adjacent oceanic waters of Bass Strait; however, in view of S. spallanzanii's current high abundance, fecundity and extended spawning periodicity, there is a high risk of future range expansions, mediated by shipping, into other temperate-water ports. Received: 17 November 1998 / Accepted: 6 January 2000     

Biological and environmental influences on the trophic ecology of leatherback turtles in the northwest Atlantic Ocean

Understanding the causes and consequences of variability in trophic status is important for interpreting population dynamics and for identifying important habitats for protected species like marine turtles. In the northwest Atlantic Ocean, many leatherback turtles (Dermochelys coriacea) from distinct breeding stocks throughout the Wider Caribbean region migrate to Canadian waters seasonally to feed, but their trophic status during the migratory and breeding cycle and its implications have not yet been described. In this study, we used stable carbon and nitrogen isotope analyses of bulk skin to characterize the trophic status of leatherbacks in Atlantic Canadian waters by identifying trophic patterns among turtles and the factors influencing those patterns. δ¹⁵N values of adult males and females were significantly higher than those of turtles of unknown gender (i.e., presumed to be subadults), and δ¹⁵N increased significantly with body size. We found no significant differences among average stable isotope values of turtles according to breeding stock origin. Significant inter-annual variation in δ¹⁵N among cohorts probably reflects broad-scale oceanographic variability that drives fluctuations in stable isotope values of nutrient sources transferred through several trophic positions to leatherbacks, variation in baseline isotope values among different overwintering habitats used by leatherbacks, or a combination of both. Our results demonstrate that understanding effects of demographic and physiological factors, as well as oceanographic conditions, on trophic status is key to explaining observed patterns in population dynamics and for identifying important habitats for widely distributed, long-lived species like leatherbacks.     

The tunicate Salpa thompsoni ecology in the Southern Ocean. I. Distribution, biomass, demography and feeding ecophysiology

Distribution, density, and feeding dynamics of the pelagic tunicate Salpa thompsoni have been investigated during the expedition ANTARKTIS XVIII/5b to the Eastern Bellingshausen Sea on board RV Polarstern in April 2001. This expedition was the German contribution to the field campaign of the Southern Ocean Global Ocean Ecosystems Dynamics Study (SO-GLOBEC). Salps were found at 31% of all RMT-8 and Bongo stations. Their densities in the RMT-8 samples were low and did not exceed 4.8 ind m⁻² and 7.4 mg C m⁻². However, maximum salp densities sampled with the Bongo net reached 56 ind m⁻² and 341 mg C m⁻². A bimodal salp length frequency distribution was recorded over the shelf, and suggested two recent budding events. This was also confirmed by the developmental stage composition of solitary forms. Ingestion rates of aggregate forms increased from 2.8 to 13.9 μg (pig) ind⁻¹ day⁻¹ or from 0.25 to 2.38 mg C ind⁻¹ day⁻¹ in salps from 10 to 40 mm oral-atrial length, accounting for 25–75% of body carbon per day. Faecal pellet production rates were on average 0.08 pellet ind⁻¹ h⁻¹ with a pronounced diel pattern. Daily individual egestion rates in 13 and 30 mm aggregates ranged from 0.6 to 4.8 μg (pig) day⁻¹ or from 164 to 239 μg C day⁻¹. Assimilation efficiency ranged from 73 to 90% and from 65 to 76% in 13 and 30 mm aggregates, respectively. S. thompsoni exhibited similar ingestion and egestion rates previously estimated for low Antarctic (~50°S) habitats. It has been suggested that the salp population was able to develop in the Eastern Bellingshausen Sea due to an intrusion into the area of the warm Upper Circumpolar Deep Water     

Relationships between diet and body size, mouth morphology, habitat and movements of six sillaginid species in coastal waters: implications for resource partitioning

The dietary compositions and breadths of sequential 50 mm size classes of the six whiting species found in nearshore (<1.5 m), shallow inner-shelf (5 to 15 m) and/or deep inner-shelf (20 to 35 m) waters of the lower west coast of Australia were determined. Comparisons between the results of principal components analysis of head and mouth dimensions and the dietary compositions of Sillago bassensis, S. vittata, S. burrus, S. schomburgkii, S. robusta and Sillaginodespunctata suggests that any differences in the dietary composition of similar-sized representatives of different species, when they occur in the same habitat, are more likely to be due to differences in foraging behaviour than mouth morphology. Classification, ordination and Schoener's overlap indices showed that, in nearshore waters, the juveniles of Sillago bassensis, which colonise relatively exposed areas, have a different diet to those of the smallest representatives of the other whiting species that occupy more sheltered habitats. S. bassensis consumes mainly amphipods, whereas the smaller representatives of S. vittata, S. burrus, S. schomburgkii and Sillaginodes punctata ingest large volumes of copepods, which are typically abundant in protected nearshore waters. Although the mouth dimensions of S. punctata tend to be smaller than those of Sillago schomburgkii, the larger individuals of the former species ingest greater quantities of larger prey, such as crabs and carid shrimps. As S. bassensis, S. vittata and S. burrus increase in size and migrate out into shallow inner-shelf waters, the latter two species tend to concentrate more on benthic prey, while the former species ingests fauna that is more epibenthic. The largest S. bassensis subsequently migrate out into deep inner-shelf waters, where they co-occur with S. robusta, which is restricted to those waters. In these waters, S. bassensis feeds to a far greater extent on large benthic prey, whereas S. robusta consumes a greater quantity of small epibenthic crustaceans, differences that reflect the far larger lengths of the former species in that region. The above data emphasise that the distribution and ontogenetic movements of the six abundant species of whiting play a major role in facilitating a partitioning of food resources amongst these species found in coastal waters of the lower west coast of Australia. Received: 7 October 1996 / Accepted: 31 January 1997     

Population dynamics and production of the planktonic copepods in a eutrophic inlet of the Inland Sea of Japan. I. Centropages abdominalis

Population dynamics and production of the calanoid copepod Centropages abdominalis were studied from November 1986 to November 1987 in Fukuyama Harbor, in the central part of the Inland Sea of Japan. This species was present in the plankton during a cold-water period from November to June (temperature range: 8.9 to 21.1 °C), with a peak abundance (23 600 ind m^–3) in February. During this period, six generations could be detected, and each generation time agreed well with that predicted from food-satiated laboratory experiments, indicating that the natural population was not food-limited. The population suffered extremely high mortality during the period from egg to naupliar stage (N) II: only 0.02 to 4% of the eggs survived to NII. However, the mortality in stages older than NII was almost negligible. The growth rate of C. abdominalis increased exponentially with increasing temperature. Its biomass and production rate showed marked seasonal variations largely in parallel with numerical abundance. The estimated production between 7 November 1986 and 29 May 1987 was 355 mg C m^–3 or 2.66 g C m^–2, 95% of which occurred during February and March. The daily production rate to biomass ratio increased exponentially with temperature from 0.18 at 8.9°C to 0.37 at 19°C.     

Distribution,life history,and production of three species of <Emphasis Type="Italic">Neomysis</Emphasis> in Akkeshi-ko estuary,northern Japan

Spatial and seasonal distribution pattern, life history and production of three species of Neomysis (Mysidacea) which commonly occur in northwestern subarctic Pacific coastal waters, were investigated throughout the year in the Akkeshi-ko estuary, northern Japan. The most abundant species Neomysis awatschensis (annual mean density: 179.8 inds. m⁻², biomass: 108.8 mg DW m⁻²) occurred at the inner part of the estuary including low salinity areas with no clear preference for the seagrass bed. The second most abundant Neomysis mirabilis (mean density: 95.8 inds. m⁻², biomass: 90.1 mg DW m⁻²) occurred at relatively saline seagrass site throughout the year. Occurrence of Neomysis czerniawskii in the estuary was limited to the seagrass bed during summer when their population mainly consisted of juveniles, suggesting that this species is a seasonal migrant between the estuary and the marine environment. Both N. awatschensis and N. mirabilis populations were composed of two generation types, a larger sized overwintering and smaller sized spring/summer generations; however, each species had a different reproductive strategy. N. awatschensis was characterized by fast growth to maturity at a smaller size than N. mirabilis with a relatively high fecundity during warm season, suggesting that this species is an r-strategist which can utilize opportunistically a wide variety of habitats. In contrast, the seagrass bed resident N. mirabilis was a K-strategist which matures at a larger size producing fewer but larger offspring. The annual production of N. awatschensis (0.57–0.70 g DW m⁻², mean of the whole estuary) and N. mirabilis (0.58–0.68 g DW m⁻², mean of the seagrass bed) at their respective habitats was comparable. Consequently, species-specific life history and distribution pattern are concluded to allow Neomysis spp. to coexist in the estuary and the high carrying capacity of seagrass bed is suggested to contribute to maintain their high biomass level.     

Induction of sexual reproduction and resting egg production in Brachionus plicatilis by a diet of salt-grown Nannochloris oculata

Rates of population increase in early spring and the sizes of overwintering stocks were calculated for the planktonic copepods Pseudocalanus elongatus and Acartia clausi for a set of areas covering the open waters of the north-east Atlantic Ocean and the North Sea for the period 1948 to 1979. For both species, the rates of population increase were higher in the open ocean than in the North Sea and appear to be related to temperature. The overwintering stocks in the North Sea were larger than those in the open ocean and are probably related to phytoplanton concentration. P. elongatus shows higher overwintering stocks and lower rates of population increase than A. clausi, resulting in different levels of persistence in the stocks of the two species. It is suggested that this difference in persistence is responsible for differences between the two species with respect to geographical distribution in summer and different patterns of year-to-year fluctuations in abundance.     

Vertical distribution and migration of fishes of the lower mesopelagic zone off Oregon

An exceptionally large midwater trawl (50 m² mouth area) with 5 opening and closing codends was towed horizontally in the lower mesopelagic zone at depths of 500, 650, 800 and 1000 m off Oregon (USA) from 1–6 September, 1978. In comparison to more conventional trawls, ours collected more fish, including rare species and large individuals of common species. Comparison of collections made by day and by night revealed that 12 of the 15 most common species probably migrated vertically. Bathylagus milleri evidently migrates from 650 m during the day to 500 m at night. Cyclothone acclinidens and C. atraria were more abundant by night than by day at 800 m, possibly due to an upward migration from deeper depths at night. C. pseudopallida, C. signata, Chauliodus macouni, Tactostoma macropus and Stenobrachius leucopsarus were more abundant by day than by night at 500 m, suggesting that they migrated out of this depth horizon at night. Lampanyctus regalis, and large individuals of B. pacificus were more abundant by night than by day at 500 m, possibly because they migrated upward from near 650 m. Many species exhibited trends of increasing or decreasing size with depth, and several species showed changes in migratory behavior with size. For example, only small (<240 mm) T. macropus migrated vertically, whereas only large (>110 mm) B. pacificus appeared to migrate. Depths of maximum abundance of congeneric species were usually separated. B. milleri and B. pacificus had similar distributions by day, but the former was shallower at night. S. leucopsarus tended to live shallower than S. nannochir both day and night. Congeners always occurring at the same depth were Cyclothone pseudopallida and C. signata (both most abundant at 500 m) and C. acclinidens and C. atraria (both most abundant at 800 m).     

Plankton of the fladen ground during FLEX 76 II. Population dynamics and production of Thysanoessa inermis (Crustacea: Euphausiacea)

Samples taken in the northern North Sea with the Continuous Plankton Recorder (CPR), the Undulating Oceanographic Recorder (UOR) and the Longhurst-Hardy Plankton Recorder (LHPR) during the Fladen Ground Experiment in 1976 (FLEX 76) are used to describe the vertical distribution and population dynamics of Thysanoessa inermis (Krøyer) and to provide estimates of the production and carbon budget of the population from 19th March to 3 June 1976. Spawning occurred in late April and early May, in near synchronisation with the start of the spring bloom of phytoplankton. Eggs, nauplii and calyptopes reached maximum abundance in succession, and furciliae were numerous when sampling ceased in early June. Adults increased in length from a mean of 12.1 mm in mid-March to 17.5 mm in early June and the estimated production was 2.40 mg m^-3 over the 74 d period. Total carbon ingested by the population of T. inermis was estimated to be 10 mg C m^-2 d^-1 in the upper 100m which was only 1.5% of the daily primary production of 0.68 gC m^-2 measured over the FLEX period 26 March to 4 June 1976. The grazing by T. inermis on the phytoplankton population was assumed to have little effect on the control and depletion of the spring phytoplankton bloom during FLEX 77.JOSDAP Contribution No. 50     

Primary production and environmental factors in an oligotrophic biome in the Aegean Sea

The rate of the primary production of the phytoplankton community in the Petalion Gulf, Aegean Sea, was studied from January 1970 to May 1971, at a station situated at approximately Latitude 37°54N; Longitude 24°11E. A variety of physical and chemical parameters such as chlorophyll, primary nutrients (N,P,Si), temperature, salinity, oxygen and light penetration were also studied simultaneously. The rate of the gross primary production varied from 40 to 200 mg C m^-2 day^-1, with a mean value of 90 mg C m^-2 day^-1. The annual gross primary production was calculated to be 33 g C m^-2, which is the minimum known value in the Aegean and Mediterranean Seas. Maximum production was found at the depth of 20 m on the average, mainly due to high light intensities. Petalion Gulf supports a small photosynthetic biomass, as indicated by the low seasonal values of chlorophyll a (0.01 to 0.18 mg m^-2), the highest values being found in the summer. The low production rate noted may have been due to the low nutrient concentrations found: N, 0.04 to 0.32 g-at/1; P, 0.00 to 0.15 g-at/1; Si, 0.45 to 2.25 g-at/1. It is suggested that inorganic phosphorus and nitrogen may alternate in limiting primary production rates in these oligotrophic waters. The temperate waters of the Petalion Gulf are stratified in summer (15.5° to 24.7°C) and well-mixed in winter (12.9° to 15.0°C); they are oxygen-saturated throughout the year, and of high transparency, with 86 m depth for the euphotic zone on the average yearly. The Petalion Gulf is therefore characterized as a typical oligotrophic biome in the Aegean and Eastern Mediterranean Seas.     

Variation in Vitamin C content of sprat larvae (Sprattus sprattus) in the Irish Sea

Sprat (Sprattus sprattus) eggs and larvae were sampled from plankton and the Irish Sea in 1988 and 1989 and analysed forl-ascorbic acid (Vitamin C) content, which is considered an index of the nutritional well being and thus indicative of the status of the population in relation to environmental (physical and biological) structures. In one month, the Vitamin C content of larvae in different developmental stages decreased from 800 to 300µg g^–1 in the youngest larvae (4 to 14 mm) and to 250µg g^–1 in the oldest larvae (14 to 28 mm). No significant differences in the Vitamin C content per unit weight were found between larvae collected at four sites located in western stratified waters, central stratified, central mixed and eastern mixed waters. The mean Vitamin C content per larva, as well as mean length and wet weight of larvae were lowest in central mixed and eastern mixed waters in May–June. The estimated increases in Vitamin C, length and weight of individuals in the population of larvae varied significantly from April to June and between western stratified and eastern mixed areas. Highest rates coincided with stratified water conditions and with suitable quantity and quality of food, which seemed to constitute the most favourable environmental conditions for abundance and growth of sprat larvae.     

Population dynamics and production of euphausiids II. Thysanoessa inermis and T. raschi in the North Sea and American Coastal Waters

Results from the Continuous Plankton Recorder (CPR) survey for 1966 and 1967 are used to describe seasonal changes in abundance, size and aspects of the population structure of Thysanoessa inermis (Krøyer) and T. raschi (M. Sars) at a depth of 10 m in the North Sea and in American coastal waters from the Grand Banks to the Gulf of Maine. Production and dry weight were estimated from these data. Two year-groups were usually present in the breeding population, the proportion surviving into a second year being higher in American waters than in the North Sea. Annual production for each species was within the range 0.69 to 4.66 mg m^-3 and the ratio between production and biomass (P:B) was between 1.3 and 4.2; values outside these ranges were obtained only for American coastal waters in 1967, when the frequency of sampling was low.     

Distribution patterns,abundance and population dynamics of the euphausiidsNyctiphanes capensis andEuphausia hanseni in the northern Benguela upwelling system

Distribution patterns, population structure and biomass of the euphausiidsNyctiphanes capensis andEuphausia hanseni were examined off the coast of Namibia, southwest Africa, in relation to temperature, depth and season, from data collected on nine surveys from September 1982 to March 1984. High densities ofN. capensis were found in the shallow coastal waters (<200 m), with the biomass of adults ranging from 675 to 5 706 mg dry wt m^–2. For adultE. hanseni, the biomass was an order of magnitude lower, ranging from 65 to 505 mg dry wt m^–2, with most specimens occurring over the shelf break at depths of 200 to 1000 m. These distribution patterns remained relatively constant throughout the year, despite seasonal differences in upwelling events. Both species displayed continuous breeding, with 43 to 82% of the adult femaleE. hanseni being fertilized, while a much lower proportion ofN. capensis females were reproductively active (0.5 to 26%). Different breeding strategies were adopted by these two euphausiid species, withE. hanseni producing frequent broods (14.8 broods in 6 mo) consisting of relatively large eggs which are released into the sea, andN. capensis exhibiting a lower frequency of spawning, with broods consisting of large numbers of relatively small eggs, protected by a brood pouch. These strategies enable both species to maintain high densities throughout the year in a fluctuating physical environment. Growth rate estimated from size-frequency distributions were 0.003 to 0.063 mm d^–1 forN. capensis and 0.077 to 0.083 mm d^–1 forE. hanseni, suggesting an adult lifespan of approximately 6 mo for both species. Maximum sizes were attained in September, withN. capensis reaching a total length of 21 mm (in contrast to all previous studies onN. capensis, where the maximum size recorded was only 13 mm total length) andE. hanseni a total length of 33 mm.