Aug 29, 1998 - 2 Station Marine, Université de Lille I, B.P. 80, 62 930 Wimereux, France ... For marine benthic invertebrates exhibiting complex life cycles, ...
Hydrobiologia 375/376: 165–176, 1998. S. Baden, L. Pihl, R. Rosenberg, J.-O. Strömberg, I. Svane & P. Tiselius (eds), Recruitment, Colonization and Physical–Chemical Forcing in Marine Biological Systems. © 1998 Kluwer Academic Publishers. Printed in Belgium.
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Do hydrodynamic factors affect the recruitment of marine invertebrates in a macrotidal area? The case study of Pectinaria koreni (Polychaeta) in the Bay of Seine (English Channel) E. Thi´ebaut1 , Y. Lagadeuc2 , F. Olivier3, J. C. Dauvin2 & C. Reti`ere3 1
Laboratoire d’Oc´eanographie Biologique et d’Ecologie du Plancton Marin, URA CNRS 2077, Universit´e Pierre et Marie Curie – Paris VI, Bâtiment A, Case 6, 4 Place Jussieu, 75252 Paris cedex 05, France 2 Station Marine, Universit´e de Lille I, B.P. 80, 62 930 Wimereux, France 3 Mus´ eum National d’Histoire Naturelle, Laboratoire Maritime de Dinard, URA CNRS 699, 17 Avenue Georges V, BP 28, 35801 Dinard cedex, France
Key words: larval dispersal, postlarval drifting, recruitment, Pectinaria koreni, English Channel
Abstract For marine benthic invertebrates exhibiting complex life cycles, changes in populations’ distribution and abundance are governed by a large variety of physical, chemical and biological processes. From field observations in the Bay of Seine and laboratory experiments conducted since 1987 on the polychaete Pectinaria koreni, the present study highlights the relative importance of hydrodynamical and biological factors which affect individuals within both the planktonic and benthic phases at different scales of space and time in a macrotidal area. Pectinaria koreni is one of the main macrofaunal component of the Abra alba muddy fine sand community of the eastern Bay of Seine. Despite a highly advective and diffusive environment, a relative larval retention near adult population was reported due to some local hydrodynamics features (e.g. tidal residual circulation, Seine river plume front) and the interaction between the vertical current structure and the larval vertical migration. Although larval retention could be disrupted by wind induced currents, multiple spawning events over the reproductive period increase the likelihood that at least one larval cohort ensures a high recruitment during the life-span. Following a massive settlement whatever the sediment grain size, the newly settled larvae exhibited a high immediate decrease of their densities as a result of postlarval mortality and migration. Postlarval drifting was induced by a combination of physical factors (i.e. tidal currents and swell) and postlarval behaviour in response to sediment texture and adult/settlers interactions. According to the hydrodynamics of the bay, this process may generate a postlarval transport from offshore bottoms to coastal suitable habitats and counteract the demographic effects of larval dispersal. A conceptual model of factors governing the recruitment and population maintenance of Pectinaria koreni is proposed and discussed in comparison with results obtained on another polychaete, Owenia fusiformis, in the same area.
Introduction In marine coastal zones, the majority of benthic invertebrates exhibit a complex life cycle including a planktonic larval phase and two bottom-dwelling juvenile and adult phases. For such species, the recruitment, defined as the supply of early juveniles to established adult populations, includes both pre- and post-settlement processes (Connell, 1985). It shows large variations on a variety of temporal and spatial scales so that the success or the failure of the
recruitment can regulate ultimately populations’ distribution and abundance (Rougharden et al., 1988; Gaines & Bertness, 1992). The successive processes which governed the recruitment are mainly: (1) egg fertilization and offspring production which is controlled by both organisms characteristics (e.g. degree of aggregation of conspecific adults) and the external physical environment (e.g. current velocity) (Levitan et al., 1992); (2) larval dispersal which is determined by local circulation features (i.e. eddy-diffusion and advection)
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166 and larval attributes (e.g. duration of planktonic larval life and vertical migrations) (Scheltema, 1986; Hill, 1991); (3) metamorphosis and larval settlement which depends on both substratum encouter rate related to the near-bottom concentration of competent larvae and bottom boundary-layer flows, and the substratum acceptance or rejection (Butman, 1987); (4) post-settlement events including juvenile mortality (Olafsson et al., 1994; Gosselin & Qian, 1997) and juvenile migration which results from passive resuspension and/or active dispersal behaviour (Armonies, 1994). Thus, the recruitment depends on a large variety of hydrodynamical and biological properties as well as their interactions which affect the probability that an individual of any stage make the transition to the next one (Eckman, 1996). Within the English Channel, characterized by intense tidal currents average velocity of which is about 1 m s−1 (Salomon, 1991), the macrotidal hydrodynamical regime can greatly affect the achievement of benthic marine invertebrates life cycle. While currents may advecte and disperse eggs, larvae and post-larvae, they also determine adult benthic communities distribution related to superficial sediment composition, so that characteristic species of fine sand communities form isolated insular populations which are confined in bays and estuaries (Cabioch et al., 1982). Since 1987, field observations in the eastern Bay of Seine and laboratory experiments were conducted on a target-species, the polychaete Pectinaria koreni, to determine which factors govern the recruitment of marine invertebrates in a macrotidal area. The aim of the present study is to highlight the relative importance of hydrodynamical and biological processes which affect individuals within both the planktonic and benthic phases. A conceptual model of factors controlling the recruitment and population maintenance of Pectinaria koreni is proposed and discussed in comparison with results obtained on other benthic invertebrates in the same area.
Study area The Bay of Seine is a quadrilateral embayment, ca. 65 by 140 km wide and 5 to 30 m deep, located along the French coasts of the eastern English Channel. The major freshwater inputs into the bay are due to the Seine River, discharge of which varies seasonally from
a maximum of up to 2000 m3 s−1 in winter to a minimum of 100–200 m3 s−1 in summer. Maximum tidal current velocities decrease from 1.8 to 0.5 m s−1 from the north-western part to the eastern part of the bay (Chabert d’Hières, 1986). In the latter area, tidal flows are asymmetrical so that flood currents are stronger but shorter in duration than ebb currents. Residual circulation depends on 3 factors: (1) tide, (2) horizontal density gradients due to the Seine freshwater input and (3) meteorological factors, especially wind (Le Hir et al., 1986) (Figure 1). Tide induces a general drift of water masses to the east (up to 6– 7 cm s−1 ) in the northern part of the bay and to the west along the coasts of Calvados. It also generates anticyclonic gyres which can locally increase the residence time of particles. The gyre off the Cap d’Antifer favours the Seine freshwater flow in a north-westerly direction. Because of horizontal density gradients, the eastern part of the bay acts as a partially well-mixed estuary with a 2-layer circulation. The diluted surface layer exhibits a net seaward residual flow which reaches 5 cm s−1 while the deeper saline layer is characterised by a net landward residual flow which varies between 2 and 5 cm s−1 . At coastal shallower depths, wind action causes an increase of residual current velocity which can reach 30 cm s−1 during strong winds (> 15 m s−1 ). In the eastern part of the bay, benthic communities are distributed in response to hydrodynamic and sedimentary gradients, so that four different communities are distinguished from the coast to the open sea: a muddy Macoma balthica community, a muddy fine sand Abra alba-Pectinaria koreni community, a medium sand Ophelia borealis community and a sandy gravel Glycymeris glycymeris community (Cabioch & Gentil, 1975). The target species: Pectinaria koreni The tubicolous polychaete Pectinaria koreni, which behaves as a sub-surface deposit feeder, is one of the most common species of the Abra alba muddy-fine sand community (Thiébaut et al., 1997). Its densities show strong seasonal variations with a minimum in winter (i.e. few hundred ind. m−2 ) and a maximum in summer (i.e. up to 3000 ind. m−2 ) after the recruitment (Gentil et al., 1986). Between 1986 and 1991, the adult population exhibited weak fluctuations in terms of abundance and distribution so that highest densities were regularly reported off the Seine estuary, off Deauville and Cabourg, and in a coastal zone from the
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Figure 1. Schematic representation of the residual circulation in the Bay of Seine (modified from Le Hir et al., 1986).
Figure 2. Schematic representation of Pectinaria koreni larval distribution at 3-m depth in relation to wind direction and intensity. (A) Distribution of a 10 d old larval cohort on 25 May 1987 under the influence of northern and north-eastern wind. (B) Distribution of a 10 d old larval cohort on 5 June 1987 under the influence of western and south-western wind (from Lagadeuc, 1992b). Larval densities are expressed as ind. m−3 . Dots indicate sampling stations. Arrows indicate wind direction. The averaged wind velocity over the 10 d preceding the sampling is given.
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168 Table 1. Mean densities of Pectinaria koreni settlers at differents stations of the eastern Bay of Seine sampled on 9–12 June 1987 during the peak-settling (from Lambert, 1991). Sediments characteristics were defined from samples collected in February 1988. Station Latitude Longitude Depth (m)
R1 49◦ 25.080 N 0◦ 07.080 E 1.5
R2 49◦ 26.280 N 0◦ 03.200 E 2
E 49◦ 27.130 N 0◦ 02.950 E 8
A 49◦ 27.200 N 0◦ 01.500 E 10
B 49◦ 27.300 N 0◦ 00.200 E 9
R5 49◦ 28.180 N 0◦ 02.820 W 9.5
R6 49◦ 29.050 N 0◦ 04.380 W 12
R7 49◦ 30.000 N 0◦ 07.090 W 14
R8 49◦ 30.960 N 0◦ 08.970 W 17.5
Sediment characteristics substrate type % < 63 µm % ≥ 500 µm
mud – –
mud 4 0
mud 16 8
sandy mud 34 2
muddy fine sand 2 10
clean fine sand 0 4
clean fine sand 0 6
clean med. sand – –
clean med. sand 0 45
Settlers density (ind m−2 )
1118
529
11301
28409
16481
176
647
2060
0
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169 Cap de La Hève to the Cap d’Antifer (Thiébaut et al., 1997). Pectinaria koreni is an univoltine species with a short life-span of 12–18 months (Elkaïm & Irlinger, 1987). It has two main spawning periods in March–April and May–June and exhibits a high fecundity ranged from 10 000 to 12 000 ovocytes per female (Elkaïm & Irlinger, 1987; Irlinger et al., 1991). Following an external fertilization, the pelagic larval development which was estimated to be about 11–13 d from in situ observations, included two trochophore stages, three metatrochophore stages and one aulophore stage (Lagadeuc & Retière, 1993). The larval life span of the successive trochophore and metatrochophore stages are 1, 2–3, 4–5, 2 and 1 d respectively; the planktonic period of the aulophore stage is undetermined.
Seine estuary and off the Cap de La Hève (> 5000 ind. m−3 ) and, were dispersed northward in response to westerly and south-westerly winds (Figure 2B). Larval vertical distribution is related to water column stratification (Lagadeuc, 1992a). When vertical stratification is strong, larval vertical distribution varies according to larval development stages. Trochophore stages are mainly reported in surface waters at the level of the halocline while metatrochophore stages are sampled below the halocline (Figure 3B). The aulophore stage is confined within near-bottom waters. Conversely, when the stratification is low in response to an increase of vertical turbulence, all larval stages are distributed evenly over the whole water column (Figure 3A).
Larval dispersal
The metamorphosis, defined as an irreversible set of anatomical and physiological changes of individuals (Butman, 1987), begins in the water column by the transformation of metatrochophore into aulophore larvae, i.e. competent larval stage (Lambert et al., 1996). It is a gradual process which continues over about two weeks after the settlement by the purchase of all morphological structures of juveniles. Although Pectinaria koreni aulophores have lost their typically larval morphological characteristics, they do not exhibit all the organs of the juveniles even as buds and are not well adapted to benthic life when they reach the bottom. Under still water conditions, Pectinaria koreni postlarvae settle preferentially on muds and sandy muds rather than on muddy fine sands or clean fine sands, and are able to actively select a particular substrate by crawling at the sediment surface on a microspatial scale (i.e. rate of displacement: 0.36 to 1.90 cm d−1 ) (Desroy et al., 1997). Although this mechanism may be involved in the microscale distribution of postlarvae, field observations at the scale of the eastern Bay of Seine reveal that larvae may settle over a large edaphic spectrum, from muds to clean medium sands, regardless of substrate suitability for adults (Table 1). Following the settlement, postlarvae exhibit a high mortality rate ranging from 41% to 100% over two weeks (Lambert, 1991; Lambert et al., 1996). This large decrease in postlarval densities may be explained by (1) metamorphosis as a critical period for survival, (2) post-settlement processes like predation and (3) postlarval dispersal.
Horizontal transport of Pectinaria koreni larvae is mainly governed by tidal residual circulation and wind-induced currents so that larvae can be advected up to 40 nautical miles from the adult population (Lagadeuc, 1990). In accordance with the tidal residual circulation, a general drift of larvae occurs principally in westerly and north-westerly directions, and to a lesser extent, in north-easterly direction (Lagadeuc, 1992b). Although such a transport can generate a sizeable loss of larvae outside the eastern Bay of Seine, the low intensity of tidal residual currents off the Seine estuary is sufficient to promote larval retention near the adult population. Moreover, in the north-easterly direction, the frontal structure of the plume acts as a physical boundary which limits offshore export of larvae (Thiébaut, 1996). Episodic wind events can modify the predictive pattern of tidal dispersal of larvae and disrupt larval retention (Lagadeuc, 1992b). During Pectinaria koreni reproductive period, from March to July, main wind directions are north/north-east and west/south-west (Table 2). Larval distribution and transport in relation to wind-induced circulation are given for two different 10 d old larval cohorts sampled in May and June 1987 (Figure 2). On 25 May, larval population was transported westward under the influence of north-easterly winds: maximal larval densities (> 2000 ind. m−3 ) were located off Deauville and off Ouistreham while larval densities in the north of the Cap de La Hève were low (< 20 ind. m−3 ) (Figure 2A). Conversely, on 5 June, larvae exhibited maximal densities off the
Metamorphosis and settlement
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170 Table 2. Variations of climatic characteristics (i.e. wind regime and Seine freshwater discharge) from 1967 to 1996 in the eastern Bay of Seine during the Pectinaria koreni reproductive period. Wind characteristics (i.e. 8 values per day) were measured at the signal station of the Cap de la H`eve (Meteofrance data). Daily Seine freshwater discharge was measured at a lock-gate located 150 km upstream of Le Havre Harbour (data from the Navigation Department of Rouen Harbour).
Frequency of main wind direction (%) Monthly wind speed (m s−1 )
Nb of days with wind speed ≥ 7 m s−1
Seine freshwater discharge (m3 . s−1 )
N-NE W-SW Average Min. Max. Average Min. Max. Average Min. Max.
March
April
May
June
July
29.2 31.8 6.5 4.3 9.1 12.1 1 20 625 312 1480
41.7 23.2 5.9 4.3 8.8 9.3 2 21 586 256 1280
37.9 25.3 5.4 3.9 7.6 7.0 0 15 436 178 1110
38.4 29.6 5.1 4.1 6.5 5.7 1 12 332 125 673
34.7 32.3 5.1 3.3 6.7 5.4 0 12 265 121 517
Figure 3. Vertical distribution of Pectinaria koreni larval stages at one station off the Seine estuary (49◦ 28.180 N–0◦ 02.820 W) in relation to water column stratification. (A) Vertical homogeneity (28 May 1987). (B) Vertical stratification (3 June 1987). (from Lagadeuc, 1992a).
Postlarval dispersal Postlarval dispersal results from combined action of hydrodynamical factors in the bottom boundary layer and postlarval behaviour. Flume experiments have shown that postlarvae and juveniles can actively leave the substrate by the secretion of a long mucous thread which increases the hydrodynamic drag on the organisms and enables them to be resuspended in the
water column (Lambert, 1991; Olivier et al., 1996b; Desroy et al., 1997). Such a behaviour is induced by unfavourable ecological conditions like the occurrence of an unsuitable substrate or high densities of conspecific adults (Olivier et al., 1996b). Adults effects on postlarval dispersal result either from direct action on postlarvae or from sediment disturbance generated by bioturbation.
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Figure 4. Temporal evolution of Pectinaria koreni drifting postlarvae in relation to semi-diurnal tidal cycle and bottom shear stress at one station off the Seine estuary (49◦ 26.600 N–0◦ 01.300 E). (A) Depth-averaged postlarval densities (ind. m−2 ) from 26 to 28 May 1992 (from Thi´ebaut et al., 1996). (B) Near-bottom postlarval densities (ind. 400 m−3 ) from 1 to 2 June 1992 (from Olivier et al., 1996a). (C) Near-bottom postlarval densities (ind. 400 m−3 ) on 13 June 1992 (from Olivier et al., 1996a).
172 From different time-series carried out in May-June 1992 at one station seaward of the Seine estuary, Olivier et al. (1996a) and Thiébaut et al. (1996) demonstrated that instantaneous tidal currents and swell are the main hydrodynamical factors involved in postlarval dispersal in the field (Figure 4). Observations with pump samples collected over the whole water column in calm weather conditions (mean wind velocity of ∼ 5.2 m s−1 ) highlight cyclic fluctuations of depth-averaged densities of drifting postlarvae in relation to semi-diurnal tidal cycle (Figure 4A). Maximum densities (i.e. > 100 ind. m−2 ) were consistently observed at the beginning of the flood when the bottom shear stress exceeded a threshold value of 0.1 N m−2 . Furthermore, data collected using a suprabenthic sledge which samples the water column from 0.10 to 1.45 m above the sea-bed reveal that the importance of tidal currents on postlarval dispersal depends on weather conditions. In calm weather conditions (mean wind velocity of 2.1 m s−1 ), maximum density of drifting postlarvae (i.e. 2000 ind. 400 m−3 ) was also reported during the flood even if cyclic variations are not clearly apparent (Figure 4B). Conversely, with moderate winds (mean wind velocity of 5.8 m s−1 ), swell increases significantly the bottom shear stress, which exceeded 0.2 N m−2 and reached 1 N m−2 , and induces a near permanent resuspension of postlarvae (Figure 4C).
Discussion Over the past two decades, spawning, larval dispersal, settlement and early settlers mortality of marine invertebrates have been commonly studied to understand how these processes determine the structure and abundance of adult benthic populations. However, most studies have focused only on one life stage and have emphasized its role. For instance, Gaines & Rougharden (1985) argued that larval supply in combination with hydrodynamical factors play a crucial role in generating recruitment patterns. Conversely, from a synthetic review of major processes regulating the recruitment of marine soft-bottom invertebrates, Olafsson et al. (1994) have shown that post-settlement mortality could be the dominant determinant. The present study on the recruitment of Pectinaria koreni in the eastern Bay of Seine reveals that the spatial and temporal maintenance of the adult population could not be explained by one or few keyfactors but is a result of a combination of a large
variety of favourable and repressive factors which include biological and hydrodynamical properties acting on different life stages (Figure 5). Pectinaria koreni larval dispersal is caused by its biological properties and by local hydrodynamic features which could favour either larval retention near the parental population or larval export to open sea (Figure 5). In relation to the two-layer estuarine circulation, the ontogenic vertical migration of larvae could be one of the dominant factor that ensures their retention off the Seine estuary. Younger larvae, mainly located in the diluted surface layer are carried downstream whereas older larvae present in the deeper saline layer are advected upstream to the area of adult population. As the influence of Seine freshwater was estimated to extend around about 15 nautical miles from the mouth of the estuary (Thiébaut et al., 1992), the ontogenic vertical migration is efficient in terms of larval retention only if larvae can benefit of the two-layer circulation and are not transported too far offshore. The low tidal residual circulation and the Seine River plume front may ensure that high larval densities remain in the vicinity of the Seine estuary. Conversely, wind effects induce a large variability in larval dispersal and may disrupt larval retention at the proximity of adult population resulting of the processes cited above: (1) When wind velocity exceeds 7 m s−1 , ontogenic vertical migration is not reported and larvae are evenly distributed over the whole water column so that no differential transport occurs for the different larval stages (Lagadeuc, 1992a); (2) Moderate north-easterly winds are able to break up the Seine River plume front and generate a dilution in surface of the plume waters and larvae (Thiébaut, 1996); (3) Wind-induced currents modify over the few days of larval life duration the direction of larval transport; they also increase residual currents velocity in the coastal zones and consequently the larval export to offshore waters (Lagadeuc, 1992b). As wind events cause an unpredictable hydrodynamical environment at a short temporal scale, multiple spawning events over the 4 months of Pectinaria koreni reproductive period could prevent the loss of all offspring in response to a catastrophic event and increase the likelihood that at least one larval cohort allows a significant recruitment each year. Due to the variations of climatic characteristics in the eastern Bay of Seine from March to July, the relative importance of wind events on larval dispersal and recruitment should
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Figure 5. Conceptual model of biological and hydrodynamical factors governing Pectinaria koreni recruitment in the eastern Bay of Seine. Solid oval: life-cycle stages. Dashed oval: processes governing the recruitment. Solid rectangle: hydrodynamical and biological factors involved in the recruitment.
vary with time. Thus, higher monthly wind speed and higher number of days with a wind speed ≥ 7 m s−1 in March–April should decrease the probability of larval retention at the beginning of the reproductive period, even if stronger Seine freshwater discharge at this time could strengthen the two-layer estuarine circulation (Table 2). Pectinaria koreni postlarval drifting depends on both active behaviour (i.e. the secretion of a mucous thread) in response to an unfavourable biotic and abiotic environment and the bottom shear stress generated by tidal currents and swell (Figure 5). According to the size of drifted postlarvae and juveniles reported from field studies, which varied between 0.9 and 62.5 mm (Lambert et al., 1996; Olivier et al., 1996a), it appears that postlarvae and juveniles retain the ability to be transported by thread drifting over few months, independent of the adaptations of organisms to the benthic life. While crawling at the sediment surface induces a postlarval motion at microscale (i.e. cm), thread drift-
Table 3. Distance covered by a drifted postlarva over a single resuspension event for different size classes and heights above the sea-bed. Postlarval fall velocity has been measured experimentally by Olivier (1997). Horizontal current velocity is assumed to be constant and equal to 0.4 m s−1 . Size Height Fall classes above the velocity (mm) seabed (cm) (cm s−1 ) 1–2
2–4
4–10
300 200 100 150 100 50 100 50 25
0.383–0.506
0.880–1.904
4.190–5.359
Covered distance (m) 313–237 209–158 104–79 68–31.5 45.5–21.0 22.7–10.5 9.5–7.5 4.8–3.7 2.4–1.9
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174 ing in the water column may represent a significant dispersal mechanism for the postlarval population at mesoscale (i.e. m to km) and change substantially the primary settlement pattern following larval dispersal (Günther, 1992). The consequences of this mechanism on Pectinaria koreni recruitment should depend on both the distance covered by postlarvae and juveniles, and the direction of the transport. The distance covered by an individual over a single resuspension event (D, in m) is a function of the postlarval position in the water column (h, in cm), its fall velocity (w, in cm s−1 ) and advective current velocity (v, in m s−1 ), and can be calculated as follows (Orth et al., 1994): D = (h/w) v. The different parameters of this equation have been estimated from field and laboratory experiments. Younger and smaller drifting postlarvae are mainly located within the 3 m above the bottom while the older and larger ones are confined within the last meter above the seabed (Olivier et al., 1996a; Thiébaut et al., 1996). The averaged postlarval fall velocity ranges between 0.383 cm s−1 for smaller organisms and 5.359 cm s−1 for larger ones (Olivier, 1997). Assuming an advective current velocity of 0.4 m s−1 (i.e. averaged tidal current velocity during the flood-peak), the distances covered by a juvenile over a single resuspension event are estimated for different size classes (Table 3). These calculations highlight that spatial scales of postlarval drifting vary greatly from few hundreds meters for smaller postlarvae to few meters for larger juveniles. Thus, just young postlarvae may be subject to a consequent transport by thread drifting over successive resuspension events at the scale of the eastern Bay of Seine. For old ones, this mechanism may only be involved in the exploration of prospective habitats or in response to disturbance at local scale. The direction of the transport changes according to the main hydrodynamical factors involved in postlarval drifting. In calm weather conditions, the relationship between postlarval resuspension and semi-diurnal tidal currents would favour a regular and landward transport during the flood and could lead over several tidal cycles to a migration of young postlarvae from unsuitable offshore bottoms towards coastal muddy fine sands, on a distance of several km. Such a migration could act as a feed-back mechanism which partly counteracts the demographic effects of larval dispersal on the recruitment. Conversely, the swell related to moderate or strong winds induces a near permanent resuspension of postlarvae independently of the tidal cycle and generates an episodic transport which some-
times strengthens or sometimes is opposed to the tidal effect. Notwithstanding the macrotidal regime of the Bay of Seine, it is noteworthy that wind events are the main source of variability of Pectinaria koreni recruitment, acting both on larval and postlarval phases. Although their episodic effects seem insufficient to prevent the persistence of the adult population, they could partly disrupt larval retention or induce an uncertain postlarval dispersal. The crucial role of wind events on recruitment in a macrotidal area has been already suggested by Luczak et al. (1993) in the Southern Bight of the North Sea. These authors argued that larvae of the american jack knife clam Ensis directus have colonised the Dover Strait from the Belgian or Dutch coasts in 1991. Larvae were transported to the south-west after a period of predominantly northerly winds, in the opposite direction to the tidal residual circulation which reach about 10 cm s−1 in this region. In the eastern Bay of Seine, the persistence of adult populations with a low year-to-year variability of their abundance has also been reported for most species of the Abra alba-Pectinaria koreni community like the polychaete Owenia fusiformis which remained the dominant species of the community between 1986 and 1991 (Dauvin & Gillet, 1991; Thiébaut et al., 1997). The comparison of biological properties between Pectinaria koreni and Owenia fusiformis reveals how different processes involved in the recruitment may ensure the maintenance of adult populations in the same hydrodynamical environment (Table 4). Although wind can greatly modify the scheme of residual circulation, its frequent changes in speed and direction vanish partly its effects on long time scales in comparison with the permanent tide action (Salomon, 1991). Therefore, its consequences on larval dispersal should depend on larval life duration so that larval transport is principally governed by wind for Pectinaria koreni and by tidal residual circulation for Owenia fusiformis. The dominant role of tide for the latter species would limit the likelihood of a total export of larval population to offshore unsuitable substrates (Thiébaut et al., 1994). Nevertheless, the analysis of Owenia fusiformis population structure between 1986 and 1988 have shown significant year-to-year variability in juvenile abundance so that yearly high recruitment alternated with yearly moderate or low recruitment (Dauvin & Gillet, 1991). Thus, the long life-span and the multivoltine reproduction of Owenia fusiformis may be considered as analogous to the multiple spawning events per year of Pectinaria koreni to
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175 Table 4. Comparison of main biological properties of Pectinaria koreni and Owenia fusiformis larvae, postlarvae and adults, and their consequences on the recruitment in relation to hydrodynamical factors. 1 M´enard et al. (1989); 2 Gentil et al. (1990); 3 Thi´ebaut et al. (1992); 4 Dauvin (1992). Biological properties
Pectinaria koreni
Owenia fusiformis
Consequences on the recruitment
Life-span Reproduction Nb of spawning events/year
12–18 months univoltine high (> 3–4)
3–4 y1 multivoltine2 low (∼ 1–2) 2
Larval life duration
11–13 d
∼ 1 month 3
Larval vertical migration
ontogenic
ontogenic3
Larval habitat selection
no
no4
Postlarval behaviour
postlarval drifting
no postlarval drifting4
Multiple spawning events over one or several years for P. koreni and O. fusiformis respectively increase the probability that at least one larval cohort generates a high recruitment during the life-span. As the effects of wind-induced currents on the residual circulation decreases with time, dispersal of P. koreni larvae is more sensitive to wind events than dispersal of O. fusiformis larvae. For both species, interactions between ontogenic vertical migration of larvae and two-layer estuarine circulation could favour larval retention off the Seine estuary. At the scale of eastern Bay of Seine, primary settlement of both the species is related to larval supply and results of a passive deposition; it is independent of sediment grain size. While O. fusiformis settlement is near definitive, behaviour of P. koreni postlarvae induces a postlarval drifting which may change primary settlement pattern and generate a net landward transport.
increase the probability that at least one larval cohort permit a high recruitment during the life-span of both the species.
Acknowledgements This study forms a part of the contribution of the ‘GDR Manche’ framework to the French Program on the Determinism of the Recruitment (PNDR-GLOBEC France). It was supported by the CNRS and IFREMER.
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