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C. Finmarchicus and C. Helgolandicus MARINE ECOLOGY PROGRESS SERIES l Vol. 134: 101-109,1996 Published April 25 Mar Ecol Prog Ser Calanus and environment in the eastern North Atlantic. I. Spatial and temporal patterns of C. finmarchicus and C. helgolandicus Benjamin Planq~e'~~~*,Jean-Marc ~romentin~ 'Sir Alister Hardy Foundation for Ocean Science, The Laboratory, Citadel Hill, Plymouth PLl 2PB, United Kingdom 'Laboratoire dlOceanographie Biologique et dPEcologiedu Plancton Marin, URA2077, Station Zoologique. BP 28, F-06230 Villeiranche-sur-Mer. France ABSTRACT Spatial and temporal patterns of Calanus finmarchicus and C. helgolandicus (Copepoda, Calanoida) were investigated in the northeast Atlantic and the North Sea from 1962 to 1992. The seasonal cycle of C. finmarchicus is characterised by a single peak of abundance in spring, whereas the seasonal cycle of C. helgolandicus shows 2 abundance maxlma, one in spring and one in autumn. The former species mainly occurs in northern regions (limited by the 55' N parallel in the North Sea and by the 50' N parallel in the open ocean). The latter species shows 2 types of spatial patterns, occurring in the Celtic Sea during spring and in the Celtic Sea plus the North Sea in autumn. Differences in seasonal spatial patterns of Calanus specles may result from different responses to the environment, ultimately due to different life cyde strategies, different vertical distributions, opposite temperature dffinities and interspecific competition. Futhermore, results reveal that annual means of abundance are closely related to annual maxima and to spatial extensions of the species. It also appears that the long-term trends of the 2 Calanus species are opposite: C. finn~arch~cusshows a clear downward trend in abun- dance, while C. helgolandlcus presents an upward one KEY WORDS: Calanus - Long-term changes Seasonal spatial patterns. Eastern North Atlantlc . North Sea Continuous Plankton Recorder survey INTRODUCTION In most of these works, zooplankton was considered as a whole and the behaviour of total zooplankton was The high variability at low frequencies of plankton principally studied. Our purpose was to determine the comn~unitieshas been quite recently described by specific responses of individual zooplankton species means of long-term monitoring (Russell et al. 1971, that have different physiological and biological prop- Cushing & Dickson 1976, Bernal 1979, 1981). Long- erties to environmental changes. We studied 2 calanoid term fluctuations of zooplankton appear to result from copepod species, Calanus finmarchlcus and C. helgo- environmental changes, rather than purely biological landicus. These species were chosen because: (1) they processes (Colebrook 1978, Chelton et al. 1982).Recent constitute the major component of the northeast studies demonstrated the importance of environmental Atlantic and North Sea zooplankton in terms of bio- factors such as sea temperature, oceanic currents, tur- mass, numbers and trophic role (Williams et al. 1994, bulence, wind stress and nutrients on plankton changes Gislason & Astthorsson 1995); (2) they are closely (Colebrook & Taylor 1979,1984, Colebrook 1982,1985, related, play a similar role in the ecosystem and their 1986, Dickson et al. 1988, Radach et al. 1990, Fransz biology and physiology have been largely described et al. 1991). (Marshal1 & Orr 1972, Williams 1985). To understand the specific responses of these 2 Calanus species to their environment, we first have to describe their spatial and temporal patterns. The pur- O Inter-Research 1996 Resale of full article not permitted 102 Mar Ecoi Prog Ser 134: 101-109, 1996 pose of this article is to identify and compare seasonal In order to assess variations in the spatial extension of cycles, spatial patterns, and long-term fluctuations in each species, we defined a variable called 'spatial cov- abundance of the 2 specles, as well as their potential erage' For each monthly map, pixels with at least l in- link to environmental variables. d.ividua1were coded 1 and others were coded 0. Spatial coverage of each monthly map was defined by the ratio of pixels coded 1 to the total number of pixels and was METHODS given as a percentage (0% indicates that there is no pixel with at least 1 individual, 100% ~ndicatesthat Sampling. Data were provided by the Continuous there is at least 1 individual in every pixel). Plankton Recorder (CPR) survey. The CPR is a high- Annual abundances correspond to the mean of speed plankton sampler designed to be towed from abundance from January to December. Annual max- commercially operated 'ships of opportunity' over long ima of abundance are defined as the hlghest monthly distances. CPR are towed in the surface layer (7 to 8 m abundances of each year. Seasonal cycles are calcu- depth) and, due to mixing induced by the ship towing lated from the means of each month of the 1962-92 the CPR, the 0 to 20 m layer is sampled (H. G. Hunt series. pers. comm.).Plankton samples are collected every 20 Comparison of long-term series was done using the nautical miles (37 km), corresponding to about 3 m3 of Pearson correlation on origlnal series and detrended filtered water. The survey started in 1931 in the North ones. Correlations on detrended series were used to Sea, and was extended over the North Atlantic at the reveal relationships that were not due to long-term end of the 1950s (Warner & Hays 1994). However, trends. Cross-correlation was tested by a Monte-Carlo Calanus finmarchicus and C, helgolandicus species approach (5000 simulations). were not routinely distinguished until the late 1950s, thus we have only used the data collected during the period 1962 to 1992. Only stage V copepodites and RESULTS adults are routinely identified to species level i.n the CPR survey, and we present here data on these stages Seasonal cycles only. We focused on the eastern North Atlantic, where the sampling was constant and regular during 1962-92 The mean seasonal cycle of Calanus finmarchicus (see Warner & Hays 1994) and where the 2 species abundance, calculated from the 1962-92 series, shows commonly occur (Oceanographic Laboratory Edin- burgh 1973, Colebrook 1986). Plankton data consist of about 72 000 samples (-2300 per year). The CPR survey provides information on the sub- surface distribution of species. Therefore, changes in abundance presented in this study only refer to the sub-surface layer. However, Calanus species undergo ontogenetic vertical migrations (Williams & Conway 1980, Hirche 1983, Williams 1985) and consequences of this behaviour on sub-surface changes in Calanus abundance will be discussed. Analyses. Species abundances in each sample were log-transformed using the log(x+l) function (Cole- brook 1975). Monthly data were interpolated in order to obtain a series of regular grids. Interpolation was performed by kriging, the most suitable method for a nonstandard sampling (Cressie 1993). A total of 372 grids were calculated, 1 for each month from 1962 to 1992, and resulted in 372 maps. In the eastern North Atlantic, grid nodes (map pixels) with a minimum of 70% of data during 1962-92 were selected. Remaining missing values were estimated using the ZET method (Zagoruiko & Yolkina 1982), an iterative multiple re- Fig. l Study area. Selection of pixels in this area was based gression model. The final grid consisted of 224 nodes on the regularity of the Continuous Plankton Recorder (CPR) representing a map of 224 pixels of 8575 km2 surface survey from 1962 to 1992 and on the numbers of missing each (Fig. 1). values by pixels Planque & Fromc,ntin: Spatial and temporal patterns of ~dldla11~1~species 103 a clear peak of abundance during spring, i.e. April to annual abundance maximum appears clearly on the June, and a plateau from July to September (Fig 2a). May-June map. The spatial distribution is characterised The seasonal cycle of spatial coverage follows the by 2 areas of very high abundances, south of Iceland same pattern, indicating that these 2 variables are and south of Norway. The southern limit is around the closely related and that the number of organisms 55" N parallel in the North Sea and the 50" N parallel per surface unit is constant all through the year in the open ocean. During summer, there is a general (Fig. 2a). decline in abundance and spatlal extension decreases The mean seasonal cycle of Calanus helgolandicus in the open ocean (southern limit of the distribution abundance is quite different (Fig. 2b). There are 2 moves north to around the 55" N parallel). In autumn, abundance maxima during the year The first one rather high abundance always occurs in the northern occurs from May to June, and the second and major North Sea and spatial restriction continues, organisms one from September to October. Contrary to C. fin- being confined further north than in summer. In marchicus, abundance and spatial coverage are not November-December, there is a drop in abundance totally simllar (Fig. 2b). The shapes of the 2 curves are and Calanus finmarchicus is limlted to the northern close, but spatial coverage is at a hlgher level during North Sea. autumn. This result indicates either that the mean number of organisms per surface unit is lower during this period or that the species spatial dispersal is higher Calanus helgolandicus or that there has been a reorganisation of the species vertical distribution. During winter, the species distribution is confined The seasonal pattern of Calanus finmarchicus is mainly in the south of the area studied and in the reproducible from year to year; in 90% of the cases eastern part of the North Sea (Fig. 3b). At the begin- the annual maximum of abundance occurs around ning of spring, there is a rise in abundance. Popula- May. For C. helgolandicus the situation is a bit more tions extend further south into the Bay of Biscay and complicated, with annual maximum abundance gen- disappear from the North Sea surface waters.
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