The vertical distribution of plankton is described for 3 stations representative of stratified. frontal and vertically mixed regions of the western English Channel in summer. All components of organic carbon, representing dissolved and particulate organic carbon, phytoplankton, bacteria. protozoans, micro-and meso-zooplankton, were estimated independently. Major differences were found In the abundance and species composition of the phytoplankton, and in the relative proportions of different groups of heterotrophs. In the frontal region the phytoplankton (26.5 g C m-2) was composed of an essentially monospecific, surface population of the dinoflagellate Gj~odinium aureolum; by contrast, under well-stratified cond~tions small naked flagellates (0.42 g C m-2) forming a sub-surface chlorophyll maximum were dominant, and the tidally mixed waters were characterised by diatoms (7.91 g C m-'). At each station the estimated standing stock of heterotrophs was between 2.3 and 3.2 g C m-' , 10 to 30 % of which consisted of bacteria. Hence the phytoplankton was the dominant compartment in the frontal and mixed regions, whereas the zooplankton biomass considerably exceeded that of the phytoplankton in the well-stratified water. The ecological implications of these carbon distribution patterns are discussed.

The reproductive biology of Calanus finmarchjcus was investigated at a permanent station in the Norwegian Sea (Weathership Stn M, 66" N, 2" E) during a time series between March and June 1997 The temporal development of female abundance, egg production rate and gonad development stage in relation to the phytoplankton production cycle is described Abundance of females, copepodite stage 5 and males as well as female gonad morphology were examlned from MrP2 net samples taken daily from the upper 100 m. Daily egg production rate and number of spawn.ing females we]-e determined from 50 individual females placed in multiwells or beakers. Once a week a multinet haul from 1000-500-250-100-50-0 m was performed to study the depth d~stribution of females and gonad development stages. Results show that the reproductive period of C. finmarchicusin the Atlantic rcqion of the Norwegian Sea can be subdivided in 3 periods in relation to phytoplankton devrlopmt,nt.(1) During the prebloom over a period of 49 d mean egg production rate was 8 eggs female-' d ' and an average of 47 of the females were mature. (2) Coincident with the bloom in mid May the e g g production rate increased up to a maximum of 44 eggs female-' d-' while more than 80% of the females were mature. (3) After the bloom at the beginning of June, egg production decreased, and mature females were only rarely found. Feedlng expenments indicate that food quantity limited egg production prior to the bloom, while presumably food quality was not sufficient dunng postbloom. However, due to high female abundance the total population egg production prior to the bloom was the same as during the bloom. This implies that the reproduction of C. finmarchicus in the Norwegian Sea is to some extent decoupled from the phytoplankton bloom.

Copepods are able to discriminate between different foods on the basis of particle size and nutritional quality. However, the extent of selective feeding behavior and the mechanisms controlling it in the field are still poorly understood. In this study, we investigated selective feeding behavior and egg production for Calanus helgolandicus feeding on natural phytoplankton (using high-performance liquid chromatography techniques), and egg production, at a coastal station off Plymouth with the annual phytoplankton cycle from July 1996 to June 1997. The phytoplankton succession included biomass peaks of dinoflagellates, prymnesiophytes, and diatoms. C. helgolandicus showed little selective feeding behavior throughout the study with a slight preference for diatoms. The influence of the diet composition on egg production was analyzed using forward stepwise regression methods. Prymnesiophytes and diatoms were shown to have positive effects whereas the effect of dinoflagellates was negative. The effect of the different phytoplankton peaks is analyzed and discussed in relation to the phytoplankton taxonomic composition and dietary diversity.Since the initial controversies (see Harvey 1937) the ability of calanoid copepods to discriminate between different particles, either as a function of size (Frost 1972) or of food quality (Huntley et al. 1983;Cowles et al. 1988) has been clearly demonstrated in laboratory experiments using phytoplankton cultures. Video techniques have also helped us to understand the mechanisms used by copepods to detect, capture, and manipulate different particles as a function of their characteristics, their concentration, or the physical environment (Paffenhöfer 1988;Price 1988;Kiørboe and Saiz 1995).However, field studies are less conclusive (Poulet 1978;Huntley 1981) and the factors, other than the size (Cowles 1979), governing selective feeding from natural particulate assemblages are still doubtful. Optimal foraging theory has been advanced as an explanation of selectivity in the field (DeMott and Moxter 1991;DeMott 1993DeMott , 1995a, hypothesizing that the intensity of selection should depend on the concentration of high-quality food. This has been demonstrated in the laboratory with pairs of particles (DeMott 1993) and in the field with the same food (DeMott 1995a,b). However, other studies provide evidence for nonselective feeding by copepods on natural particle mixtures (Huntley 1981;Turner and Tester 1989).On the other hand, the fact that different phytoplankton Acknowledgments This research was supported by funding from the European Commission through the TASC project, Contract MAS3-CT95-0039. B. Meyer-Harms was supported by a EU grant (TMR, MAS3-CT96-5032). Thanks are due to the captains and crew of the RV Squilla and RV Sepia for collecting the samples. We also would like to thank W. DeMott and an anonymous reviewer for their helpful comments and suggestions.

A parallel is shown to exist between the development of phytoplankton distributions and the establishment and breakdown of the seasonal thermocline, which in turn is determined largely by tidal streaming. The spring phytoplankton outburst was first observed as the thermocline became established in the area of weak tides south of Nymphe bank in the Northern Celtic Sea. In the summer, dense blooms of phytoplankton were found in the frontal regions between well-stratified shelf waters and more coastal areas where strong tides maintained well-mixed conditions. In the autumn the retreating thermocline was followed back to the area of weak tides south of Nymphe bank. Despite the greater availability of surface nutrients at this time, the increased mixed layer depth and the reduced light levels did not permit such high surface chlorophyll 'a' values.

Selective feeding by the calanoid copepod Calanus finmarchicus was investigated during a 3-month study in spring 1997 at a permanent station in the Norwegian Sea (Sta. M, 66ЊN, 02ЊE). Phytoplankton biomass increased from 317 ng chlorophyll a (Chl a) liter Ϫ1 in the prebloom phase to 2,095 ng liter Ϫ1 during the bloom and declined after the bloom to 1,260 ng Chl a liter Ϫ1 . In the prebloom phase, clearance rates of C. finmarchicus females were between 22 and 100 ml copepod Ϫ1 d Ϫ1 , while during the bloom, they ranged from 75 to 92 ml copepod Ϫ1 d Ϫ1, with a decline in the postbloom phase (41 ml copepod Ϫ1 d Ϫ1). After the phytoplankton bloom, the C. finmarchicus population was dominated by copepodid stages CIV and CV, with clearance rates ranging from 7 to 23 ml copepod. Grazing rates of adult females on the phytoplankton standing stock were low in the prebloom phases (5-23 Chl a copepod Ϫ1 d Ϫ1 , respectively), increased during the bloom from 82 to 219 ng Chl a copepod Ϫ1 d Ϫ1 , and declined after the bloom (64 ng Chl a copepod Ϫ1 d Ϫ1 for adult females and 7-27 ng Chl a copepod Ϫ1 d Ϫ1 for copepodid stages CIV and CV). C. finmarchicus showed a selection for diatoms throughout the study period and for dinoflagellates, before and after the spring bloom, despite the low concentration of both groups in the pre-and postbloom phases. During the postbloom period, no differences were observed in the selective feeding behavior of the copepodid stages compared to the adults. The contribution of diatoms to the overall phytoplankton biomass was 8 and 14% in the pre-and postbloom periods, respectively, while dinoflagellates were Յ3%. Haptophytes (dominated by Phaeocystis pouchetii) and cryptophytes were ingested according to their abundance. Avoidance of cyanobacteria (Synechococcus spp.), pelagophytes, and ''green algae'' was observed throughout the study period.The calanoid copepod C. finmarchicus dominates zooplankton biomass in the North Atlantic (Williams et al. 1994;Cowles and Fessenden 1995). It is a key biological component of temperate ecosystems and plays an intermediary role between phytoplankton and fisheries variability in this area (Runge 1988). The Marshall and Orr monograph (1955a), the review by Huntley (1988) and, more recently, Harris (1996) have all considered the feeding biology of Calanus. Much of the earlier literature on copepod feeding behavior focused on size selection of phytoplankton in the field (e.g., Poulet 1973Poulet , 1974Gamble 1978) and more recently, on small-scale interactions between feeding behavior and anatomical structure (Paffenhöfer 1988). From recent investigations (e.g., Støttrup and Jensen 1990; Jónasdóttir 1994; Jónasdóttir et al. 1995;Pond et al. 1996), it has been established that food quality affects copepod production and ultimately, production at higher trophic levels. A detailed understanding of food quality will depend on an appreciation of the role of individual groups, and even species, of phytoplankton in nutrition (Harris 1996). Up to now, little in- Ackn...

The feeding ecology of Calanus finmarchicus was investigated during spring 1997 at a permanent station in the Norwegian Sea (Stn M, 66"N, 2"E) as part of the TASC (Trans Atlantic Study of Calanus finmarchicus) project time-series investigations. The phytoplankton bloom began in mid May, coinciding m t h the onset of the stratification, and was mainly composed of d~atoms and prymnesiophytes. Daily Ingestion of phytoplankton by C. finmarchicus was about 2% of body carbon for the long period before the bloom, 30% during the bloom and 14 % after the bloom. Due to their low abundance, ingestion of ciliates represented only 2 to 6 O/o of the phytoplankton ingestion. Body carbon and carbon/nitrogen ratio decreased significantly before the bloom indicating a deficit in carbon ingestion and the use of storage lipid during this period.

The rations removed by adult and stage V Calanus helgolandicus (Claus) feeding on large cells of the diatom Biddulphia sinensis Grev. were measured using an apparatus especially designed to keep the plant cells in suspension and estimated to reach a maximum of 1800 cells/animal/day at a food concentrationof 11,300 cells/1. A continual flow method was used to study feeding at very low algal concentrations and it was found that the animals still captured Biddulphia at a food level of only 270 cells/1.The animals in the feeding experiments were used at a very low population density (6 copepods/1350 ml.) and the maximum value found for the volume of sea water swept clear – 700 ml./animal/day - was unusually high.The maximum daily rations consumed by each animal were also high, being equivalent to 47–5 % of the body nitrogen and 46–4 % of the body phosphorus. The number of faecal pellets released by the animals increased with the size of ration captured, but the percentage of the ration lost as faecal pellets was fairly constant. In terms of dietary nitrogen the average value was 65–9% and that for dietary phosphorus 59·6%.The percentage of the daily ration of nitrogen excreted in soluble form was 266% and that of phosphorus 41–2%, these values being significantly higher than those found using unfed animals. Nearly 90 % of the nitrogen excreted by the animals, whether feeding or unfed, was in the form of ammonia. The amounts of excreted ammonia and total soluble phosphorus increased significantly when the animals fed; but the small quantities of nitrogen released in forms other than ammonia did not change.

Nauplii of Calanus helgolandicus were raised from eggs, laid within a 12 h period, to Copepodite Stage I (CI) on 5 different species of algae at high concentrations at 15°C. The diets used were Isochrysis galbana (5 µm spherical diameter), Rhodomonas baltica (7 µm), the coccolithophorid Pleurochrysis carterae (12 µm), the diatom Thalassiosira weissflogii (14 µm) and the dinoflagellate Prorocentrum micans (30 µm). Each day a sample was taken and preserved for later cohort analysis. Growth was estimated from CHN samples collected almost daily, from which naupliar stages were also distinguished. Ingestion was measured for each naupliar feeding stage. The fastest development was obtained with I. galbana and P. micans. We found the highest value of carbon and nitrogen content of Naupliar Stages NV to CI for individuals reared on the smallest algae, I. galbana and R. baltica. However, ingestion rate in terms of carbon or nitrogen was lowest with these same (smallest) algae. Therefore, the gross growth efficiency was highest for the smallest algae. These results suggest the following: Firstly, that factors influencing development time and weight in stage are different; weight in stage is negatively related to algal size, whereas development time is independent of it. The quality of the algal biochemical components could be the factor influencing development. Secondly, that small algae are fully assimilated in the gut whereas larger cells, i.e. those with indigestible components around the cell (theca, frustule, calcium layer) are only partly assimilated.KEY WORDS: Calanus helgolandicus · Nauplii · Growth · Development · Ingestion · Efficiency · Food quality Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 216: [151][152][153][154][155][156][157][158][159][160][161][162][163][164][165] 2001 Some studies have found consumption of diatoms by females to result in low hatching success (Ianora & Poulet 1993, Poulet et al. 1994, Chaudron et al. 1996, Ban et al. 1997), but this is still open to discussion (Jónasdóttir & Kiørboe 1996, Jónas-dóttir et al. 1998, Irigoien et al. 2000b.Development, body size and weight of nauplii and copepodites have been negatively related to temperature (Thompson 1982, Peterson 1986, Hopcroft & Roff 1998. Ontogenetic differences in the physiological response to temperature in copepods have also been noted (Pedersen & Tande 1992). The effect of food quantity on growth processes has often been unclear (Hart 1990, Hopcroft & Roff 1998, but some studies have clearly established an effect of food limitation on wild nauplii (Lopez 1991, Melle 1998, and this has been further substantiated by laboratory studies (Green et al. 1991, Lopez 1996.Food quality is another factor which has to be considered when examining variability in naupliar growth (Mullin & Brooks 1970, Paffenhöfer 1971, 1976, Fernández 1979a,b, Diel & Klein Breteler 1986, Verity & Smayda 1989. The major algal characteristics of importance are biochemical content, morphology, and di...