The vertical distribution of zooplankton in the western Irish Sea

Estuarine. Coastal and Shelf Science (1986) 22,785-802

The Vertical the Western

Sherard Marine

Distribution Irish Sea

Scrape-Howe Laboratories,

Science

of Zooplankton

and David

in

A. Jones

Menai Bridge, Anglesey, Gwynedd, U.K.

Received 11 April 1984 and in revised form 24 May 1985

Keywords: Irish Sea.

Zooplankton;

vertical migration;

chlorophyll

distribution;

western

Zooplankton die1 vertical migration is evident on the mixed isothermal side of the western Irish Sea frontal system but is often influenced by large tides and persistent geostrophic currents. On the stratified side of the front, temperature acts as a controlling factor with most of the zooplankton occurring above the thermocline and carrying out pronounced vertical migration when chlorophyll a levels are low and diffise. At higher chlorophyll levels, when discrete chlorophyll a maxima form, zooplankton vertical movement may be greatly modified with a large number of species and stages concentrating within these maxima at all times of the die1 light cycle.

Introduction Details of the general distribution and abundance relatively well known (Herdman et al., 1908-1921; 1952; 1956a,b; 1963; 1975; Khan & Williamson,

of zooplankton in the Irish Sea are Johnstone et al., 1924; Williamson, 1970; Leigh, 1977; Floodgate et al.,

1981; Scrape-Howe & Jones, 1985), but with the exception of Lee & Williamson (1975), there is a notable absenceof studies on die1vertical migration in these waters. The physical oceanography of the western Irish Sea has been studied by Simpson (197 1) who developed the tidal mixing theory, predicting that the occurrence of stratification is determined by the parameter H/U3 where H is the depth of the water column and U the amplitude of the tidal stream. Simpson (1971) observed that during the summer there is an area of strong thermal stratification off the Irish coast, separated from tidally

mixed

isothermal

water

to the east by a well defined

frontal

system. During

recent

multi-disciplinary research into the biological activity of this region (Fogg et ul., 1985) it was found that die1 vertical migrations of zooplankton on the stratified side of this front may be modified by the presence of high concentrations of chlorophyll a during seasonal phytoplankton blooms. The aggregation of migrating zooplankters at sharply discontinuous phytoplankton maxima has been found elsewhere in seasonallystratified waters (Beers & Stewart, 1967; McLaren, 1969; Anderson et al., 1972; Mullin & Brooks, 1972; Hobson & Lorenzen, 1972; Chester, 1975; Haury, 1976; Bird, 1983; Sameoto, 1984; Townsend et al., 1984; Paffenhofer

et al., 1984), and also in permanently

stratified,

tropical

oceans (Venrick

785 0272-7?14/86/060785+

18 $03.00/O

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1986

Academic Press Inc. (London)

Limited

et

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-53O .

- 52’N

I

Figure

1. Locations

of sampling

stations

along the main western

Irish

:Ow Sea transect.

al., 1973; Youngbluth, 1975; Gunderson et al., 1976) where a complex relationship exists between the zooplankton and deep chlorophyll maxima (Longhurst, 1976; Ortner et al., 1980; Longhurst 8z Herman, 1981). The presence of a discrete phytoplankton layer together with an indication of chlorophyll u levels required to constrain the die1vertical migration of zooplankton is reported here for the first time in a temperate shallow shelf seaenvironment. Methods Samples were taken on a series of cruises by the U.C.N.W. research vessel ‘Prince Madog ’ at stations, fixed by Decca coordinates, along a standard survey transect in the western Irish Sea described by Fogg et al. (1985) and shown in Figure 1. Depth profiles of temperature and conductivity were obtained at each station using a CTD probe (Plessey Environmental Systems model 9400) periodically calibrated by NIO water bottle casts.

Vertical distribution

787

of zooDlankton

TABLE 1. The time, geographical position (station water mass location and type of station for the four Time A B C D

series

Date Jun 1980 May 1981 Jull980 May 1981

Station 9 9 1

2

numbers in reference time series mentioned Water

mass

Mixed Mixed Stratified Stratified

to Figure l), in the text Station

type

Drogue Anchor Drogue Anchor

Zooplankton samples were collected with a plankton pumping system with a 6 cm internal diameter hose which was lowered vertically to precise depths determined by a pressure transducer (Shape model S300). Water was discharged by a modified, submersible, electric, centrifugal pump (Flygt model B2051) through a fine mesh conical plankton net (142 microns aperture). The pump produced a measured flowrate (Kent flowmeter model PS40-300) of 250 1min- r to yield a sample of 1m- 3 for each stratum. The discrete depth intervals sampled represent a compromise between a desire for total vertical coverage and the practical constraints imposed by up to 45 hours of continuous sampling. Time series(seeTable 1) were carried out either while the ship was at anchor or by keeping station with a subsurface drogue tracking system. The drogue was a fixed geometry cruciform model, recommended by Vachon (1977), consisting of two intersecting rectangles, each 1 m wide by 2 m high, of plastic canvas suspended beneath a near-surface buoy with a small surface float located by flag and flashing light (Fogg et al., 1985). The fresh sampleswere immediately preserved in 5% neutral formalin seawater solution. These were subsequently analysed back in the laboratory where zooplankton density was determined by volumetric subsampling with replacement, and all groups identified to speciesand stage levels from samplesof at least 100 individuals (Winsor 8z Walford, 1936). Data for all the day-time samplesand all the night-time sampleswere tallied separately for each of the four time series listed in Table 1. The percentage per sampled depth of these totals for day and night periods were then calculated to show the relative vertical distribution of both the whole zooplankton population and the individual species(Pugh, 1984). To determine if there were any differences in abundance of speciesbetween day and night, unpaired ’ t ’ tests (Sameoto, 1984) were carried out. Nine out of 55 tested categories showed a significant difference (P
788

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2.The migration behaviour exhibited by the species and stages (a =adult; c= copepodite; n = nauplii) of zooplankton examined during the four time series listed in Table 1. Four main types were categorized: (1) those that showed upward movement at night towards the surface; (2) those that showed a downward movement at night; (3) those that showed different vertical movements within the population or moved towards a particular intermediate depth in the water column at night; (4) those that showed no apparent vertical movement (-indicates that a species or state was absent)

TABLE

Time Taxonomic ivow P. elongatus (a) P. elongatus (c) P. elongatus (n) A. clausi (a) A. clausi (c) A. clausi (n) T. Zongicornis (a) T. longicornis (c) T. longicornis’(n) 0. sin&s M. pusilhs M. hens lamellibranch veligers gastropod veligers echinoderm plutei polychaete larvae M. membranacea 0. dioica P. leuckarti E. nordmanni

All data storage, manipulation tern-10 computer.

series

A

B

C

D

1 3 1 1 2 2 3 2 3 3 3 -

1 3 2 3 2 2 3 2 2 3

1 1 1 1 1

1 1 4 3 1 1 1 1 3 4 4 3 3

1 4

3

1 3 2 2

3 -

-

-

3 2 3 3

and analysis was carried out on the U.C.N.W.

DECsys-

Mixed isothermal water-Time series A Analysis of the vertical distribution samples collected during a 12-hour time series in June 1980 at station 9 (see Figure 1 for station positions) are shown in Figure 2. These result from pumping at 4 discrete depths in the water column (surface, bottom and two intermediate levels) at 3-hour intervals and using the cruciform drogue system to fix the ship’s position in the same water body. Figure 2(a) shows the day and night depth distribution of the total zooplankton population shown as percentages of total numbers found in the water column by day and by night (Pugh, 1984). The 11 species and stages (Table 2) commonly encountered during this time series are represented in the same manner as Figure 2&l). The sampling was carried out during a period of slack tides and low wind velocity so the drogue was able to track the same water body effectively as shown by the relatively consistent temperature and salinity observed throughout the water column during this time series (Fogg et al., 1985). Unpaired t-tests showed that there was no significant difference (P>O.O5) between numbers sampled during the day and night

--

Vertical

distribution

of zooplankton

a

percentage

40

0

b 51) 2.5

0

2.5

789

5.0

;;(x;$*

0

20

40

d

’4jyig

Figure 2. The day and night (black) depth distributions of zooplankton sampled during the mixed water time series A in June 1980 shown as percentages of the total numbers found in the water column, by day and by night. These totals are given for each profile. 0 indicates no individuals were observed at that particular depth. (a) total zooplankton; (b) P. elagatus adults; (c) I? elongufus copepodites; (d) P. elongutm naupiii; (e) A. c2uu.Gadults; (f) A. clausi copepodites; (g) A. clausi nauplii; (h) T. Zongicornis adults; (i) T. Zongicornis copepodites; (j) M. membrunaceu; (k) lamellibranch veligers; (1) 0. similis.

periods. Figure 2(a) shows a small general upward movement in the total zooplankton population (although the highest concentrations still remain around the deeper depths of the water column) with a 5% increase in the total zooplankton at the surface at night. Of the 11 different species or stages 3 [Pseudocalanus elongatusadults (2b), P. elongatus

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nauplii (2d) and Acartiu cluusi adults (2e)] showed movement towards the surface at night; 3 [A. clausi copepodites (2f), A. clausi nauplii (2g) and Temora longicornis copepodites (2i)] showed a downward movement; and 5 appeared to exhibit different vertical movements within the population or to move towards a particular intermediate depth in the water column during the night [P. elongatus copepodites (2~) and the cyphonaute larvae of Membranipora membranacea (2j) appeared to concentrate at 10m and towards the bottom, T. Zongicornis adults (2h) and lamellibranch veligers (2k) tended to move towards the surface and bottom while Oithona similis (2 1) concentrated at 10 m]. Mixed

isothermal

water-Time

series B

Figure 3 shows the results from a 40-hour time seriestaken in May 1981 when the ship was anchored at station 9. On this occasion 6 discrete depths were sampled in the water column (surface, bottom and four intermediate levels) every 2 hours for the first 24 hours, followed by a further 20-hour period with 3-hour sampling intervals. As in Figure 2, Figure 3(a) showsthe day and night depth distribution of the total zooplankton population and Figure 3(b-m) show the day and night distributions for 12 speciesand stagescommonly encountered (Table 2). This was an anchor station, rather than a free drifting one, as the main purpose of the cruise was to study diurnal changes in phytoplankton physiology using firmly moored bag enclosures (Heath, 1982). This, coupled with the fact that sampling was carried out during a period of strong spring tides, would imply that there would be a certain amount of patchiness in the plankton due to advection of water past the anchored vessel (Sameoto, 1978). Unpaired t-tests showed that for 50% of the speciesand stages,and indeed for the total zooplankton, there was a significant difference (P c 0.05) in numbers between day and night with more individuals sampled during the day. Figure 3(a) shows a general concentration around 20 m during the night with movement actually away from the surface. Of the 12 different speciesor stagesonly 1 [P. elongatus adults (3b)] showed a general movement towards the surface at night, 5 [P. elongatus nauplii (3d), A. cluusi copepodites (3f’), A. cluusi nauplii (3g), T. Iongicornis copepodites (3i) and T. Zongicornis nauplii (3j)] showed a downward movement; and 6 appeared to show different vertical movements within the population or to move towards a particular intermediate depth in the water column during the night [I’. elongatus copepodites (3c), M. membranacea (31) and lamellibranch veligers (3m) appeared to move towards the surface and bottom, the adults of A. clausi (3e) and T. Zongicornis (3h) concentrated at the surface and the middle of the water column and 0. similis (3k) was found to concentrate around 20 m]. Thus in spite of the different sampling conditions encountered during the 2 mixed water time seriesonly I’. elongatus nauplii and A. cluusi adults out of eleven speciesor stagesfound in both time series showed different types of migration on each occasion. The chlorophyll a levels measured during both time series were never higher than 1.6 ug 1-r and always tended to be uniformly distributed with depth (Heath, 1982; Fogg et al., 1985). Compared with the stratified side of the front, the mixed isothermal water contained much lower concentrations of both chlorophyll a (Fogg et al., 1985) and zooplankton (Scrape-Howe 81Jones, 1985). Stratified

water-Time

series C

Figure 4 shows the results from a diurnal study in the thermally stratified water at station 1 in July 1980using the samecruciform drogue position fixing technique asin time series A. Pumping was carried out at 4 discrete depths (surface, above, in and below the

Vertical distribution

791

ojzooplankton

percentage

d

g

I

2

40 IF

0

20

40

m

20

‘e 511

,270

633

0

20

40

I59

s

s

i%

Figure 3. The day and night (black) depth distributions of zooplankton sampled during the mixed water time series B in May 1981 shown as percentages of the total numbers found in the water column, by day and by night. These totals are given for each profile. 0 indicates no individuals were observed at that particular depth. S indicates there were significantly (PcO.05) more animals observed during the day than at night as measured with an unpaired ‘ t ’ test. (a) total zooplankton; (b) P. elongutur adults; (c) P. eiongurus copepodites; (d) F. elongutti nauplii; (e) A. clausi adults; (f) A. clausi copepodites; (g) A. clausi nauplii; (h) T. longicomis adults; (i) T. Zongicornis copepodites; (j) T. hngicotnis nauplii; (k) 0. similis; (1) M. membrunuceu; (m) lamellibranch veligers.

792

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percentage

Figure 4. The day and night (black) depth distributions of zooplankton sampled during the stratified wafer time series C in July 1980 shown as percentages of the total numbers found in the water column, by day and by night. These totals are given for each profile. The tbermocline was situated at 20 m. 0 indicates no individuals were observed at that particular depth * indicates that there was no significant (Fz 0.05) vertical movement between day and night populations as measured by a chi-square test. (a) total zooplankton; (b) P. elongutus adults; (c) P. elagatus copepodites; (d) P. elongatus nauplii; (e) A. cluusa’adults; (f) A. chusi copepodites; (g) 0. similis; (h) M. pusilhs; (i) lamellibranch veligers; (j) gastropod veligers; (k) echinoderm plutei; (1) polycbaere larvae. thermocline) determined from CTD cast data and was continued for 15 hours, sampling at 2 hour intervals. The thermocline was situated at 20 m (Fogg et al., 1985). As previously, Figure 4(a) shows the day and night total zooplankton vertical distribution and Figure 4&-l) shows the same for the 11 common species or stages listed in Table 2. It would appear from temperature profiles taken during the time series (Fogg et al., 1985) that there

Vertical distribution

of zooplankton

793

may have been somelateral advection into the sampledwater column below the deployed depth of the drogue. However, unpaired t-tests showed that there was no significant difference (P> 0.05) between numbers sampled during the day and night periods. Also chi-squared tests showed that in the majority of speciesor stagesthere were significant changes in depth distribution between day and night (P
water-Time

series D

Figure 5 shows the results from a 36-hour study taken in May 1981 when the ship was anchored at station 2 in the stratified water. Six depths were chosen so as to include samplesfrom the surface, the depth of maximum chlorophyll a concentration, a depth above, in and below the thermocline (which was situated at 30 m) and one from near the bottom. Sampling of this profile was carried out every 2 hours. The reasonfor an anchor, rather than a drogue, station was the sameas explained above and thus the problem of patchiness due to the advection of different water past an anchored vessel is also the same.However, since the tidal flow and general currents are much lessin this area than in the mixed water (Simpson, 1971) it would be expected that their effect on zooplankton distributions would be appreciably less. Indeed, unpaired t-tests showed that there was no significant difference (P>O*O5) between numbers sampled during the day and the night. Figure 5(a) shows the day and night depth distribution for the total zooplankton and also gives an indication of the general temperature structure of the water column obtained from CTD data and the depth range of the chlorophyll a maxima layer which was situated around 15 m (Heath, personal communication). The relatively low, uniform distribution of zooplankton numbers below the thermocline during both day and night is similar to that shown in Figure 4. There is a small general movement upwards at night to give a 5% increase in total zooplankton at the surface which is far lessmarked than that seenin Figure 4. In contrast to the vertical movements shown in Figure 4, the majority of the total zooplankton (40%) are strongly concentrated around the 15 m stratum, corresponding to the chlorophyll a maxima, and remain there throughout day and night. Figure 5&-r) show the day and night depth distribution for the 17 different species or stages encountered (Table 2). Of these, 6 [I’. elongutus adults (5b), P. elongatus copepodites (5c), A. clausi copepodites (5f), A. cluusi nauplii (5g), T. longicornis adults

794

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OC

%

a

4020

0

0 40

6

10

14

0

.C .c

d

b 0 15 30

m

Figure

5. For legend,

see opposite.

(5h) and T. longicornis copepodites (5i)] show general movements upwards towards the surface at night while the appendicularian Oikopleura dioica (5r) was the only species observed to move downward in the water column with a small shift from the chlorophyll a maxima to the thermocline at night. Seven species or stages showed movements towards a particular intermediate depth in the water column. A. clausi adults (.5e), T. longicornis nauplii (5j), lamellibranch veligers (5m) and Podon Zeuckarti (5q) showed movement into the chlorophyll a layer at night and Ewadne nordmanni (5~) tended to move towards the thermocline. M. membranacea (50), which had greater numbers below the thermocline, moved towards the bottom, although a very small percentage found above the thermocline actually moved towards the surface at night. Metridiu lucens (5m) wasonly found below the thermocline and tended to move off the bottom at night to give

Vertical

distribution

of zooplankton

percentage

n

0 70

35

0

35

70

0 15 30

‘z I c n w 0 -1

P 0 15 30

80

Figure 5. The day and night (black) depth distributions of zooplankton sampled during the stratified water time series D in h4ay 1981 shown as percentages of the total numbers found in the water column, by day and by night. These totals are given for each profile. In (a) the general temperature structure of the water column is provided (the thermocline was situated at 30 m) and the horizontal lines marked with a C represent the depth range of the chlorophyll a layer (15 m). 0 indicates no individuals were observed at that particular depth. t indicates that there was no significant (P> 0.05) vertical movement between day and night populations as measured by a cm-square test. (a) total zooplankton; (b) P. ekmgatus adults; (c) P. elongatus copepodites; (d) P. elongarus nauplii; (e) A. clausi adults; (f) A. clausi copepodites; (g) A. cluusi nauplii; (h) T. L~&ornis adults; (i) T. longicowris copepodites; (j) T. longicornis nauplii; (k) 0. similir; (1) M. pu.riZlus; (m) M. lucens; (n) lamellibranch veligers; (0) M. membrunacea; (p) E. nordmanni; (q) P. leuckarti; (r) 0. dioica.

a more even distribution in that part of the water column. Three species or stages exhibited no apparent vertical movement. Around 37% of the P. elongatusnauplii (Sd) remained in the chlorophyll a layer, 0. sirniZi~(5k) remained more concentrated around the thermocline and M. pusillus (51) remained fairly evenly distributed beneath the thermocline. Chlorophyll a and zooplankton distributions Regression analysis of log zooplankton distribution against log chlorophyll a concentrations from the stratified water studies (Fogg et al., 1985; Heath, personal

796

S. Scrape-Howe

& D. A. Jones

0 0 g

5.0-

0

0

l

A

0

4.51.6

I 1.7

I 1.8

Log,,

Chlorophyll

I 2.0

I 1.9 a concentration

Figure 6. Relationship between logi (Lc31-V.

0

, 2.1

I 2.:

(pg C hl a I-’ 1

zooplankton (no. mm3) and log,, chlorophyll a

TABLE 3. The concentration of chlorophyll a (ugl-i) recorded by other authors as being a subsurface maxima with associated peaks in vertical zooplankton distribution

Location Ba8=m Island Oregon Coast California coast Gulf of Mexico California Current W. English Channel Peruvian coast BalIin Bay Gulf of Mexico Florida coast Arctic Nova Scotia coast Gulf of Maine

(pg Chl n 1- i) 6.0

1.0 4.0 0.6 0.3

50.0 12.0 5.0 4.5

12.0 5.0 2.0 7.0

Reference McLaren ( 1969) Anderson ez al. (1972) Mullin & Brooks (1972) Hobson & Lorenzen (1972) Haury (1976) Holligan (1978) Sameoto (1982) Herman (1983) Bird (1983) Paffenhofer er al. ( 1984) Longhurst et al. (1984) Sameoto (1984) Townsend er al. (1984)

communication) show that zooplankton distribution as a function of the total range of chlorophyll a concentration (l-9 ug l- ‘) gives a significant relationship with r=0.480 and P > 0.05. When the chlorophyll data are separated into lower (14 pg 1- ’ ) and higher (5-9 pg 1-r) concentration levels within the total range and regressed against the corresponding zooplankton concentrations as before, it appears that for low chlorophyll a levels the regression analysis shows no significant relationship with the zooplankton (r = 0.125 and P < O*OS).At these low chlorophyll levels zooplankton may be observed to continue to demonstrate expected patterns of vertical migration above the thermocline (Figure 4). However, as the chlorophyll a concentrations increase above this level the relationship with zooplankton distribution (Figure 6) becomes significant (r = 0.484 and P> 0.05) and die1 vertical movements cease for a large part of the zooplankton population which remains in the chlorophyll a rich areaof the water column (Figure 5a). The regression line in Figure 6 is very similar to the one obtained by Mullin

Vertical distribution

of zooplankton

797

& Brooks (1972) who found that juvenile Calanus tended to be more abundant in the sampleswith the highest chlorophyll a levels. Table 3 shows the levels of chlorophyll a reported by other authors as being chlorophyll maxima associated with peaks of zooplankton abundance. Only 4 authors quote chlorophyll a maxima with levels of < 4 l.tg chlorophyll a l- i . Around 50% of the speciesand stagesobserved during time series D (Figure 5) had the highest proportion of their population in the chlorophyll a layer at all times (i.e. during both day and night). This increasesto 70% if night sample data are taken separately based on an assumption that the effect of upward vertical migration during this period would mean a maximum number of zooplankton being present in or around the chlorophyll a layer (Sameoto, 1984). The only exceptions to this were 0. similis with around 50?,, of the population associated with the thermocline during both day and night periods, and M. pusillus, M. lucens and M. membrunuceu which were either confined solely beneath the thermocline or had the greater majority of their populations in this area of the water column. Discussion Station 9 (Figures 2 and 3) is relatively shallow (around 50 m) and subject to persistent currents asit is positioned in the path of the main northward movement of water flowing from St George’s Channel to the North Channel (Khan & Williamson, 1970; Hunter, 1972). As a result this region to the east of the western Irish Sea frontal boundary is isothermal and subject to complete vertical mixing (Simpson et al., 1977). In addition Heath (1982) observed a high amount of suspendedinorganic particulate material mixed into the water column by turbulence during time series B (the anchor station), which was also subject to strong spring tides. This severely hampered the counting of phytoplankton cells which were found to contain a large percentage of benthic diatoms. In view of the persistent currents, the complete vertical mixing and, as in the caseof time series B, strong spring tides it would be reasonable to expect a relatively uniform vertical distribution of zooplankton in the water column at all times asfound by Turner & Dagg (1983) and Holligan et al. (1984). Certainly there was very little fluctuation at any depth and no indication of any die1 rhythms for all other measured characteristics of the mixed water sampled during time series A (Fogg et al., 1985). However, the zooplankton distributions (Figures 2 and 3) show someevidence that die1vertical movements take place. Chi-squared tests on the total zooplankton and all speciesand stages encountered during both mixed isothermal water time series indicate that, without exception, vertical movement took place in every category tested. Die1 vertical movements of zooplankton in mixed waters have also been reported by Pearre (1973), Lee & Williamson (1975) and Southward & Barrett (1983). Stations 1 and 2 (Figure 1) on the stratified side of the frontal boundary are situated close to the deepest part of the Irish Sea, and are practically unaffected by tidal currents (Lee & Williamson, 1975). It is evident from the time seriescarried out at these stratified stations (Figures 4 and 5) that the thermocline significantly influences the vertical distribution of the zooplankton. The majority of the population occurred in the warmer water above the thermocline, with only a small part of the total zooplankton present in the water column resident below or actually in the discontinuity layer. There are numerous reports on zooplankton concentrations in relation to the thermocline. Vucetic (1961) reports observations on zooplankton confined beneath the thermocline, Miller et al.

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(1963) and Minoda & Osawa (1967) within the thermocline itself, and different distributions in relation to the thermocline have been shown by various specieswithin a population (Hansen, 1951; Angel, 1968; Southward & Barrett, 1983; Holligan et al., 1984). Higher concentrations of zooplankton occurring above the thermocline, asin the present observations, have been found by Boyd (1973) and Peter & Nair (1978). Zooplankton aggregation above the discontinuity layer is most probably a response to high levels of primary productivity caused by enhanced rates of biological activity in this part of the water column (Banse, 1964; Fogg et al., 1985). It would appear that when chlorophyll a levels above the thermocline are relatively low and diffuse, the zooplankton in this study carried out normal die1 vertical movements with respect to the light cycle without crossing the thermocline into the cooler water below. However, when chlorophyll u levels were high and became concentrated into discrete subsurface layers, the zooplankton ceasedvertical migration and aggregated within these maxima. Several similar observations of an active, rather than passive or coincidental, aggregation by copepods within seasonalsubsurface chlorophyll maxima have been made (Beers & Stewart, 1967; McLaren, 1969; Anderson et al., 1972; Mullin & Brooks, 1972; Hobson & Lorenzen, 1972; Holligan, 1978; Parsons et al., 1981; Paffenhofer, 1983; Longhurst et al., 1984; Sameoto, 1984; Townsend et al., 1984; Paffenhofer et al., 1984). Ciliate micro-zooplankton (Chester, 1975), planktonic Foraminifera (Fairbanks et al., 1980) and fish larvae (Sameoto, 1982) have also been shown to concentrate in phytoplankton rich layers. In the open ocean, where stratification can be a permanent feature, there is a complex relationship between deep chlorophyll maxima, the discontinuity layer and the vertical distribution of zooplankton, which has recently been a point of much dispute (Longhurst & Herman, 1981). Youngbluth (1975), Gunderson et al. (1976), Ortner et al. (1980) and Pugh & Boxshall (1984) describe zooplankton concentrating directly within the deep chlorophyll maximum, whereas Venrick et al. (1973), Longhurst (1976) and Herman (1983) found that zooplankton abundance was always highest above this layer. Longhurst (1976) states that zooplankton tend to aggregate closer to the zone of maximum phytoplankton production (determined by Cl4 uptake rates), 10 m above the layer where maximum chlorophyll standing stock occurs, and that this is a predictable feature of many marine environments. Ortner et uE.(1981) regard this difference of opinion as being largely due to ‘ technological ’ reasons and suggest that there may be significant differences between permanently and seasonally stratified regions. Certainly in present observations there are no separate production layers at station 2 as described by Longhurst (1976), and highest in situ production rates were associated with the chlorophyll a maxima itself (Heath, personal communication). Pseudoculunus elongutus was the dominant copepod speciesin the western Irish Sea during these studies (Scrape-Howe & Jones, 1985). Regardlessof underestimations due to possible avoidance, the adults of this specieswere the only category that exhibited the samemigration behaviour during all four time series, always moving towards the surface at night. The copepodites showed different nocturnal behaviour between the two water masses,moving to the surface with the adults in the stratified water and showing a bimodal distribution in the mixed water. This vertical pattern of abundance in the mixed water may be due to unsynchronized vertical migration as suggestedby Pearre (1979). The nauplii moved upwards at night in both stratified and mixed regions during June/ July 1980 but actually showed downward movement at night during time seriesC. It was this stage that constituted the majority of the total zooplankton concentrated in the

Vertical distribution

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chlorophyll a maxima during time seriesD. McLaren (1969) also found a strong correlation between Pseudocalunussp. and the highest summer chlorophyll concentrations in Ogac Lake on BafIln Island. Townsend et al. (1984) likewise found evidence of Pseudoculunus nauplii being maximal in a subsurface chlorophyll maximum in the Gulf of Maine with ‘ post-naupliar ’ copepods more abundant in the surface waters. Acurtiu cluusi adults moved upwards to the surface in both water massesat night in June/July 1980, showed a bimodal distribution in mixed water in May 1981 and moved into the chlorophyll a maxima at night in time seriesD. The copepodites and nauplii of this species(and the copepodites of T. longicornis) showed the samevertical distribution differences between water masses,moving downward at night in mixed water and upward towards the surface at night in stratified water. The adults of T. Zongicornis had a similar distribution in the different water massesto P. elongutus copepodites, whilst T. longicornis nauplii moved downwards in the mixed water at night and concentrated in the chlorophyll a maxima during time seriesD. Different migration behaviour amongst the samespeciesfound in two adjacent, but physically different, water masseshas also been reported by Lee & Williamson (1975), Turner & Dagg (1983) and Sameoto (1984). Oithonu similis tended to concentrate around the 10 m and 20 m depth in the mixed water but wasfound closely associatedwith the thermocline in stratified water. Turner & Dagg (1983) and Sameoto (1984) found that this speciesdid not appear to undergo any vertical migration. McLaren (1969) observed a similar distribution to present observations in Ogac Lake, with 0. similis associatedmore with the thermocline than with the chlorophyll a maxima. He related this to the more eclectic diet of this speciesand also to the smaller influence of temperature on growth. Microculunus pusillus occurred as a non-migrator below the thermocline and towards the bottom of the water column in this study asexpected, for this copepod has long been considered a deep water species(Wiborg, 1954). Herdman et al. (1908-1921) regard M. pusillus asan oceanic specieswhich enters the Irish Seain the deep water of the channel between the Isle of Man and Ireland, spreading into shallower areas. This specieswas confined to the stratified region during this study (Scrape-Howe & Jones, 1985). Metridiu hens was also found in the deeper water below the thermocline when it occurred in time seriesD, which contrasts with the distribution for this speciesreported by Sameoto (1984) and Holligan et al. (1984) who found it concentrated in the thermocline at all times of the day and night. The cyphonaute larvae of M. membrunuceu, which showed a bimodal distribution in mixed water, also seemedto be found mostly below the thermocline in stratified water. Of the other non-copepod zooplankton, echinoderm plutei and polychaete larvae moved downwards towards the thermocline at night (time seriesC) in the stratified water, and gastropod veligers showed abimodal noctural movement, but still concentrated around the 10 m stratum during both day and night. Lamellibranch veligers showed a bimodal distribution in mixed water but moved towards the surface at night during time seriesC in the stratified water. During the night in time seriesD lamellibranch veligers moved into the chlorophyll a maxima as did the appendicularians and cladocerans. Southward & Barrett (1983) found lamellibranch veligers and appendicularians showing signsof aggregation in the thermocline and chlorophyll maximum which were closely associatedoff Plymouth. It has been shown in the present work that, although die1vertical migrations take place, zooplankton distributions are often influenced by large tides and persistent geostrophic currents in the mixed isothermal water to the eastthe western Irish Seafrontal boundary. However, on the stratified side of the front, where die1vertical migration appears to be a

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strong feature, these migratibns can be modified by the presence of high chlorophyll levels above 5 pg 1- 1 in the water column.

a

Acknowledgements This research was supported by NERC Grant GR3/3938. We thank Dr M. R. Heath of DAFS Marine Laboratory, Aberdeen, for chlorophyll data from the cruise in May 1981 and the officers and crew of the R.V. ‘ Prince Madog ’ for their help at sea during the course of this work. References Anderson, G. C., Frost, B. W. & Peterson, W. K. 1972 On the vertical distribution of zooplankton in relation to chlorophyll concentration. In Biological Oceanography of the Northern North Pacific Ocean (Takenouti, A. Y., ed.). Idemitsu Shoten. pp. 341-345. Angel, M. V. 1968 The thermocline as an ecological boundary. Sarsia 34,299-312. Banse, K. 1964 On the vertical distribution of zooplankton in the sea. Progress in Oceanography 2,X3-125. Beers, J. R. & Stewart, G. L. 1967 Microzooplankton in the euphotic zone at five locations across the California current. Journal of the Fisheries Research Board of Canada 24,2053-2068. Bird, J. L. 1983 Relationships between particle-grazing zooplankton and vertical phytoplankton distributions on the Texas continental shelf. Estwrine, Coastal and Shelf Science 16,131-144. Boyd, C. M. 1973 Small scale spatial patterns of marine zooplankton examined by an in situ zooplankton detecting device. NerherlandsJournal of Sea Research 7, 103-l 11. Chester, N. L. 1975 Ciliate microzooplankton distribution relative to a sub-surface chlorophyll maximum off the Washington coast. Master’s Thesis, Washington University. Fairbanks, R. S., Wiebe, P. H. & Be,-A. W. H. 1980 Vertical distribution and isotopic composition of living planktonic Foraminifera in the Western North Atlantic. Science 207,61-63. Floodgate, G. D., Fogg, G. E., Jones, D. A., Lochte, K. & Turley, C. M. 1981 Microbial and zooplankton activity at a front in Liverpool Bay. Nature 290,133-136. Fogg, G. E., Egan, B., Hoy, S., Lochte, K., Scrape-Howe, S. & Turley, C. M. 1985 Biological studies in the vicinity of a shallow-sea tidal mixing front. I. Physical and chemical background. Philosophical Transactions of rhe Royal Society, London, Series B 310,407-433. Gunderson, K. R., Corbin, J. S., Hanson, C. L., Hanson, M. L., Hanson, R. B., Russel, D. J., Stollar, A. & Yamada, 0. 1976 Structure and biological dynamics of the oligotrophic ocean photic zone off the Hawaiian Islands. Pacific Science 30,45-68. Hansen, K. V. 1951 On the diurnal migration of zooplankton in relation to the discontinuity layer. Journal du conseil. Conseilpermanent international pour PExploration de la Mer, Copenhague 17,231-241. Haury, L. R. 1976 Small-scale pattern of a California current zooplankton assemblage. Marine Biology 37, 137-157. Heath, M. R. 1982 Some preliminary results from a new method for studying phytoplankton physiology in the field. Marine Biology Letters 3,173185. Herdman, W. A. and various co-workers 1908-1921 An intensive study of the marine plankton around the south end of the Isle of Man. Proceedings and Transactions of the Liverpool Biological Society 1-13. Herman, A. W. 1983 Vertical distribution patterns of copepods, chlorophyll and production in the northeastern BafXn Bay. Limnology and Oceanography 28,709-719. Hobson, L. A. & Lorenzen, C. J. 1972 Relationship of chlorophyll maxima to density structure in the Atlantic Ocean and the Gulf of Mexico. Deep Sea Research 19,297-306. Holligan, P. M. 1978 Patchiness in subsurface phytoplankton populations on the Northwest European continental shelf. In Spatial Pattern in Plankton Communities (Steele, J. H., ed.). Plenum Press, New York. pp. 181-220. Holligan, P. M., Harris, R. P., Newell, R. C., Harbour, D. S., Head, R. N., Linley, E. A. S., Lucas, M. I., Tranter, I’. R. G. & Weekley, C. M. 1984 Vertical distribution and partitioning of organic carbon in mixed, frontal and stratified waters of the English Channel. Marine Ecology-Progress Series 14, 111-127. Hunter, J. R. 1972 An investigation into the circulation of the Irish Sea. Ph.D. Thesis, University College of North Wales. Johnstone, J., Scott, A. & Chadwick, H. C. 1924 The marine plankton, with special reference to investigations made at Port Erin, Isle of Man, during 1907-1914. Liverpool University Press. Khan, M. A. & Williamson, D. I. 1970 Seasonal changes in the distribution of Chaetognatha and other plankton in the Eastern Irish Sea. Journal of Experimental Marine Biology and Ecology $285-303.

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