Thèse de doctorat de l'Université Pierre et Marie Curie Paris VI




НазваниеThèse de doctorat de l'Université Pierre et Marie Curie Paris VI
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Abstract

Even without human perturbations, marine ecosystems are highly dynamic on time-scales ranging from hours to millions of years. Although the North Atlantic Ocean and the North Sea have been intensively studied during the last few decades, the mechanisms leading to interannual variability and fluctuation in biogeographical boundaries of major planktonic species in these basins are still not well understood.

A major component of the North Atlantic zooplankton in terms of biomass and numbers is constituted by the copepod genus Calanus. Calanus species are widely distributed and play a key role in the marine trophic web, both as phytoplankton grazers and as prey for higher trophic levels and especially commercial fish larvae, such as cod. In temperate waters, the two dominant Calanus species are C. finmarchicus and C. helgolandicus. Their comparative ecology should provide important information about the effects of regional environment and climate changes on plankton distribution and abundance, as well as about the impact of zooplankton changes on the sustainability of marine fish stocks.

The present work is a contribution to this general goal and its purpose is (1) to identify the dominant features of the large-scale distribution patterns of Calanus species and of their temporal fluctuations, and (2) to point out the connections between major climatic/oceanic changes and Calanus distribution and abundance.

The Continuous Plankton Recorder (CPR) survey is a unique multi-decadal pan-Atlantic monitoring program with over 60 years of sub-surface plankton abundance data. Calanus species are well sampled by CPRs and, therefore, the CPR database provides the most extensive collection of samples available to study spatial and temporal fluctuations in Calanus abundance in the North Atlantic.

Data analysis procedures were applied to the CPR data set in order to describe the spatial dependency of Calanus species, to model their geographical distribution, to identify the dominant patterns in their temporal fluctuations and to define the global spatio-temporal distribution.

A new CPR mapping procedure, based on geostatistical theory, provides an accurate large-scale representation of the sub-surface distribution of C. finmarchicus and C. helgolandicus in the North Atlantic. The spatial distribution of C. finmarchicus is characterised by the contrast between the northwestern region where the species is abundant and the southeastern region where abundance is low. The distribution pattern of C. helgolandicus is the reverse, with low numbers (or absence) in the northwestern part of the survey and high values in the southeastern part.

The seasonal cycle of C. finmarchicus is highly variable between distinct oceanic areas, with an early peak of abundance around Newfoundland, an 'average' timing of maximum abundance in the centre of the North Atlantic basin, and a late peak to the southwest of Greenland. In the northeast Atlantic and the North Sea, similar regional changes in the timing and duration of seasonal peaks of abundance are observed for both C. helgolandicus and C. finmarchicus.

The year-to-year fluctuations of C. finmarchicus abundance also vary between oceanic regions ranging from 400 to 1000 km wide. Data on C. finmarchicus in the eastern North Atlantic follow the general zooplankton decline observed during the 1950s, 1960s and 1970s by the CPR survey. However, in other areas the species abundance does not show any clear decline for the period 1962-1974.

In contrast to C. finmarchicus, the long-term changes in abundance of C. helgolandicus in the northeast Atlantic have followed an increase during the last three decades. A detailed study of the spatial, seasonal, and long-term changes of the two species in the northeast Atlantic reveals that, although they are almost morphologically identical, they have adopted different strategies for their spatial and temporal distribution, which results in reduced interspecific competition.

Finally, a comparative study of Calanus abundance and large-scale changes in weather patterns reveals that the long-term variations in abundance of these two species are closely associated with the state of the North Atlantic Oscillation (NAO), a large-scale atmospheric pressure gradient change. Furthermore, changes in abundance of C. helgolandicus are possibly related to the latitudinal displacements of the Gulf Stream North Wall position (GSNW). It is suggested that the relationship between the NAO, GSNW and the two Calanus species can result from the combination of several processes: (1) changes in the availability of phytoplankton resources, (2) variations in the competitive balance between the two species induced by temperature changes, (3) regional biogeographical displacements of Calanus populations, (4) turbulence-induced fluctuations in the predator forcing, (5) changes in vertical distribution patterns, and (6) changes in surface currents strength and direction.

To test the validity of these hypotheses, proposed research is outlined.

Introduction

Even without human perturbations, ecosystems are highly dynamic on time-scales ranging from hours to millions of years. This is primarily a response to environmental changes which also occur on a wide range of time-scales. Environmental changes occurring at certain time-scales are also generally found to be related to specific spatial scales. Haury et al. (1978) summarised the dominant time-space scales relevant to plankton studies in the so-called Stommel Diagram (Fig. 1).

Environmental policies aim to reduce human-induced effects on ecosystems. Therefore, understanding the natural variability of the marine ecosystem at time-scales ranging from decades to millennia is currently a important issue, since human effects can only be perceived as a departure from a 'natural variability baseline'.

Since fish is a major alimentary resource for human beings, changes in fish stocks have been intensively studied and great efforts have been made to relate these changes to either natural variability or intensive fishing consequences. However, because of the complexity of interactions between various fish stocks and the stages of their early life history, Hempel (1978) stated that it was not possible to quantify the effects of man-made and natural factors on fisheries.

Planktonic organisms, principally copepods, constitute a major food resource for many commercial fish species (i.e. herring and cod), and changes in their populations mainly depend on natural factors (although they can sometimes be influenced by human activities, i.e. by pollution effects). Assessing plankton variability and determining its causes is hence a major issue of current environmental research.

The copepod genus Calanus is widely distributed over the global ocean and, consequently, Calanus species are found in almost all marine environments, from tropical to arctic regions. In the North Atlantic the zooplankton is largely dominated by copepods and among them by the genus Calanus which constitutes a major resource to higher trophic levels and a potential strong grazer for phytoplankton. The comparative ecology of Calanus species in the North Atlantic should provide a better understanding about the links between changes in regional environment and climate and plankton distribution and abundance. It will also increase our knowledge about the impact of zooplankton changes on the sustainability of marine fish stocks in this basin.

Phylogeny of Calanus

Calanus finmarchicus was the first free-living copepod to be described when Gunnerus (1770) identified it as Monoculus finmarchicus. For the following century there was considerable difficulty with its name, until Giesbrecht (1892) reviewed the nomenclature carefully. Seventy-five years later, Matthews (1967b), after reviewing a series of taxonomic works (Sars, 1903; Brodsky, 1948; Rees, 1949; Jaschnov, 1955; Brodsky, 1959; Brodsky, 1961; Brodsky, 1965), identified seven species, five from the northern hemisphere (C. finmarchicus, C. glacialis, C. helgolandicus, C. pacificus, C. sinicus) and two from the southern hemisphere (C. chilensis and C. australis).

The morphological similarity and the overlapping geographical range of Calanus species have resulted in persistent problems in their identification, despite their ecological importance. As a consequence, the systematics of the genus remained unclear for a long time and, since Matthews’ work, a series of publications have been dedicated to Calanus systematics and phylogeny: Matthews (1967a), Frost (1971; 1974), Brodsky (1972), Bradford and Jillett (1974), Jaschnov (1975), Fleminger and Hulsemann (1977), Shih (1984), Bradford (1988), DeDecker et al. (1991), Hulsemann (1991), Bucklin et al. (1992), Bucklin and Lajeunesse (1994), Bucklin et al. (1995).

Although Calanus species exhibit a high level of morphological similarities, they are quite distinct in their genetic characters (Fig. 2). Recent development of molecular techniques dedicated to species identification have been shown to be powerful in separating congeneric species (Bucklin et al., 1995). Genetic techniques are especially useful for differentiating young stages that are difficult to identify only by their morphological characters.

Following the taxonomic classification given by Bucklin et al. (1995), the genus Calanus comprises 14 species divided into two main groups and three additional species. The finmarchicus group comprises C. finmarchicus, C. glacialis and C. marshallae. The helgolandicus group comprises C. helgolandicus, C. pacificus, C. orientalis, C. australis, C. sinicus, C. chilensis, C. euxinus and C. agulhensi,. Three species are not related to these two groups: C. propinquus, C. simillimus and C. hyperboreus. Table 1 indicates the authority and the geographical location of the different species, as well as some species previously in the genus Calanus which were placed in different genera by Bradford (1988).

Although the taxonomy of Calanus is becoming clearer as more data become available on the genus, for certain species, doubts still remain on their affiliation. Another classification based on historical bibliographic work by Razouls (1995) distinguishes 19 Calanus species. However, some of these species have not been reliably recorded, since their initial description.

Life Cycle

The life cycle of Calanus consists of 13 stages, including the egg, six naupliar stages, and six copepodite stages including the adult (Lebour, 1916). Since C. finmarchicus populations are distributed over a wide geographical range, they are submitted to a wide range of environmental conditions including important sea temperature gradients. Calanus growth, development and mortality rates are affected by temperature (Carlotti et al., 1993) and temperature regime (Tande, 1988). The length of the ‘biological summer’ (i.e. period of high primary production) and hence the period of food availability also strongly depends on the temperature regime. Finally, the number of generations per year, which depends on development rate and duration of the food availability period, is highly variable in regions between which temperatures gradients are important. For example, life cycle duration is one year or more in cold waters of northern Norway (Diel and Tande, 1992); in Nova Scotia and Gulf of Maine, the life cycle is about 2.5 months and there are two generations per year (Sameoto and Herman, 1990); in temperate waters around the British Isles, there is time for three or even four generations (Marshall and Orr, 1972). C. helgolandicus completes up to five generations from February to August in the warm-temperate waters of the Celtic Sea (Williams, 1985).

A major feature in the life cycle of C. finmarchicus is the strategy adopted by the species during winter (Fig. 3). At the end of the summer period, Calanus copepodites stage V (CV) load their oil sac with lipids and descend into deep waters, generally greater than 200m and around 500-1000m where they overwinter (Williams and Conway, 1988; Miller et al., 1991). Overwintering CVs are in a low activity state with empty guts (Hallberg and Hirche, 1980), low digestive enzyme activity (Tande and Slagstad, 1982; Hirche, 1989) and low respiration rates (Hirche, 1983). During winter, their dry body weight decreases, as a result of fasting and reserves catabolism (Båmsted and Ervik, 1984; Hopkins et al., 1984). The migration of C. finmarchicus into deeper, darker and colder water probably reduces predation risks, and low catabolism rates associated with high lipid reserves increase survival probability in an environment were food is sparse or absent. However, not all C. finmarchicus migrate into deep water and sometimes two distinct overwintering populations can be found within different depth strata (Hirche, 1991; Pedersen et al., 1995). Although overwintering CVs do not feed under natural conditions, CVs with feeding and respiratory activity during winter have been observed in shallow waters (Marshall and Orr, 1958; Butler et al., 1970; Corner et al., 1974).

At the end of winter, resting CVs moult into adults and vertically migrate to the surface where the females spawn at a time that generally coincides with the spring increase of phytoplankton. Colebrook (1979) suggested that the timing of the spring phytoplankton outbreak might induce the spawning of copepods. Results from Dickson et al. (1988), by assuming that the timing of stratification and/or temperature increase in early spring are the determinant factors for zooplankton success, supported this hypothesis. On the other hand, observations made during the late winter period (Hopkins et al., 1984; Williams, 1985) suggest that the upward vertical migration, development and gonad maturation of the overwintering populations of Calanus starts when phytoplankton is absent and the water column not yet stratified. Thus, it is more likely that factors such as increase in the late winter photoperiod or an endogeneous long-range timer might determine the end of the Calanus overwintering period (Miller et al., 1991), whereas the phytoplankton outbreak would induce spawning, as suggested by Tande and Hopkins (1981). Since temperature, light, and food conditions vary with oceanic regions, the timing of induction and termination of the diapause period may also vary geographically.

Although C. finmarchicus generally undergoes vertical migration to deep cold waters at the beginning of winter, the overwintering habitat of the species is highly variable, with depths ranging from 2000 m to near surface and temperature from -1 to +11°C. Consequently, Kaartvedt (1996) argued that the timing of vertical migration and the selection of an overwintering habitat might be modulated primarily by predator forcing rather than by environmental requirements.

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