EGG PRODUCTION AND ABUNDANCE

D 452

NORTH SEA, SUMMER 1991.

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Netherlands Institute tor Sea Research; P.O.Box 59, 1790 AB Texel, The Netherlands.

EGG PRODUCTION AND ABUNDANCE OF COPEPODS IN THE NORTH SEA, SUMMER 1991.

I.A.Flameling

Nether/ands /nstitute tor Sea Research; P.O.Box 59, 1790 AB Texe/, The Nether/ands.

Abstract

Egg production ofthe calanoid copepod species Temora /ongicornis, Centropages hamatus and Acartia c/ausi was measured in the Marsdiep in the season of 1991 by 24 h incubation of mature females. Egg production of T./ongicornis increased from about 5 eggs per female per dav in the beg inning of March to 50 in April, and then decreased to about 15 in May-July. It showed a significant positive correlation to chlorophyll-a concentration. Egg production of the other 2 species was only measured in June and July, and fluctuated between 10 and 60 eggs.

Some experiments at 15°C showed a rather constant rate of egg laying over one 24 h period, but egg production seemed to stop after 24 hours. Data on the abundance of 11 copepod species and on the egg production of 7 of these species were collected during a North Sea cruise in August of the same year.

Adults of T./ongicornis, A.c/ausi and Pseudoca/anus e/ongatus were most abundant, and occurred especially in areas not deeper than 50 m. Other neritic, but less abundant species were Centropages hamatus and /sias c/avipes.

Centropages typicus, Ca/anus finmarchicus and Anoma/ocera patersoni we re virtually absent in the Southern Bight. Oithona similis and Metridia /ucens we re mostly found north of 56^oN. Ca/anus he/go/andicus, sparsely occurring in all areas, was seldomly caught together with C.finmarchicus. Egg production of T./ongicornis, A.c/ausi, and C.finmarchicus was generally low; mean values for C.hamatus and C.typicus were higher. Fluorescence shows a significant positive correlation to egg production of the 3 smallest, neritic species. Egg production of C.he/go/andicus and A.patersoni was rather constant during 24 hours of incubation at 15°C, and showed no diurnal rythm. The results are compared to literature data.

1. Introduction

Copepods form generally the most abundant part of the smaller zooplankton in the North Sea. At certain times (especially in spring), their biomass can be up to 90% of the entire zooplankton biomass (WILLIAMS & LlNDLEY, 1980). Most copepods are omnivoruous and feed mainly on diatoms and similar-sized algae, detritus and microzooplankton; although their diet is species-specific. They are a crucial food souree for young fish, and thus form an important link in the food chain by converting energy and material from micro- to macroscopical levels.

Distribution patterns of copepods in the North Sea are reported to depend mainly on water mass characteristics, with species diversity increasing from coastal areas to regions with oceanic influence (e.g. BOWMAN, 1971; COLEBROOK et al., 1961). It is assumed that some calanoid copepod species (e.g. Calanus finmarchicus) are allochthonous in character in the North Sea, and occur here only due to regular replenishment with individuals from the production centres in the North Atlantic and on the shelf edges (JASHNOV, 1970). In contrast, some typical neritic species (e.g.

Temora longicornis and Acartia claus/) are found in the shallower areas of the North Sea.

Species-specific adaptation to temperature and food conditions plays an important role in determining distribution patterns of calanoid copepods in the North Sea.

Temperature is a principal physical factor determining copepod development rates in all life stages (KLEIN BRETELER & GONZALEZ, 1988). Food concentration has a comparable effect on development rat es of laboratory cultures of several copepod species, (KLEIN BRETELER et al., 1982). Food concentration may be a limiting factor for copepod growth in the North Sea, especially in summer (FRANSZ & GIESKES, 1984; FRANSZ, 1975). Grazing rat es may be very low in this time of the year, pointing to unsuitable algae (BAARS & FRANSZ, 1984). In periods of algal food shortage, copepods probably switch to microzooplankton as a food source, and even cannibalism on copepod eggs and nauplius larvae occurs in those circumstances (DAAN, 1988). Temperature and food conditions also have a pronounced effect on the final body size of the adult stage (e.g. HARRIS & PAFFENHÖFER, 1976; DURBIN

& DURBIN, 1978).

Copepod egg production rate is shown to respond rapidly to changes in temperature and the concentration or quality of food. Egg production rates can be used as a simple means to make grazing rate inferences for adult copepods and larger copepodids (KICJ)RBOE et al., 1985a,b). Egg production by adult females seems to be equivalent with growth of juvenile stages for several copepod species (CORKETT

& MCLAREN, 1978; SEKIGUCHI et al., 1980; MCLAREN & CORKETT, 1981;

BERGGREEN et al., 1988), which might be a great convenience for predicting population changes and estimating rates of secundary production. However, care should be taken when applying ratios found under laboratory circumstances to field conditions. Egg production in the field is generally influenced by several undetermined factors, including life history of the females and temperature-differences due to vertical migration (KICJ)RBOE & JOHANSEN, 1986). Besides, food availability under field circumstances is a very complicated factor because of the variety in the suitability for different life stages.

This study presents a time series of egg production of 3 calanoid copepod species

in the Marsdiep in June-July 1991 and some additional experiments on the course of egg production during 30 h, as weil as the results of a zooplankton sampling programme during a North Sea cruise by R.R.S. "Challenger" in August 1991. During this cruise, the abundance of adult copepods of 11 species was determined and the egg production and body size of 7 species was measured. Results are correlated with physical factors and food conditions.

2. Material and methods

2.1 Copepod distribution and mesozooplankton volume.

Samples for measuring copepod biomass and distribution were taken at 68 net stations on the North Sea, during a cruise with the British R.R.S. "Challenger" in August 1991 (Fig.1). Samples were obtained by taking vertical hauls with a 300 J-L

vertical net from the botlom to the surface. A general description of this net can be found in BAARS et al. (1990). The version of the net used had a cone opening of 70 cm. The ratio of the filtration area to the area of the cone opening is 5: 1. Values for abundance and volume of zoo plankton we re calculated per m². The same net was used during cruises with the R.R.S. "Challenger" in the Southern North Sea in 1988/1989, so the results for th is region in 1991 are fully comparable to the data obtained in August 1988 and August 1989. The net catches were split up in two parts;

one was stored in formalin for determination of copepod abundance and species composition. (The other part was stored at -80 °C for RNNDNA measurements on copepods).

A number of stations in August 1991 were occupied for 24 hours. At these stations sampling was repeated every 5-6 hours, to obtain an impression of the short-time variation in numbers and species composition at a geographically fixed position.

The formalin samples were counted 6-8 weeks later under a dissection microscope (magnification 20-30x). Adult individuals of the main North Sea copepod species were counted in subsamples of 0.5-20 %, containing at least 50 adults. The number per m² was calculated by dividing the estimated total number of copepods in the net catch through the surface area of the cone opening.

For measurements of volume and total number of particles of the mesozooplankton (size range 300-2000 J-L), a zoo plankton counting tube designed at the NIOZ (S.Oosterhuis) was used. The samples we re measured after 6-10 weeks of storage in formalin, so the volume may be sligthly biased.

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2.2 Production of eggs in the /aboratory and on board.

For measurements of the egg production in the Marsdiep, adult female copepods were collected 3-4 times per week during flood or high tide from the NlOZ-jetty, or, if possible, by boat (Fig.2). For collection, a 200 J1. zooplanktonnet was used. The species concerned were Temora /ongicornis,

Centropages hamatus and Acartia c/ausi. The egg production series lasted 6 weeks, from the beginning of June till half July. Additional data on Marsdiep egg production of Temora /ongicornis in spring were kindfully supplied by S.Gonzalez (NIaZ).

On the R.R.S. "Challenger", adult females we re collected by vertical net hauls, as described before, at the same net stations (Tabie ... ).

Species concerned were Temora /ongicornis, Centropages hamatus and C.typicus, Acartia c/ausi, Ca/anus he/go/andicus and C.finmarchicus, and Anoma/ocera patersoni.

After anaesthetization with 100 J1.1 ms 222/ 10 mi seawater, adult females of different species were

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sorted out with a pipette under a dissection

Fig.2 Location of sampling station NIOZ

microscope (magnification 15-20 x). The females jetty (A). (after CADËE & HEGEMAN, 1991)

were placed in 2 I plastic bottles filled with fresh

50 IJ. filtered seawater and incubated during 20-27 h. During incubation at the NIOZ- laboratory the bottles were kept at seawater temperature (12°C in the beginning and 15°C at the end of the series), and a 16/8 LID cycle was maintained. On board the bottles we re kept in dimmed natural light. The temperature was 15 ± 1 ° C. The seawater surface temperature range during the cruise was 13-20 ° C, with a mean value of 16.3 °C. (Appendix I). To avoid predation on eggs, densities were kept low (10 femalesjbottle for the smaller species and 5 femalesjbottle for Ca/anus and A.patersom). The number of incubations per species was 1-5, depending on the availability of adult females. After incubation, the contents of each bottle were filtered over a 50 J1. filter. After checking if the females had survived the experiment, females and eggs we re killed with a few drops of formalin. Females and eggs were counted in a petri dish under a dissection microscope (magnification 10-40 x). Crumpled egg shells were counted as eggs. If immediate counting aboard was impossible, the contents of the bottles were, after filtration and checking, stored in 4 % formalin and

counted 4-6 weeks later.

After storage, the cephalothorax lengths of all the females in the egg production experiments were measured under a dissection microscope (magnification 50x for the smaller species, 32x for Calanus and Apatersom). For the assessment of fresh length values, a correction factor of -7% was applied, since killing of copepodids in formalin appears to increase their thorax length by ± 7% (KLEIN BRETELER, 1980). A formalin shrinkage percentage, usually applied to many other zooplankton groups, does not have to be applied to copepods (DURBIN & DURBIN, 1978).

The mean egg production per female in 24 h was calculated according to the equation: P = n x 24/ N x t, werein P is the egg production per female during 24 h, n is the total number of eggs produced, N is the number of females alive after the incubation and t is the duration of the experiment in hours. Since it was found by SEKUCHI et al. (1980) that the egg production of Ac/ausi females stops at least 24 hours before they die of age, it was assumed that females that were found dead at the end of the experiment had not contributed to the egg production.

In this equation, the assumption is made that the egg production remains constant along the duration of the experiment. To test this assumption, experiments were carried out to estimate the course of the individual egg production and the eventual presence of a diurnal rhythm. Single females of Acartia c/ausii, Temora longicornis and Centropagus hamatus were placed in 6 or 12 cm

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or 50 IJ. filtered seawater, and incubated for 30 h at 15.0 ⁰ C. During incubation a 16/8 LID cycle was maintained. Egg counts were made at intervals of 2-7 h. Eggs we re not removed, so the possibility of predation on eggs was not excluded. Empty egg shells were counted as eggs. On board of the R.R.S. "Challenger", similar experiments were carried out on C.helgolandicus at station 43 and Apatersoni at station 12. 15, respectively 9 replicate incubations of 5 females of C.helgolandicus and Apatersoni were started simultaneously in plastic 2 I bottles filled with 50 IJ. filtered sea water.

After 8, 16 and 24 hours 5, respectively 3 incubations were finished and the eggs were counted. Cumulative egg numbers at different points of time are, therefore, laid by different groups of females.

2.3 Physical and chemical parameters and numbers of algae.

CTD plots and surface data on fluorescense, oxygen, salinity and temperature of the net stations (Appendix I) were kindfully supplied by the scientific crew of the R.R.S.

"Challenger" (N.Owens, PML). Dr G. Cadée (NIOZ) supplied data on chlorophyll-a, phaeopigment and numbers of diatoms, flagellates and Phaeocystis spec. in the Marsdiep during the egg production series.

3. Results

3.1 Egg production in the Marsdiep in spring 1991.

Mortality of females used in the experiments was generally low: ±6%/day for T./ongicornis; ±6%/day for Ac/ausi and ±3%/day for C.hamatus. Egg production of the 3 species in the Marsdiep in spring 1991 is shown in Figs.3-5. T./ongicornis had a maximum egg production of ± 50 eggs female"l day"l in April. Later in spring, the egg production sharply decreased to values of even 10 eggs female"l day"l, and thereafter remained at ±20 till July. The egg production of C.hamatus and Ac/ausi, that has only been measured in the period 4 june - 17 july, was highly variabie. Mean values of both species were higher than of T./ongicornis during th is period.

Concentrations of chlorophyll-a and phaeopigment, and cell concentrations of diatoms, Phaeocystis spec. and flagellates (other than Phaeocystis) in spring 1991 are displayed in Figs.6-9. These parameters may reflect copepod food availability.

Seawater temperature is shown in Fig.1 O. Regression analysis revealed that egg production of T./ongicornis showed a significant positive correlation to chlorophyll-a concentration (R=0.454; P=0.013). Stepwise multiple regression analysis did nor reveal other significant relationships, even if all variables were averaged over periods of 10 days.

However, comparing the different types of algae available may provide some indication about their suitability as a food source for T./ongicornis. Egg production increased after a bioom of diatoms. Maximum egg production corresponded with a sharp increase in numbers of Phaeocystis, while diatom numbers were still fairly high in this period. The sharp decrease in egg production corresponded with a similar decrease in diatom numbers, while Phaeocystis reached its peak concentration. This results indicate that diatoms form possibly the most important food source for T./ongicornis.

Since egg production of C.hamatus and Ac/ausi was only measured during a short period and showed no clear trend, it was not compared to food availibility.

Cumulative egg numbers per female of T./ongicornis, Ac/ausi and C.hamatus during 24 h incubation in 10 mi petri-dishes are shown in Figs.11-13. In the case of C.

hamatus and Ac/ausi, crumpled egg shells were found within 5 h, while no nauplii emerged, indicating predation on eggs. Since hatching of Ac/ausi nauplii took sometimes place in 20 hours, nauplii were also counted. There proved to be large individual differences in egg production. 20-40 % of the mature females did not produce any eggs. Other females started egg production after up to 18 h, or produced a large number of eggs in the beginning of the incubation and stopped

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egg production afterwards. Eggs of T./ongicornis and C.hamatus were often attached to each other in double strings of up to 40 eggs, which may indicate that egg production in these species is not a constant process.

The mean egg production ra te of 10 or more females proved to be reasonably constant for all species during 24 hours of incubation. There also seemed to be no diurnal rhythm in egg production. Egg numbers did not increase after 24 hours, suggesting that after one day confinement in petri-dishes filled with 50 J1. filtered seawater the food supply was exhausted and/or deteriorated and egg production stopped.

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3.2 Classification of the North Sea into different areas, August 1991.

The net stations were classified according to a cluster analysis (Euclidean distance) in the program SYSTAT, based on standardised values of depth and mean salinity and temperature in the layer above the thermocline, or, in not-stratified

waters, in the entire water column. In a second cluster analysis, the presence of stratification, value- and depth of the fluorescence maximum, value- and depth of the oxygen concentration maximum and surface fluorescence were included.

Fluorescense is a rough measure of the concentration of chlorophyll in the water.

Based on the results of these cluster analyses, the North Sea was divided into 5 areas (Fig.14): Area I: the English side of the Southern Bight, area 11: the Dutch side of the Southern Bight plus the German Bight, area 111: the English/Scottish waters from Vork up to the Shetlands, area IV: the central and northern North Sea and area V: the Skagerak and the Norwegian channel. Table 1 shows for the 5 North Sea areas the total number of stations in the area, the mean values and ranges of depth and surface temperature, salinity and fluorescense, as weil as the % of stations where

Table 1. Means and ranges of surface depth, -temperature, -salinity and -fluorescense and % of stations with stratification in the 5 North Sea areas.

11 111 IV V

stations 8 13 12 14 4

depth (m) 33.2(19.1-45.5) 29.5(16.2-41.0) 80.6(31.5-144.0) 77.4(23.6-165.0) 109.5(46.8-256.0)

temp. (oC) 16.4(14.9-17.7) 18.1 (16.7-20.0) 14.6(12.9-17.2) 16.1 (13.8-17.8) 16.3(15.7-16.8)

salinity (%0) 34.40(33.94-34.74) 33.39(31.26-35.04) 34.43(34.10-34.85) 34.41 (34.10-34.95) 32.94(31.75-34.30)

fluor. (V) 2.35(2.06-2.66) 2.62(1.67-3.11) 2.05(1.39-3.44) 1.56(1.01-2.25) 1.65(1.40-2.10)

strat. (%) 0 0 80 90 100

stratification of the water column was present. Area land 11 were shallow, stratified areas with high fluorescense values, the latter probably being the result of high nutrient values caused by inflow of rivers. Coastal influences seemed to be somewhat stronger in area 11 than in area I: Area 11 had a higher surface temperature, higher surface fluorescense and lower surface salinity as a result of the outflow of several continental rivers in this area. Area 111 and IV we re of intermediate depth. Mean surface fluorescense in area 111 was higher than in area IV, probably because area 111 had some coastal stations. In addition, a very high fluorescense value was found at station

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Because of the outflow of brackish Baltic water on top of the North Sea water, surface salinity in this area was low. Fluorescese was equally low, probably because of the lack of river inflow in this area.

3.3 Adu/t copepod distribution and mesozoop/ankton volume.

Adults of 11 species of copepods were recorded in the samples, viz. of the calanoids Temora /ongicornis, Acartia c/ausi, Centropages hamatus, C.typicus, Pseudoca/anus e/ongatus, Ca/anus finmarchicus, C.he/go/andicus, Anome/ocera patersoni,

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c/avipes, Candacia pachydacty/a and Metridia /ucens and of the cyclopoid Oithona similis. Since male copepodids-V and male adults of A.c/ausi and P.e/ongatus can only be distinguished by a small difference in their antenna, the numbers of adult males are less accurate in these than in other species. Male adults of O.similis were rare and hard to distinguish from younger stages, so for this species only females were recorded. Mean abundances of females and males per m² in the 5 North Sea areas are presented in Table 2 (except for C.pachydacty/a, which was found only sporadically). Net hauls at the same station (e.g. during the 24-hour stations) were averaged.

Numbers of adult copepods in different net hauls at 24-hour stations at a fixed geographical position are displayed in Fig.15. Variation in total adult copepods/m² was generally <50%. Variation in species composition was relatively low, except for station 24, in an area with strong tidal currents, where an increase in T. /ongicornis occurred over 24 hours, suggesting that not the very same plankton population was sampled during this time span.

Fig.16 shows the total number of adult copepods at each station. Zooplankton biomass in the fraction 300-2000 M, measured with the zooplankton counting tube, is shown in Fig.17. These two parameters rather showed an inverse relationship. In the Southern Bight (areas I, 11 and the southern part of area 111) total numbers of adult copepods were high, while zooplankton biomass reached its highest values in the central and northern North Sea and the Norwegian Channel (areas IV and V). This can be attributed to the fact that the larger copepod species (e.g. C.typicus and Ca/anus spec.) dominated in these more oceanic waters. Besides adult copepods, the samples contained juvenile copepods and other kinds of zooplankton (e.g.

chaetognaths, mysids, euphausids, amphipods, pteropods and hydrozoa).

Estimates of the number of adults per m² of the separate copepod species at each station are displayed in the Figs.18-28 (see also Appendix 11). Values of 0-100 adults/m²,

Table 2. Zooplankton volume, copepod numbers and copepod e99 production In 5 different North Sea areas.

11 111 IV V

particles/m² (x10⁴⁾ 4.3 5.5 6.0 6.5 6.3

zooplanktonvol./m² (mI) 5.7 6.7 13.7 12.5 19.3

T./ongicornis

9/m² (x10³) 4.7 5.7 1.0 3.7 1.1

d/m² (x10³⁾ 4.6 5.7 1.0 4.2 1.1 eggs/9/day 3.1 ±2.2 7.4 ±1.8 5.3 ±1.0 2.7 ±2.0 7.9±2.4

body length (IJ.) 760 806 776 779 869

A.c/ausi

9/m² (x10³⁾ 13.1 3.6 13.3 0.6 0.3

d/m² (x10³) 1.5 1.1 4.4 0.3 0.1 eggs/9/day 3.0 ±0.8 4.6 ±0.9 1.5 ±0.7 3.4 ±0.6 8.0 ±2.6

body length (IJ.) 768 742 786 767 768

C.hamatus

9/m² (x10³) 1.9 1.7 0.2 0.3 0.01

d/m² (x10³) 1.2 1.5 0.1 0.4 0.04

eggs/9/day 10.4 ±2.1 19.7 ±4.6 9.7 ±1.8

body length (IJ.) 868 885 876

C.typicus

9/m² (x10³⁾ 0.0 0.3 0.0 0.1 0.1

d/m² (x10³⁾ 0.0 0.03 0.0 0.1 0.1

eggs/9/day 32.6 ±18.7 34.3 ±12.5 30.7 ±9.1

body length (IJ.) 1129 1126 1107

C.finmarchicus

9/m² (x10³⁾ 0.0 0.0 0.8 0.6 0.6

d/m² (x10³⁾ 0.0 0.0 0.3 0.05 0.1

eggs/9/day 11.4 ±9.6 13.0 ±7.2 7.3 ±3.1

body length (IJ.) 2272 2292 2240

P.e/ongatus

9/m² (x10³⁾ 3.0 7.0 4.5 7.1 5.1

d/m² (x10³⁾ 0.3 0.8 0.3 0.1 0.9

Table 2. (continued).

11 111 IV V

G.he/go/andicus

91m

dlm

/.c/avipes

91m

dlm

Apatersoni

91m

dlm

O.similis

91m

M./ucens

adults/m² (x10³) 0.00 0.00 0.1 0.05 0.00

TOTAL

Adult copepods/m² (x10³)

30.5 27.6 27.0 19.6 9.9

as represented by the open circles, indicate th at the species was not found in the examined subsample.

Gommon species in the Southern- and the German Bight (areas land 11) were T./ongicornis, Ac/ausi and P.e/ongatus (Figs.18-20). Ac/ausi was also abundant in area 111, whereas P.e/ongatus occurred throughout the whole North Sea.

G.hamatus (Fig.21) occurred in rather large quantities in areas land 11, although it was less abundant than the 3 dominant species. G.typicus (Fig.22) occurred in sm all quantities north of 54⁰N and along the Norwegian Ghannel (areas IV and V).

G.he/go/andicus (Fig.23) occurred in the Southern Bight and on some scattered points troughout the rest of the North Sea. This species was especially abundant at station 45, north of the Shetland Islands. G.finmarchicus (Fig.24) was found

throughout the entire northern Nort Sea. These 2 species were seldomly caught together. Station 47, south of the Shetland Islands, was the only station where both species were found.

/.c/avipes (Fig.25) was found in small quantities in the Southern Bight (areas land 11).

O.similis and M./ucens (Figs.26 and 27) seem to be typically oceanic species, having their main distribution in the northern parts of area 111 and IV. A.patersoni was found at two stations in the North Sea with mean numbers above 100/m² (Fig.28), and at some other stations in lower numbers. Abundance of the adults of this neuston species was probably underestimated, due to net avoidance.

34 32 30 28 26 24 22

N'i 20

E"

,,~ 18

" "

~] ¹⁶

<!:: 14

12 10 8 6 4 2 0

.1.12 .1.24 .1.43 .1.50

Fig. 15. Numbers of adult copepods in different net hauls at 24-hour stations at a fixed geographical position. [Z2]: T./ongicornis; cs::sJ :A.c/ausi; [22Z3 P.elongatus; bSSSl :C.hamatus; [K6J :C.finmarchicus.

61 ⁰

51 ⁰

- "

... ... \ I I I

'"

".

".

".

,'"

,.,) ,.. _ _ _ - - l ... ,

, - r_' ,

".

,

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•

...

• >10000 _ >20000

50000

•

, ,

•

"-

, , ,

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\ I f I I I

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,

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\

Fig. 16. Total adult copepods in the North Sea in August 1991.

,

•

1 ⁰

9⁰

1 °

,go

53°

51 °

"

"

4°

"

I

m

"

"

\

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\

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11m

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•

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0 2.5 5 10 20

0°

Fig. 17. Mesozooplankton biomass in the North Sea in August 1991.

1⁰

9°

61 ⁰

59⁰

51 ⁰

40~ ________ ~2~0J-________ -=0_0L-________ ~2~0J-_________ 4 __ 07-____ ~ ___ 6_0~ _________ 8~o~ __ ~

_

,. ....

,

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el ^.,

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• > 100

• > 1000 e > 5000

.>10000

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•

,.

... ~, ( / I

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I

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o ... ^\ o o

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-- -

,

Fig. 18. Distribution of Temora longicornis in the North Sea in August 1991.

1 0

9°

,1 °

53°

51 °

2 0 00 20 40 60 8°

4°~ ____ ~ ______ ~ ____ ~~ ____ ~+-

__

______

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,

4°

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e

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dults/m²

'.

a ₀ ~ 0 ~

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e>

.>10000 20000

2° 0° 2° 4°

Fig. 19. Distribution of Acartia clausi in the North Sea in August 1991.

6° 8°

I °

9°

3°

1 °

61 ⁰

59⁰

55⁰

510

4^{0 ~} __________ L -__________ L -__________ L -__________ ~ _ _ _ _ ~----L---L---⁸

....

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• > 100

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~:

1 0

9°

~ __________ ^~ __________ - . ______ ^c-~~~~ I' ________ , -__________ - . __________ ^~ ____ ^~~1 0

8⁰

Fig. 20. Distribution of Pseudocalanus elongatus in the North Sea in August 1991.

01 0

I 0

40~ ________ =2_0L-________ O~0L-______ ~2~0 __________ 4~O~----~--_6~O~---_8_0L-__ ~ ,

~

., \

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r_'

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,

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adults/m²

o ~ 0

• > 100

• > 1000 e > 5000

.>10000

20000

,

I o o

"\

\ I1

~ f ....

\. ,,... Ij

•

Fig. 21. Distribution of Centropages hamatus in the North Sea in August 1991.

1 0

go

61 °

57°

55°

51 °

40 ^~ ______ ~L-2° ________ 0° ~ ________ 2° - L ________ ~ _ _ _ _ , -_ _ ~ ________ ~ _ _ _ _ ,

,.

o 1 \ I /

"

\

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r _ J

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,

adults/1l 2

o ~ 0

• > 100

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e > 5000 .>10000

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1 0

9°

7°

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+---~~---_.---L~~_r~---._---_,r_---~----_L51 °

2° 4° 6° 8°

Fig. 22. Distribution of Centropages typicus in the North Sea in August 1991.

61 ⁰

59⁰

1 ⁰

,.. ...

I

,

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I

,~

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---) "

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• > 100

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1 ⁰

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Fig. 23. Distribution of Ca/anus he/go/andicus in the North Sea in August 1991.

61 ⁰

51 ⁰

- '

... \

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)

"

"

"

I

/ /

\

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....

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adults/m²

o ~ 0

• > 100

• > 1000 e > 5000 .>10000 20000

(\

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,~ •

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Fig, 24, Distribution of Ca/anus finmarchicus in the North Sea in August 1991,

1 0

go