The zooplankton community structure in relation to ... - Oxford Journals

Journal of Plankton Research Vol.21 no.6 pp.1133–1152, 1999

The zooplankton community structure in relation to its biological and physical environment on the Faroe shelf, 1989–1997 Eilif Gaard Fisheries Laboratory of the Faroes, Nóatún, PO Box 3051, FO-110 Tórshavn, Faroe Islands Abstract. The Faroe shelf water is separated from the offshore water by a persistent tidal front, which surrounds the islands. This shelf water contains a neritic zooplankton community, which, regarding species composition, production, seasonal development and environmental conditions, is quite different from that in the surrounding ocean. While during spring and summer the zooplankton in the oceanic environment are dominated by the copepod Calanus finmarchicus, the zooplankton in the shelf water are largely dominated by neritic copepods, mainly Acartia longiremis and Temora longicornis. Calanus finmarchicus occurs in interannually highly variable abundance in the Faroe shelf ecosystem. Meroplanktonic larvae, mainly Balanus spp. and decapod larvae, are also common in the shelf water during spring and summer. During the period presented (1989–1997), the Faroe shelf ecosystem has undergone very large changes in abundance of different zooplankton species. The midsummer abundance of C.finmarchicus, which originally is advected into the shelf from the open ocean, fluctuated from ~400 copepods m–3 in 1989 to ~25 copepods m–3 in 1994, and at the same time the neritic zooplankton increased from ~120 m–3 in 1989 to 450 m–3 in 1994. Consequently, the midsummer biomass in the shelf fluctuated by a factor of 10 during the same period. It is presumed that this variability between oceanic- and neritic-dominated zooplankton, their sizes and their biomass has greatly affected the entire pelagic ecosystem.

Introduction The Faroe shelf is located on the Scotland–Iceland Ridge and is separated from other shallow areas by a depth exceeding 500 m in all directions (Figure 1). The upper layers (above ~500 m depth) are dominated by Atlantic water from the North Atlantic Current (Hansen et al., 1998). The shallow part of the shelf (above 100– 130 m bottom depth) is ~60 3 80 nautical miles and the area is ~8000–10 000 km2. Despite its small size, the Faroe shelf has its own plankton community, which is relatively well isolated from those in the surrounding oceanic environment. Both the phytoplankton (Gaard, 1994, 1996a; Gaard et al., 1998) and the zooplankton on the Faroe shelf (Gaard, 1994, 1996b) are quite different from those of the oceanic environment with regard to production, abundance and species composition. The phytoplankton in the oceanic environment outside the shelf are dominated by small flagellates, while those on the shelf are more composed of neritic species (Gaard, 1996a; Gaard et al., 1998). The zooplankton in the oceanic environment outside the Faroe shelf are mainly dominated by the copepod Calanus finmarchicus, but on the shelf they are usually dominated by a large number of neritic zooplankton species, mainly the copepods Temora longicornis and Acartia longiremis (Gaard, 1994, 1996b; Gaard and Reinert, 1996). The seasonal development and production of the phyto- and zooplankton in these two areas are also relatively independent from each other (Gaard, 1994, 1996a). Hence, the Faroe shelf has its own plankton ecosystem, which regarding both phyto- and zooplankton species composition, is independent of that in the © Oxford University Press

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Fig. 1. Topography and main features of the flow field around the Faroes. The dotted line indicates a typical position of the tidal front that separates the shelf water from the open ocean. The black dots with letters refer to sampling stations.

oceanic environment. The Faroe shelf is also the domicile for a large number of fish stocks, which spawn on or close to the shelf. Their eggs and larvae are dispersed over the shelf area and they grow within the system. The reason for this has to be found in the current system on the Faroe shelf. Because of very strong tidal currents, the shelf water is well mixed from surface to bottom, usually with no stratification of the water column during summer. This water mass is separated from offshore waters by a tidal front, which is usually situated between the 100 and 150 m isobath, and the well-mixed shelf water is maintained as a separate water mass by a persistent anticyclonic circulation (Figure 1) (Hansen, 1992). This current system prevents, to some extent, the neritic holoplankton and meroplankton from drifting off the shelf, and the oceanic plankton from advecting into the shelf current system. Thus, it forms the basis for production and trophic interactions within the system. However, some interactions between the shelf and the surrounding offshore area do occur and may affect the conditions on the shelf. This is especially the case for the lowest trophic levels (Gaard, 1994; Gaard et al., 1998). Data on plankton composition, productivity and abundance on the Faroe shelf are only available for the last decade. These data indicate quite large variations, 1134

Zooplankton community structure on Faroe shelf, 1989–1997

with very low primary productivity during the beginning of the 1990s; however, the time series are too short to trace variability on a longer time scale. Landing statistics of cod and haddock, which go back to the beginning of this century, generally do not show large fluctuations. This indicates that, in the long view, productivity on the Faroe shelf may be relatively stable between years. However, during the early 1990s, the landings decreased drastically and were the lowest recorded in this century. This decrease covered most of the fish stocks that inhabit the shelf. The Faroe shelf area is a very suitable area for plankton ecological studies. It is well defined, relatively uniform and its small size makes it possible to cover the whole area within a reasonable time. The aim of this paper is to give an overview of the dominant zooplankton abundance and species composition, to describe the seasonal development and interannual variability in the Faroe shelf ecosystem in relation to the environment, and to describe variability in advection of zooplankton from the surrounding area into the Faroe shelf and possible consequences on the ecosystem. Method The investigations were carried out on the Faroe Plateau with the Faroese research vessel ‘Magnus Heinason’ on several cruises during the period 1989–1997. Regular samplings are made on the shelf stations S1 and S2 and on the offshore stations O1 and O2 just outside the tidal front 5–8 times per year (Figure 1). Two closely located shelf stations (marked T on Figure 1) were visited 23 times between January 1997 and January 1998, and in addition zooplankton samples were collected from 40–50 stations which are distributed all over the shelf and just outside the front in late June each year. Samples were taken to describe the hydrography, in situ fluorescence, chlorophyll a, and zooplankton abundance and composition. The salinity was measured with a CTD. Prior to May 1995, an EG&G CTD was used and after May 1995 a Seabird Electronics SBE 911+ CTD. Both instruments were equipped with a rosette sampler and Niskin water bottles, and the salinity was calibrated against water bottles analysed on an Autosal Model 8400A salinometer. In situ fluorescence was measured with Sea Tech fluorometers interfaced to the CTDs and fluorescence was calibrated against selected samples that were analysed for chlorophyll a. The chlorophyll a measurements were carried out using the methods described by Baltic Marine Biologists (1979) with a difference in that the homogenization was carried out with a Soniprep 140 ultrasound homogenizer. When computing the results, the equation of Jeffrey and Humprey (1975) was applied. Mesozooplankton were sampled by vertical hauls from 50 m depth to the surface. A Hensen net was used in 1989–1991 and a WP2 net in 1992–1997. Both nets had a mesh size of 200 µm and the towing speed was 0.3–0.5 m s–1. The samples were preserved in 4% formaldehyde. In the laboratory, subsamples were taken out with a plankton splitter and were then identified and counted. The samples were then dried at 65°C until they reached constant weight. 1135

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Results Hydrography Owing to precipitation and the shallow bottom depth, the salinity in the Faroe shelf water is always somewhat lower than that in the surrounding ocean (Figure 2). Based on the isohalines, the front between the oceanic and the shelf water can be identified. It usually follows the bottom contour, and is usually situated between the 100 and 150 isobath (see also Figure 1). However, there is always some advection of oceanic water into the shelf. This advection is due to the heterogeneous bottom topography of the shelf and the different current forces generated in the slope areas. Species composition and distribution The hydrographic conditions greatly affect the zooplankton species composition and distribution on the Faroe shelf. During midsummer, the copepod C.finmarchicus, which was in the open ocean that surrounds the shelf, occurred in significantly lower concentrations inside the tidal front. On the other hand, the shelf water was dominated by neritic copepod species, mainly Acartia spp. and T.longicornis (Figure 2a). In general, the highest abundance of neritic species was found in areas with the lowest salinity. Also, meroplanktonic larvae are common in the shelf water. Balanus larvae especially may be abundant during spring and early summer inside the tidal front, but are very rare outside the front (Figure 2b). The examples presented above clearly illustrate the difference in the species composition between the Faroe shelf area and the surrounding oceanic environment. The shelf is basically a neritic zooplankton ecosystem, although there is some advection of oceanic species into the system. It will, however, be shown later in this paper that this advection—and hence the neritic versus oceanic composition—may vary significantly interannually. A more detailed illustration of the species composition in midsummer 1994–1997 (Figure 3) shows that at this time of the year copepods dominate among the mesozooplankton in the Faroe shelf ecosystem and generally they represent (by number) ~90% of the total mesozooplankton. Generally, the neritic copepods (i.e. Acartia, Temora and a substantial part of the group ‘other copepods’) dominated in the areas with bottom depths shallower than 100–150 m, while C.finmarchicus dominated at the deeper stations. Oithona similis is frequently found on the oceanic stations, but since this species is much smaller than C.finmarchicus, it contributed little to the total zooplankton biomass. The group ‘other copepods’ contained mainly Pseudocalanus elongatus, Microcalanus sp. and Centropages hamatus, while the chaetognath Sagitta elegans, the larvacean Oikopleura, decapod larvae, euphausiids, bivalve veliger larvae, the shelled gastropod Limacina, cladocerans and Balanus nauplius and veliger larvae were commonly found at variable abundance. The abundance of C.finmarchicus on the shelf varied a lot between these 4 years from 1994 to 1997 (Table I). During summer of 1994 and 1995, the species occurred only in very small concentrations and the shelf ecosystem was totally 1136


Fig. 2. Salinity and abundance of C.finmarchicus, Acartia spp. and T.longicornis around the Faroe shelf, 23 June–2 July 1994 (a), and salinity and abundance of Balanus nauplii and cyprids, 2–7 April 1997 (b). The salinity is at 50 m depth and the zooplankton are presented in mean numbers m–3 in the upper 50 m of the water column. Note the difference in scale in the panels.

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Fig. 3. Abundance (mean number m–3 in the upper 50 m of the water column) of the total zooplankton (left) and copepods (right) plotted against bottom depth on the Faroe shelf and the surrounding area in late June 1994–1997. The results are based on 31–46 sampling stations each year, distributed all over the shelf and through the tidal front.

dominated by the copepods Acartia and Temora. However, in 1996 and 1997, C.finmarchicus occurred in substantially higher numbers, although the abundance of the neritic copepods was about the same as in the two previous years. The distribution of C.finmarchicus may also be horizontally variable, possibly 1138


Table I. Abundance (mean number m–3 in the upper 50 m of the water column) of zooplankton at different bottom depth intervals on the Faroe shelf and the surrounding area in late June 1994–1997. The stations are distributed all over the shelf and through the tidal front Year

Bottom No. of depth (m) stations

1994

60–79 80–99 100–119 120–139 140–159 160–179 ≥180 60–79 80–99 100–119 120–139 140–159 160–179 ≥180 60–79 80–99 100–119 120–139 140–159 160–179 ≥180 60–79 80–99 100–119 120–139 140–159 160–179 ≥180

1995

1996

1997

Calanus Oithona Temora

Acartia

Other Balanus Other copepods

5 7 5 2 5 4 3 7 11 5

29 42 116 364 224 980 998 50 86 174

12 12 52 45 71 134 115 10 12 49

115 91 104 98 120 28 8 38 45 59

315 444 346 209 145 20 15 151 163 172

111 100 74 62 44 15 26 43 34 65

43 70 37 40 45 1 0 43 40 25

40 57 56 50 27 24 15 50 58 67

5 6 4 9 9 8 3 7

251 452 1303 164 682 487 652 1106

58 66 156 28 127 65 153 56

61 101 104 116 234 275 154 173

90 34 67 233 513 252 411 297

47 109 87 10 46 15 26 29

15 4 10 59 92 72 60 20

52 93 115 37 73 76 63 132

7 6 14 5 7 6 4 3

1302 563 125 699 366 797 1121 899

349 55 35 84 80 395 492 299

399 655 174 309 246 302 201 204

148 278 168 121 105 74 127 61

29 58 40 63 51 159 174 135

18 103 57 18 27 25 10 7

100 267 61 200 164 236 275 186

depending on the advective forces in the frontal regions. Thus, the ability of C.finmarchicus to invade the shelf region is apparently temporally and spatially variable. Seasonal development Frequent samplings on the central Faroe shelf during 1997 demonstrate large seasonal variations in the zooplankton abundance (Figure 4). This seasonal development of the zooplankton is a consequence of the seasonal variations of primary production and phytoplankton abundance. Also, the species composition of zooplankton varies seasonally. During the winter of 1997, only a few copepods were present, but in early spring Balanus larvae increased substantially in numbers and dominated (by number) among the zooplankton during most of the spring period (Figure 5). In late spring and during the summer, copepods gradually became more abundant, but decreased again in autumn. A group (called ‘other’) occurring in lower numbers is a mixture of many different zooplankton, of which the most common were decapod larvae, Oikopleura, Sagitta, Limacina and bivalve veliger larvae. 1139

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Fig. 4. Mean zooplankton abundance and chlorophyll a on station T, January 1997–January 1998.

With the exception of the spring period, the copepods were the largest group among the zooplankton (Figure 5). During winter, few copepods were present in the shelf water, and the dominant species was Pseudocalanus. It was never abundant, but was found in more or less constant concentrations during the whole year. Thus, during autumn and early winter of 1997–1998, when the other copepod species decreased in numbers, its relative importance increased basically. Calanus finmarchicus was almost totally absent from the shelf during winter, but increased substantially (in numbers) during spring 1997. Already in July–August, it decreased again to very low numbers and almost disappeared from the shelf in autumn. With the exception of the spring period, the species was not dominant among the copepods by numbers, but represented only a small fraction of the total copepods on the Faroe shelf. However, since this species is much larger than the other species which were present in the ecosystem, it affected the total biomass and the prey availability when present on the shelf. The most abundant copepod species during the productive period were Acartia spp. (mainly A.longiremis) and T.longicornis. Especially during summer, they increased substantially in numbers and almost totally outcompeted the other zooplankton. The second group (Figure 5, left panel), Balanus larvae, started to increase in number in spring before the copepods and at that time they were even more abundant than all the copepods together. By midsummer, the abundance of Balanus larvae was substantially lower and they were almost absent from September onwards. Less frequent, but comparable sampling series have been made on both sides of the tidal front in previous years (Figure 6). These results confirm the general trend in the seasonal development within the shelf region, previously shown, although some variations did indeed occur between the years. In the oceanic environment, outside the front, both the species composition and their abundance were quite different than inside the front. Here, the diversity was lower than in the shelf water and the two copepod species C.finmarchicus and O.similis 1140

Fig. 5. Absolute (upper) and relative (lower) abundance of dominant mesozooplankton (left) and copepods (right) on station T (see Figure 1) on the Faroe shelf during 1997. Copepod nauplii are not included.


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Fig. 6. Seasonal development of the dominant zooplankton (copepods and Balanus larvae) in the upper 50 m of the water column on two shelf stations (S1 and S2) and two offshore stations (O1 and O2) during 1994–1996. The stations are shown in Figure 1.

prevailed. During summer, C.finmarchicus dominates both in abundance and in biomass, while O.similis is relatively more important during winter. Interannual variability Since 1989, which is when systematic plankton studies started, quite large changes have been observed in the Faroe shelf ecosystem. During the first years of the 1142


period, neritic zooplankton species were of minor importance. The system was dominated by C.finmarchicus, and the differences between the shelf area and the surrounding offshore area were generally small. However, during the early 1990s, the species composition of the Faroe shelf changed and the area gradually became more and more neritic (Figure 7). Calanus finmarchicus, which was still the dominant copepod outside the shelf front, became less and less abundant inside the front. Instead, other copepod species (mainly A.longiremis and T.longicornis) increased in number. Especially during the period 1993–1995, C.finmarchicus were scarce in the shelf water, but in 1996 and 1997 they again occurred in somewhat higher abundances. Hence, C.finmarchicus, which is originally advected into the shelf from outside, decreased during the period 1989–1994 from ~400 to ~40 individuals m–3 (or, relatively, from ~73% in May 1989 to ~12% in May 1994 and from ~65% in June 1989 to ~6% in June 1994 of the total copepod abundance). These changes are so profound that they obviously must mirror some quite large changes in the physical or biological environment with potential substantial impact on the entire pelagic ecosystem on the Faroe shelf. Another and apparently even more pronounced change during this period is in the abundance of Balanus larvae. This species, as shown Figures 2 and 6, is a representative for true neritic meroplankton, since it is only abundant inside the shelf. In the beginning of the presented period, the species was very rare on the shelf, but during the early 1990s it gradually increased in number and became a dominant zooplankton species (in numbers) during the spring period (Figure 8). This increase in the abundance of Balanus larvae coincided with the (absolute and relative) increase in neritic copepod species (mainly Acartia and Temora). Adding the changes in Figure 8 to the changes in the copepod composition (Figure 7), the changes in the total zooplankton composition during the 9 year period become even clearer. Owing to the size of C.finmarchicus, the abundance fluctuations of this species are the main reason for the seasonal and interannual variability in the total zooplankton biomass on the Faroe shelf (Figure 9). This shows a peak in biomass in midsummer in years when C.finmarchicus was abundant and no peak when it was scarce. Outside the shelf front, the biomass reached high concentrations in midsummer prior to the descent of copepodites in stages IV and V. The variability in C.finmarchicus abundance is also entirely responsible for the large interannual fluctuation in zooplankton biomass on the shelf in summer (Figure 10). It decreased from ~60 mg dry weight m–3 in 1991 to ~8 mg dry weight m–3 in 1994 and 1995, with a slight recovery in 1996 and 1997. Outside the shelf, the biomass did not change considerably, but remained at a relatively constant and high level (Figure 10). Hence, the figure in reality illustrates the differences in C.finmarchicus abundance on both sides of the tidal front during midsummer. Discussion Species composition and distribution The species composition on the shelf compared to the surrounding oceanic environment clearly demonstrates that the Faroe shelf contains its own plankton 1143

Fig. 7. Absolute (upper) and relative (lower) abundance of the dominant copepods on stations S1 and S2 on the Faroe shelf in May (left) and June (right), 1989–1997. The stations are shown in Figure 1.

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Fig. 8. Abundance of Balanus (a), and relative abundance of Balanus compared to copepods (b), on stations S1 and S2 in May 1989–1997. In 1990 and 1992, only relative abundance is available, and in 1991 no data are available. The station sites are shown in Figure 1.

ecosystem. Despite highly variable influences by oceanic species, in most years it is basically a neritic ecosystem. The mesozooplankton on the shelf can be divided into three major groups. First are the neritic copepod species, of which T.longicornis and Acartia spp. are the most important. The second group is the meroplankton. A large number of different species are found in very variable concentrations, of which Balanus larvae are by far the most abundant. Other commonly found species are decapod larvae, bivalve veliger larvae and polychaeta larvae, but these larvae never reach nearly as high concentrations as the Balanus larvae. In addition to this are ichthyoplankton, of which the most abundant species are cod, haddock, saith, sandeel and Norway pout. After spawning, their eggs and larvae are dispersed by currents throughout the 1145

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Fig. 9. Average seasonal variability in zooplankton biomass in the upper 50 m on the shelf stations S1 and S2 during 1992, 1994 and 1996, and the offshore stations O1 and O2 during 1994 and 1996. The station sites are shown in Figure 1.

Fig. 10. Mean zooplankton dry weight in the upper 50 m of the water column on the Faroe shelf (150 m bottom depth), respectively, in June 1991–1997. The results are based on ~30–40 sampling stations each year, distributed all over the shelf and the surrounding area.

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shelf area. This group is mainly abundant during spring and early summer when they prey on the zooplankton on the shelf. The third group contains species which were originally advected from the oceanic environment into the shelf current system, but they may grow and reproduce very well on the shelf. Mainly two copepod species belong to this category, C.finmarchicus and O.similis, of which C.finmarchicus is by far the most important. During winter, the species is almost totally absent from the shelf current system as well as the upper layer in the ocean outside the shelf. They spend the winter in diapause in deep cold water, mostly below 400–500 m depth, and an important overwintering area is the deep Norwegian Sea. In March and early April, they migrate towards the surface, moult and spawn (Gaard, 1994). The new generation grows in the upper layer until midsummer. At that time, most of the animals migrate downwards again as they reach copepodite stages IV or V, but a few animals may remain in the upper layer and produce a new generation (Gaard, 1996b). The Faroe shelf is located in an area with high abundances of C.finmarchicus, both in the upper layers during summer (Gaard and Hansen, 1991; Gaard, 1996b) and in deeper water during winter (Gaard, 1994). To the north of the shelf, cold water is found from ~300 to 500 m depth on the shelf slope and downwards (Hansen et al., 1998). To the east, south and west of the shelf is an overflow of deep Norwegian Sea water through the Faroes–Shetland Channel and further through the Faroe Bank Channel (Borenäs and Lundberg, 1988; Johnson and Sanford, 1992; Hansen and Kristiansen, 1998; Hansen et al., 1998). This overflow water carries high quantities of overwintering C.finmarchicus. It is, therefore, not surprising that some of the copepods are advected into the shelf and mix with the neritic species. Seasonal development Although the abundance of the different species varies a lot between years, the seasonal development of the zooplankton follows a regular pattern. The seasonal development of the copepods depends greatly on the timing and intensity of primary production in the area. Food availability is a prerequisite for high egg production and especially the large phytoplankton, although other food items may be important (e.g. Berggreen et al., 1988; Kiørboe et al., 1990; Kiørboe and Nielsen, 1994; Nielsen and Hansen, 1995; Irigoien et al., 1998). The diatom production during spring is, therefore, of importance for the timing and intensity of the copepod spawning. The primary production on both sides of the tidal front develop quite independently of each other. Outside the shelf front, the spring bloom cannot start before the summer thermocline is established. The time when this happens may vary between years, depending on the weather conditions, but is usually in mid or late May (Zeitzschel, 1986; E.Gaard, unpublished data). This is, however, not the case in the shelf water. The strong tidal currents in this area prevent stratification of the water column during summer and the water usually is mixed from surface to bottom. Therefore, the spring bloom on the mixed shelf water can only develop when the critical depth becomes deeper than 1147

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the bottom depth (Sverdrup, 1953), which on average is ~80 m. The spring bloom may therefore start in the shelf water before development of a summer thermocline makes it possible in the surrounding area outside the front. An earlier occurrence of the phytoplankton spring bloom on the Faroe shelf than outside the front has often been observed (Gaard, 1994, 1996a). It may not develop all over the shelf at the same time, but it often seems to start in the central and northern part of the shelf (Gaard, 1996a; E.Gaard, unpublished data). Thus, the phytoplankton biomass may increase significantly earlier on the shelf than in the surrounding oceanic environment. This may affect the seasonal development of the zooplankton in these two water masses significantly (Gaard, 1994). It should, however, be noted that some zooplankton reproduction already occurs during the pre-bloom phase. This may be more pronounced for some species than others (Figures 4 and 5). The winter community on the Faroe shelf is composed mainly of small copepods and at least in some years Pseudocalanus is a quantitatively important species. It sustains slow growth and a low level of production. At this time of the year, primary production is very low and the few algae that are present are mainly small flagellates (E.Gaard, unpublished data). These may be a main food source for the copepods during winter. When the phytoplankton (diatom) biomass gradually increases in spring, the abundance of Pseudocalanus may also increase, but so far the species has not been found in high concentrations on the shelf during the studied period, possibly because it loses in its competition with the other copepods. During spring, Balanus larvae also appear on the shelf. Also, other copepods, mainly Acartia and Calanus, increase in numbers during spring and start their reproduction during the pre-bloom phase. The other dominant species, Temora, reproduces somewhat later than the former species. It is rarely found in winter and early spring, and increases in number after the spring bloom has developed. It should, however, be emphasized that the plankton are collected with a 200 µm net and the smallest copepods might have been lost through the mesh. These copepods are mainly Acartia, Oithona, the youngest stages of Temora and Pseudocalanus, and perhaps also some of the youngest stages of Calanus. It should also be kept in mind that some of the abundance data are derived from only two stations, therefore the abundance is only approximate. The generation times of copepods depend greatly on the temperature. The mean temperature of the Faroe shelf ranges from around 6°C in March to 10°C in August (Smed, 1952). Based on studies by Corkett et al. (1986) and Miller and Tande (1993), it can be calculated that the life cycle of C.finmarchicus that are spawned in May is completed ~45–50 days later, which is in midsummer. This coincides well with the biomass peak in years when C.finmarchicus was abundant. According to Corkett and McLaren (1970), the time from spawning to copepodite stage I can be estimated as ~25 days for T.longicornis and A.clausi. Taking this into account, Acartia may produce up to five generations between April and October, and Temora may produce up to four generations between May and October. However, their abundance is highly variable during this period and generally peaks in late summer, during the time window of decreasing C.finmarchicus abundance and the decrease in primary production. 1148


Interannual variability The very high interannual variability in zooplankton species composition and abundance, which is observed on the Faroe shelf during the 9 year period, can be divided into two categories. One category is variable abundance of Calanus and the other is variable abundance of neritic species, of which Temora, Acartia and Balanus larvae are the most important. In the first years of the presented period, Calanus occurred in high abundance and at the same time the neritic zooplankton were only of minor importance, but as Calanus decreased in numbers, the neritic zooplankton increased. The total mesozooplankton numbers in May and June were about the same, but the community had a much more neritic species composition and the diversity increased greatly. Since the neritic zooplankton species are much smaller than Calanus, the total biomass also decreased significantly. Taking into account a potential undersampling of the smallest copepods with the 200 µm nets, the true increase in abundance of neritic copepods was probably even higher than indicated by Figure 7. It is not known whether these changes (decreased Calanus and increased neritic abundance) were consequences of the same processes in the environment, but it seems possible. Gaard et al. (1998) showed that during this same period primary production increased significantly and also started earlier in spring, and they suggested that this increase in primary production and earlier appearance in spring could be due to decreased grazing on the phytoplankton by Calanus. At present, we can only speculate to what extent an increased primary productivity (particularly in early spring) may have affected the observed increase in the neritic plankton species (mainly Acartia, Temora and Balanus) or whether other (e.g. advective) factors may have been involved. Neither is it known at present what may have caused the high interannual variability of the abundance of Calanus on the Faroe shelf during the presented period. One possibility is that advection of individuals from outside the shelf has decreased due to variable physical forces (currents, wind). Another possibility is variable predation pressure on Calanus. Whatever the reasons might be, such dramatic variability in primary production and zooplankton species composition and abundance has dramatic influences on the entire pelagic ecosystem. Since the series of zooplankton abundance and composition on the Faroe shelf only covers 9 years, it is too short to indicate whether such profound variability, as was observed, is common and occurs frequently or what a typical Faroe shelf zooplankton community looks like. However, data on fisheries catch and the individual fish growth rates indicate that the entire shelf ecosystem may have been in a quite unusual condition during the first years of the presented period (1989–1992). Landing statistics from the Faroe shelf are available back to 1903, and have shown that cod and haddock fisheries have usually been stable. The annual catches have usually fluctuated between 20 000 and 40 000 tonnes of cod, and between 15 000 and 25 000 tonnes of haddock (Jákupsstovu and Reinert, 1994). However, between the late 1980s and the early 1990s, the catches of both species decreased to by far the lowest in this century, and in 1993 were only ~6000 and 4000 tonnes, respectively. Also, the individual fish growth decreased to the 1149

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lowest values ever recorded. Although high fishing mortality has undoubtedly influenced the decrease in fish catches, a main reason was recruitment failure, which was apparently induced by bad environmental conditions for the recruits. Sandeel (Ammodytes sp.) also apparently had recruitment failure during these years. This species is not caught commercially, but is important as a food source for fish and seabirds on the shelf. The fish recruitment failure affected most of the fish species which spawned on the Faroe shelf during the years between the late 1980s and the beginning of the 1990s. However, since 1993, their recruitment, as well as their individual growth, has improved significantly and in 1996 and 1997 the cod and haddock landings had increased to ~34 000 and 18 000 tonnes, respectively, which is above average. Also, the individual fish growth improved a lot during this period, and the mean weight at age for 3 to 7 year-old cod and haddock increased by a factor of 1.5 from 1991 to 1996 (ICES, 1998). Hence, the Faroe shelf ecosystem has been in a quite unusual period with an extensive and general fish recruitment failure during the first years of the presented zooplankton data (1989–1992). It is, therefore, most likely that the zooplankton community, as observed during these years, has also been unusual, while the latter years of the presented period may look more like typical years. Many environmental factors may affect fish recruitment success. Drifting of fish eggs and larvae off the shelf during early spring may be important (Hansen et al., 1994), and food limitations are also known to be a main factor (e.g. Ellertsen et al., 1980; Cushing, 1990; Leggett and DeBlois, 1994). During the first feeding period, fish larvae depend on small zooplankton and, as they grow, their preferred food size also increases (e.g. Thorisson, 1989; McLaren and Avendaño, 1995; Gaard and Reinert, 1996). Cod, haddock and sandeel on the Faroe shelf spawn mainly between February and April, the first feeding larvae are abundant in April and at that time they depend on high concentrations of small-sized zooplankton. The apparently unusual zooplankton composition during the first years of the presented period coincides with the general and extensive fish recruitment failure, and one may hypothesize to what extent the fish recruitment failure may have been affected by (apparently unusual) low abundance of small-sized prey during their larval stage. Bearing in mind that the 200-µm-mesh nets have most likely undersampled the small-sized zooplankton, the true interannual variability in the small-sized zooplankton in spring during the first years of the presented period may have been even higher than observed. Hence, the food availability of small-sized zooplankton during spring may have improved even more during the first years of the presented period than the samples show. This complex topic will, however, require detailed studies of fish larvae and food distribution and abundance on the shelf. Acknowledgements I want to thank Karina Nattestad, Regin Kristiansen and Meinhard Poulsen for technical assistance, Dr Bogi Hansen for processing hydrographic data and for hydrographic advice, and Dr Kurt Tande and the referees for their constructive comments on the manuscript. The study was supported in part by the 1150


Trans-Atlantic Studies of Calanus finmarchicus (TASC) project, MAST No. MAS3-CT95-0039.

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