Annual population development and production by Calanus finmarchicus, C. glacialis and C. hyperboreus in Disko Bay, western Greenland

The populations of the copepod species Calanus finmarchicus, C. glacialis and C. hyperboreus were investigated in Disko Bay during a 14-month period in 1996-1997. The three species were predominant in the copepod community. The biomass reached a maximum at the beginning of June (127 mg C m^-3). From the end of July until the end of April the following year, the biomass was <1-6 mg C m^-3. All three species showed seasonal ontogenetic migration. The spring ascent for all three species was just prior to or in association with the break-up of sea ice and the development of the spring bloom, whereas descent occurred over a larger time span during summer. The main overwintering stages were CV for C. finmarchicus, CIV and CV for C. glacialis and C. hyperboreus. Peak abundance of juvenile copepodites, representing the new generation, was in August for C. finmarchicus, in July for C. glacialis and in May/June for C. hyperboreus. From the timing of reproduction and the population development, the life cycles were deduced to be 1 year for C. finmarchicus and at least 2 years for C. glacialis and C. hyperboreus. Secondary production and potential grazing impact of the Calanus community were estimated by two methods based on specific egg-production rates and temperature-dependent production. The Calanus community was not able to control the primary producers during the spring bloom but probably did during post-bloom. The estimates also indicated that grazing on ciliates and heterotrophic dinoflagellates contributes as an essential food source in the post-bloom period.     

Population structure of two Antarctic ice-associated copepods, Drescheriella glacialis and Paralabidocera antarctica, in winter sea ice

The spatial distribution and population structure of two dominant ice-associated copepods, Drescheriella glacialis and Paralabidocera antarctica, were studied during winter at nine locations in east Antarctic fast ice. These species accounted for at least 90% of the total metazoan abundance at each location. Abundances were high, reaching 175 individuals l^-1 (190,000 m^-2) for D. glacialis and 660 l^-1 (901,000 m^-2) for P. antarctica. These abundances were probably partly supported by the high biomass of ice-algae (Pearson correlation coefficient, r=0.75), as indicated by chlorophyll-a concentrations (1.7-10.1 µg l^-1). The population structures of each species suggested very different life-history strategies. All developmental stages of D. glacialis were isolated from the ice cores, including females with egg sacs, supporting the hypothesis that this species reproduces in the sea ice during winter. This strategy might assist D. glacialis in leading a continually colonising existence, whereby it responds opportunistically to the availability of favourable habitat patches. The populations of P. antarctica were composed primarily of nauplii (>99%), consistent with past observations of a synchronised life cycle for this species. The strong coupling of the developmental cycle of P. antarctica to the growth and decay of sea ice suggests that local extinctions might occur in areas where ice break-out is unpredictable.     

Mesozooplankton community structure, abundance and biomass in the central Arctic Ocean

During the "International Arctic Ocean Expedition 1991" (20 August-21 September 1991) mesozooplankton was sampled at six stations in the Nansen, Amundsen and Makarov Basins of the central Arctic Ocean from 1,500 m depth to the surface by multiple opening/closing net hauls. Total mesozooplankton abundance decreased from 268 ind. m^-3 in the surface layer (0-50 m) to <25 ind. m^-3 below 200 m depth. The small copepods Oithona similis and Microcalanus pygmaeus, as well as copepod nauplii, were most abundant close to the surface, while Oncaea borealis and Spinocalanus spp. frequently occurred at greater depth. Mesozooplankton dry mass (DM) integrated over the upper 1,500 m of the water column was surprisingly stable throughout the investigation area and measured 2.0ǂ.3 g DM m^-2. Dry mass in the upper 50 m measured 20.9 mg m^-3 and was dominated by Calanus hyperboreus (57.4%) and C. glacialis (21.1%). C. finmarchicus was very abundant only in the Nansen Basin. Below 200 m the calanoid copepods Metridia longa, Microcalanus pygmaeus and Pareuchaeta spp., the decapod Hymenodora glacialis and chaetognaths of the genus Eukrohnia were the principal contributors to biomass values of <1 mg DM m^-3. Hence, vertical changes in abundance, biomass and species composition were much more pronounced than regional differences between the basins. Three different mesozooplankton communities were differentiated according to their faunistic composition and are discussed in context with the major water masses: Polar Surface Water, Atlantic Layer and Arctic Deep Water.     

Reproductive cycles of three dominant Calanus species in Disko Bay, West Greenland

This study describes the annual reproductive cycles of the three dominant Calanus species, C. finmarchicus, C. glacialis and C. hyperboreus, in Disko Bay (West Greenland) in relation to seasonal phytoplankton development. Relative abundance of females, copepodite stage V (CV) and males, and the developmental stage of the female gonad were examined from plankton samples collected at weekly to monthly intervals from May 1996 to June 1997 with a WP2 net or a pump. During spring and summer, egg production rates were determined. Females of all three species were present year round. Maximum relative abundance was reached by C. hyperboreus females at the beginning of February, by C. glacialis in mid-February, and by C. finmarchicus in April. All three species reproduced successfully in Disko Bay. Their reproductive cycles were considerably different with respect to the timing of final gonad maturation and spawning, and hence in their relation to seasonal phytoplankton development. In all three species, early gonad development took place during winter, before living food became plentiful, suggesting that these processes were largely food independent. Final gonad maturation and spawning in C. finmarchicus was related to the phytoplankton concentration, reflecting that final gonad maturation processes are food dependent in this species. C. glacialis females matured and spawned prior to the spring bloom. Our results indicate that first internal lipid stores and later ice alga grazing supplied final gonad maturation and egg production. Maximum egg production rates of C. glacialis were found in spring and summer, when the chlorophyll a concentration was high. Mature female C. hyperboreus were found from February until mid-April, when the chlorophyll a concentration was still low. In this species, reproductive activity was decoupled from phytoplankton development, and final maturation processes and spawning were solely fuelled by internal energy stores.     

Seasonal variation in ecophysiological patterns in macroalgae from an Arctic fjord. I. Sensitivity of photosynthesis to ultraviolet radiation

Marine macroalgae inhabiting Arctic coastal ecosystems are exposed to pronounced seasonal variations in the radiation regime, including harmful UVB radiation. This study presents the first data on the seasonal changes in the sensitivity of macroalgal photosynthesis towards UV exposure by comparing under-ice, clear-water and turbid-water conditions characteristic for late winter, spring and summer. Various brown (Laminaria saccharina, L. digitata, L. solidungula, Saccorhiza dermatodea, Desmarestia aculeata), red (Palmaria palmata, Devaleraea ramentacea) and one green macroalgal species (Monostroma aff. arcticum) were collected at the same water depth throughout the seasons in the Kongsfjord (Spitsbergen, Svalbard, Norway). Maximum quantum yield (F_v/F_m) and maximum photosynthetic electron transport rates (ETR_max) were determined immediately after collection, after 2 h exposure to artificial UV radiation and after 18 h recovery in dim white light. Photosynthesis of the studied species showed different responses depending on their morpho-functional and physiological characteristics, their life strategies, phenology and depth distribution. Within the genus Laminaria, maximum quantum yield of adult specimens of the deep-water species L. solidungula was most strongly UV sensitive. Adult L. saccharina exhibited a lower UV sensitivity than a 6-month-old specimen. Inhibition of photosynthesis after UV exposure remained at the same level throughout the study period, both in adult L. saccharina and S. dermatodea. However, adult specimens of L. saccharina collected in May showed partial recovery only, whereas photosynthesis of specimens from both species collected later recovered fully. D. aculeata exhibited a remarkable decrease of UV sensitivity during the study period. Photosynthesis of specimens collected under the ice was strongly inhibited by UV, but the degree of inhibition decreased during spring and summer. Concomitantly ETR_max values were low after UV exposure in specimens collected in June, but increased later in the season. P. palmata exhibited a relatively flexible response. Photosynthesis in specimens collected under the ice in June or in turbid water in July/August was relatively strongly inhibited; specimens collected during sunny periods and in clear water in spring showed a much lower degree of photoinhibition after UV exposure. The seasonal pattern of low/high ETR_max values in spring/summer is probably a characteristic of the life strategy of this species. The UV sensitivity of D. ramentacea exhibited a similar seasonal pattern. In M. aff. arcticum, UV sensitivity increased and ETR_max values decreased during the study period, reflecting the life strategy of this annual late winter/spring species. The physiological basis for the seasonal changes in UV sensitivity of photosynthesis is presented in a companion paper (this issue).     

In situ feeding rates and grazing impact of zooplankton in a South African temporarily open estuary

Recent studies in temporarily open estuaries of South Africa have shown that phytoplankton biomass is at times low, when compared to the high standing stock of the grazers. In situ grazing rates of the dominant zooplankton species were estimated at the Mpenjati Estuary once during the winter closed phase, in August 1999, and once during the summer open phase, in February 2000. The study aimed at determining what proportion of the energetic demands of the dominant grazers of the estuary is met by the available phytoplankton. Results show that the gut of all species exhibited higher pigment concentrations during the night than during the day, both in winter and summer. Gut pigment contents ranged from 0.27 to 5.38 ng pigm individual^-1 in the mysid Gastrosaccus brevifissura, from 0.16 to 1.63 ng pigm individual^-1 in the copepod Pseudodiaptomus hessei, from 0.12 to 0.45 ng pigm individual^-1 in the copepod Acartia natalensis, and from 0.8 to 5.44 ng pigm individual^-1 in the caridean Palaemon sp. [where pigm is the sum of chlorophyll-a (chl-a) and phaeopigments]. During the winter closed phase, gut evacuation rates for G. brevifissura, P. hessei, and A. natalensis were 0.62, 0.42, and 0.46 h^-1, respectively. In summer, gut evacuation rates were 0.68, 0.48, and 0.46 h^-1 for G. brevifissura, P. hessei, and Palaemon sp., respectively. The rate of gut pigment destruction for G. brevifissura was 99.6% of the total ingested, one of the highest values ever recorded for any crustacean. A gut pigment destruction of 79.0% was measured for Palaemon sp., 95.7% for P. hessei, and 93.8% for A. natalensis. During winter the total grazing impact of the dominant zooplankton species ranged from 5.05 to 22.7 mg chl-a m^-2 day^-1 and accounted for 34-69% of the available chl-a in the water column. During summer, the grazing impact ranged between 0.45 and 0.65 mg chl-a m^-2 day^-1, accounting for 17-41% of the available chl-a in the water column. This shows that the dominant zooplankton species of the Mpenjati have a very high grazing impact on algal cells. At times this may exceed 100% of the available phytoplankton production, suggesting that the zooplankton community may often resort to other food sources to meet all its energetic demands.     

Studies on pigments,proteins and photosynthetic rates in some mangroves and mangrove associates from Bhitarkanika,Orissa

Pigment contents, proteins and net photosynthesis were investigated in fully developed leaf of 1-year-old seedlings of six mangroves (Bruguiera gymnorrhiza, Rhizophora apiculata) and mangrove associates (Caesalpinia bonduc, Cerbera manghas, Derris heterophylla, Thespesia populnea), collected from Bhitarkanika, located on the east coast of India. Large variations in the photosynthetic rates (P_N) among the six species were observed, ranging from 10.16 µmol CO₂ m^-2 s^-1 in C. bonduc to 15.28 µmol CO₂ m^-2 s^-1 in R. apiculata. The total leaf protein content ranged from 12.09 mg g^-1 dry wt in T. populnea to 51.89 mg g^-1 dry wt in B. gymnorrhiza. The chlorophyll a/b ratio was typically about 3.0 in all the studied species, except C. bonduc (2.8). Photosynthetic rates and chl a/b ratio in the leaves were found to be correlated. Analysis of chlorophyll and xanthophyll spectra suggested: (1) variations in different forms and amounts of carotenes as well as xanthophylls and (2) the presence of high amounts of near-UV-absorbing substances in leaves, particularly in the two mangroves (B. gymnorrhiza, R. apiculata) and a mangrove associate (T. populnea), which appears to be an adaptive feature. Estimation of the chl a/b ratios in isolated thylakoids yielded a low value of 1.8 for R. apiculata and >2.6 for other species. The total protein/chlorophyll ratios in thylakoids varied considerably from 3.14 (D. heterophylla) to 10.88 (T. populnea) among the mangrove associates and from 16.09 to 18.88 between the members of the Rhizophoraceae. The chlorophyll/carotenoid ratios in thylakoids of the six species were more or less similar. The absorption spectra for washed thylakoids of C. manghas and D. heterophylla exhibited absorption characteristics typical for C₃-plant thylakoids. However, thylakoids isolated from R. apiculata, B. gymnorrhiza, C. bonduc and T. populnea exhibited an unusual increase in absorption in the blue region (380-410 nm) of the absorption spectrum. The presence of high-absorbing (in the short-wavelength, near-UV region) pigments appears to be closely associated with the thylakoids in R. apiculata and T. populnea. Our results, therefore, suggest a wide range of variation, not only in protein and pigment contents of photosynthetic tissues, but also in the spectral characteristics and composition of the pigments in mangrove species. An understanding of the nature of these pigments in mangroves and their associates, under their natural conditions and especially in relation to eco-physiological adaptations, is necessary, not only in relation to conservation, but also to allow propagation under different salinity conditions.     

Biomass, photosynthesis and growth of Laminaria saccharina in a high-arctic fjord, NE Greenland

Biomass, photosynthesis and growth of the large, perennial brown alga Laminaria saccharina (L.) Lamour. were examined along a depth gradient in a high-arctic fjord, Young Sound, NE Greenland (74°18'N; 20°14'W), in order to evaluate how well the species is adapted to the extreme climatic conditions. The area is covered by up to 1.6-m-thick ice during 10 months of the year, and bottom water temperature is <0°C all year round. L. saccharina occurred from 2.5 m depth to a lower depth limit of about 20 m receiving 0.7% of surface irradiance. Specimen density and biomass were low, likely, because of heavy ice scouring in shallow water and intensive feeding activity from walruses in deeper areas. The largest specimens were >4 m long and older than 4 years. In contrast to temperate stands of L. saccharina, old leaf blades (2-3 years old) remained attached to the new blades. The old tissues maintained their photosynthetic capacity thereby contributing importantly to algal carbon balance. The photosynthetic characteristics of new tissues reflected a high capacity for adaptation to different light regimes. At low light under ice, or in deep water, the chlorophyll a content and photosynthetic efficiency (!) were high, while light compensation (E_c) and saturation (E_k) points were low. An E_c of 2.0 µmol photons m^-2 s^-1 under ice allowed photosynthesis to almost balance, and sometimes exceed, respiratory costs during the period with thick ice cover but high surface irradiance, from April through July. Rates of respiration were lower than usually found for macroalgae. Annual elongation rates of leaf blades (70-90 cm) were only slightly lower than for temperate L. saccharina, but specific growth rates (0.48-0.58 year^-1) were substantially lower, because the old blades remained attached. L. saccharina comprised between 5% and 10% of total macroalgal biomass in the area, and the annual contribution to primary production was only between 0.1 and 1.6 g C m^-2 year^-1.     

Large viruses and infected microeukaryotes in Ross Sea summer pack ice habitats

A variety of Ross Sea summer pack ice habitats between 66 and 75°S were examined for viruses 𔒦 nm capsid diameter. Maximum abundances of these viruses likely to infect eukaryotes were 10⁶-10⁷ ml^-1 brine in surface, interior, and bottom habitats and constituted up to 18% of the total (all sizes) viruses. There is abundant ultrastructural evidence for infection of a variety of microheterotrophs and some autotrophs. One station exhibited the classical characteristics of a lower latitude algal bloom with potential viral control. The blooming alga, Pyramimonas tychotreta Daugbjerg 2000, was infected, as were two abundant heterotrophs, Cryothecomonas spp. and an unidentified flagellate, that fed on P. tychotreta. Infections were observed in only one life history stage (multiflagellate cells) of P. tychotreta, suggesting a relationship among virus-induced lysis, life-history stages, physiology, and environmental factors regulating the life cycle. There is good evidence that diatoms are not a likely source of the large viruses, and viruses in general are not a major food source for ice microheterotrophs in summer.     

Long-term trends in abundance of cladocerans in the Central Baltic Sea

Long-term dynamics (1960-1997) of the cladoceran species Bosmina coregoni maritima, Evadne nordmanni and Podon spp. are described for the Gdansk Deep and the Gotland Basin (Central Baltic Sea). By using correlation analyses on seasonal time-series, the influence of temperature and salinity on the abundance of cladoceran species was investigated. A clear affinity to higher temperature was found for B. coregoni maritima in summer as well as for E. nordmanni and Podon spp. in spring. In addition to temperature, association tests with salinity revealed besides species-specific preferences, regional and temporal differences. Contrary to B. coregoni maritima, both other species were positively associated to salinity in summer and autumn in the Gdansk Deep. In the Gotland Basin only E. nordmanni was positively correlated to salinity in autumn. Differences in the response to hydrographic variables are possibly stage specific, i.e. between resting eggs and adults, or due to a different adaptation to the abiotic environment.     

Effects of exposure to toxic and non-toxic dinoflagellates on oxygen consumption and locomotion in stage 1 larvae of the crabs Cancer oregonensis and C. magister

Sublethal effects on larval crabs upon exposure to toxic dinoflagellates were examined in the laboratory in early 1999. Specifically, oxygen consumption rates and geotaxis responses were determined for stage 1 larvae of the crabs Cancer oregonensis (Dana) and C. magister Dana that were exposed to non-toxic (Alexandrium tamarense, strain 115) or toxic (A. fundyense, strain 1719) dinoflagellates or to freshly hatched nauplii of the brine shrimp Artemia sp. In C. oregonensis, larvae exposed to the toxic dinoflagellate showed reduced rates of oxygen consumption compared to those exposed to non-toxic dinoflagellates or brine shrimp nauplii. Larvae exposed to a filtrate of the non-toxic dinoflagellate showed no change in oxygen consumption, but a reduced rate when exposed to filtrate from the toxic alga at densities >5᎒² cells ml^-1. In C. magister, larvae exposed to the non-toxic A. tamarense or the toxic A. fundyense had reduced oxygen consumption rates. Larvae exposed to filtrates of non-toxic and toxic dinoflagellates had no change in oxygen consumption. In geotaxis tests, C. oregonensis larvae exposed for 1 day to the toxic A. fundyense reduced their level of locomotion compared to those exposed to non-toxic A. tamarense or to brine shrimp nauplii. C. magister larvae showed no change in activity after a 1-day exposure to the toxic A. fundyense. After a 4-day exposure to A. fundyense, C. magister larvae had much reduced locomotion. Reduced locomotory activity in larvae exposed to toxic algae is consistent with the changes in oxygen consumption rates. Responding to exposure to toxic algae by reducing locomotion may affect vertical migration in these negatively buoyant crab larvae, resulting in sinking below a toxic alga bloom, at least temporarily.     

Characterization of urease activity in three marine phytoplankton species,<Emphasis Type="Italic"> Aureococcus anophagefferens</Emphasis>,<Emphasis Type="Italic"> Prorocentrum minimum</Emphasis>, and<Emphasis Type="Italic"> Thalassiosira weissflogii</Emphasis>

The availability of different forms of nitrogen in coastal and estuarine waters may be important in determining the abundance and productivity of different phytoplankton species. Although urea has been shown to contribute as much as 50% of the nitrogen for phytoplankton nutrition, relatively little is known of the activity and expression of urease in phytoplankton. Using an in vitro enzyme assay, urease activities were examined in laboratory cultures of three species: Aureococcus anophagefferens Hargraves et Sieburth, Prorocentrum minimum (Pavillard) Schiller, and Thalassiosira weissflogii (Grunow) Fryxell et Hasle. Cultures of P. minimum and T. weissflogii were grown on three nitrogen sources (NO₃^m, NH₄⁺, and urea), while A. anophagefferens was grown only on NO₃^m and urea. Urease was found to be constitutive in all cultures, but activity varied with growth rate and assay temperature for the different cultures. For A. anophagefferens, urease activity varied positively with growth rate regardless of the N source, while for P. minimum, urease activity varied positively with growth rate only for cultures grown on urea and NH₄⁺. In contrast, for T. weissflogii, activity did not vary with growth rate for any of the N sources. For all species, urease activity increased with assay temperature, but with different apparent temperature optima. For A. anophagefferens, in vitro activity increased from near 0-30°C, and remained stable to 50°C, while for P. minimum, increased in vitro activity was noted from near 0-20°C, but constant activity was observed between 20°C and 50°C. For T. weissfloggii, while activity also increased from 0°C to 20°C, subsequent decreases were noted when temperature was elevated above 20°C. Urease activity had a half-saturation constant of 120-165 wg atom N l^у in all three species. On both an hourly and daily basis, urease activity in A. anophagefferens exceeded nitrogen demand for growth. In P. minimum, urease activity on an hourly basis matched the nitrogen demand, but was less than the demand on a daily basis. For T. weissflogii, urease activity was always less than the nitrogen demand. These patterns in urease activity in three different species demonstrate that while apparently constitutive, the regulation of activity was substantially different in the diatom. These differences in the physiological regulation of urease activity, as well as other enzymes, may play a role in their ecological success in different environments.     

Population dynamics and secondary production of the wedge clam <Emphasis Type="Italic">Donax</Emphasis><Emphasis Type="Italic">hanleyanus</Emphasis> (Bivalvia: Donacidae) on a high-energy,subtropical beach of Brazil

The population biology of Donax hanleyanus (Philippi, 1845) (Bivalvia: Donacidae) was studied by monthly sampling from June 1998 through June 2000, at Restinga da Marambaia Beach, Brazil. Two transects were established and divided into ten strata parallel to the waterline, and five replicates were taken with a 0.04 m² sampler in each stratum. The highest densities of D. hanleyanus were recorded in winter (September 1998, July 1999) and autumn (April 2000). A stratified distribution was observed: recruits were found mainly in the middle swash zone, while juveniles and adults occurred across the tidal gradient up to the retention zone. Mortality rates did not differ significantly between years. Annual production (in ash-free dry mass) ranged from 0.76 g AFDM m^-2 year^-1 (1998) to 3.67 g AFDM m^-2 year^-1 (1999), while the production-to-biomass (P/B) ratio varied from 1.45 to 1.59. Life span was ca. 17 months. Of all variables tested, only two, one biological and one physical, seem to have influenced the population dynamics of this species. A significant negative correlation between the densities of the suspension-feeders D. hanleyanus and Emerita brasiliensis (Crustacea: Decapoda) indicated possible intraguild competition, and there was a significant exponential correlation between beach slope and the mean across-shore position of D. hanleyanus. Data compiled from literature suggests a latitudinal gradient in population parameters, with mortality and renewal rate (P/B) of different species of Donax increasing and life span decreasing from temperate to tropical regions. Other factors influencing population dynamics, such as food availability, and the contributions of individual and community biomass to energy and nutrient cycling are also discussed.     

Nutritional quality of phloem sap in relation to host plant-alternation in the bird cherry-oat aphid

Summary. Host alternation in aphids has been attributed to complementary growth of host plants, or more specifically to seasonal changes in the nitrogen quality of the phloem sap. In this report, seasonal fluctuation of free amino acids in phloem of the winter and summer host plants (Prunus padus, bird cherry and Hordeum vulgare, barley) of Rhopalosiphum padi (the bird cherry-oat aphid) were investigated in the context of aphid growth and behaviour. Phloem was collected from the cut stylets of aphids taken from plants that were grown outdoors. The total concentration of amino acids in P. padus phloem increased between bud break and late flush (spring), decreased in mature leaves (summer), and increased again in early senescent leaves (autumn). In H. vulgare, however, amino acid concentration fluctuated less from seedlings to flowering. Spring aphids from P. padus grew rapidly on this host from bud break to late flush, but died on mature and early senescent leaves. Summer aphids from H. vulgare grew as fast on this host as spring aphids did on flush leaves of P. padus. Sexual females grew more slowly than other generations and nearly as well on mature as on early senescent P. padus leaves. As judged by aphid growth and phloem nitrogen quality, P. padus during spring equals the summer host H. vulgare. However, the lower growth rates of R. padi on mature and senescent leaves of P. padus appear only loosely correlated with phloem amino acid concentrations. Therefore, factors influencing aphid nutrition, or ecology, other than seasonal changes in phloem sap amino acid concentration may explain the existence of host alternation in R. padi.     

Cross-shelf variation in calanoid copepod production during summer 1996 off the Oregon coast, USA

The fecundity of nine species of adult female calanoid copepods, and molting rates for copepodite stages of Calanus marshallae were measured in 24 h shipboard incubations from samples taken during the upwelling season off the Oregon coast. Hydrographic and chlorophyll measurements were made at approximately 300 stations, and living zooplankton were collected at 36 stations on the continental shelf (<150 m depth) and 37 stations offshore of the shelf (>150 m depth) for experimental work. In our experiments, maximum egg production rates (EPR) were observed only for Calanus pacificus and Pseudocalanus mimus, 65.7 and 3.9 eggs fem^-1 day^-1 respectively, about 95% of the maximum rates known from published laboratory observations. EPR of all other copepod species (e.g., C. marshallae, Acartia longiremis and Eucalanus californicus) ranged from 3% to 65% of maximum published rates. Fecundity was not significantly related to body weight or temperature, but was significantly correlated with chlorophyll a concentration for all species except Paracalanus parvus and A. longiremis. Copepod biomass and production in on-shelf waters was dominated by female P. mimus and C. marshallae, accounting for 93% of the adult biomass (3.1 mg C m^-3) and 81% of the adult production (0.19 mg C m^-3 day^-1). Biomass in the off-shelf environment was dominated by female E. californicus, P. mimus, and C. pacificus, accounting for 95% of the adult biomass (2.2 mg C m^-3) and 95% of the adult production (0.08 mg C m^-3 day^-1). Copepodite (C1-C5) production was estimated to be 2.1 mg C m^-3 day^-1 (on-shelf waters) and 1.2 mg C m^-3 day^-1 (off-shelf water). Total adult + juvenile production averaged 2.3 mg C m^-3 day^-1 (on-shelf waters) and 1.3 mg C m^-3 day^-1 (off-shelf waters). We compared our measured female weight-specific growth rates to those predicted from the empirical models of copepod growth rates of Huntley and Lopez [Am Nat (1992) 140:201-242] and Hirst and Lampitt [Mar Biol (1998) 132:247-257]. Most of our measured values were lower than those predicted from the equation of Huntley and Lopez. We found good agreement with Hirst and Lampitt for growth rates <0.10 day^-1 but found that their empirical equations underestimated growth at rates >0.10 day^-1. The mismatch with Hirst and Lampitt resulted because some of our species were growing at maximum rates whereas their composite empirical equations predict "global" averages that do not represent maximum growth rates.     

Life-history traits of Plesionika martia (Decapoda: Caridea) from the eastern-central Mediterranean Sea

Life-history traits of Plesionika martia (Milne Edwards, 1883) were studied through data collected during six seasonal trawl surveys carried out in the Ionian Sea (eastern-central Mediterranean) between July 1997 and September 1998. P. martia was found at between 304 and 676 m depth, with the highest density in the 400-600 m range. Intraspecific, size-related depth segregation was shown. Recruitment occurred in summer at the shallowest depths. Juveniles moved to the deepest grounds as they grew. The largest female and male were 26 and 25 mm carapace length, respectively. The sex ratio was slightly in favour of females at depths >400 m. Although a seasonal spawning peak was shown, the reproduction appears to be rather prolonged throughout the year. Females with ripe gonads were found from spring to autumn. Ovigerous females with eggs in late maturity stage were found year round. Large females could spawn more than one time within their annual reproductive cycle. The size at first maturity (50% of the ovigerous females) was 15.5 mm CL. Average brood size of eggs with a well-developed embryo was 2,966ǃ,521. Iteroparity, low fecundity and large egg size patterns were observed. Brood size increased according to the carapace length. Two main annual groups were found in the field population of the Ionian Sea. Estimates of the Von Bertalanffy growth parameters are: L_X=30.5 mm, k=0.44 year^-1 in females; L_X=28.0 mm, k=0.50 year^-1 in males. A negative allometry was detected mostly in the ovigerous females. The life cycle of P. martia is discussed in the light of life-history adaptations shown in other deep-water shrimp species.     

Diet composition and quality for <Emphasis Type="Italic">Calanus finmarchicus</Emphasis> egg production and hatching success off south-west Iceland

Egg production and hatching success of the copepod Calanus finmarchicus was measured during spring and summer in the waters south-west of Iceland. Egg-production rates varied greatly, both temporally and spatially, with highest average rates found at a station with low chlorophyll-a concentrations (0.4 mg m^-3). Excluding this high production rate from statistical analysis, the remaining egg-production rates were found to be positively correlated with phytoplankton biomass, as well as with parameters representing healthy phytoplankton condition, food quality and diatom-type fatty acids. Hatching success of eggs was negatively correlated with some saturated and monounsaturated fatty acids related to phytoplankton senescence.     

Acoustical identification of the concentration layer of a copepod species,Calanus euxinus

Swimming trajectories of Calanus euxinus Hulsemann in the Black Sea were studied using an echosounder at 120 and 200 kHz. C. euxinus were acoustically discriminated with respect to vertical migration and swimming speed, according to dissolved oxygen (DO) concentration and the timing of migrations. Species became torpid in water with DO values <0.5 mg l^у. The time spent swimming under DO conditions between 2 and 5 mg l^у was insignificant, and varied greatly from the 10% to 25% of total time spent swimming under normoxic conditions (5-10 mg l^у). C. euxinus formed a concentration layer in the water of 1-3 m thickness. Upward migration was completed in about 3.5 h, starting 2.5 h before and ending 1 h after sunset (average rate: 0.95 cm s^у) in summer. Species ascended discretely from the suboxic to the lower boundry of the cold intermediate layer (CIL) at 0.82 cm s^у, and passed up the CIL and thermocline fast (2.3 cm s^у). Downward migration took less time (2 h), starting ~1 h before and ending ~1 h after sunrise. Swimming speed within the thermocline and CIL was 2.7 cm s^у; copepods subsequently returned to daylight depth at a sinking speed of 0.57 cm s^у. Total time for C. euxinus to settle to their nocturnal depth layer was about 5 h.     

Winter studies on zooplankton in Arctic seas: the Storfjord (Svalbard) and adjacent ice-covered Barents Sea

Zooplankton was sampled in the Storfjord and ice-covered Barents Sea during March 2003. Environmental conditions represented a typical winter situation with low air temperatures, close pack ice, and extremely low chlorophyll concentrations. Polar water dominated the hydrographic regime in the upper layers. Zooplankton distribution reflected spatial variability of hydrography. The copepods Pseudocalanus spp., Oithona similis, Microsetella norvegica together with gastropod larvae were most numerous. Biomass averaged for the entire water column varied from 3.3 to 14.3 mg dry mass m⁻³, Calanus glacialis and Parasagitta elegans contributed most, followed by C. finmarchicus, Oithona similis, and Pseudocalanus spp. Various holoplankters showed reproductive activity, especially cyclopoid copepods and chaetognaths. A few C. glacialis females laid eggs in situ, but when fed diatom cultures rapidly increased their egg production. Meroplankton including larvae of nudibranchia, bivalvia, ophiuroida, polychaeta, and bryozoa were also present. Our data demonstrate that the pelagic community of the seasonally ice-covered Barents Sea was not in a “sleeping” state at the end of the winter, but in addition to dormant stages, a portion of mainly omnivorous and several carnivorous species was reproducing.     

Estimating the influence of prawn stocking density and seagrass type on the growth of juvenile tiger prawns (Penaeus semisulcatus): results from field experiments in small enclosures

Fine mesh enclosures (0.9 m² in basal area, 1 m high, with 100 µm mesh) and a jet-net retrieval system were developed to test the influence of juvenile prawn stocking density on growth rates in (1) different months (April and October/November) and (2) different types of intertidal seagrass beds in the Embley River estuary of tropical Australia. Small juvenile tiger prawns (3-6 mm in carapace length, CL) were stocked in enclosures at densities of 4-32 prawns per enclosure (4.4-35.5 prawns m^-2) on a high biomass seagrass bed (about 70 g m^-2 of mostly Enhalus acoroides) and one with low biomass (about 10 g m^-2 of mostly Halodule uninervis). After 2-3 weeks in the enclosures, recovery rates, and hence possibly survival, were greater on the high biomass Enhalus than on the low biomass Halodule. However, not all fish and crustaceans could be excluded from the enclosures. Growth rates were twice as fast on the high biomass Enhalus than on the low biomass Halodule. It is likely that the high biomass Enhalus, with its greater surface area, supported more epiphytic flora and fauna and reduced the potential for interference competition between prawns, compared with the low biomass Halodule. Growth rates on Enhalus were significantly faster at a stocking density of 4 prawns per enclosure (1.3 mm CL week^-1) than at a stocking density of 16 and 32 prawns per enclosure (both 0.8 mm CL week^-1), and did not differ significantly between April and October/November (temperatures were about 30°C at both times). The mean growth rate at 8 prawns per enclosure (1.1 mm CL week^-1) did not differ significantly from those at 4, 16 and 32 prawns per enclosure. These results from two seagrass beds suggest that the carrying capacity for juvenile tiger prawns was greater in the high biomass Enhalus than the low biomass Halodule bed.