... Hydrobiologia 440: 331-338, 2000. 331 M.B. Jones, J.M.N Azevedo, A.!. Neta, A.C. Costa & A.M. Frias Martins (eds), Island, Ocean and Deep-Sea Biology. '' © 2000 Kluwer Academic Publishers. Annual growth of the ciliatum in the Norwegian Arctic (Svalbard area)

Minna Elisabeth Tallqvist1 & Jan Henry Sundet2 1Department ofBiology & Husă Biologica! Station, Ăbo Akademi University, FIN-20500 Ăbo, Finland 2Norwegian Institute of Fisheries and Aquaculture, N-9002 TromsrjJ, Norway

Key words: , growth, Arctic, , Svalbard

Abstract The Svalbard Islands are influenced by warm Atlantic water in the south and west, and cold Arctic water in the east. lce cover, and hence the location of the highly productive marginal ice zone, varies both intra and interannually. Part of the primary production accumulates on the bottom and is utilized by the benthos. In this study, the annual growth of the cockle Clinocardium ciliatum (Fabricius, 1780) from three sites in Svalbard waters is reported. Moffen, the site in the north (80° Ol' N, 13° 48' E) is located in the northernmost areas influenced by Atlantic water. The Storfjorden site (77° 10' N, 20° 09' E) is situated in cold Arctic water masses, and the Bear Island site (74° 50' N, 18° 54' E) is in the Polar front area where Atlantic and Arctic water masses meet. Annual growth of cockles was ana1ysed retrospectively by measuring externa! growth increments, which gave annual growth records from the 1970s to 1996. Shell height for age for different year classes was highest at the Storfjorden site, and lowest at Bear Island. Periods of high growth occurred at Storfjorden and Bear Island during the 1980s while the beginning of 1990s was characterized by low growth. At Moffen, growth was more variable between single years. Severa! factors are influencing the growth of C. ciliatum in the Svalbard area and growth cannot be coupled to only one environmental factor like ice cover.

Introduction In the Arctic, suspension feeders are subjected to large seasonal and annual variations in food supply, Generally, bivalve growth is dependent upon age and and the marginal ice zone is considered to have a size, but is affected also by environmental factors such great impact on the primary production in these areas as temperature, current velocity and food availability (Wassman et al., 1991). In our study, we sampled (Richardson et al., 1980; Appeldoorn, 1981; Mac Don• cockles from three sites with different characteristics ald & Bourne, 1987; Jones et al., 1989; Navarro et al., of ice coverage and water masses. Moffen, in the north 1992; Arsenault et al. 1997; Witbaard et al., 1997), and of Spitsbergen Island, is located in the marginal ice biotic factors such as competition (Peterson & Andre, zone where ice conditions vary even over short peri• 1980). In Arctic areas, food supply is probably one of ods of time (days/weeks); Storfjorden, on the east of the most important factors affecting growth of benthic Spitsbergen Island, has the longest period of ice cover and species are forced to adjust to a short and is situated in Arctic water masses; Bear Island growth season (Picken, 1979; Sagar, 1980; Clarke, is situated in the Polar front area where Atlantic and 1988). Polar bivalves grow slower than similar spe• Arctic water masses meet (Fig. 1). We hypothesized cies in more temperate areas (Ralph & Maxwell, 1977; that long periods of annual ice may decrease growth Clark, 1983) and the growth in winter can be close to in benthic organisms due to depressed primary produc• zero (Richardson, 1979). In a study of the bivalve Ma• tion compared with areas with less ice. Consequently, coma balthica, growth was more dependent on food sites influenced by Atlantic water will show higher supply than on temperature (Beukema et al., 1985) and growth rates than sites in Arctic water masses. It has the same has been shown for other species living in been suggested that there is a strong coupling between environments with marked seasonality (Clarke, 1988). the pelagic and the benthic environment in the Arctic 332

below 5 o C. Together with another common cockle species in this area, Serripes groenlandicus, it is im• portant as food for walrus and seals (Fay & Burnes, 1988). 80.J Description of sampling sites

Generally, Arctic waters are characterized by high sea• sonality (due to the light regime) with a short growth season and annual variation in ice coverage. The polar night (i.e. when the sun is constantly under the ho• rizon) is from 22nd October to 20th February at the northernmost site and from 6th November to 5th Feb• ruary at the southernmost site. The midnight sun (i.e. when the sun is constantly above the horizon) is from 15th April to 27th August at the northernmost site and from 30th April to 12th August at the southernmost site. In the Svalbard area, the maximum ice extent is in March and the minimum in September. Moffen (80° OI' N, 13° 48' E) was sampled on 7th September 1997 at 112-130 m water depth (Fig. l ). This site is dominated by gravei and scallop beds, and C. ciliatum were found only in hauls where clay was the domin• ant sediment. Of the 64 individuals sampled, 63 were intact and used in the analyses. Probability for ice in March is about 70-80% (Vinje & Kvambekk, 1991). The Storfjorden site (77° 1O' N, 20° 09' E) was 700 sampled on 14th September 1997 at a sampling depth of 80-118 m. This site was dominated by clay and 162 cockles were sampled, of which 154 were intact Figure 1. A map of the study area. Ali sites (indicated with black and used in the analyses. The probability of ice cover dots) are situated in Svalbard lsland waters with Moffen in the in March at Storfjorden is 100% (Vinje & Kvambekk, north, Storfjorden in the east of the island Spitsbergen, and the 1991). southernmost site north of Bear Island. Bear Island site (74° 50' N, 18° 54' E) was sampled on 1lth November 1997 at a depth range of 100- due to a slow response of zooplankton to increased 130 m. The substratum was clay with a fraction of primary production in the spring (Grebmeier et al., empty cockles; 146 cockles were sampled, of which 1988; Peinert et al., 1989). 143 were included in the analyses. The probability of In this paper, the annual growth of the com• ice cover in March at Bear Island is about 30% (Vinje mon Arctic cockle, Clinocardium ciliatum (Fabricius, & Kvambekk, 1991). 1780), is compared between three Arctic sites in the Svalbard area. At each sampling site, bivalve growth patterns are linked to ice coverage. Clinocardium cili• Materials and methods atum is one of the common larger bivalve species in the Svalbard area (Rozycki, 1987). It prefers clay Bivalves were sampled with a triangulardredge (open• sediment (Ockelmann, 1958) and is found in a wide ing 1 m*1 m*1 m, inner net size 1 cm) from the depth range from the intertidal down to about 500 m research vessel 'Jan Mayen' (Troms0 University) dur• (Bernard, 1979). Maximum size may be up to 70 mm ing two cruises in 1997. Sediment was sieved through (Bernard, 1979), but cockles of up to 40 mm are most a 5 mm mesh and C. ciliatum were picked out. Cockles common in our study area. It is a circumpolar species were washed and shell height was measured with ver• of Pacific origin and is found at water temperatures nier callipers to the nearest 0.01 mm. The body was 333

mean. In this way, different year classes of bivalves a) 30 can be analysed together and growth at different calen• dar years can be compared. Standardization of growth 20 rings is a method widely used in dendrochronology 10 (Fritts, 1976; Schweingruber, 1989) and has also been applied in growth analysis of marine bivalves (Wit• o ~ baard, 1996a,b ). Standardization is do ne by comparing b) ~ 30 expected values with actual values. Expected values v" of growth (increment size) can be calculated by dif• '- 20 o ferent methods, most of them based on curve fitting. .,~ .o 10 The time series of indices describe higher or lower E z" growth than expected for different years. The stand• o ardization was done only from the 6th increment (i.e. c) 30 the largest increment). Therefore, the standardization was done only for ages with decreasing increment size 20 and the increase in growth in young ages (0- 6) was 10 excluded from the analyses. Studies in dendrochrono• logy show that the period of years with increasing o growth at younger ages is usually not very sensit• 7 9 Il 13 15 17 19 21 23 25 27 29 31 33 ive to environmental factors and these years are often Age (years) excluded to facilitate the standardization calculations (Fritts, 1976). We present indices only for the time Figure 2. Age distribution of Clinocardium ciliatum from (a) Mof• fen, (b) Storfjorden and (c) Bear lsland. period 1980- 1994, excluding the 1970s because of the low number of cockles representing this time period. The ice data are presented as the number of days separated from the shell and the shells were stored with more than 70% ice coverage at the sites between for further analyses. Age determination was done by 15th March to 1st October (1980-1994). counting externa! growth rings on the shells. This species has very marked externa! growth rings, and these were assumed to be formed annually due to the Results pronounced seasonality in the area and on the basis of results from other polar mollusc species (Ralph The age distribution of the bivalves sampled varied & Maxwell, 1977; Richardson, 1979; Picken, 1980; between sites (Fig. 2). The sample from Moffen was Bray & Hain, 1992; Brey & Mackensen, 1997). Incre• dominated by the 1983 year class (the 14-year-olds), mental widths were measured with callipers and the with other year classes sparsely represented. Clinocar• height from umbo to each increment was also meas• dium ciliatum from Storfjorden were distributed over ured. Incremental width data were used in Figures 3 severa) year classes, and, at Bear Island, the bivalves and 4, and height to each increment data was used in were evenly distributed over the year classes 1972- results presented in Figure 5. 1982 ( 15-25-year-old individuals). The youngest year To remove the ontogenetic growth trend, all in• classes were absent at all sites, which could be due to cremental widths were standardized according to Wit• the sampling method. baard (1996a), with the modification that we used 5 Incremental size at different ages is shown in Fig• year averages instead of 7 due to the shorter life span ure 3 for all sites. In this figure, all bivalves are of C. ciliatum compared with Arctica islandica in Wit• included independent of year class. Annual incre• baard's (1996a) study. The standardized increments mental width increased with age up to about 6 years, were, therefore, calculated as after which there is a decrease with age. To com• pare the growth of the bivalves at different sites, we It = (w, - W"t - 2,t+2) / s.d.t- 2,t+2· plotted height against age for different year classes where 1 = the standardized increment, w= increment (Fig. 4). This was done for year classes where we width, w- = average of increment width over the adja• had appropriate cockles from at least two sites. For cent 5-year period and s.d. = standard deviation of the the year classes 1979 and 1980, all three sites were 334

6 a) 5

,--.._ 4 -d 3 1 u.i 2 •• îî1Btiîî -+1 1 ;::: o î !îJ ~j î ~ ~ ~JJJ f V"' 5 6 8 5 8 4 b)tH1H1t ~ 3 -~ 2 lîÎî -;:: 1 ÎîîLH:2:2:2:su:o o V o 8 V.... (.) 6 c) ;::: 5 - 4 3 2 ,tnHHttHh 1 o Îitf!E.: .....

o IO 20 30 Age (years)

Figure 3. Width of the annual growth incremeni (mean ± s. d.) at different ages of Clinocardium ciliatum from (a) Moffen, (b) Storfjorden and, (c) Bear Island. compared. The results show that the Storfjorden site the beginning of the 1990s, no trend in ice coverage has the highest height at age, Bear Island the lowest can be distinguished and the variation is large between for severa! year classes, and Moffen has a height at years. The Storfjorden ice data indicate that the 1980s age between these two. To analyse growth in different was a period with a high number of days with ice and calendar years, independent of the age of the bivalves, the 1990s had almost half the number of days with ice we standardized the material to get time series of mean coverage compared to the 1980s. Bear Island showed standardized indices (Fig. 5). High or low mean index peaks with a high number of days with ice (compared values indicates that most of the cockles showed the to other years at the same site) at 1982 and 1989. The same growth trend (i.e. higher or lower growth than beginning of 1980s is characterized by a low number expected). For Storfjorden and Bear Island, the values of days with ice, and the end of 1980s and the be• do not vary much from O, due to low similarity in the ginning of 1990s have a high number of days with growth pattern of the cockles. At Moffen, most of the ice. cockles showed either positive or negative index val• ues at certain years. These results are, however, based on one dominant year class of C. ciliatum. From the Storfjorden and Bear Island results, we can distinguish Discussion some periods of years with higher or lower growth than expected. Storfjorden has a period of high growth The age distribution of the bivalve samples shows that in the beginning of and in the late 1980s, and a period there is a lack of younger specimens at ali sites. The of low growth in the beginning of the 1990s. Bear Is• triangular dredge is quite robust and with a minimum land C. ciliatum show a period of high growth from inner mesh size of 1 cm, small specimens can be lost. 1982 to 1985 and a period of low growth from 1989 to When the dredge is hauled up, the uppermost sediment 1992. layer is lost with the small bivalves inhabiting this hab• The ice data (Fig. 6) show that there was a high itat. Another possibility is that the younger specimens number of days with ice coverage at the Moffen site in were not present in the hauls because of the small area the beginning of 1980s. In the late 1980s and during sampled. There is also the possibility that there is a 335

50 1982 1981 40 30 20 10 o 1980 1979 40 30 20 10 â o s 1978 1977 '-'..... 40 ...c:: ·vbfJ 30 ...c:: 20 Q) ...c:: (/) 10 o 1976 40 30 20 10 o 40 30 20 10

0~~-rrrTO~rrTToo~rrTO~rr~«~rr~o-rrTOoo-rrr~«-r~ o 2 4 6 8 10 12 14 16 18 20 22 24 o 2 4 6 8 10 12 14 16 18 20 22 24 Age

Fi[Jure 4. Shell height at age for Clinocardium ciliatum of different year classes from Moffen (-{}-), Storfjorden (-()-) and Bear Island (--+---). lack of younger specimens at ali three sites but more Storfjorden has the smallest variability in incre• sampling is needed to support this hypothesis. mental width at different ages, indicating that the From incremental width results, it is clear that C. environmental conditions inftuencing growth are more ciliatum has a nonlinear growth pattern with increase stable here than at the two other sites. Ice data support in growth until an age of 6 years after which annual the growth variation data, as the variation in number of growth decreases with age. This break point in growth days with ice cover between years is low at Storfjorden may be linked to other morphological changes and be compared to the two other sites. coupled to different factors, such as maturity. No in• To compare the annual growth at the three sites, formation of the age of maturation of this species in the height at different ages for separate year classes the Svalbard area is available at present. was analysed (Fig. 4). Interestingly, Storfjorden has 336

150 1 a) 0,8 a) 0,6 100 0,4 0.2 o ... 50 -0.2 V ;;>o -0.4 (,) Q) o -0.6 -~ -0.8 .:; -1 - ~ 100 1 ;;>, b) "'~ 0,8 "''- 50 0,6 o... 0,4 Q) .D 0,2 E X ::s o -oQ) o z c) .= -0,2 -0,4 100 -0,6 -0,8 -1 50 1 c) 0,8 o 0,6 0,4 0,2 o Years -0,2 Figure 6. Number of days with ice cover at (a) Moffen, (b) Stor• -0,4 fjorden and (c) Bear Island from the 15th of March to the l st of -0,6 October for the period 1980 to 1994. -0,8 -1 o ..... \0 ao c ao .... ao ao ao o- ao o- o- "' "' o- ~ Clarke, 1995). Resuspension of benthic microfl ora can also be an important source of food during this period Years of time with low cholorophyll content in the water Figure 5. Mean standardized indices for Clinocardium ciliatum column (Ahn, 1993). Storfjorden is also characterized from (a) Moffen, (b) Storfjorden and (c) Bear Island for the period by a high frequency of polynyas covering a significant 1980 to 1994. part of the area (Vinje & K vambekk, 199 1), which can be an important factor affecting the production patterns found here. the highest annual growth rate (i.e. highest height at Water depth can affect the growth of C. ciliatum age) and Bear Island the lowest. The longer period of and the growth is higher at shallower depths (H0pner• annual ice cover at Storfjorden, compared with Mof• Petersen, 1978). H0pner-Petersen (1978) also showed fen and Bear Island, does not appear to affect the that there may be large variations in growth between total annual growth of the species. Studies from the different sites clase to each other (all samples taken Antarctic show that seasonality and ice cover have an in Disco Bay, West Greenland). Variation in height effect on the feeding and growth of suspension feed• at the age of 5 varied from 1O mm in deep waters ers, with lower growth during the dark, ice-covered (1 00 m) to about 25 mm height at the same age at winter period (Picken, 1979, 1980; Clarke, 1988; shallower depths (25-40 m). The height at age in our Barnes & Clarke, 1995). This does not mean that populations were almost the same values as found by the growth is zero during the dark periods and it has H0pner-Petersen (1978) at the deeper stations. Sub• been suggested that suspension feeders with low feed• arctic populations of C. ciliatum grow slower than ing thresholds can use the low number of algal cells Arctic populations and the growth has been shown to present in the water column for feeding (Barnes & be related to both temperature and salinity (Andrews, 337

1972). Andrews ( 1972) ana1ysed both recent and fossil food supply are needed to be able to study the rela• growth rate of C. ciliatum in the high Arctic and height tionship between production in the water column and at age for the recent populations were higher than our growth of benthic animals. In one study on the growth records, but consistent with the results of H0pner• of an Antarctic bryozoan species, Cellarinella watersi, Petersen (1978) from the shallower sites [the sampling positive correlation could be detected between annual depth is not mentioned by Andrews (1972)]. growth and summer plankton blooms (Barnes, 1995). To be able to compare growth at different years, It has also been shown that the period of growth de• independent of bivalve year classes, we standardized creases at higher latitudes resulting in lower growth the annual growth data. The standardized indices are rates of the mollusc Protothaca (Harrington, 1986, in presented as mean index values at different years for Clarke, 1988). the different sites but we did not do any further cor• Ice coverage is known to affect the production relations or other statistica! analyses on the data. The in the water and can be considered as one of sev• comparison with ice data is subjective and not based era! factors determining the growing environment for on any analysis. At the Moffen site, most of the benthic animals in polar areas. Storfjorden and Bear cockles showed the same growth variation at differ• Island seem to have a better growth in the 1980s ent years (i.e. either negative or positive index values) than in the beginning of the 1990s. Ice data from demonstrated by the higher mean index values than the Storfjorde site show more days of ice coverage at the two other sites. This is probably due to the in the 1980s than in the 1990s, whilst ice data from fact that Moffen is dominated by just one year class Bear Island do not match C. ciliatum growth. We can• of bivalves, minimizing the variability between year not see any match between growth and ice coverage classes. At Storfjorden and Bear Island, the synchrony on the Moffen site either. When comparing the three between index values was lower, demonstrated by the sites, Storfjorden and Bear Island show similar an• index values not varying much around zero. Witbaard nual growth trends in mean standardized indices, with ( 1996a) found high synchrony between index values Moffen differing from these two. for the bivalve Arctica islandica, with severa! years We have no quantitative data on the potential com• with 100% of the bivalves showing the same growth petitors of C. ciliatum at the three sites, but species that variation (i.e. either positive or negative values). Our were caught together with C. ciliatum were the cockle results indicate that there are severa! factors inftuen• Serripes groenlandicus and the scallop Chlamys is• cing the growth of C. ciliatum in the Svalbard area landica. These two species were low in numbers and that different year classes, and different speci• but are common at all three sites. Water depth and mens within a year class, can respond differently to substratum seemed to determine the distribution of these factors. Variability in growth within and between these three species at the sites, and C. ciliatum was sites, and within and between year classes, can be dominating in soft clay substrata. an interesting factor that must be studied in more de• Our results indicate that higher Arctic latitudes tai! to be able to explain the growth patterns found. (Moffen) and Arctic water masses (Storfjorden) do Witbaard (1996a) correlated standardized growth in• not seem to reduce growth of the bivalve C. ciliatum crements with phytoplankton data, and data on influx compared to the more southern site (Bear Island). of Atlantic water with the Fair Isle current and the East The most Arctic site (Storfjorden) had the highest Shetland Atlantic Inftow, but did not get any signi• annual growth, indicating that factors other than dura• ficant correlations. The feeding activity of Antarctic tion of ice cover and water temperature are important, bryozoan species could not be related to environ• and that generally high Arctic areas should not be mental cues like ice cover, temperature, chlorophyll considered less productive than other areas. or sedimentation, underlining further the difficulty of linking growth to specific environmental parameters in the field, (Barnes & Clarke, 1994). 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