Copepod Diet Research Articles

Energy demand of larval and juvenile Cape horse mackerels, Trachurus capensis, and indications of hypoxia tolerance as benefit in a changing environment

Trachurus capensis is an important fisheries resource in the degraded Namibian upwelling ecosystem. Food supply and shoaling of hypoxic zones are hypothesised to influence the species’ recruitment success. This paper is the first to quantify energy requirements and hypoxia tolerance of larval and juvenile stages of a Trachurus species. We measured normoxic respiration rates of T. capensis with a size range from 0.001 to 20.8 g wet mass (WM) collected off Cape Town (33.9°S, 18.5°E, 12/2009) and in the northern Benguela (17–24°S, 11–15°E, 02/2011). Routine metabolic rate (RMR) and standard routine rate (SRR) (mg O2 h−1) followed the allometric functions RMR = 0.418 WM0.774 and SRR = 0.275 WM0.855, respectively. Larvae and juveniles had comparatively high metabolic rates, and the energy demand of juveniles at the upper end of the size range appeared too high to be fuelled by a copepod diet alone. T. capensis’ early life stages showed a high tolerance to hypoxic conditions. RMR in larvae did not change until 30 % O2sat at 18 °C. In juveniles, critical oxygen saturation levels were low (PC for SRR = 11.2 ± 1.7 % O2sat and PC for RMR = 13.2 ± 1.6 % O2sat at 20 °C) and oxy-regulation effective (regulation index = 0.78 ± 0.09). A high hypoxia tolerance may facilitate the retention of larvae in near-shore waters providing favourable feeding conditions and allowing juveniles to exploit food resources in the oxygen minimum zone. These mechanisms seem to well adapt T. capensis to a habitat affected by spreading hypoxic zones and probably enhance its recruitment success.

Influence of diet on chitobiase-based production rates for the harpacticoid copepod Tigriopus californicus

The influence of diet on copepod production is important when examining energy transfer within marine food webs, yet a direct link between diet and copepod production is lacking. Recently, measurements of the crustacean moulting enzyme chitobiase have allowed for rapid estimates of production rates for zooplankton communities. We tested the sensitivity of this method as a tool for measuring the productivity response of copepods to different diets by rearing the harpacticoid splash-pool copepod, Tigriopus californicus, on single and mixed species phytoplankton diets. Chitobiase-based productivity measurements demonstrate that a single species algal diet of the diatom Thalassiosira weissflogii was the best food item for T. californicus. In contrast, the dinoflagellate Amphidinium carterae was likely toxic given that copepods reared on this diet spent more time in early developmental stages and ultimately failed to survive. Copepod production and daily growth rates of T. californicus were higher on the T. weissflogii and natural food assemblage diets compared with the A. carterae or mixed (A. carterae/T. weissflogii) diets. Our use of the chitobiase method in a lab setting demonstrates the potential utility and sensitivity of this approach for field studies examining the impact and significance of short-term shifts in food quality on entire crustacean zooplankton communities.

First record of killer whales (Orcinus orca) feeding on Atlantic salmon (Salmo salar) in northern Norway suggest a multi-prey feeding type

Killer whales (Orcinus orca) are widely distributed throughout the world’s oceans, where they forage on a large variety of different prey species ranging from fish to marine mammals and birds (Forney & Wade, 2007). It has been shown that feeding ecology is of central importance to the killer whales’ social network. Food availability has a direct influence on group structure, and it is under debate to what extent the sociality of killer whales is flexible enough to adapt to local ecological conditions (Beck et al., 2012; Foster et al., 2012). In the north-east Pacific some highly specialized sympatric populations have evolved, with resident groups foraging on salmonids, transient groups feeding on marine mammals, and offshore groups feeding on fish such as sharks (Ford et al., 1998). Often these specialized populations show high site and time fidelity, such as the northern and southern resident lines in the north-east Pacific which feed primarily on Chinook salmon (Oncorhynchus tshawytscha) (Ford & Ellis, 2006). The survival of these killer whales seems to depend on the Chinook salmon’s year-round abundance (Ford et al., 2010). In the north-east Atlantic at least two different populations have been recently identified, including a herring feeding population in Norway and Iceland (Foote et al., 2011). In addition, two ecotypes of North Atlantic killer whales with morphological differences were determined: Type 1, with severe apical tooth wear, a generalist type with a length of up to 6.6 m, presumably feeding on fish and to some extent on seals; and Type 2, with no apical tooth wear, a highly specialized type with a length of up to 8.5 m, presumably feeding on other whales (Foote et al., 2009). Further, marine mammal–feeding killer whales were described from Scottish inshore waters (Bolt et al., 2009). In Irish waters, killer whales mostly feed on fish; stomach contents revealed a diet of siphonostomatoid copepods (Cecrops latreilli), ocean sunfish (Mola mola), mullet (Chelon labrosus) and salmon (Salmo salar) (Ryan & Wilson, 2003; McHugh et al., 2007, Ryan & Holmes, 2012), and in the 1970s a killer whale was observed hunting salmon in the Lough Foyle Estuary (Wilson & Pitcher, 1979). In the waters around the British Isles, herring and salmon were identified as prey species (Evans, 1988). Thus, we are only starting to understand the variety of the North Atlantic killer whales’ feeding ecology, especially the degree of prey specializations. Killer whales studied in Norway feed mostly on herring, with tagged animals showing high site and time fidelity to Fig. 1. Salmon caught by killer whale in Oksfjord, northern Norway, 2011. Photograph kindly provided by Geir Notnes.

Zooplankton distribution and feeding in the Arctic Ocean during a Phaeocystis pouchetii bloom

In early summer 2007 we determined the vertical distribution of mesozooplankton (>200μm) and assessed the copepod feeding rates in 19 stations distributed along the East Greenland Current and the Fram Strait. The study coincided with a bloom of the haptophyte Phaeocystis pouchetii in the colonial form. Copepods dominated the zooplankton community numerically, and were mainly distributed within the upper 150m (except for Metridia longa and Oithona spp., that inhabited deeper waters), without showing a clear avoidance of the P. pouchetii layer. Copepod diet was diverse, ciliates having a relevant share (40% of the diet). Copepods also displayed active grazing upon the colonies of P. pouchetii. In general, feeding rates were low (on average, daily ration was 1.6% of body carbon), likely due to the scarcity of nano and microplankton during the study (<100μgCL−1). Consequently, the trophic impacts on both the nano- and microplankton standing stocks and on primary production were negligible. These results suggest that during the period of study the transfer of carbon and energy from lower trophic levels towards copepods was low.

Natural abundance stable isotopes and dual isotope tracer additions help to resolve resources supporting a saltmarsh food web

We used a combination of natural abundance stable isotopes (NASI) and multiple isotope additions to examine the importance of local and tidally imported basal resources to infauna in a tidal, saltmarsh creek within the Plum Island Estuary, Massachusetts, U.S.A. NASI analysis was conducted on primary producers and infauna prior to isotope additions. While algal resources as a group had distinct NASI values compared to Spartina spp. resources, various algal resources [sediment-associated algae (local) and phytoplankton (tidally imported)] had similar NASI leading to low dietary resolution with mixing models. To separate primary producer isotope values, we conducted a large-scale, 42 day 15N-addition to the whole saltmarsh creek, which enriched tidally imported (i.e., phytoplankton) and local algae resources. We also conducted small-scale additions of enriched 13C to 1-m 2 plots within the creek receiving the 15N addition to separately label local resources. Enriched 15N and 13C were taken up by filamentous algae and associated epiphytic diatoms and benthic microalgae creating distinct isotope values in local algal resources. In addition, changes in 15N-tracer uptake by phytoplankton during the first week of the 15N-addition provided a unique enrichment pattern that was followed through the food web. Both NASIs and isotope additions indicated that Spartina spp. was not an important basal food resource to infauna, lending credence to the notion that saltmarsh food webs don't often rely on macrophyte detritus. Isotope additions indicated that various types of algae, both local and tidally imported, were important to the diets of infauna, including polychaetes, tanaids and harpacticoid copepods. Isotope additions increased diet resolution and revealed less accurate basal resource contributions determined with NASI alone. While many food web studies rely on NASI, we feel that the combination of NASI and isotope additions is needed in systems with similar primary producer values to more accurately determine resource contributions, to increase resolution of dietary contributions and to determine the dietary importance of local basal resources.

Role of midwater chaetognaths in Southern Ocean pelagic energy flow

We estimate the energy flow through mesoand bathypelagic chaetognaths in the Atlantic sector of the Southern Ocean from (1) depth-structured chaetognath abundance and body mass data, (2) a general chaetognath respiration model driven by body mass, temperature, water depth and taxon, and (3) published relationships between respiration, production and consumption in chaetognaths. In the 500 to 2000 m depth layer chaetognaths have a mean biomass of 0.109 mg C m–3 in summer and 0.146 mg C m–3 in winter. Chaetognaths respiration and consumption amount to 282 and 563 mg C m–2 yr–1, respectively. Thus, Antarctic midwater chaetognaths consume 0.05% of the copepod standing stock per day or 1% of the daily copepod production in summer. About 2.8% (= 1.9 g C m–2 yr–1) of the net annual primary production is required to fuel the midwater chaetognath community via herbivorous copepods. When assuming a 1:1 diet of herbivorous and carnivorous copepods, this share increases to 6.1% (= 4.1 g C m–2 yr–1) of annual primary production. It is estimated that chaetognath consumption for the whole water column is 1.4 g C m–2 yr–1. This corresponds to 7.1% (= 4.8 g C m–2 yr–1) and 15.5% (= 10.4 g C m–2 yr–1) of the primary production channeled through herbivorous copepods and through herbivorous and carnivorous copepods, respectively. Hence, chaetognaths represent an important link between lower and higher trophic levels. To further unravel their role in the ecosystem, additional studies on the mesoand bathypelagic zooplankton community are needed.

Copepod feeding in the ocean: scaling patterns, composition of their diet and the bias of estimates due to microzooplankton grazing during incubations

Here, we report insights from the compilation and analysis of data on marine calanoid copepod feeding rates in the ocean. Our study shows that food availability and body weight are major factors shaping copepod feeding rates in the field, with a relatively minor role of temperature. Although the maximal feeding rates of copepods that are observed in the field agree with the well-known 3/4 of body size scaling rule for animals, copepod feeding in the oceans is typically limited and departs from this rule. Ciliates and dinoflagellates appear to be highly relevant in the composition of copepod diets, and this represents an indirect increase in the flux of primary production that is likely to reach the upper trophic levels; this contribution is higher in the less productive systems and may help to explain accounts of proportionally higher standing stocks of copepods supported per unit of primary producer biomass in oligotrophic environments. Contrary to common belief, diatoms emerge from our dataset as small contributors to the diet of copepods, except in some very productive ecosystems. We have also evaluated the bias in the estimation of copepod grazing rates due to within-bottle trophic cascade effects caused by the removal of microheterotrophs by copepods. This release of microzooplankton grazing pressure accounts for a relevant, but moderate, increase in copepod grazing estimates (ca. 20–30%); this bias has an effect on both the carbon flux budgets through copepods and on our view of their diet composition. However, caution is recommended against the indiscriminate use of corrections because they may turn out to be overestimates of the bias. We advise that both uncorrected and corrected grazing rates should be provided in future studies, as they probably correspond to the lower and upper boundaries of the true grazing rates.

Diets of crustacean zooplankton, inferred from stable carbon and nitrogen isotope analyses, in lakes with varying allochthonous dissolved organic carbon content

We used stable carbon and nitrogen isotope analyses to estimate the relative proportions of three putative food sources (1) algae, (2) allochthonous organic matter (but including also heterotrophic bacteria and green-sulphur bacteria having similar isotopic values) and (3) methane-oxidizing bacteria (MOB) in the diets of crustacean zooplankton in five small boreal lakes representing a gradient of dissolved organic carbon (DOC) concentration from ca. 5 to 40 mg C l−1. The lakes were sampled in May, after establishment of stratification, and again in October during autumnal mixing of the water column. IsoSource mixing model outputs indicated that the proportion of algae in the diets of zooplankton was generally higher in May than in October, and that bacteria contributed to the diets of both cladocerans and copepods. Our results indicate that bacteria, especially MOB, can make an appreciable contribution to zooplankton diets in these small lakes, even in those with relatively low DOC concentrations.

Grazing by the copepods Calanus helgolandicus and Acartia clausi on the protozooplankton community at station L4 in the Western English Channel

C. helgolandicus and A. clausi, respectively. Calanus helgolandicus obtained 2 – 27% of its daily ration as a percentage of body carbon from phytoplankton and 2 – 17% of its daily ration from PZP. Acartia clausi obtained 22– 161% of its daily ration from phytoplankton and 3 – 44% daily ration from PZP. Calanus helgolandicus and A. clausi populations together grazed up to 21% of the total PZP standing stock daily. Both C. helgolandicus and A. clausi demonstrated selective feeding on large choreotrich ciliates and heterotrophc dinoflagellates. Our results support the hypothesis that PZP comprise a significant proportion of the copepod diet after the spring bloom when the phytoplankton community is dominated by small cells.

Carbon transfer in a herbivore- and microbial loop-dominated pelagic food webs in the southern Barents Sea during spring and summer

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 398:93-107 (2010) - DOI: https://doi.org/10.3354/meps08335 Carbon transfer in a herbivore- and microbial loop-dominated pelagic food webs in the southern Barents Sea during spring and summer Frederik De Laender*, Dick Van Oevelen, Karline Soetaert, Jack J. Middelburg NIOO-KNAW, Centre for Estuarine and Marine Ecology, Korringaweg 7, PO Box 140, 4401 Yerseke, The Netherlands *Email: frederik.delaender@ugent.be ABSTRACT: We compared carbon budgets between a herbivore-dominated and a microbial loop-dominated food web and examined the implications of food web structure for fish production. We used the southern Barents Sea as a case study and inverse modelling as an analysis method. In spring, when the system was dominated by the herbivorous web, the diet of protozoa consisted of similar amounts of bacteria and phytoplankton. Copepods showed no clear preference for protozoa. Cod Gadus morhua, a predatory fish preying on copepods and on copepod-feeding capelin Mallotus villosus in spring, moderately depended on the microbial loop in spring, as only 20 to 60% of its food passed through the microbial loop. In summer, when the food web was dominated by the microbial loop, protozoa ingested 4 times more bacteria than phytoplankton and protozoa formed 80 to 90% of the copepod diet. Because of this strong link between the microbial loop and copepods (the young cod’s main prey item) young cod (<3 yr) depended more on the microbial loop than on any other food web compartment, as >60% of its food passed through the microbial loop in summer. Adult cod (≤3 yr) relied far less on the microbial loop than young cod as it preyed on strictly herbivorous krill in summer. Food web efficiency for fish production was comparable between seasons (~5 × 10–4) and 2 times higher in summer (5 × 10–2) than in spring for copepod production. KEY WORDS: Food web · Microbial loop · Protozoa · Copepods · Gadus morhua Full text in pdf format Supplementary Material PreviousNextCite this article as: De Laender F, Van Oevelen D, Soetaert K, Middelburg JJ (2010) Carbon transfer in a herbivore- and microbial loop-dominated pelagic food webs in the southern Barents Sea during spring and summer. Mar Ecol Prog Ser 398:93-107. https://doi.org/10.3354/meps08335Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 398. Online publication date: January 05, 2010 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2010 Inter-Research.

Variability in fatty acids of two marine copepods upon changing food supply in the coastal upwelling zone off Chile: importance of the picoplankton and nanoplankton fractions

Fatty acids composition of two marine copepods,Acartia tonsaandCentropages brachiatus, and lipid profiles of natural food assemblages were studied during the austral summer 2006 at three upwelling sites in the coastal upwelling zone off Chile, along with oceanographic conditions. Fatty acids of food supply were assessed for the picoplankton, nanoplankton and microplankton size fractions. There were marked differences in upwelling conditions among locations, as well as in their food supply in terms of quantity and quality. Differences in fatty acid composition were also found, both among food assemblages and between copepod species. Essential polyunsaturated fatty acids (PUFA; linoleic acid) and monounsaturated fatty acids (MUFA; oleic acid) dominated the picoplankton and nanoplankton size fractions of food, and they were highly represented in both species of copepods, indicating these size fractions were the major contributors to their diet. These fatty acids can thus be considered as useful trophic markers for copepods. Variation in lipid profiles between species depended on sampling sites, whereas differences in lipid composition among sampling sites were attributed to distinct upwelling conditions, which drive the changes in food quality, such that trophic response is highly dependent on food offer. Variation in fatty acids compositions of copepods may thus act as an indicator of upwelling variability. Our findings suggest that lipid transfer from primary producers to primary consumers can have a crucial role for carbon cycling in the marine food web, and that picoplankton and nanoplankton fractions are the key items of copepod diet in this upwelling system.

Differential routing of ‘new’ nitrogen toward higher trophic levels within the marine food web of the Gulf of Aqaba, Northern Red Sea

Mesozooplankton communities in the meso-oligotrophic Gulf of Aqaba, Northern Red Sea, were investigated over a 2 years period (2005–2007) with emphasis on the trophodynamic relations among different taxonomic groups ranging from primary consumers to carnivorous predators. Based on stable isotope analyses, we present evidence for a strong contribution of ‘new’ nitrogen mainly derived from the utilization of aerosol nitrate by unicellular cyanobacteria especially during summer stratification and the propagation of exceptionally low δ15N onto higher trophic levels. In contrast, N2-fixation by diazotrophs seemed to play a minor role, while the utilization of deep water nitrate by cyanobacteria and eukaryotic algae might be of importance during winter mixing. Based on 15N enrichment of consumers, clear differences between exclusively herbivorous organisms (doliolids, appendicularians, pteropods) and those with omnivorous feeding modes were detected. The category of omnivores comprised a large variety of taxons ranging from small meroplanktonic larvae to non-calanoid copepods (harpacticoids, cyclopoids and poecilostomatoids) that together form a diverse and complex community with overlapping feeding modes. In addition, distinct seasonality patterns in δ15N of copepods were found showing elevated trophic positions during periods of winter mixing, which were most pronounced for non-calanoid copepods. In general, feeding modes of omnivores appeared rather unselective, and relative contributions of heterotrophic protists and degraded material to the diets of non-calanoid copepods are discussed. At elevated trophic positions, four groups of carnivore predators were identified, while calanoid copepods and meroplanktonic predators showing lowest 15N enrichment within the carnivores. The direct link between ‘new’ nitrogen utilization by primary producers and the 15N enrichment of consumers in the planktonic food web of the Gulf of Aqaba emphasizes the significant contribution of ‘new’ nitrogen to the nitrogen budget and ecosystem functions in subtropical and tropical oligotrophic oceans.

Seasonal succession and composition of the zooplankton community along an eutrophication and salinity gradient exemplified by Danish waters

The seasonal distribution of the major components of the zooplankton community, protozooplankton, copepods and cladocerans, along a eutrophication gradient were examined in order to establish if eutrophication through increases in phytoplankton biomass and productivity has an impact on biomass and composition of the zooplankton community. Data on salinity, temperature, inorganic nutrients, chlorophyll a, phytoplankton biomass and production, protozooplankton and mesozoo-plankton biomass were collected during routine sampling at seven stations representing a eutrophication gradient from open to estuarine waters. Mesozooplankton biomass varied seasonally in all areas with a unimodal pattern characterized by a peak between mid-spring and mid-summer. Total mesozooplankton biomass as well as copepod and cladoceran biomass, decreased with DIN concentration. Salinity and the type of ecoystem (well-mixed shallow estuaries versus stratified open waters) were the major factors controlling the biomass proportion of copepod species. Annual production of the total copepod community as well as the total grazing impact of copepods on primary production was higher in open waters than in estuarine waters. In estuarine type ecosystems, phytoplankton production is underexploited by copepod grazing whereas in the open type ecosystems, the phytoplankton production alone could not satisfy the carbon demand of copepods stressing the potential importance of protozoans in the copepod diet.

Contribution of auto- and heterotrophic protozoa to the diet of copepods in the Ulleung Basin, East Sea/Japan Sea

We estimated the ingestion rates and prey selectivity of two calanoid copepods, Calanus sinicus and Neocalanus plumchrus, on auto- and heterotrophic protozoa in the Ulleung Basin in the East Sea/Japan Sea. Both copepod species preferentially ingested auto- and heterotrophic protozoa. The heterotrophic protozoa comprised between 42.9 and 86.7% of the total carbon ration ingested by the copepods, although they comprised on average 21.5% of the total available prey carbon. In particular, ciliates comprised the major dietary component for the copepods in the study area. The copepod consumption of ciliates was on average 38.2% of the total daily carbon ration of auto-and heterotrophic protozoa. Auto- and heterotrophic protozoa in the 20―50 μm size were the fraction that was most actively consumed by copepods and they comprised on average 45.1% of the total carbon ration ingested. In all experiments, the copepods selected the ciliate and heterotrophic dinoflagellate (HDF) groups over the other protozoa. Our results indicate that the copepod feeding regime was influenced by the composition and size of the potential food and that, in this study area, feeding behavior of copepods may control the populations of ciliates and HDFs that are larger than 10 μm in size. Thus, the selective feeding patterns and higher grazing pressure of copepods on heterotrophic protozoa, compared with autotrophic protozoa, indicate a trophic coupling between copepods and the microbial food web in the Ulleung Basin.

Contrasting trophic interactions of microbial and copepod communities in a fjord ecosystem, Chilean Patagonia

Trophic interactions between microbial and copepod communities were studied during winter and under spring-bloom conditions in the Reloncavi Fjord, Chile, and adjacent channels. Grazing by heterotrophic nanoflagellates (HNF) and microzooplankton was estimated using the size- fractionation method. Simultaneously, copepod grazing experiments using naturally occurring plank- ton assemblages were performed. Contrasting food environments for planktonic consumers were found between winter and spring, with biomasses of prey organisms 50% d -1 ). During spring, small and large copepods exhibited prey ingestion rates of 2 to 3 and ca. 6 to 10 μg C ind. -1 d -1 , respectively. In such a contrasting food environment, copepods have to alleviate the effects of food scarcity either by modifying their metabolic demands or by switching their diet to microbial organisms that are available during periods of low diatom biomass. The pre- sent study reveals that, even under productive spring-bloom conditions, the less-abundant heterotrophic protists constitute a substantial proportion (ca. 30% of the daily consumption) of the copepod diet in fjord ecosystems.

Fatty acid profiles of algae mark the development and composition of harpacticoid copepods

Summary1. The value of algal fatty acids (FA) as diet biomarkers for benthic harpacticoid copepods was investigated. A high proportion of 18:1ω9 and 18:2ω6 FA was observed in the lipid reserve fraction of copepods fed with cyanobacteria. In contrast, a high proportion of 16:1ω7 and ω3 FA (including eicosapentaenoic) was present in the lipid reserve fraction of copepods grown on diatoms.2. Copepods that were grown on cyanobacteria showed reduced survival and took 26% more time to develop from the first copepodid stage to adult than copepods that were grown on diatoms. Copepods feeding on the cyanobacteria showed reduced FA content when compared with animals fed with diatoms. This reduction in FA content was more pronounced in the apolar lipid fraction (mainly reserve lipids) than in the polar (mainly structural) lipid fraction.3. The FA profiles of algae were used to calculate a function discriminating between diatoms and cyanobacteria. This function was applied to the FA profiles in the reserve lipid fraction of copepods and correctly classified copepod diet. 16:1ω7, 18:2ω6 and 20:5ω3 were the most important FA in the discriminant function. The suitability of this chemometric method to infer copepod diet was further tested by using algal class FA data from literature to derive the discriminant functions. The correct classification of the diet when the functions were applied to FA composition of the copepod reserve lipids suggests that this method may be employed in trophic web studies. 18:3ω3, 18:1ω9 and 16:1ω7 were the most important FA in the functions discriminating diatoms, cyanobacteria and green algae. The identification and quantification of the whole suit of 16:1ω7, 18:1ω9, 18:2ω6, 18:3ω3 and 20:5ω3 in trophic web studies is therefore of paramount importance to infer diet origin of aquatic herbivores.4. The FA profile of copepod polar lipids did not reflect that of the diet. The presence of long chain polyunsaturated FAs in the polar lipid fraction of copepods feeding on the cyanobacterium suggests that C18 FAs from the diet may be elongated and desaturated by the copepod. The ability to elongate and desaturated FAs may reduce the importance of some FAs as diet biomarkers while it may turn the copepods into valuable trophic intermediaries in transferring organic matter from microorganisms to higher trophic levels.

Quantitative PCR to estimate copepod feeding

Copepods play a central role in marine food webs as grazers of plankton and as key prey for many predators. Therefore, quantifying their specific trophic interactions is critical for understanding the role of copepods in ocean processes. However, because of methodological constraints, it remains difficult to investigate in situ copepod feeding without reliance on laborious intrusive and potentially biased incubation approaches. Recent advances in PCR-based methodologies have demonstrated the feasibility of directly identifying copepod diets based on prey DNA sequences. Yet, obtaining quantitative information from these approaches remains challenging. This study presents results of systematic efforts to develop a quantitative PCR (qPCR) assay targeted to 18S rRNA gene fragments to estimate copepod gut content of specific species of prey algae. These results were first compared to gut content estimates based on fluorescence in the copepod Calanus finmarchicus fed monocultures of two different microalgae species in controlled laboratory studies. In subsequent field studies, we compared feeding rates obtained by microscopy and qPCR for Temora longicornis and Acartia clausi feeding on the haptophyte Phaeocystis globosa in natural blooms. These investigations demonstrate a semi-quantitative relationship between gut content estimates derived from qPCR, gut pigment, and direct microscopy. However, absolute estimates of gut content based on qPCR methodology were consistently lower than expected. This did not appear to be explained by the extraction methods used, or interference by non-target (predator) DNA in the PCR reactions, instead suggesting digestion of prey-specific nucleic acids. Furthermore, the 18S rDNA target gene copy number of the phytoplankton varied with growth phase. Nonetheless, when prey target gene copy number in the ambient water is quantified, the qPCR-approach can be compared to other methods, and then used to semi-quantitatively estimate relative copepod grazing on specific prey in situ without involving further incubations. A distinct advantage of a DNA-based molecular approach compared to gut fluorescence and direct microscopic observation, is the ability to detect non-pigmented and macerated prey. Future studies should aim to correct for breakdown in prey DNA and perform extensive calibrations to other methods in order to achieve a quantitative measure of feeding rates in situ.

Grazing by Calanus helgolandicus and Para-Pseudocalanus spp. on phytoplankton and protozooplankton during the spring bloom in the Celtic Sea

Feeding rates and selectivity of the calanoid copepods Calanus helgolandicus and Para-Pseudocalanus spp. on natural assemblages of microplankton were evaluated in the English Channel and western Celtic Sea during non-bloom and bloom conditions in April 2002. Ingestion rates of total chlorophyll- a were low at non-bloom stations where the phytoplankton community was dominated by cells < 5 μm in length and higher during the bloom when the > 5 μm size fraction was dominant. Protozooplankton contributed to the copepod diet in all experiments, C. helgolandicus clearance and ingestion rates were highest for the ciliate Myrionecta rubra (626–1347 ml cop − 1 d − 1 ; 0.3–27 μg C cop − 1 d − 1 ). C. helgolandicus ingested between 1 and 18 μg C cop − 1 d − 1 (1–12% body C) from phytoplankton + protozooplankton food sources. The total carbon ingested by Para-Pseudocalanus spp. was lower (0.5–6 μg cop − 1 d − 1 ) but this was equivalent to between 6 and 78% of body carbon being ingested daily. Our data suggest that C. helgolandicus selected prey according to size; this was not the case for Para-Pseudocalanus spp. which became more selective as chlorophyll- a concentration increased. Grazing impact of the entire copepod community on protozooplankton was assessed. We found that at non-bloom stations between 12 and 17% of the protozooplankton community was being removed daily by the copepod community, whereas during the peak of the bloom the proportion being removed daily was only 2%. We conclude that during the spring bloom period copepods gained the majority of their carbon from phytoplankton ingestion but during non-bloom periods, protozooplankton and the ciliate M. rubra made a significant contribution to copepod diet.

High reproduction of Calanus finmarchicus during a diatom-dominated spring bloom

Feeding, egg production, hatching success and early naupliar development of Calanus finmarchicus were measured in three north Norwegian fjords during a spring bloom dominated by diatoms and the haptophyte Phaeocystis pouchetii. Majority of the copepod diet consisted of diatoms, mainly Thalassiosira spp. and Chaetoceros spp., with clearance rates up to 10 ml ind−1 h−1 for individual algae species/groups. Egg production rates were high, ranging from ca 40 up to 90 eggs f−1 d−1, with a hatching success of 70–85%, and fast naupliar development through the first non-feeding stages. There was no correlation between the egg or nauplii production and diatom abundance, but the hatching success was slightly negatively correlated with diatom biomass. However, the overall high reproductive rates suggested that the main food items were not harmful for C. finmarchicus reproduction in the area, although direct chemical measurements were not conducted. The high population egg production (>1,20,000 eggs m−2 d−1) indicated that a large part of the annual reproduction took place during the investigation, which stresses the importance of diatom-dominated spring phytoplankton bloom for population recruitment of C. finmarchicus in these northern ecosystems.

Overwintering habitats of migratory juvenile American shad in Chesapeake Bay

We describe overwintering habitats of age-0 American shad in the lower Chesapeake Bay estuary through analyses of multiple, complementary data sets, including bottom-trawls of the Virginia portion of Chesapeake Bay and its tributaries, stable isotope analysis of American shad and common prey items, and stomach content analysis. This is the first detailed description of overwintering habitats used by young American shad during their first migration to the Atlantic Ocean. American shad generally migrated from their freshwater rearing habitat during November and December and migrated to the ocean during February through March. American shad were captured in all of Virginia’s tributaries and along Chesapeake Bay’s western coast. These fish were caught in relatively cool waters (5–9°C) over a wide range of salinities (0.1–27.5). Strong selection for certain temperatures or salinities was not apparent. Stomach content and stable isotope analyses demonstrated that juveniles were feeding in the estuary, growing on a diet of estuarine calanoid copepods, mysid shrimps, and larval fishes. The stable isotope data were used to describe temperature- and size-cued migration from fresh water. Temperature was an important cue affecting both the timing and the rate of migration. Further, American shad exhibited at least three different size-related migration behaviors: most juveniles emigrated from the freshwater rearing habitat at 2–5 g (ca. 55–75 mm fork length); other juveniles emigrated at a size of 2 g or less and rapidly moved into the lower estuary; and finally, a few juveniles remained in the upper estuary and did not emigrate until they were 5 g or larger. A few American shad were captured with anomalous stable isotope signatures, which may be explained by migration into the Chesapeake Bay estuary from an adjacent system.