Differential grazing on protozoan microplankton by developmental stages of the calanoid copepod Eurytemora affinis Poppe

Nauplii and adults of the copepod Eurytemora affinis Poppe collectedfrom the Multiscale Experimental Ecosystem Research Center (MEERC)mesocosms and from the Choptank River (a subestuary of ChesapeakeBay) reduced ciliate and dinoflagellate microplankton densitiessignificantly during grazing experiments. Protozoan microplanktongenerally were consumed in proportion to their availability,although both adult copepod and naupliar clearance rates werehigher for larger prey ({small tilde}40µm). Ingestionof ciliate microplankton was confirmed by examining copepodgut fluorescence after brief (1 h) incubations with ciliateslabeled with a fluorescent vital stain (5-chloromethylfluoresceindiacetate). In addition, both adults and nauplii of E.affiniscleared protozoan microplankton at considerably higher ratesthan chlorophyll a. Naupliar clearance rates were generallyan order of magnitude lower than adult rates, but given naupliarabundances in copepod assemblages, they should contribute substantiallyto total grazing on protozoan microplankton. Grazing by thetotal copepod assemblage in mesocosms and in the field may beunderestimated if juvenile stages are ignored.     

Effects of prey motility and concentration on feeding in Acartia tonsa and Temora longicornis: the importance of feeding modes

Feeding experiments were conducted with the ambush-feeding copepodAcartia tonsa and the feeding-current-generating copepod Temoralongicornis. The copepods were offered a mixed diet of the dinoflagellateHeterocapsa triquetra and the ciliate Balanion comatum of similarcell size. The dinoflagellate was offered at a constant concentrationof 10–15 cells mL^–1, whereas the ciliate was offeredat a variety of concentrations, ranging from 7 to 57 cells mL^–1.Copepods with different feeding modes possess different mechanismsfor prey detection, suggesting that the two copepods would responddifferently to the two prey types. Both copepods had significantlyhigher clearance rates on the highly motile ciliate than onthe less motile dinoflagellate. In encounters between A. tonsaand its prey, we argue that this is due to the higher hydromechanicalsignal generated by the ciliate. The advection feeding copepodT. longicornis fed on the two prey according to their relativeconcentrations; in this case, we suggest that although B. comatumis capable of detecting feeding-current-generating predators,the feeding current velocity generated by T. longicornis isgreater than the escape velocity of this ciliate.     

Mesozooplankton Grazing on Picocyanobacteria in the Baltic Sea as Inferred from Molecular Diet Analysis

Our current knowledge on the microbial component of zooplankton diet is limited, and it is generally assumed that bacteria-sized prey is not directly consumed by most mesozooplankton grazers in the marine food webs. We questioned this assumption and conducted field and laboratory studies to examine picocyanobacteria contribution to the diets of Baltic Sea zooplankton, including copepods. First, qPCR targeting ITS-1 rDNA sequence of the picocyanobacteria Synechococcus spp. was used to examine picocyanobacterial DNA occurrence in the guts of Baltic zooplankton (copepods, cladocerans and rotifers). All field-collected zooplankton were found to consume picocyanobacteria in substantial quantities. In terms of Synechococcus quantity, the individual gut content was highest in cladocerans, whereas biomass-specific gut content was highest in rotifers and copepod nauplii. Moreover, the gut content in copepods was positively related to the picocyanobacteria abundance and negatively to the total phytoplankton abundance in the water column at the time of sampling. This indicates that increased availability of picocyanobacteria resulted in the increased intake of this prey and that copepods may rely more on picoplankton when food in the preferred size range declines. Second, a feeding experiments with a laboratory reared copepod Acartia tonsa fed a mixture of the picocyanobacterium Synechococcus bacillaris and microalga Rhodomonas salina confirmed that copepods ingested Synechococcus, even when the alternative food was plentiful. Finally, palatability of the picocyanobacteria for A. tonsa was demonstrated using uptake of ¹³C by the copepods as a proxy for carbon uptake in feeding experiment with ¹³C-labeled S. bacillaris. These findings suggest that, if abundant, picoplankton may become an important component of mesozooplankton diet, which needs to be accounted for in food web models and productivity assessments.     

Contrasting patterns of mussel abundance at neighbouring sites: does recruitment limitation explain the absence of mussels on Cook Strait (New Zealand) shores?

Wellington Harbour (New Zealand) supports large populations of mussels (Aulacomya maoriana, Mytilus galloprovincialis and Perna canaliculus), whereas these species are absent from Cook Strait shores only a few km away. The density of planktonic mussel larvae and their recruitment rates to artificial substrates were investigated at harbour (with mussels) and Cook Strait (no mussels) sites to determine if a diminished or a zero larval supply and/or settlement explains the absence of mussels from Cook Strait shores. At both locations, larvae were collected from the plankton approximately monthly between September 1998 and February 2000, and recruitment rates to artificial substrates were estimated between March 2000 and February 2001. Planktonic larval densities were almost an order of magnitude greater within the harbour than at coastal sites (mean (±S.D.) density was 982 m⁻³ (±1478) with a peak density in September 1998 of 4207 m⁻³, compared with 106 (±94) and 381 m⁻³, respectively, in March 1999). Larval recruitment at harbour sites was also significantly greater than at coastal sites (mean (±S.D.) recruitment density was 2169 m⁻² (±4207) with a peak of ca. 211,425 m⁻² in July 2000, compared with 88 m⁻² (±86) and ca. 3700 m⁻², respectively, in February 2001). It has been suggested that “bottom up” regulation of community structure, principally via a diet of particulates low in organic matter, is the explanation for the absence of suspension feeding mussels from Cook Strait sites [Helson, J. G., 2001. An investigation into the absence of mussels (Perna canalicus, Aulacomya maoriana and Mytilus galloprovincialis) from the South Coast of Wellington, New Zealand. Unpublished PhD thesis, Victoria University of Wellington, 183 pp.], but given that planktonic larval supply and recruitment rates are much reduced at coastal sites, these data may also be important in explaining the absence. Whether current levels of recruitment are sufficient to maintain an adult population is at present unknown and requires further examination.     

Effects of seston variability on the clearance rate and absorption efficiency of the mussels Aulacomya maoriana, Mytilus galloprovincialis and Perna canaliculus from New Zealand

The clearance rates (CR in l g⁻¹ h⁻¹) and net absorption efficiencies (AE) of three co-occurring mytilid species were estimated to determine the effects of variation in ambient seston quantity (total particulate matter, TPM, mg l⁻¹) and quality (particulate organic matter, POM, mg l⁻¹; percent organic matter, PCOM=POM/TPM) on these physiological functions. CR_S estimates were significantly different among the species (Mytilus galloprovincialis>Perna canaliculus=Aulacomya maoriana), but were not correlated with differences in species-dependent mean body size (P. canaliculus>M. galloprovincialis>A. maoriana). AE estimates were independent of mean body weight, and did not differ among the species. For all three species, CR responded in a simple linear manner to seston organic content, either in terms of POM (for A. maoriana and M. galloprovincialis), or PCOM (for P. canaliculus). The AE response, although also of a simple linear type, was species-dependent and involved interactions of two or three seston components: TPM, POM and PCOM for A. maoriana; POM and PCOM for M. galloprovincialis; and TPM and PCOM for P. canaliculus. For all three species, seston variation explained 15-20% of the variation in CR and 52-59% of the variation in AE. Comparisons among the species indicated that two very different responses to variation in seston quantity and quality exist. First, significant differences in CR result from species-dependent differences in the magnitude of the response to seston variation. Second, species-dependent responses to variation in seston variation resulted in the similarity of the AE responses. Thus, the three species appear to have evolved different strategies for dealing with seston variation, but with the end result that their AE responses do not differ. Finally, no evidence was found of a negative association between CR and AE for any of the three species, suggesting that under the seston conditions experienced in this work, these species do not have the physiological compensatory capacity to reduce CR in order to increase AE. The importance of a comparative (i.e., multi-species) approach utilising ambient seston is emphasised by the findings of this research if we are to understand better the feeding and digestive physiologies of suspension-feeding organisms such as mussels.     

Feeding, respiration and life history of the hyperiid amphipod Themisto libellula in the Arctic marginal ice zone of the Greenland Sea

Daily ingestion rates of the pelagic hyperiid amphipod Themisto libellula were studied in the marginal ice zone of the Arctic Fram Strait by feeding experiments, respiration measurements and an allometric approach based on body mass. Amphipods were collected by stratified multiple opening/closing net hauls and Rectangular Midwater Trawl (RMT 8) in August 2000 during the expedition ARK XVI/2 of R/V “Polarstern”. T. libellula occurred with abundances of 0.043 and 0.015 ind. m⁻³ in the upper 30 m of the water column at two RMT 8 stations. Based on respiration data, the daily ingestion necessary to cover metabolic energy demands measured 1.9±0.6% of body carbon per day. Actual prey consumption during feeding experiments with Calanus copepodids as prey was very similar and accounted for 1.9±1.5% day⁻¹, indicating that feeding on Calanus can meet the energy demands of T. libellula. In general, experimental results were slightly lower than the maximum potential ingestion (2% day⁻¹ for an individual of median body dry mass of 32 mg) estimated by an allometric equation based on body mass, but feeding experiments showed a strong variability. Reduced metabolism and low ingestion rates of T. libellula are consistent with low ambient temperature, large body size, slow growth and long life span of this polar species. The effect of the active pelagic life style of T. libellula on metabolism and ingestion rate is discussed in comparison to the sympagic (i.e. ice-associated) amphipod Gammarus wilkitzkii of similar body size living in the same environment. In relation to the mesozooplankton biomass in the investigation area, the predation impact by T. libellula was low. However, high-Arctic conditions also limit the secondary production of principal prey species, such as Calanus glacialis and Calanus hyperboreus, so that even low predation rates may affect the growth of prey populations.     

Individual to population level effects of South Louisiana crude oil water accommodated hydrocarbon fraction (WAF) on a marine meiobenthic copepod

Acute toxicities of crude oil and crude oil water accommodated hydrocarbon fraction (WAF) are relatively well documented, but data on the biological effects of chronic exposures to WAF on species and populations are scarce. South Louisiana Sweet crude oil was used to assess the effects of crude oil WAF on the copepod Amphiascus tenuiremis' survival, development and reproduction. Effects were evaluated using a 96-well microplate full life-cycle toxicity test, a test that allows tracking of individuals from the nauplius stage to sexual maturation and reproduction. Briefly, 24-h hatched nauplii were followed to adulthood (n_i = ≥ 120 nauplii/treatment) in individual glass-coated microplate wells containing 200 μL of seawater solution. Treatments consisted of 10%, 30%, 50% and 100% Louisiana WAF, with seawater used as control. Nauplii were monitored through development to adulthood, and sexually mature virgin copepods were mated pairwise in wells containing original rearing treatments. Nauplius-to-copepodite survival was reduced by 57% in exposures to 100% WAF, relative to controls (88 ± 3%), and copepodite-to-adult survival was reduced by 18% in the 50% WAF, relative to controls (98 ± 3%). Analysis of development curves showed that nauplii in the 10% WAF developed significantly faster into copepodites, while nauplii in the 50% WAF developed significantly slower than controls. Although the naupliar developmental rate in the 100% WAF was not significantly different from the control, these nauplii showed an average 1.4 day delay in development into copepodites. Similarly, copepodite development into mature females and males was significantly enhanced by 1.2 to 1.8 days and delayed by 1.9 to 2.2 days (p < 0.05) in the 10% and 50% WAFs, respectively, compared to controls. Although the copepodite developmental rate in the 100% WAF was not significantly different from the control, these copepodites still showed an average 1.5 and 2.1 day delay in development into females and males, respectively. Analysis of reproductive endpoints showed that fertility was the only endpoint negatively affected by WAFs; reproductive failure increased by 30% and 41% in exposures to 30% and 100% WAF, respectively, compared to controls (3.33 ± 4.71%). Leslie matrix population projections based on empirical microplate data indicated lower production rates through three generations of exposure to WAFs. Furthermore, a comparison between NIST and Louisiana crude oil WAFs using the same life-cycle approach indicated a greater chronic toxicity for the Louisiana WAF and an overall developmental delay in exposures to high WAFs (50% and 100% WAFs) from both crude oil types.     

Are the diatoms Navicula sp. and Thalassiosira fluviatilis suitable to be fed to the benthic harpacticoid copepod Tisbe biminiensis?

The objective of this study was to compare the influence of the diets of two diatoms, Navicula sp. (benthic) and Thalassiosira fluviatilis (planctonic), on the development, fecundity and survival of the harpacticoid copepod Tisbe biminiensis. In order to determine the optimal concentration of food, 35 egg-bearing females were submitted to six algal concentrations and controls (without food). After 24 h, the content of the recipients was fixed with 4% formalin and then fecal pellets produced by each female were counted and measured. The larval development was studied by surveying 50 nauplii on each diet individually until the adult stage, at intervals of 6 h. The cast exoskeletons were removed to count the number of segments and for measurement. The fecundity was obtained counting the naupliar production every 48 h of groups containing 10 females in different algal concentrations in both diets. The diet influence on fecundity was tested by submitting four groups of 10 females fed on optimal algal concentrations based on the fecal pellet experiments. In the Navicula sp. concentration of 0.4 μg Chl-a/ml, considered to be optimal for fecal pellets production, the diatoms were shown toxic, resulting in a low survival rate and inhibiting the egg production of copepods. The optimal concentration considering fecundity was estimated to be 0.1 μg Chl-a/ml for both diets. Copepods fed on Navicula sp. presented a faster development rate and higher naupliar production compared to copepods fed on T. fluviatilis. Size and survival did not vary significantly among diets. The algal concentration interfered significantly in the reproductive success of females. Both very low and very high algal concentrations reduced reproductive success. Concluding, the benthic diatom Navicula sp. was more favorable to the copepod T. biminiensis than the planktonic diatom T. fluviatilis.     

Hypoxia tolerance in the copepod Calanoides carinatus and the effect of an intermediate oxygen minimum layer on copepod vertical distribution in the northern Benguela Current upwelling system and the Angola-Benguela Front

There is a growing concern that hypoxic and anoxic areas in the sea spread in extent and intensity, posing a severe risk to marine ecosystems and fisheries. Hypoxia may affect fish stocks directly or via detrimental effects on important prey species, such as zooplankton. A unique feature of the northern Benguela Current upwelling region and Angola-Benguela frontal system is a pronounced intermediate oxygen minimum layer (IOML) at 60-500 m depth with oxygen concentrations ≤ 1.4 mg O₂l^− 1 (minimum < 0.7 mg O₂l^− 1). Field studies during February-March 2002 demonstrated that the abundance of calanoid copepods and the biomass of mesozooplankton in general were severely reduced within the IOML. The dominant copepod Calanoides carinatus showed a bimodal vertical distribution with parts of the population either comprising all developmental stages concentrated in the surface layer (0-60 m), or copepodids C5 diapausing below 400 m depth apparently avoiding the IOML. Accordingly, abundances of other calanoid copepods were higher at the surface and below 300 m than in the centre of the IOML. The scarcity of planktonic life within the IOML raises the question whether this layer represents an effective barrier for zooplankton vertical migrations. Especially in C. carinatus, ontogenetic vertical migration plays a key role in the retention of the population within the productive upwelling region and for the rapid re-colonisation of plumes of newly upwelled water. To address this issue, the hypoxia tolerance of C. carinatus was determined in a series of laboratory-based, closed-bottle experiments in January 2005. Copepods were kept in gas-tight bottles and the decreasing oxygen concentrations were monitored to establish their minimum oxygen demands. Although copepodids survived apparently unharmed at surprisingly low oxygen concentrations of ca. 1.5 mg O₂l^− 1, they could not tolerate oxygen levels < 1.1 mg O₂l^− 1, implying that the core of the IOML, where O₂ concentrations are below this threshold, is uninhabitable for C. carinatus. In contrast, the IOML may represent a refuge from competition and predation for other copepod species specifically adapted to hypoxic environments.     

New evidence for the involvement of Paracartia grani (Copepoda,Calanoida) in the life cycle of Marteilia refringens (Paramyxea)

The dynamics of the protozoan parasite Marteilia refringens was studied in Thau lagoon, an important French shellfish site, for 1 year in three potential hosts: the Mediterranean mussel Mytilus galloprovincialis (Mytiliidae), the grooved carpet shell Ruditapes decussatus (Veneriidae) and the copepod Paracartia grani (Acartiidae). Parasite DNA was detected by PCR in R. decussatus. In situ hybridisation showed necrotic cells of M. refringens in the digestive epithelia of some R. decussatus suggesting the non-involvement of this species in the parasite life cycle. In contrast, the detection of M. refringens in mussels using PCR appeared bimodal with two peaks in spring and autumn. Histological observations of PCR-positive mussels revealed the presence of different parasite stages including mature sporangia in spring and autumn. These results suggest that the parasite has two cycles per year in the Thau lagoon and that mussels release parasites into the water column during these two periods. Moreover, PCR detection of the parasite in the copepodid stages of P. grani between June and November supports the hypothesis of the transmission of the parasite from mussels to copepods and conversely. In situ hybridisation performed on copepodites showed labeling in some sections. Unusual M. refringens cells were observed in the digestive tract and the gonad from the third copepodid stage, suggesting that the parasite could infect a copepod by ingestion and be released through the gonad. This hypothesis is supported by the PCR detection of parasite DNA in copepod eggs from PCR-positive females, which suggests that eggs could contribute to the parasite spreading in the water and could allow overwintering of M. refringens. Finally, in order to understand the interactions between mussels and copepods, mussel retention efficiency (number of copepods retained by a mussel) was measured for all P. grani developmental stages. Results showed that all copepod stages could contribute to the transmission of the parasite, especially eggs and nauplii which were retained by up to 90%.     

Feeding response of a benthic copepod to ciliate prey type,prey concentration and habitat complexity

1. Protozoans are important consumers within microbial food webs and, in turn, they represent potential prey for small metazoans. However, feeding interactions within these food webs are rarely characterised and this is especially true for freshwater sediments. 2. We aimed to quantify the feeding links between a freshwater meiofaunal copepod and ciliates in two laboratory experiments. The first experiment addressed the response of Eucyclops serrulatus towards ciliate density and type (two ciliate species of the same genus differing in terms of body size). A second experiment assessed the effect of habitat structure on feeding rates by introducing different structural complexity into the feeding arena. In contrast to the first experiment, which was run only for one time period, this experiment also tested three different total feeding times (4, 7 and 9 h). 3. Eucyclops serrulatus exhibited high ingestion rates, with 3–69 ciliates copepod^?1 h^?1 consumed depending on food concentration, food type and habitat complexity. Copepods exhibited a preference for the smaller ciliate when total ciliate concentration was low, but selected both ciliates equally when food concentrations were medium or high. However, at very high food concentration, Eucyclops preferred the larger ciliate (which was 1/3 of its own body size), suggesting that the longer handling times of the larger prey are rewarding when the large prey is present in high numbers. In terms of total numbers consumed, copepods fed on more small ciliates, but in terms of carbon units both ciliates were selected equally when total prey concentration was low or medium. However, copepods derived more carbon from the larger prey at high and very high prey concentrations (up to 0.7 μgC out of a maximum of 1.1 μgC copepod^?1 h^?1). Habitat complexity influenced the feeding of copepods when it was observed over time. 4. The copepod–ciliate link is well known from the pelagic zone of both marine and freshwater habitats. We have shown its potential importance within the benthos, where it can be influenced by food identity, food quantity and possibly by habitat complexity.     

The production and ingestion of faecal pellets by nauplii of marine calanoid copepods

The downward transport of organic matter as zooplankton faecalmaterial is influenced by copepods which fragment, ingest andrecycle some of the pellet contents. Most of this activity hasbeen attributed to the later copepodite stages and the adults,but little is known about the role of nauplii. Stage-relateddefaecation rates during the naupliar development of two speciesof copepod, Calanus helgolandicus and Pseudocalanus elongatus,were quantified in a series of laboratory experiments. The productionof faecal material commenced soon after the appearance of theNIII in both species and increased throughout naupliar development.The causes of the increase were the formation of larger pelletsby later stages in Calanus and an increased rate of productionby Pseudocalanus. Calanus nauplii, when supplied with algalfood at concentrations that would support full naupliar development,ingested or broke up the pellets of the smaller Pseudocalanusspecies at rates of 1.15 pellets nauplius^–1 h^–1This consumption increased to 2.96 pellets nauplius^–1h^–1 when the concentration of algal food was reduced toa limiting level. Pseudocalanus was not able to consume thepellets of Calanus. Ingestion of Pseudocalanus faecal pelletsby Calanus could supply a nutritional benefit to a food-limitednauplius.     

Different diatoms induce contrasting effects on the reproductive success of the copepod Temora stylifera

Four diatoms, Thalassiosira rotula (THA), Thalassiosira weissflogii (TWEI), Skeletonema costatum (SKE) and Phaeodactylum tricornutum (PHA), and one dinoflagellate Prorocentrum minimum (PRO, control diet), were tested during the course of 14-day laboratory experiments examining the effects of these algae on the fecundity and egg viability in the copepod Temora stylifera. All algae were provided at the same mean carbon concentrations of 0.98 μg C ml⁻¹ to normalize the effects due to differences in diatom size and nutrient concentrations, at least with regards to carbon, so that variations in egg production and hatching viability were presumably due to other chemical constituents of the cells. Our results show that only PRO supported high reproductive success, with stable egg production rates and egg viability close to 100%. By contrast, all four diatoms negatively impacted egg production and hatching success, with varying degrees of suppression. The two diets SKE and PHA induced dramatic effects on reproductive success; SKE was the worst diet with naupliar production ceasing altogether within only 3 days. With PHA, fecundity never reached zero, but at the end of the experiments, initial egg production rates had dropped to 6.6% of initial rates, and viability had declined to zero within 7 days. The two Thalassiosira species mainly affected egg viability, so that after 14 days of feeding, a decrease of 90% had occurred. However, egg production rates were high and were reduced only by 15-24% towards the end of the experiments. In terms of potential recruitment rates, even though PRO induced high mean naupliar production, a slight decrease was observed during the first 5 days, likely due to the time required for T. stylifera to acclimate to PRO. After day 5, potential recruitment rate was higher, comparable to in situ values. THA and TWEI inhibited potential recruitment up to 90-100%, but these effects were not evident over a short-term period, denoting the importance of introducing the time factor when evaluating diatom effects on copepod production. With the other two diatom diets, SKE and PHA, recruitment was zero after 2 and 6 days, respectively. Experiments testing the effects of unsaturated aldehydes in THA (2-trans-4-trans-decadienal) and SKE (2-trans-4-trans-octadienal and 2-trans-4-trans-heptadienal) on the hatching viability of T. stylifera indicate that diatom-derived aldehydes may differ in terms of biological activity. Of the three molecules tested, decadienal had somewhat stronger effects on hatching success, with total blockage occurring at concentrations of 2.0 μg ml⁻¹; the concentrations to induce total blockage by octadienal and heptadienal were somewhat higher and ≥2.5 μg ml⁻¹. We suggest that the greater biological activity of SKE in terms of cell blockage may thus be due to the presence of antimitotic compounds other than these unsaturated aldehydes.     

Evidence of cannibalism and bentho-pelagic coupling within the life cycle of the mussel, Perna canaliculus

The feeding ecology of the green-lipped mussel, Perna canaliculus, was investigated within three intertidal mussel beds along Ninety Mile Beach, northern New Zealand, between August 2000 and March 2001. Adult mussels of different sizes (45-105 mm in shell length) were collected from the intertidal sites about 30 min after being submerged by the incoming tide for gut content analyses. Results of these analyses indicate that mussels consume a variety of phytoplankton, micro- and mesozooplankton, including mussel larvae and post-larvae. Cannibalism of juveniles of up to 620 μm was recorded for intertidal mussels, and conspecifics of up to 2.4 mm were found within the stomachs of additional mussels collected in August 2000 from a nearby subtidal site. For all three intertidal populations, mussel larvae and juveniles contribute about 70% of the food particle consumption during the spawning peak in August, while phytoplankton and other zooplankton constitute the majority of the food source (about 99%) in March, during gametogenesis. Larger intertidal mussels tended to have more food particles in their stomachs than smaller mussels within all three populations. Distinctive differences in food consumption among intertidal populations directly coincide with variations in total particulate matter (TPM), particulate organic matter (POM) and percent organic matter (OM) in the adjacent seawater.Separate experiments designed to test the feeding behavior of mussels feeding at different times during the incoming tide were conducted at one of the intertidal sites during August 2000 and March 2001. Results from these experiments indicate a marked shift in food consumption from bivalves to other mesozooplankton in August, and from phytoplankton to mesozooplankton in March. The observed combination of mussel predatory and grazing behavior over the incoming tide and through the year provides evidence for a strong food-web link between the benthic and pelagic life stages of this species. Furthermore, the high rate of cannibalism during some months of the year suggests that this source of food may significantly contribute to the energy budget of wild populations, with potential implications for evolutionary adaptive success.     

Biological interactions in enclosed plankton communities including Alexandrium catenella and copepods: Role of phosphorus

A microcosm approach was used to test whether: a) growth under unbalanced nutrient conditions (varying N:P ratios) affected the susceptibility of a phytoplankton community including the dinoflagellate Alexandrium catenella (a paralytic shellfish toxin producer) to mesozooplankton grazing, and b) the potential effects of unbalanced nutrient conditions were mediated by changes in toxicity of A. catenella or by other mechanisms. The experimental setup consisted of fifteen 30 l microcosms, filled with water from the Barcelona Harbour and subjected to treatments combining nutrient inputs at three different N:P ratios (Redfield N:P ratio or nutrient-balanced, high N:P and low N:P), addition or omission of A. catenella (an estimated initial concentration of 38 A. catenella cells ml^− 1, a value typical for blooms in harbours of the Catalan coast), and selective addition of a cultured population of Acartia grani. P sufficiency had a strong positive effect on the growth of A. grani, both with or without A. catenella addition, presumably due to enhanced food quality of the prey community. The presence of this copepod resulted in lower concentrations of ciliates, A. catenella, and other dinoflagellates, suggesting active grazing by the copepods. No noxious effects of A. catenella on the copepods were detected at the relatively low cell concentrations of that dinoflagellate used in the experiment.     

Interactions between Zooplankton and Crude Oil: Toxic Effects and Bioaccumulation of Polycyclic Aromatic Hydrocarbons

We conducted ship-, shore- and laboratory-based crude oil exposure experiments to investigate (1) the effects of crude oil (Louisiana light sweet oil) on survival and bioaccumulation of polycyclic aromatic hydrocarbons (PAHs) in mesozooplankton communities, (2) the lethal effects of dispersant (Corexit 9500A) and dispersant-treated oil on mesozooplankton, (3) the influence of UVB radiation/sunlight exposure on the toxicity of dispersed crude oil to mesozooplankton, and (4) the role of marine protozoans on the sublethal effects of crude oil and in the bioaccumulation of PAHs in the copepod Acartia tonsa. Mortality of mesozooplankton increased with increasing oil concentration following a sigmoid model with a median lethal concentration of 32.4 µl L⁻¹ in 16 h. At the ratio of dispersant to oil commonly used in the treatment of oil spills (i.e. 1∶20), dispersant (0.25 µl L⁻¹) and dispersant- treated oil were 2.3 and 3.4 times more toxic, respectively, than crude oil alone (5 µl L⁻¹) to mesozooplankton. UVB radiation increased the lethal effects of dispersed crude oil in mesozooplankton communities by 35%. We observed selective bioaccumulation of five PAHs, fluoranthene, phenanthrene, pyrene, chrysene and benzo[b]fluoranthene in both mesozooplankton communities and in the copepod A. tonsa. The presence of the protozoan Oxyrrhis marina reduced sublethal effects of oil on A. tonsa and was related to lower accumulations of PAHs in tissues and fecal pellets, suggesting that protozoa may be important in mitigating the harmful effects of crude oil exposure in copepods and the transfer of PAHs to higher trophic levels. Overall, our results indicate that the negative impact of oil spills on mesozooplankton may be increased by the use of chemical dispersant and UV radiation, but attenuated by crude oil-microbial food webs interactions, and that both mesozooplankton and protozoans may play an important role in fate of PAHs in marine environments.     

Growth and development of Calanus chilensis nauplii reared under laboratory conditions: testing the effects of temperature and food resources

We assessed growth and development of naupliar stages of Calanus chilensis Brodsky 1959, under a combination of three temperatures and two food levels in laboratory conditions. Both food supply and temperature significantly affected naupliar growth and development. High food, measured as chlorophyll a (Chl a) concentration, was 40 μg l⁻¹, on average, and yielded temperature-dependent growth rates in the range of 0.13-0.17 day⁻¹. Low food was about 1.2 μg chlorophyll a l⁻¹ and retarded or arrested development and drastically reduced the growth rate to the range of 0.05-0.09 day⁻¹. To test whether these experimental results were consistent with field data, we used published information on temperature and chlorophyll a variability in northern Chile and developed a combined temperature/food-dependent model to diagnose naupliar growth in the field through a 2-year seasonal cycle including the 1997-1998 El Niño conditions. We concluded that in the upwelling region off northern Chile, C. chilensis might seldom encounter conditions of food shortage, as those applied in the laboratory. Thus, naupliar growth of this species may be primarily controlled by environmental temperature and this might also be the case for the dynamics of the entire population inhabiting the coastal upwelling zone.     

The paradox high zooplankton biomass-low vegetal particulate organic matter in high turbidity zones: What way for energy transfer?

Estuarine ecosystems are characterized by high zooplanktonic biomasses, essentially constituted by copepods and mysids whose nutritional requirements are mainly provided by phytoplankton, an easily available carbon form. The Gironde estuary is characterized by high turbidities which limit light penetration in the water column and therefore primary production. Consequently, primary production is low and its availability for higher trophic level is very limited. The main goal of this study was to characterize the total vegetal particulate organic matter (POM) in high turbidity zones of the Gironde estuary during summer (a critical period characterized by high heterotrophic bacterial degradation and high zooplanktonic biomasses) and to analyse its utilization by zooplankton, using prey/predator experiments and trophic biomarkers (fatty acids). The specific goals were to define (i) how vegetal POM was exploited by the different zooplanktonic groups (protozoa, copepods and mysids) and (ii) which alternative preys could be used when vegetal POM was not sufficient to ensure their nutritional requirements.Chlorophyll biomass was very low in the MTZ during summer 2002 (0.48 ± 0.03 mg m^− 3). Total zooplankton grazing was low (19% d^− 1) probably due to a large contribution of detritus originating from terrestrial plants in vegetal POM compared to phytoplankton. The highest grazing pressure was exercised by the mysid Mesopodopsis slabberi due to its high abundances and by its almost entirely herbivorous diet (phytoplankton and small terrestrial detritus). Grazing rates (19.7 ± 4.2 and 9.6 μgC cop^− 1 d^− 1 for juveniles and adults, respectively) seemed to be sufficient to satisfy their daily carbon requirement. Grazing rate of the copepod Eurytemora affinis (139 ngC cop^− 1 d^− 1) seemed to be insufficient to cover its nutritional requirements and the copepods probably needed to complete a great part of their diet from protozoa. Grazing rates of the mysid Neomysis integer (24.7 ± 0.01 and 20.89 ± 8.45 μgC cop^− 1 d^− 1 for juveniles and adults, respectively) were higher than those of M. slabberi when feeding only on phytoplankton. However, when other preys were introduced in its environment, N. integer only fed on the copepod E. affinis with a preference for nauplii. The study revealed the great importance of protozoa and bacteria in the trophic transfers between vegetal POM and zooplankton in the MTZ during summer, despite the low protozoa grazing pressure on vegetal POM (3.1%). The detritic food chain probably implies various trophic transfers with little direct relationships between vegetal POM and zooplankton.     

Effect of culture density and antioxidants on naupliar production and gene expression of the cyclopoid copepod, Paracyclopina nana

Although attempts have been made to use mass cultures of marine copepods as live foods in marine aquaculture, some limitations such as low density culture still exist. The brackish water cyclopoid copepod, Paracyclopina nana has the potential for mass culturing as live food. In this study, we not only investigated the effect of culture density on the naupliar production and specific gene expressions of P. nana, but also the effect of several antioxidants under the conditions of a high density culture. The naupliar production of the copepod decreased with increasing culture density. The expression of glutathione reductase (GR), selenium-dependent glutathione peroxidase (SeGPx), glutathione S-transferase kappa (GST kappa), heat shock protein 40 (Hsp40), and Hsp70 genes of P. nana increased in the high density treatment but vitellogenin genes (Vg1 and Vg2) showed downregulation. In the condition with 20 inds./mL, vitamin C had a significant decrease but sodium selenite induced the naupliar production of P. nana greatly. The expressions of GR, SeGPx, Hsp70, and Vg genes increased with the vitamin C treatment. Sodium selenite caused a decrease of SeGPx and Hsp40 but GST kappa increased in the treatment with 20 inds./mL. These results suggest that sodium selenite is a positive antioxidant which can increase the culture efficiency of the copepod.     

Hypoxia and seasonal temperature: Short-term effects and long-term implications for Acartia tonsa dana

Far more attention has been given to the short-term lethal impacts of reduced dissolved oxygen on commercially important fish and crabs than to the long-term sublethal impacts on these same species, or on lower trophic levels. This study demonstrates that chronic, sublethal effects of hypoxia on the copepod Acartia tonsa, a critical component of many pelagic coastal food webs, can lead to significant decreases in population growth. The results of laboratory experiments conducted at 15 °C (winter) and 25 °C (summer), under conditions of normoxia (Controls), sublethal hypoxia (1.5 ml l^− 1) and lethal hypoxia (0.7 ml l^− 1) show that egg production female^− 1 day^− 1 was significantly lower at 0.7 ml l^− 1 compared to Controls at both temperatures, while egg production female^− 1 day^− 1 was significantly lower at 1.5 ml l^− 1 compared to controls in both summer experiments and in one of the two winter experiments. Survival was significantly decreased in the 0.7 ml l^− 1 treatment compared to Controls and the 1.5 ml l^− 1 treatment. Copepods developed more slowly and matured at smaller adult body sizes at both temperatures under both lethal and sublethal hypoxia compared to normoxia. Under summer temperatures, egg production was reduced by hypoxia exposure on two counts: (1) exposure to hypoxia during development resulted in smaller adults, which translated into lower egg production, and (2) egg production was still significantly lower in hypoxia treatments compared to Controls even when differences in body size were taken into account. While copepods collected in winter and exposed to winter temperatures and hypoxia also matured at smaller body sizes than copepods exposed to normoxia, egg production in winter was almost entirely attributable to this reduction in body size. These results suggest that coastal hypoxia may have a significantly greater impact in the summer months, when copepod populations are most abundant and growing at their most rapid rate of the year. With the anticipated increases in global temperatures, hypoxia may have even greater impacts on pelagic food webs.