Impact of freshwater on a subarctic coastal ecosystem under seasonal sea ice (southeastern Hudson Bay, Canada). III. Feeding success of marine fish larvae

We monitored the feeding success (percent feeding incidence at length and mean feeding ratio at length) of Arctic cod (Boreogadus saida) and sand lance (Ammodytes sp.) larvae in relation to prey density, light, temperature and potential predator density under the ice cover of southeastern Hudson Bay in the spring of 1988, 1989 and 1990. Both prey density and light limited larval fish feeding. The relationship between feeding success and actual food availability (nauplii density X irradiance) was adequately described by an Ivlev function which explained 64 and 76% of the variance in Arctic cod and sand lance feeding success respectively. By affecting both prey density and irradiance, the thickness of the Great Whale River plume (as defined by the depth of the 25 isohaline) was the main determinant of prey availability. Arctic cod and sand lance larvae stopped feeding when the depth of the 25 isohaline exceeded 9 m. Limitation of feeding success attributable to freshwater inputs occurred exclusively in 1988, the only time when the depth of the 25 isohaline exceeded the 9 m threshold. The close dependence of larval fish feeding success on the timing of the freshet and plume dynamics suggests a direct link between climate and survival of Arctic cod and sand lance larvae. The actual impact of climate fluctuations and/or hydro-electric developments on recruitment will depend on the fraction of the larval dispersal area of the two species that is affected by river plumes.     

Feeding strategies of planktonic cyclopoids in lacustrine ecosystems

Present knowledge of feeding tactics and behaviour, food composition and consumption, and feeding strategies of planktonic cyclopoid copepods is synthesized. Planktonic freshwater cyclopoids consume both plant and animal food. Predatory feeding is highly selective: prey species differ in their size, defense structures, the distance at which they are recognized by the cyclopoid, defensive behaviour when attacked, and their occurrence in the same space as the predator. Within a prey species, cyclopoids select for smaller individuals. The impact of cyclopoid predation on the other zooplankton may be an important source of mortality. However, algal material is consumed to a large extent by the adult and later developmental stages and is a necessary food source for the youngest stages, which have to compete with other planktonic herbivores. Some implications of these food requirements for life strategies of planktonic cyclopoid species in the seasonally changing environments are discussed.     

Annual cycle of distribution of three ice-associated copepods along the coast near Dumont d'Urville, Terre Adélie (Antarctica)

In polar regions sea ice is a site of enhanced primary production during winter and provides important habitat for small grazers, such as copepods. We sampled zooplankton from the sea ice and water column throughout 2005, near Dumont d'Urville station (Terre Adélie, Antarctica). Three species of ice-associated copepods were found: two calanoid copepods Paralabidocera antarctica and Stephos longipes and the harpacticoid copepod Drescheriella glacialis. P. antarctica was the most abundant of the three and was closely associated with the sea ice during most of the year. This species had a one year life cycle with a probable over-wintering period in the sea ice as nauplii and a short copepodite phase in spring. Reproduction and spawning occurred in early summer. A comparison with two other populations (near Syowa and Davis stations) along the east coast of Antarctica showed that there was a temporal shift in the life cycles of the three populations, which was linked to variability in sea ice conditions. D. glacialis was the second most abundant copepod and was more common during the winter than during summer, indicating its preference for the sea ice habitat. In autumn, the presence of D. glacialis in the deeper part of the water column suggested that this species colonised the sea ice from the benthos. S. longipes was found only in the water column near Dumont d'Urville and was not very abundant. In Terre Adélie particular environmental conditions, such as the absence of a permanent sea ice zone throughout the year, a longer time of open water, strong katabatic winds and the presence of polynyas, have influenced both the abundance and distribution of the three common ice-associated copepods.     

A model study with light-dependent mortality rates of copepod stages

This paper is based on an advanced ecosystem model of the Baltic Sea (ERGOM [J. Mar. Sys. 25 (3–4) (2005) 405]), but with an increased resolution of the zooplankton stage variable [J. Plankton Res. 23 (2001) 1217; ICES Marine Science 219 (2003) 208]. The model copepods are represented by five stages: eggs, an aggregated variable of nauplii, two aggregated groups of copepodites and adults. The transfer among the stages, i.e., hatching, molting and reproduction, is controlled by food availability and temperature.As usual, the model food web is truncated at the level of zooplankton. The study explores the effects of different parametrization of zooplankton mortality and looks in particular on light-dependent rates. The light climate may serve a proxy for the effects of visual feeding of fish larvae and fish. Different choices of the mortality parameters can result in remarkable differences in abundances and biomass of the model zooplankton and in the timing of its development.It is found that the different choices of mortality affect the development of populations in several ways: Relative small initial differences of abundances at the beginning of the spring bloom are important for the development of the model populations. Higher mortality rates are less important at food rich conditions than at scarce resources. At low phytoplankton levels, the individual development of the copepods through the stages can be faster for elevated mortality rates because then less animals have to share the available food.     

A numerical investigation of the impact of turbulence on the feeding rates of Oithona davisae

Individual based numerical simulations of the copepod, Oithona davisae, feeding on motile prey, Oxyrrhis marina, under variable turbulent conditions are performed. These simulations correspond to laboratory observations conducted by Saiz et al. [Saiz, E., Calbet, A., and Broglio, E., 2003. Effects of small-scale turbulence on copepods: the case of Oithona Davisae. Limnol. Oceanogr., 48:1304–1311.].The flow field in the simulation is reconstructed by a kinematic simulation whose characteristic scales are derived from the grid mesh and the dissipation rates of the laboratory experiments. The kinematic simulation provides a simplified model, which while not fully realistic, captures the basic relevant feature of turbulence. A hop and sink swimming behaviour is prescribed for O. davisae, while O. marina moves along helical paths with random changes of directions.Three possible effects are tested: the existence of a time threshold in the duration of the contacts between predator and prey, a progressive reduction of the perceptive distance with increasing turbulence level and an abrupt reduction in feeding of O. davisae when the flow speed, in relation to the copepod position, is higher than a prescribed threshold. This last approach introduces an intermittency in the feeding which depends on the variations of velocity both in space and time within the numerical box.The introduction of the time threshold causes a dome-shaped relationship between the simulated enhancement factor and the dissipation rate, while with the other two effects, a monotonic decrease in the enhancement factor is observed, with values reasonably close to the ones observed in the laboratory experiment. In all the cases, the use of realistic values of biological parameters (e.g. swimming behaviour) reproduces response curves in the range of the observations.     

The role of pelagic fish as forage for the demersal fish community in the southern Bay of Biscay

The small pelagic fish represents the link between nekton and demersal communities, and they are an important food source for some demersal fish species. In the autumns between 1993 and 2002, 74,869 stomach contents of 25 demersal fish species were analysed during the scientific surveys organized by the Instituto Español de Oceanografía. These species represent the demersal fish community of the southern Bay of Biscay. An important part of their diet (39% by volume) was composed of pelagic fish species (e.g. Engraulis encrasicolus, Gadiculus argenteus, Micromesistius poutassou, Trachurus trachurus, Sardina pilchardus, Scomber scombrus). Among all these prey species, the relevance of M. poutassou and G. argenteus stood out because of their high abundance both in the diets and during the scientific surveys. The relevance of pelagic fish as prey increased with predator size, reaching more than 60% of the diet by volume in the length range 25–29 cm. However, fish predators larger than 50 cm depended less on pelagic fish (33%), since they were also able to feed on other sources such as megafaunal invertebrates, mainly cephalopods. The pelagic fish resource was primarily exploited by 12 demersal fish species, with Merluccius merluccius and Zeus faber being the main ones feeding on small pelagics, which was related to the vertical movement of predators and prey through the water column. Survey abundance indices were used as indicators of prey abundance in the ecosystem, both for biomass and number. There was evidence for density-dependant feeding by predators on E. encrasicolus, G. argenteus and T. trachurus, while the main discrepancies between abundance in the stomachs and in the surveys were due to differential availability of prey length classes in the environment. Small prey species and individuals were exploited by most demersal fish while large prey species were less accessible to predators.     

Life cycle strategies of free-living copepods in fresh waters

Freshwater copepods live in habitats characterized by a high degree of instability. To survive occasional deterioration of their environment copepods have evolved adaptive mechanisms like dormancy or migration in order to avoid lethal conditions and to synchronize growth and reproduction with favourable abiotic and biotic conditions. Typical life cycles of harpacticoid, calanoid and cyclopoid copepods are presented to show strategies that have evolved to survive threatening environmental conditions.     

New species of the genus Xanthocalanus (Copepoda, Calanoida, Phaennidae) from the Laptev Sea

Sampling of the near-bottom calanoid copepods was performed on board of the research vessel Polarstern in 1993 in the Laptev Sea (Siberian Arctic). Three new species of Xanthocalanus were identified as such, and are described and figured here: X. spinodenticulatus sp. nov., X. laptevorum sp. nov. and X. polarsternae sp. nov.     

Seasonal carbon distribution of copepods in the eastern Weddell Sea, Antarctica

Copepods were sampled by a multiple opening-closing net in the eastern Weddell Sea during various seasons (late winter/early spring, summer, autumn). Total copepod biomass integrated over the upper 1000 m varied seasonally between 1.7 mg C m⁻³ in late winter/early spring and 3.7 mg C m⁻³ in autumn. After the dark season the copepods were rather evenly distributed vertically and highest biomass levels were found in the mid-water layers between about 200 m and 500 m. By contrast, especially in summer but also in autumn copepod biomass concentrated in the uppermost water layer. A total of 64 calanoid species were identified in the upper 1000 m with maximum species numbers in the deepest layer. The large calanoids Calanus propinquus, Calanoides acutus, Metridia gerlachei, Euchaeta antarctica and the small calanoid Microcalanus pygmaeus prevailed and accounted for 60–70% of total copepod biomass, while the small poecilostomatoid Oncaea and the cyclopoid Oithona species comprised about 20%. Hence, the distribution pattern of the entire copepod biomass is strongly influenced by the life cycles of a few dominant species.     

Linking mercury exposure to habitat and feeding behaviour in Beaufort Sea beluga whales

Mercury (Hg) levels in the Beaufort Sea beluga population have been increasing since the 1990's. Ultimately, it is the Hg content of prey that determines beluga Hg levels. However, the Beaufort Sea beluga diet is not understood, and little is known about the diet Hg sources in their summer habitat. During the summer, they segregate into social groups based on habitat use leading to the hypothesis that they may feed in different food webs explaining Hg dietary sources. Methyl mercury (MeHg) and total mercury (THg) levels were measured in the estuarine-shelf, Amundsen Gulf and epibenthic food webs in the western Canadian Arctic collected during the Canadian Arctic Shelf Exchange Study (CASES) to assess their dietary Hg contribution. To our knowledge, this is the first study to report MeHg levels in estuarine fish and epibenthic invertebrates from the Arctic Ocean. Although the Mackenzie River is a large source of Hg, the estuarine-shelf prey items had the lowest MeHg levels, ranging from 0.1 to 0.27 μg/g dry weight (dw) in arctic cisco (Coregonus autumnalis) and saffron cod (Eleginus gracilis) respectively. Highest MeHg levels occurred in fourhorn sculpin (Myoxocephalus quadricornis) (0.5 μg/g dw) from the epibenthic food web. Beluga hypothesized to feed in the epibenthic and Amundsen Gulf food webs had the highest Hg levels matching with high Hg levels in associated food webs, and estuarine-shelf belugas had the lowest Hg levels (2.6 μg/g dw), corresponding with the low food web Hg levels, supporting the variation in dietary Hg uptake. The trophic level transfer of Hg was similar among the food webs, highlighting the importance of Hg sources at the bottom of the food web as well as food web length. We propose that future biomagnification studies incorporate predator behaviour with food web structure to assist in the evaluation of dietary Hg sources.     

Evidence of episodic changes in deep-sea mesozooplankton abundance and composition in the Levantine Sea (Eastern Mediterranean)

Mesozooplankton samples from the 4270-m deep Ierapetra Basin in the oligotrophic Levantine Sea have revealed a strong temporal variability in abundance and composition hitherto unknown for the deep sea pelagic zone. The phenomenon reflected by a survey in June 1993 as compared to January 1987 is assessed by (i) the increase of the mesozooplankton standing crop throughout the water column, (ii) the absence of a decline in mesozooplankton abundance with increasing depth below 1000 m, (iii) the outstanding dominance of two interzonal calanoid copepods at bathypelagic and abyssopelagic depths and (iv) a substantial faunal change due to the codominance of Calanus helgolandicus, which was found for the first time in the area. The underlying hydrographical factors are addressed in the light of the Eastern Mediterranean Transient (EMT). Apart from a possible long-term faunal change, the EMT may exaggerate episodic plankton blooms and surface-abyssopelagic coupling in space and time.     

Distribution and diet of larval and juvenile Arctic cod (Boreogadus saida) in the shallow Canadian Beaufort Sea

The distribution and diet of larval and juvenile Arctic cod (Boreogadus saida) were studied during summer 2005 in the coastal Canadian Beaufort Sea. A total of 275 individuals were captured and the highest abundance was observed at station depths of 20–30 m. This corresponds well with the location of the frontal zone where the Mackenzie River plume water and open sea water meet. Diet examinations were performed on 220 Arctic cod, which were found undamaged from sampling. We observed a gradual decrease in prey number per fish and increase in prey size as larvae grew which corresponded to a shift from Rotifera and nauplii towards larger copepodid stages. However, at all sizes, the larvae remain generalists and feed on a broad range of organisms. Environmental changes due to climate warming could have a two-fold impact on fish larvae feeding in the studied region. First, the potential for increased primary production may lead to increased zooplankton production that may impact the feeding and nutrition positively. On the other hand, greater discharge of turbid water from the Mackenzie River may reduce light penetration in the water column that may negatively influence the ability of visual predators to successively forage.     

Life cycle strategies of epipelagic copepods in the Southern Ocean

Twelve epipelagic copepod species were reviewed to compare their adaptations to the short primary production season and low temperatures which characterise the Southern Ocean. The species show a spectrum of adaptations, but three broad life cycle strategies were defined: (1) herbivorous in summer, a short reproductive period and winter diapause at depth (Calanoides acutus and possibly Ctenocalanus citer); (2) predominantly omnivorous/detritivorous diet, an extended period of feeding, growth and reproduction and less reliance on diapause at depth (Metridia gerlachei, Calanus propinquus, Calanus simillimus, Oithona similis, Microcalanus pygmaeus, and possibly Oncaea curvata and Oithona frigida); (3) overwintering and feeding within sea ice as early nauplii or copepodids (Stephos longipes and Paralabidocera antarctica). The large species Rhincalanus gigas appears to be intermediate between strategies (1) and (2). Contrasting species from groups (1) and (2), namely C. acutus and O. similis, were selected for more detailed comparison. For C. acutus, maximum (probably food saturated) feeding and egg production rates are well below equivalent values for Calanus spp. at lower latitudes. Likewise, summer growth and moulting rates are slower, and the growth season of this herbivore is only 2–4 months. Therefore, both the low summer temperatures and short primary production season seem to dictate a long (1 year) life cycle for C. acutus. A collation of data on O. similis revealed that its abundance increases about tenfold from the Antarctic shelf northwards to the Polar Frontal Zone, where abundances are similar to those in temperate and tropical shelf seas. In contrast to C. acutus, O. similis appears to remain in the epipelagic and reproduce there year-round, although the food sources which sustain this are still uncertain.     

Additions to Mexican freshwater copepods with the description of the female Leptodiaptomus mexicanus (Marsh)

A survey on 10 temporary and permanent water-bodies, located at more than 2000 m above the sea level in the central part of Mexico resulted in the presence of 17 cyclopoid and five calanoid copepods. From the total taxa found, seven are new to the country: Leptodiaptomus assiniboiaensis (Anderson), Microcyclops dubitabilis Kiefer, Ectocyclops rubescens Brady, Eucyplops cf. bondi Kiefer, Eucyclops cf. prionophorus Kiefer, Eucyclops pseudoensifer Dussart, and Eucyclops cf. solitarius. The female of Leptodiaptomus mexicanus, a species known only from a single male found near Mexico City in 1929, is here described for the first time. Copepod fauna for each of the systems was highly diverse, with up to 12 species co-occurring in temporary ponds. This study revealed a mixture of north and south American taxa in the area.     

带吊舱水下机器人运动建模与仿真(英文)

To provide a simulation system platform for designing and debugging a small autonomous underwater vehicle’s (AUV) motion controller, a six-degree of freedom (6-DOF) dynamic model for AUV controlled by thruster and fins with appendages is examined. Based on the dynamic model, a simulation system for the AUV’s motion is established. The different kinds of typical motions are simulated to analyze the motion performance and the maneuverability of the AUV. In order to evaluate the influences of appendages on the motion performance of the AUV, simulations of the AUV with and without appendages are performed and compared. The results demonstrate the AUV has good maneuverability with and without appendages.     

基于重叠网格技术的主动式水翼数值模拟研究（英文）

In marine engineering, appendages such as fin stabilizers and/or T-foils are made to rotate and to reduce the motion of ships.Research on the hydrodynamics of ships fitted with active appendages has significantly improved the design and control of such ships. However, most studies focus on fixed rather than rotating appendages, thereby ignoring the hydrodynamic unsteadiness of active appendages. To enhance the reliability and precision of the numerical simulations, we propose the use of overlapping grids for simulating advanced catamarans fitted with a pair of rotating T-foils under each bow. The fundamental purpose of the overlapping grid technique is to realize information exchange via regional overlap sharing in each subdomain of the computing domain, instead of using the method of boundary sharing, thus greatly alleviating the difficulty of generating the subdomain grid;moreover, the technique guarantees the quality of the subdomain grid. Within the main computational domain, a subdomain was allocated to accommodate the T-foil. Overlapping meshes near the interface between the two domains enable information flow during the simulation; the overlapping grids are updated at every iteration step because the subdomain rotates. The instantaneous trim and sinkage responses of the catamaran to the T-foil rotation were reproduced. From the moment the active T-foil stopped moving, there was no change in the ship's sailing attitude, indicating that the response was in real time. By comparing with EFD data, the numerical results showed reasonable agreement, indicating the feasibility and effectiveness of the technique in simulating the hydrodynamics of ships fitted with active appendages.     

The influence of food quality on the nutritional acclimation of the copepod Acartia clausi

The influence of food quality on the nutritional metabolism of Acartia clausi was studied experimentally using four different diets: (1) diatom cells of Thalassiosira weissflogii, (2) detritus prepared from the same culture, (3) 50 : 50 mix on a protein basis of the two previous diets, and 4) dinoflagellate cells of Prorocentrum micans. For each trophic, ingestion, gut transit time, trypsin activity and K_m (half saturation constant) were measured at limiting and saturating concentration. Assimilation rates were also estimated for both pure diatoms and mixed live–detrital cell diets. Ingestion followed a Holling type 2 response for diets 1 and 4, a linear one for detritus and an intermediate response for diet 3. Gut transit time displayed different adaptive changes with food regime depending on protein concentration. Trypsin activity was lower for detrital food and trypsin K_m significantly decreased with increasing concentration of live diatoms. Assimilation rates were higher for live food than for mixed live–detrital food. Results illustrated that Acartia-type copepods optimize nitrogen or protein uptake. They suggest that besides chemoreception-mediated selectivity, internal controls by digestion and assimilation also regulate ingestion (feed-back).     

Quantifying copepod kinematics in a laboratory turbulence apparatus

We describe application of a new apparatus that permits simultaneous detailed observations of plankton behavior and turbulent velocities. We are able to acquire 3D trajectories amenable to statistical analyses for comparisons of copepod responses to well-quantified turbulence intensities that match those found in the coastal ocean environment. The turbulence characteristics consist of nearly isotropic and homogeneous velocity fluctuation statistics in the observation region. In the apparatus, three species of copepods, Acartia hudsonica, Temora longicornis, and Calanus finmarchicus were exposed separately to stagnant water plus four sequentially increasing levels of turbulence intensity. Copepod kinematics were quantified via several measures, including transport speed, motility number, net-to-gross displacement ratio, number of escape events, and number of animals phototactically aggregating per minute. The results suggest that these copepods could control their position and movements at low turbulence intensity. At higher turbulence intensity, the copepods movement was dominated by the water motion, although species-specific modifications due to size and swimming mode of the copepod influenced the results. Several trends support a dome-shaped variation of copepod kinematics with increasing turbulence. These species-specific trends and threshold quantities provide a data set for future comparative analyses of copepod responses to turbulence of varying duration as well as intensity.     

Analysis of the parasitic copepod species richness among Mediterranean fish

The Mediterranean ichthyofauna is composed of 652 species belonging to 405 genera and 117 families. Among these, 182 were studied for their parasitic copepods. The analysis of all the works conducted on these crustacea yielded 226 species distributed in 88 genera and 20 families. For each fish species we have established a file providing the species name of the fish, its family, its geographical distribution within the Mediterranean and some of its bio-ecological characteristics. Within each file, all the parasitic copepod species reported on each host species were listed. This allowed to know the species richness (SR) of these hosts. We thus produced 182 files within which 226 copepod species are distributed. A program was created under the Hypercard software, in order to analyse our data. Two parameters were studied. The first one is the mean species richness (MSR), which corresponds to the mean of the different SR found on the different host species. The second is the parasite–host ratio (P/H), which is the ratio of the number of copepod species by the number of host species. These parameters are calculated by our program for all the 182 species of Mediterranean fishes retained in our investigation, on the first hand, and, on the second hand, for one particular group of fish species. We used the following variables to investigate their correlations with copepod species richness: taxonomy—fish families, genera and species; biometry—maximal size of the adult fish; eco-ethology—mode of life (benthic, pelagic or nectonic), displacements (sedentary, migratory with environmental change, or migratory without environmental change), behaviour (solitary or gregarious). Other variables (colour, food, reproduction, abundance, distribution area) were also analysed but did not reveal any clear correlation. Providing that our study does not rely on quantitative (prevalence, intensity) but qualitative basis our aim was only to reveal some tendencies. These tendencies are as follows: (1) In many cases, parasite and host phylogeny seem to play an important role. There are fish families with copepods and families with few species of these parasites. The phyletic constraints could be due to the morphological characteristics of the habitat (e.g. structure of the gills) or biological/ecological characteristics that we were unable to identify. (2) It appears that the presence in a same environment of related fish species (e.g. several species of the same genus, or numerous genera of the same family) is correlated with high parasite richness. A likely explanation is that such situations favours alternated processes of lateral transfers and speciation. (3) Some eco-ethological criteria seem to favour the establishment of a large parasite species richness. It should be noted for instance that Mediterranean fishes the most often infected with copepods are generally nectonic or pelagic, migratory, and gregarious species.     

Modelling Pseudocalanus elongatus stage-structured population dynamics embedded in a water column ecosystem model for the northern North Sea

This paper outlines an approach to couple a structured zooplankton population model with state variables for eggs, nauplii, two copepodites stages and adults adapted to Pseudocalanus elongatus into the complex marine ecosystem model ECOHAM2 with 13 state variables resolving the carbon and nitrogen cycle. Different temperature and food scenarios derived from laboratory culture studies were examined to improve the process parameterisation for copepod stage dependent development processes. To study annual cycles under realistic weather and hydrographic conditions, the coupled ecosystem–zooplankton model is applied to a water column in the northern North Sea. The main ecosystem state variables were validated against observed monthly mean values. Then vertical profiles of selected state variables were compared to the physical forcing to study differences between zooplankton as one biomass state variable or partitioned into five population state variables. Simulated generation times are more affected by temperature than food conditions except during the spring phytoplankton bloom. Up to six generations within the annual cycle can be discerned in the simulation.