Spatial patterns of surface blooms and recruitment dynamics of Calanus finmarchicus in the NE Norwegian Sea

Spatial dynamics of phytoplankton blooms and Calanus finmarchicuswere analysed in a large-scale oceanic area (10274 km) in theNE Norwegian Sea (69–71°N, 12–16°E). Dataon hydrography, chlorophyll a (Chl a) and mesozooplankton (netsamples and Optical Plankton Counter) were collected in surfacewaters (0–30 m) during spring and early summer 2003. Spatialpatterns of copepodites in relation to hydrography and Chl aconcentration were analysed by constrained correspondence analysis.Distribution of phytoplankton and C. finmarchicus was highlypatchy. The overwintering generation (CV and adult females)had highest abundances in areas with phytoplankton blooms, whereasthe recruiting cohort (CII + CIII copepodites) was found inwater parcels with low Chl a concentration. Differences in recruitmentdynamics between the southern and northern Norwegian Sea arediscussed. Our data confirm the importance of phytoplanktonspring blooms in initiating the recruitment of C. finmarchicus.This opens up for a future mapping of sea surface chlorophyllto depict the large-scale variability in the demography of C.finmarchicus during the reproductive season in the NorwegianSea.     

Dynamics of coexisting Calanus finmarchicus, Calanus glacialis and Calanus hyperboreus populations in a high-Arctic fjord

We studied the population dynamics of Calanus finmarchicus, Calanus glacialis and Calanus hyperboreus in Billefjorden, Svalbard (78°40N). All three species reproduced in the fjord with different timing. The maximum abundance of Calanus spp. copepodite stages peaked on the 11th of July (29,000 ind m^–2). C. glacialis was the dominant species accounting for 60–80% of the total Calanus abundance. C. finmarchicus appear to thrive in the fjord despite the low temperatures (–1.86°C to 5°C) and accounted for 20–30% of the total population. C. hyperboreus contributed less to the total abundance (5–20%). A 1-year life cycle is suggested for C. finmarchicus and C. hyperboreus in the fjord, C. glacialis has a 1- to 2-year life cycle. Highest mortality rates were observed for copepodite stage CV in C. finmarchicus and C. glacialis (0.09 and 0.075 d^–1, respectively) and for females in C. hyperboreus (0.149 d^–1). Mortality of copepodite stages was substantially lower in C. glacialis than in the other species. This is particularly obvious in the early and numerous copepodite stages (CI + CII) during the period of recruitment to these stages. This suggests that differences in secondary production in Arctic pelagic ecosystems are controlled partly by population loss rates.     

Experimental evaluation of the avoidance reaction of Calanus finmarchicus

The avoidance reaction of Calanus finmarchicus copepodids wasobserved in the presence of quantifiable fluid mechanical signals.The experimental apparatus was a rotating cylinder of sea waterwith fixed obstacles (diameters 6 mm and 10 mm) which disturbedthe flow and stimulated avoidance. The experimental conditionsprevented unambiguous discrimination of signals; the amountand rate of fluid deformation appear to be the effective stimuli.Avoidance reactions began at a mean distance of 7 mm from thesmall obstacle and 6 mm from the large obstacle. Mean valuesfor fluid deformation for the small and large obstacles were0.12 and 0.24 mm/mm; fluid deformation rate values were 0.80and 1.0 sec^–1. There was less scatter for deformationvalues. The copepods preferentially avoided obstacles at anglesof about 60° with respect to the approach streamline. Avoidancespeeds averaged 16 cm.sec^–1; mean distance of avoidanceflights was 15 mm. ¹Contribution Number 4195 from the Woods Hole OceanographicInstitution. ²Present Address: Scripps Institution of Oceanography, La Jolla,California 92093. ³Present Address: Marathon Oil Company, Littleton, Colorado80160.     

Lipids and life strategies of Calanus finmarchicus, Calanus glacialis and Calanus hyperboreus in late autumn, Kongsfjorden, Svalbard

Stage IV and V copepodites were the dominant forms of Calanus finmarchicus, C. glacialis and C. hyperboreus in Kongsfjorden in late September 1997. Stage IV and V copepodites of C. glacialis and C. hyperboreus were rich in lipid, largely wax esters, and were well fitted to overwinter. Stage IV copepodites of C. finmarchicus were also rich in wax esters, but stage V copepodites of C. finmarchicus were less wax ester-rich. Large size increments between stage IV and V copepodites and between stage V copepodites and females were noted in C. finmarchicus. A very large increment between stage IV and V copepodites was noted for C. glacialis but the size difference between stage V copepodites and females was very small in this species. Particularly large increments were noted between stage IV and V copepodites of C. hyperboreus and also between stage V copepodites and females of this species. The very large, wax ester-rich C. hyperboreus is well adapted to survive the most extreme variations in the Arctic, in Arctic basin waters, whereas the smaller, wax ester-rich C. glacialis is adapted to survive less extreme Arctic variations, as in Arctic shelf waters. The smallest of the three, C. finmarchicus, is best adapted to survive the more predictable waters of the North Atlantic and the Barents Sea. Accepted: 3 January 2000     

Future climate‐driven shifts in distribution of Calanus finmarchicus

Calanus finmarchicus is a key‐structural species of the North Atlantic polar biome. The species plays an important trophic role in subpolar and polar ecosystems as a grazer of phytoplankton and as a prey for higher trophic levels such as the larval stages of many fish species. Here, we used a recently developed ecological niche model to assess the ecological niche (sensu Hutchinson) of C. finmarchicus and characterize its spatial distribution. This model explained about 65% of the total variance of the observed spatial distribution inferred from an independent dataset (data of the continuous plankton recorder survey). Comparisons with other types of models (structured population and ecophysiological models) revealed a clear similarity between modeled spatial distributions at the scale of the North Atlantic. Contemporary models coupled with future projections indicated a progressive reduction of the spatial habitat of the species at the southern edge and a more pronounced one in the Georges Bank, the Scotian Shelf and the North Sea and a potential increase in abundance at the northern edge of its spatial distribution, especially in the Barents Sea. These major changes will probably lead to a major alteration of the trophodynamics of North Atlantic ecosystems affecting the trophodynamics and the biological carbon pump.     

Assessment of Calanus finmarchicus growth and dormancy using the aminoacyl-tRNA synthetases method

We obtained growth rates of the copepod Calanus finmarchicusat different locations across the North Atlantic between May1998 and June 2004. Animals were incubated for 2–9 daysand fed either with natural food assemblages or with culturedalgae. During this period, we measured both somatic weight-specificgrowth rates (measured as protein change) and aminoacyl-tRNAsynthetases (AARS) activity. We found a highly significant relationshipbetween AARS activity and growth in protein content (R² = 0.55,P < 0.001). Significant AARS activity also occurred whengrowth was negative, the relationship predicting an AARS activitylevel of 8.33 nmPPi·mg protein^–1·h^–1when somatic growth is zero. This is because AARS activity isexpected even when growth is negative, owing to the continuedprotein turnover in the cells. The AARS method allowed for thefirst time the study of protein metabolism in overwinteringC. finmarchicus. Our study results showed that overwinteringcopepods had significantly lower values of AARS activity thannon-diapausing animals (t = –3.51, P < 0.002). TheAARS method opens the possibility to better understand physiologydynamics of deep-water organisms (e.g. the beginning and endof diapause).     

Some ideas about the role of lipids in the life cycle of Calanus finmarchicus

A conceptual model of the possible role of lipids in the lifecycle of Calanus finmarchicus is proposed. Lipid storage couldplay a key role at various levels: (i) by triggering diapausethrough variations in cholesterol and fatty acid derived hormonelevels; (ii) by determining the overwintering depth in relationto the convective mixed layer; and (iii) by playing an importantrole in the population adaptation to the hydrological conditionsof the basin. A number of ways to test the validity of the proposedhypotheses are proposed. Written responses to this article should be submitted to theEditorial Office within two months of publication. For furtherinformation, please see the Editorial ‘Horizons’in Journal of Plankton Research, Volume 26, Number 3, Page 257.     

Feeding and reproduction of Calanus finmarchicus during non-bloom conditions in the Irminger Sea

Simultaneous ingestion and egg production experiments were conductedwith female Calanus finmarchicus in April/May and July/August2002 in the Irminger Sea. Experimental animals were providedwith natural microplankton food assemblages and incubated underin situ conditions for 24 h. The quantity of food consumed wassignificantly related to the concentration of prey cells, withtotal daily ingestion rates ranging from 0.6 to 8.1 µgof carbon female^–1 day^–1, corresponding to carbon-specificrates of 0.6–4.7% day^–1. Egg production rates (EPRs)remained relatively low (0.3–11 eggs female^–1 day^–1)during both periods of investigation and were not influencedby food availability. The data were used to construct energeticbudgets in which the microplankton carbon ingested, includingciliates, was compared with the carbon utilized for egg productionand respiration. These budgets showed that ingestion alone couldnot provide the necessary carbon to sustain the observed demandsfor growth and metabolism. Although ciliates constituted >80%of the total material ingested at times, they were not sufficientto provide the metabolic shortfall. Indeed, the females weretypically lacking 5 µg of carbon each day, 5% of theircarbon biomass. Our study results highlight the possible importanceof internal reserves in sustaining reproduction in C. finmarchicusduring periods of food scarcity.     

The occurrence of Calanus finmarchicus (Gunnerus) and Calanus helgolandicus (Claus) in the western Irish Sea

The seasonal abundance of Calanus finmarchicus and Calanus helgolandicusin the North Channel and stratified region of the western IrishSea is reviewed using data collected between 1992 and 1996.Both species occur in the western Irish Sea, but were more abundantin the stratified region during spring. Increased abundanceduring May/June was attributed to an increase in copepoditestages. Calanus helgolandicus dominated in both regions, exhibitingspring and autumn peaks in abundance in the stratified region.It is argued that the presence of ripe females and naupliarstages in the stratified region is evidence of an in situ breedingpopulation, rather than advection of individuals from populationcentres outside the Irish Sea. The lack of geographical separationof the two species in the western Irish sea, and reports thatboth species occur in the Celtic Sea and Malin Shelf, limitthe use of either species as indicators of exchange processesbetween the Irish Sea and neighbouring waters.     

Food web interactions in a Calanus finmarchicus dominated pelagic ecosystem--a mesocosm study

The significance of nauplii versus copepodite stage V of Calanusfinmarchicus grazing and their effects on the structure of thefood web were investigated during two sampling periods of 7–8days in March and April in experimental mesocosms held in aNorwegian fjord over a 2 month period. The mesocosms were manipulatedby the addition of two different levels of inorganic nutrients(control versus enriched). During the ‘naupliar’period in March, the phytoplankton was characterized by a diatombloom while during the ‘copepodite’ period in April,it was in a post-bloom phase characterized by small-celled species,mainly Phaeocystis pouchetii. Phytoplankton, bacterial and protozooanbiomass and production rates were measured in addition to copepodbiomass. Copepod grazing was estimated by three different methods:(i) gut fluorescence; (ii) chlorophyll clearance from the water;and (iii) growth method measured as body carbon increase. Thetwo latter methods gave similar results for nauplii, but allthree gave different results for the copepodites. Independentsomatic growth, based on changes in abundance and individualcarbon content, and grazing estimates revealed an overall growthefficiency of 0.66 ± 0.20 (mean ± S.E.) for copepodites.Empirical carbon flow models were constructed, which indicatedthat the nauplii could not control either phytoplankton or protozoangrowth in either the control or in the enriched system. Ignoringrecycling and sedimentation, the fate of the primary productionfor the nauplii-dominated community was to be grazed by a diverseand abundant protozooplankton community. In the copepodite-dominatedcommunity, the copepods grazed >100% of the daily primaryproduction, and also grazed heavily on a protozooplankton communityof low biomass and diversity and presumably on detritus. Thefate of the primary production in the two different copepodscenarios followed predicted routes for ‘low meso-zooplankton’and for ‘high meso-zooplankton’ biomass systems,as suggested by Wassmann (Wassmann, 1998).     

Light primes the escape response of the calanoid copepod, Calanus finmarchicus

The timing and magnitude of an escape reaction is often the determining factor governing a copepod's success at avoiding predation. Copepods initiate rapid and directed escapes in response to fluid signals created by predators; however little is known about how copepods modulate their behavior in response to additional sensory input. This study investigates the effect of light level on the escape behavior of Calanus finmarchicus. A siphon flow was used to generate a consistent fluid signal and the behavioral threshold and magnitude of the escape response was quantified in the dark and in the light. The results show that C. finmarchicus initiated their escape reaction further from the siphon and traveled with greater speed in the light than in the dark. However, no difference was found in the escape distance. These results suggest that copepods use information derived from multiple sensory inputs to modulate the sensitivity and strength of the escape in response to an increase risk of predation. Population and IBM models that predict optimal vertical distributions of copepods in response to visual predators need to consider changes in the copepod's behavioral thresholds when predicting predation risk within the water column.     

Does turbulence play a role in feeding and reproduction of Calanus finmarchicus?

Data collected on cruises in the Norwegian Sea and the NortEast Atlantic have been re-analysed, introducing wind speedas a factor to explain gut fluorescence and egg production ofCalanus finmarchicus. Wind speed showed a negative relationwith gut fluorescence but not with egg production.     

Naupliar development times and survival of the copepods Calanus helgolandicus and Calanus finmarchicus in relation to food and temperature

We measured egg hatching times and stage specific naupliar developmenttimes of the key calanoid copepods Calanus helgolandicus andC. finmarchicus in response to temperature, food quantity andfood type. Calanus helgolandicus development times decreasedwith increasing temperature and when fed Isochrysis galbana(4.4 µm ESD) rather than Prorocentrum micans (29.5 µmESD). Nauplii needed higher food carbon concentration to developpast the first feeding stage (N3) when fed I. galbana comparedwith P. micans. At low food carbon concentrations nauplii developedmore slowly past N3 than at more saturated levels. The survivalof nauplii fed P. micans increased with temperature, but starvednauplii survival decreased at higher temperatures. We parameterizeda temperature-dependent model of development for both specieswhich fitted the observed stage durations under non-limitingfood extremely well and demonstrated that C. finmarchicus developsfaster than C. helgolandicus. Further data are needed to clarifythe effect of food-temperature interactions on development rates.     

Egg production and hatching success in the calanoid copepods Calanus helgolandicus and Calanus finmarchicus in the North Sea from March to September 2001

Spatial and seasonal egg production rates (E_r) and egg hatchingsuccess in the copepods Calanus finmarchicus and Calanus helgolandicuswere measured in the North Sea from March to September. Foodavailability was monitored by chlorophyll and protist concentrationsand three size fractions of seston fatty acids. Seasonal andspatial distribution and production differed between the species.Calanus finmarchicus was found only offshore of the 50-m isobath,with decreasing E_r (37–28 eggs female^–1 day^–1)from March to July. Calanus helgolandicus had two abundancepeaks, in spring and autumn, with a low in May during whichtime the highest E_r were observed (38 eggs female^–1 day^–1).At other times, E_r in C. helgolandicus remained lower than inC. finmarchicus (     

High dispersal potential has maintained long-term population stability in the North Atlantic copepod Calanus finmarchicus

The cool-water copepod Calanus finmarchicus is a key species in North Atlantic marine ecosystems since it represents an important food resource for the developmental stages of several fish of major economic value. Over the last 40 years, however, data from the Continuous Plankton Recorder survey have highlighted a 70 per cent reduction in C. finmarchicus biomass, coupled with a gradual northward shift in the species's distribution, which have both been linked with climate change. To determine the potential for C. finmarchicus to track changes in habitat availability and maintain stable effective population sizes, we have assessed levels of gene flow and dispersal in current populations, as well as using a coalescent approach together with palaeodistribution modelling to elucidate the historical population demography of the species over previous changes in Earth's climate. Our findings indicate high levels of dispersal and a constant effective population size over the period 359,000-566,000 BP and suggest that C. finmarchicus possesses the capacity to track changes in available habitat, a feature that may be of crucial importance to the species's ability to cope with the current period of global climate change.     

Post-bloom feeding of Calanus finmarchicus copepodites: Selection for autotrophic versus heterotrophic prey

Feeding and faecal pellet production of late copepodite stages of Calanus finmarchicus were measured in mixtures of cultured autotrophic and heterotrophic food, as well as in a natural post-bloom plankton assemblage, in order to evaluate food selection and its potential effect on sedimentation of organic matter. Calanus finmarchicus consistently selected for diatoms, both in mixtures with the heterotrophic dinoflagellate Oxyrrhis marina, and in natural seston containing dinoflagellates, ciliates and flagellates. Similarly, the filtration, ingestion and faecal pellet production rates were significantly higher feeding on diatoms than when feeding on other food species. Calanus finmarchicus selection appeared relatively inflexible, so that changes in seston composition induced large changes in diet quantity and composition. Our results support the traditional view of C. finmarchicus as a major grazer of diatoms, and suggest potentially high post-bloom faecal pellet production rates.     

Temperature compensation in the escape response of a marine copepod, Calanus finmarchicus (Crustacea)

Calanus finmarchicus, the dominant mesozooplankter of the North Atlantic, is an important food source for many fishes and other planktivores. This species, which has limited diel vertical migration, depends on its fast-start escape response to evade predators. It has myelinated neuronal axons, which contribute to its rapid and powerful escape response. The thermal environment that C. finmarchicus inhabits ranges from below 0 degrees C to 16 degrees C. Previous studies have shown that respiration, growth, and reproductive rates are strongly dependent on temperature, with Q10 > 2.5. A comparable dependence of the escape response could place the animal at higher risk for cold-compensated predators. Our work focused on the temperature dependence of the behavioral response to stimuli that mimic predatory attacks. We found that in contrast to other biological processes, all aspects of the escape response showed a low dependence on temperature, with Q10 values below 2. This low temperature dependence was consistent for escape parameters that involved neural as well as muscle components of the behavioral response. These findings are discussed in the contexts of the predator-prey relations of copepods and the thermal dependence of behavior in other taxa.