© 2003 by ICES/CIEM International Council for the Exploration of the Sea/Conseil International pour l'Exploration de la Mer
Ichthyoplankton distribution and plankton production related to the shelf break front at the Avilés Canyon
Departamento of B.O.S., Universidad de Oviedo c/Catedrático Rodrigo Uría s/n, Oviedo 33071, Spain
*Correspondence to R. González-Quirós. e-mail: rafaelg{at}correo.uniovi.es.
The overall objective of this study was to search for spatial differences in primary production and its transference towards larval fish related with the distribution of water masses at shelf break of the Avilés Canyon. High primary production and ichthyoplankton abundance were associated with a shelf break front at the Avilés Canyon. Egg distributions of Scomber scombrus, Sardina pilchardus and Trachurus trachurus, coupled with topography, the associated physical structure and phytoplankton productivity, suggested adaptive spawning strategies. The distribution of copepod stages, which are considered the trophic link between primary producers and fish larvae, was not related with the position of the front. Moreover, the egg production of two abundant species (Calanus helgolandicus and Acartia clausi) was not significantly related with phytoplankton abundance and productivity.
Keywords: shelf break, canyon, primary production, food web, ichthyoplankton
Received 22 July 2002; accepted 14 January 2003.
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Introduction |
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Fronts and and physical dynamic processes related with the shelf break have been considered as the cause of localized high plankton biomass. It has been widely debated whether this is a consequence of aggregation processes, an increase in plankton productivity, or both, caused by mesoscale physical dynamics (e.g. Le Fèvre and Frontier, 1988). Increased primary production has been related with different shelf break related mesoscale processes such as divergences between slope currents and coastal waters (Munk et al., 1995) and internal waves (Holligan et al., 1985). The distribution of different groups included in the mesozooplankton has been related with the distribution of water masses (e.g. Fernández et al., 1993; Gil et al., 2002). In addition, there is considerable evidence suggesting a relationship between the distribution pattern of ichthyoplankton and the shelf break, that for many species implies an increase in their abundance and for others a limit of their distribution (e.g. Sabatés and Masó, 1990; O'Brien and Fives, 1995).
Physical retention and food availability for larval fish have been proposed as two of the major factors that affect survival during early stages (Cushing, 1975; Iles and Sinclair, 1982). As a consequence, physical and biological processes occurring at the shelf break are thought to be important for the regulation of some fish populations, particularly because their recruitment is dependent on the dynamics during early life stages. In this context, spawning strategies of many species have been related to those processes and interpreted as adaptive responses that favour survival during early life stages (Checkley et al., 1988; Coombs et al., 1990).
The structure and dynamic of the food web are critical in the way primary production is channelled towards higher trophic levels. Prey availability for larval fish may be at least partially dependent on this transference, although other factors such as turbulence (MacKenzie and Leggett, 1991) and prey aggregation (Lasker, 1975) are also important. The different stages of copepods are the main component of the diet during the ontogenetic development of most pelagic fish (Last, 1980). Therefore, the availability of prey for fish larvae will at least partially depend on the rate of transference of primary production through this group of plankton organisms. Many studies have examined the influence of mesoscale processes on the distribution and activity of different organisms and trophic levels, but few have included primary producers, secondary producers and fish larvae (Kiørboe et al., 1988; Munk et al., 1995).
In this study we have searched for spatial patterns of trophic pathways in relation with the mesoscale distribution of water masses, based on the analysis of a variety of physical, chemical and biological variables. This approach includes the estimation of phytoplankton, copepods and larval fish abundance, but also primary production and copepod female egg production as productivity rate indices of the first and second trophic levels. In this way, we would be able to formulate hypotheses about the significance of mesoscale physical dynamics on plankton production and how this production may be channelled towards larval fish. In addition, the distribution of early life stages of fish, in view of the distribution of water masses, would provide evidences for hypothesis formulation about which physical processes may affect larval retention.
Our study was carried out at a shelf area indented by a submarine canyon. These topographic features have been related with the enhancement of primary production (Shea and Broenkow, 1982) and with the increase of abundance and biomass of several organisms from different trophic levels (e.g. Stefanescu et al., 1994). However, as far as we know, there have been no studies that include a multi-trophic data set that analyses primary production transference to higher trophic levels.
In terms of the temporal framework, this study was carried out at a late bloom or post-bloom phase. In mid latitudes, spring phytoplankton blooms reduce inorganic nutrients in the photic layer as water column stratification increases. Thereafter, primary production is limited by nutrient concentration. Maximum zooplankton biomass presents a temporal lag with respect to the spring phytoplankton bloom. At post-bloom phases, when nutrients have been already depleted in the photic layer and zooplankton biomass is high, mesoscale physical processes that enhance primary production may be of special significance on intensifying trophic pathways towards higher trophic levels (e.g. larval fish).
The overall objective of this study is to determine the relationship between the physical structure of water masses and ichthyoplankton distribution and trophic pathways that may enhance larval fish food availability at the shelf break of the Avilés Canyon.
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Materials and methods |
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Area of study
This study was carried out in the coastal waters of the central Cantabrian Sea (southern Bay of Biscay, NW Spain) in May 1996. This area is characterised by a narrow continental shelf and the presence of the Avilés Canyon (Figure 1). In spring, the hydrography is dominated by eastward slope currents of high salinity and saline stratification in coastal waters that are influenced by freshwater runoff (Botas et al., 1988). Plankton distribution is greatly influenced by the distribution of these water masses and their associated fronts (Fernández et al., 1993).
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In the context of the annual cycle, total annual primary production is primarily due to the spring bloom and secondarily to coastal upwelling events (Serret, 1997). The spring phytoplankton bloom usually occurs earlier in the season, in April (Fernández and Bode, 1991), and coastal upwelling events are frequent during the summer (Botas et al., 1990), although they may sporadically occur in spring (Serret, 1997).
In the Cantabrian Sea, ichthyoplankton are more abundant in spring and in its central area Sardina pilchardus, Scomber scombrus and Trachurus trachurus are the dominant species (Solá et al., 1990).
Hydrography and ichthyoplankton
Vertical distribution of temperature, salinity and sigma-t and abundance of ichthyoplankton were determined on a grid of 24 stations distributed along four coast–ocean transects (Figure 1) between 7 and 15 May 1996. Temperature, salinity and density were measured by a SBE25 CTD. CTD probe failed at stations 18, 19 and 24 and no physical data from these sites are available. Wind data were obtained from the Asturias Airport meteorological observatory (Instituto Nacional de Meteorología, Ministerio de Medio Ambiente, Spain), which is located 3.5 km to the southeast of station 1. Wind speed and direction were recorded at 07:00, 13:00 and 18:00, every day during the sampling period.
Ichthyoplankton was sampled with a 40 cm diameter bongo net with a 200 µm mesh net. Bongo tows were 100 or 5 m above the bottom. Calibrated flowmeters were mounted on both bongo nets to calculate volume of water filtered and a depth monitor was used to determine maximum tow depth. Ship speed during bongo tows was maintained at 3–3.5 knots. Samples were preserved in tetraborate-buffered fresh water 4% formaldehyde. Only the abundance of the eggs and the larvae of Sardina pilchardus, Scomber scombrus and T. trachurus are reported. Larvae of Sardina pilchardus and Scomber scombrus from stations 1 to 19 were measured to the nearest 0.01 mm.
Nutrient concentration and plankton abundance and production
At selected stations (Figure 1), the vertical distribution of nitrate and chlorophyll-a concentration, as well as the abundance of copepod eggs, nauplii and copepodites were determined. At these stations, phytoplankton production and copepod egg production rates of Acartia clausi and Calanus helgolandicus were also estimated. Copepod egg production rates are interpreted as an index of the transfer of primary production to the prey for fish larvae.
Water samples for nitrate and chlorophyll-a analysis were collected with 5 l Niskin bottles at 0, 10, 20, 30, 40, 50, 75, 100, 150 and 200 m. Nutrient concentration was measured with a Technicon Autoanalizer II (Grasshoff et al., 1983). To determine chlorophyll-a concentration, 200 ml were filtered through Whatman GF/F that were subsequently frozen. Chlorophyll-a concentration was measured using a Turner Designs 10 fluorometer after extraction in 90% acetone for 24 h at 4°C (Yentsch and Menzel, 1963).
Primary production was estimated at 10 m intervals from 0 to 50 m through determination of 14C uptake by phytoplankton. Three replicates of 125 ml were inoculated with 370 kBq (10 µCi) of NaH14CO3, and placed in outdoor water-cooled incubators for 24 h (14/10 light/dark) at irradiance that simulated in situ conditions. Subsequently, samples were filtered through Whatman GF/F filters that were immediately frozen and stored at –20°C until analysis. Radioactive response was measured in a liquid scintillation Packard counter, after addition of Optiphase Hi-safe scintillation liquid.
Copepod nauplii and eggs abundance was calculated from water samples collected with 5 l Niskin bottles at 0, 10 and 40 m (or maximum depth) at coastal stations and at 0, 20 and 50 m at shelf break stations. Water samples were filtered through a 30 µm mesh net and the retained material was washed with Whatman GF/F filtered sea water to a final volume of 125 ml and preserved in 2.5% tetraborate-buffered formaldehyde. Abundance of copepodites was determined from the same bongo net samples as for the ichthyoplankton. Data on the abundance of ciliates were taken from Quevedo and Anadón (2000) and their integrated abundance was calculated.
Egg production of C. helgolandicus and A. clausi was determined as an index of secondary production following the method of Runge (1985). Females were collected by vertical WP2 tows from 100 m to surface and transferred to the laboratory in an insulated box filled with surface water from the same station within 1.5 h. In the laboratory, actively swimming females of both species were selected under a stereomicroscope and incubated in 250 ml crystallising glasses with surface water from the same station of collection and filtered through a 100 µm mesh net. For C. helgolandicus there were 5–10 replicates with one female in each and for A. clausi 7–10 replicates with three females in each. Incubations were maintained in a temperature-controlled chamber at 15°C for 24 h (14/10 light/dark). At the end of the incubation, the total number of eggs in each replicate was counted.
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Wind conditions
Mean wind speed during the sampling period was 3.16 (sd=2.12) m s–1 and maximum speed (8.3 m s–1) was observed on day 13 at 18:00. Wind direction alternated between westerly and easterly winds in an approximate periodicity of 2 days (Table 1). The Cantabrian Coast presents an East–West orientation, therefore this wind alternating pattern resulted in corresponding positive and negative Ekman transport periods. Westerly winds dominated on 7–8 May (mean=2.87; sd = 1.35 m s–1) and 11–12 May (mean=1.95; sd =1.52 m s–1), whereas easterly winds dominated on 9–10 May (mean=3.10; sd =1.45 m s–1) and 13–15 May (mean=4.26; sd =2.87 m s–1).
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The vertical pattern in temperature indicated the onset of thermal stratification, about 2°C in the upper 100 m (Figure 2a–c). Low surface salinity values at the coast (stations 1 and 7) reflected the influence of fresh water runoff associated with the plume of the River Nalón (Figure 2d–f). Offshore, there were also low surface salinity values, although not as low as in coastal waters. A body of high salinity (>35.60) was observed in subsurface waters offshore with maximum values at the shelf break. A front was located at the shelf break and characterised by the presence of a dome of low temperature and high salinity. This feature separated surface saline stratified waters at the coast and offshore, and was coincident with the coastal limit of the high salinity water body. The vertical distribution of sigma-t showed a general deepening of the isopicnals towards the coast above 100 m that was more apparent at the shelf break (Figure 2g–i).
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The distribution of sigma-t at 20 m reflects the horizontal distribution of the front (Figure 3). A water mass of low density located offshore reflected the shape of the Avilés Canyon but displaced to the southeast about 2 miles and water of low density also occurred close to the coast. Between both water bodies of low density, there was water of high density with maximum values at stations 4, 9, 15 and 20.
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The eggs and the larvae of Scomber scombrus, Sardina pilchardus and T. trachurus represented the majority of the ichthyoplankton observed in this study (>77% of the eggs and >94% of the larvae). T. trachurus accounted for 66.4% of the total number of fish eggs collected and Scomber scombrus and Sardina pilchardus dominated the larvae, at 38.3 and 34.2% of total larvae, respectively. A single exception to this pattern was observed at station 19, where larvae of the family Mictophydae were more abundant.
The eggs of Scomber scombrus, Sardina pilchardus and T. trachurus were aggregated in distribution with maximum abundance at the frontal area (Figure 4a–c). Maximum abundance of Scomber scombrus eggs in each transect followed the shelf break at the Avilés Canyon (Figure 4a). The distribution of Sardina pilchardus and T. trachurus eggs was similar but slightly displaced shoreward (Figure 4b, c). The larvae of Scomber scombrus and Sardina pilchardus presented higher dispersion and they are more abundant than their respective eggs at the coast (Figure 4d, e). However, they are also abundant at the frontal area and present low abundance offshore. Low abundance of T. trachurus larvae in relation with the eggs prevents a comparison on the distribution of both stages.
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Scomber scombrus larval size ranged from 2.07 to 10.88 mm, Sardina pilchardus from 2.45 to 20.45 mm and T. trachurus from 1.88 to 7.8 mm. No clear pattern in the frequency distribution of larval size was observed in Sardina pilchardus and Scomber scombrus (Figure 5). Nonetheless, the higher proportion of larger larvae of both species was coincident in the same station. Larval size of T. trachurus larvae was not analysed because of very low abundance at several stations (Figure 4f).
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Phytoplankton production and biomass and nitrate concentration
High values of integrated primary production were observed at shelf stations at the frontal area close to the shelf break and low values at coastal stations and at station 19 over the Avilés Canyon (Figure 6). Station 1 (coastal station with high values for primary production and chlorophyll-a) was an exception to this general pattern. This station is affected by the river plume of the River Nalón, and NO3– concentrations were elevated at the surface (Figure 7a). The integrated chlorophyll-a showed a similar pattern to primary production although less obviously (Figure 6). The variables were significantly correlated (r=0.84; p<0.01; n=10). The vertical distribution of primary production indicated maximum values at the surface with the exception of stations 10 and 14 where a subsurface maximum was observed at 10 m and at 20 m for station 19 (e.g. Figure 7). Chlorophyll-a vertical distribution followed a pattern similar to the one observed for primary production for every station (e.g. Figure 7), except station 14, where chlorophyll-a concentration was fairly through out the water column.
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Low concentrations of nitrate were observed in the photic layer except at stations 1 (Figure 7a) and 7, which were affected by the river plume. Surface concentration of nitrate was below 1 µmol NO3– kg–1 everywhere except these stations. At station 14, which was also near the coast, the concentration of nutrients was below 0.5 µmol NO3– kg–1 throughout the sampled portion of water column. Offshore, there was a nutricline between 30 and 60 m (e.g. stations 4, 10, 17 and 19; Figure 7). Station 4 was the only station coincident with the dome of subsurface water where the concentration of nutrients was measured and the only station offshore where concentrations >1 µmol NO3– kg–1 were observed in the upper 20 m.
Copepod abundance and egg production of A. clausi and C. helgolandicus
The distribution of copepod eggs, nauplii and copepodites (Figure 8) was unrelated to the position of the shelf break front (Figure 3) or to the distribution of primary production and phytoplankton biomass (Figure 6). Moreover, the abundance of copepod eggs, nauplii and copepodites were highly variable among stations (Figure 8) and there was no clear pattern in their distribution. Only the abundance of copepod eggs and copepod nauplii were significantly correlated (Table 2).
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A. clausi and C. helgolandicus females accounted for more than 10% of total copepod adult females except at station 19. Their abundance (Table 3) was not related to primary production, integrated chlorophyll-a or abundance of ciliates (Table 4). In addition, the individual egg production rate of A. clausi and C. helgolandicus females (Table 3) was not related to primary production, chlorophyll-a or the abundance of ciliates (Table 4). The individual egg production rate of each species was not significantly correlated (r=0.36; p=0.31; n=10) and no spatial pattern was observed. As a consequence, the values of the population egg production at each station were not related to the position of the front, primary production, chlorophyll-a or the abundance of ciliates.
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Discussion |
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Environmental forcing
The study of the spatial distribution of physical and biological variables at the coastal edge of the Avilés Canyon required a short distance between transects and stations. Research vessel characteristics and methodological constraints (i.e. number of stations for which primary production and copepod egg production can be estimated) limited the number of stations that could be sampled per day. During the sampling period (9 days) horizontal and vertical water movements may have affected the horizontal and vertical distribution of physical and biological variables, so the described patterns in this study could differ from the real distribution during the sampling period. However, all stations, except coastal stations at transect 4 (20 and 21) were sampled within 7 days. Coastal stations were sampled from transect 1 to 3 in 2 days (May 7 and 8) and off-shore stations were sampled from transect 1 to 4 in 3 days (from May 11 to 13). In the Cantabrian Sea, May can be considered a month of transition from dominant westerly currents in spring to dominant easterly currents in summer. In fact, Gil et al. (2002) observed easterly dominant currents and Gonzalez-Quirós et al. (submitted) observed westerly dominant currents in May in different years. In our study, the deepening of the isopicnals towards the coast suggests a westerly circulation pattern. Under this situation Gonzalez-Quirós et al. (unpublished data) observed maximum geostrophic currents of 8 cm s–1. These would mean that plankton organisms could be advected from one transect to the next one in less than 1 day.
Other than geostrophic circulation, winds were weak and parallel to the coast during the study period. During the first 6 days wind speed was seldom over 5 m s–1 and its direction varied from westerly dominant to easterly dominant with a 2 day periodicity. Consequently, wind driven Ekman transport was not strong and it varied from positive to negative values. Only at the end of the survey, from day 13 to 15, there were moderate winds, most records over 5 m s–1 and with a maximum of 8.34 m s–1. These stronger easterly winds, which cause coastal upwelling in this area, may have been responsible for the observed high density water at stations 20 and 21 (reflected in the sigma-t distribution at 20 m; Figure 3) that were sampled on day 15. However, off-shore stations in transect 4 were sampled before or just at the beginning of this moderately strong wind period.
Nevertheless, the distribution pattern of water masses was consistent in transects 1 to 3 and the distribution pattern of several biological variables seemed to be related with the topography of the Avilés Canyon despite West–East advection that may have occurred during the survey. In terms of coast–ocean advection, it does not seem that the wind conditions would have affected distribution patterns to a large extent.
Aggregation of ichthyoplankton at the shelf break front
In this study, the distribution of eggs and larvae of Scomber scombrus, Sardina pilchardus and T. trachurus was spatially coincident with the shelf break front and the associated high phytoplankton biomass and productivity. The abundance of eggs and larvae of these species was within the highest values observed in the Cantabrian Sea (Solá et al., 1990, 1992) and in other spawning areas in the northeast Atlantic (Haynes and Nichols, 1994; Horstman and Fives, 1994). Other studies have shown that the general spawning pattern of these species usually presents spatial segregation in relation to the coast–ocean gradient [in this area (González-Quirós, 1999), in other areas of the Cantabrian Sea (López-Jamart et al., 1995; Porteiro et al., 1996) and in the northeast Atlantic (Arbault and Lacroix, 1971; Coombs et al., 1990; Horstman and Fives, 1994)]. Namely, Scomber scombrus spawning primarily takes place at the shelf break, T. trachurus over the shelf and Sardina pilchardus mainly close to the coast. Although fish eggs and larvae may have been advected from elsewhere or aggregated by physical processes, the similar distribution of the three species relative to the front suggests adaptative spawning strategies in response to the environmental conditions.
Several studies have suggested the role of physical processes that occur at shelf break fronts as retention mechanisms for larval fish by favouring aggregation within the spawning area (Munk et al., 1995) or drifting towards the coast (Koutsicopoulos et al., 1991; Walsh et al., 1996). The reduced spatial and temporal framework of our study does not allow determining the impact that the observed shelf break front may have on larval retention. However, the distribution of the larvae of the three species in relation to the distribution of their eggs suggests that the front may be a mechanism of retention (sensu Sinclair, 1988) that would avoid the dispersion of the larvae offshore. Although larvae of Scomber scombrus and Sardina pilchardus were more coastal in distribution than their egg stages, the patterns in larval length do not suggest larval drift towards the coast. High coastal abundance of Sardina pilchardus larvae could be a consequence of previous spawning that frequently occurs onshore (Arbault and Lacroix, 1971; González-Quirós, 1999). In the case of Scomber scombrus spawning has been shown to be more intense at the shelf break (Coombs et al., 1990; González-Quirós, 1999), although coastal spawning cannot be discounted in an area with a shelf as narrow as that observed in the Cantabrian Sea. Further studies should analyse the spatial magnitude of the shelf break front in relation to the distribution of early stages along the Cantabrian Coast, its temporal persistence and its effect on larval drifting.
Enhanced productivity at the shelf break
Our study was carried out during a post-bloom situation when surface thermal stratification had already developed and nutrients were depleted in the photic layer. Under these conditions, primary production in the photic zone is limited by the availability of nutrients. Primary production and phytoplankton biomass observed at coastal stations and offshore at the Avilés Canyon are similar to other reported values during periods of low productivity in the annual cycle (Fernández and Bode, 1991), with the exception of the coastal station directly affected by the river plume. In contrast, primary production at the frontal area is within the values observed during the spring phytoplankton bloom in this area (Fernández and Bode, 1991), although phytoplankton biomass was relatively low. The dome of high density water and the associated hydrodynamic processes at the front may enhance input of nutrients into the photic layer that are responsible for the observed high primary production. Unfortunately, the spatial resolution of nutrient sampling is insufficient to address this hypothesis, given that nutrients were only sampled at one station coincident with the dome of water of high density in relation to other stations at the shelf break area.
The upwelling of subsurface water at the front may result from a divergence caused by the displacement of two adjacent water masses with different velocities. Munk et al. (1995) observed high values of phytoplankton biomass and primary production associated with a shelf break front with a dome of subsurface water between the Norwegian Coastal Current and the Jutland Coastal Current. Checkley et al. (1988) observed elevated production near the Gulf Stream associated with upwelling of subsurface water at the shelf break. In our study, the coincidence of the dome of subsurface water at the coastal boundary of the high salinity water mass suggests that a similar process may cause the observed high primary production and phytoplankton biomass. Alternatively, high primary production may have been generated by the interaction between tidal internal waves and the shelf break. At the north of the Bay of Biscay, Pingree et al. (1986) observed that internal waves provoked an increase of vertical mixing at the shelf break that increases nutrients and decreases surface temperature, which caused high phytoplankton biomass (Holligan et al., 1985). The position of the front in relation to the Avilés Canyon suggests that this topographical feature is related to the dynamic of the front and the observed increase in primary production.
We cannot discern which dynamic process is the ultimate cause of the increase in primary production associated with the shelf break and the topography of the Avilés Canyon. Nevertheless, it may be a significant contribution to the productivity of the ecosystem that has not been considered in previous studies that have focused on the phytoplankton bloom and the coastal upwelling in this area (Botas et al., 1990; Fernández and Bode, 1991; Serret, 1997).
Transference of high primary production through the trophic chain
Copepods are considered to be the major trophic link between phytoplankton production and fish larvae, given that the different herbivorous copepod stages are the main feeding resource for most pelagic fish larvae. Le Fèvre and Frontier (1988) argued that the structure of the trophic chain, not only the general productivity of the ecosystem, is regulated by the temporal pattern of energy transference at the ergoclines. In particular, they argued that at shelf break fronts generated by internal waves with an approximate periodicity of 12 h, there is a higher transfer of energy through the classic trophic chain. However, the decoupling observed here between copepod abundance (eggs, nauplii and copepodites) and phytoplankton biomass and production, may merely represent a temporal lag between maximum peaks of abundance of phytoplankton and zooplankton in temperate seas. The observed abundance of zooplankton may be a consequence of the transfer from primary production during the phytoplankton bloom earlier in the season, particularly given that nutrients in surface waters were already depleted and that the phytoplankton bloom usually occurs in April in this area (Fernández and Bode, 1991). Moreover, other factors that are important to zooplankton population regulation, such as advection (e.g. Checkley et al., 1988) and mortality caused by predation (Kiørboe, 1998), may be partially responsible for decoupling.
It has been suggested that the rate of individual egg production of herbivorous copepods could be interpreted as an index of the rate at which primary production is channelled through this group and becomes available for fish larvae (Runge, 1988). Moreover, variability of egg production for the genera Acartia and Calanus may be highly dependent on phytoplankton availability (Runge, 1988). Under these assumptions, when these zooplankton species are dominant, a close relationship between phytoplankton biomass and the availability of food for fish larvae may be expected. Two possibilities can explain the absence of a correlation between phytoplankton biomass and egg production for both species in this study. The first is that the concentration of chlorophyll-a is not a good index of phytoplankton availability for females. Most of the phytoplankton biomass may exceed the range of particle sizes suitable for copepod feeding (Berggreen et al., 1988; Huntley, 1988). The second possibility is that availability of phytoplankton and the production of copepods may be blurred by other factors, namely: (1) egg production is temperature dependent (Huntley and Lopez, 1992); (2) microzooplankton is an important source of energy for species previously considered strict herbivorous (Runge and De Lafontaine, 1996) and (3) egg production may be dependent on energetic reserves. For example, Calanus finmarchicus egg production is partly dependent on reserves accumulated prior to diapause in the previous winter (Richardson et al., 1999). Although C. helgolandicus adults in May in the Cantabrian Sea may not correspond to cohorts that have gone through diapause, reserves may also play a significant role on a shorter time scale.
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High primary production was associated to the shelf break front and the topography of the canyon. The study of the physical process that causes this coupling, and its spatial and temporal variability, will be necessary to understand its consequences on the pelagic food web. Nevertheless, mesoscale processes that enhance primary production at post-bloom phases characterised by high zooplankton biomass may be of special significance on the energy transference to high trophic levels. In this sense, high spawning intensity coincident with high primary production at the shelf break may be interpreted as an adaptive response to favourable conditions for early life stages. On the contrary, the abundance and female egg production of copepods, the ‘trophic link’ between phytoplankton and fish (sensu classical food chain), were not related with the spatial pattern of primary production, which suggests that high production at the frontal area is not apparently channelled through the trophic chain towards larval fish. However, uncertainties about the factors that regulate copepod population dynamics and female egg production do not allow addressing this hypothesis. In addition, alternative trophic pathways may play a significant role on this transference: fish larvae consumption of appendicularians [e.g. Scomber scombrus (Last, 1980)] and protozoans (Fukami et al., 1999), which primarily graze small size phytoplankton or, alternatively, through copepod production that may also depend on protozoans (Runge, 1996) and appendicularians (López-Urrutia et al., unpublished).
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Acknowledgements |
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We want to thank the crew of the RV ‘Jose Rioja’ from the I.E.O. for their help during sampling procedures. This work and the contract for J. C. was supported by funds of FICYT Proyect PB-REC 9505 (Gobierno del Principado de Asturias; Spain). A fellowship to R. G.-Q. was provided by the Consejería de Agricultura y Pesca (Gobierno del Principado de Asturias; Spain). Critical comments of two anonymous referees and the editor, Pierre Pepin, have greatly improved the present manuscript.
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References |
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Arbault S and Lacroix N. (1971) Aires de ponte de la sardine, du sprat et de l'anchois dans le Golfe de Gascogne et sur le plateau celtique résultats de 6 annes d'étude. Revue des Travaux Institute des Pêches Maritimes 35:35–56.
Berggreen U, Hansen B, Kiørboe T. (1988) Food size spectra, ingestion and growth of the copepod Acartia tonsa during development: implications for determination of copepod production. Marine Biology 99:341–352.[CrossRef]
Botas J.A, Bode A, Fernández E, Anadón R. (1988) Descripción de una intrusión de agua de elevada salinidad en el Cantábrico central: distribución de los nutrientes inorgánicos y su relación con el fitoplancton. Investigación Pesquera 52:561–574.
Botas J.A, Fernández E, Bode A, Anadón R. (1990) A persistant upwelling off the Central Cantabrian Coast (Bay of Biscay). Estuarine Coastal and Shelf Science 30:185–199.[CrossRef]
Checkley D.M, Raman J, Maillet S.G.L, Mason K.M. (1988) Winter storm effects on the spawning and larval drift of a pelagic fish. Nature 335:346–348.[CrossRef]
Coombs S.H, Aiken J, Griffin T.D. (1990) The aetiology of mackerel spawning to the west of the British Isles. Meeresforchung 33:52–75.
Cushing D.H. (1975) Marine Ecology and Fisheries(Cambridge University Press, Cambridge) 278 pp.
Fernández E and Bode A. (1991) Seasonal patterns of primary production in the Central Cantabrian Sea (Bay of Biscay). Scientia Marina 55:629–636.
Fernández E, Cabal J, Acuña J.L, Bode A, Botas A, García-Soto C. (1993) Plankton distribution across a slope current-induced front in the southern Bay of Biscay. Journal of Plankton Research 15:619–641.
Fukami K, Watanabe A, Fujita S, Yamaoka K, Nishijima T. (1999) Predation on naked protozoan microzooplankton by fish larvae. Marine Ecology Progress Series 185:285–291.[Web of Science]
Gil J, Valdés L, Moral M, Sánchez R, García-Soto C. (2002) Mesocale variability in a high-resolution grid in the Cantabrian Sea (southern Bay of Biscay), May 1995. Deep-Sea Research I 49:1591–1607.[CrossRef]
González-Quirós R. (1999) Distribución del ictioplancton en el Cantábrico central y ecología alimentaria de Micromesistius poutassou (Risso 1826). PhD thesis, Departamento de Biología de Organismos y Sistemas, Universidad de Oviedo, Oviedo: 204 pp.
Grasshoff K, Ehrhardt M, Kremling M. (1983) Methods of Seawater Analysis 2nd edition (Verlag Chemie, Weinheim) 419 pp.
Haynes G.M and Nichols J.H. (1994) Pilchard (Sardina pilchardus, Walbaum) egg distribution in the English Channel from plankton surveys in 1978, 1981, 1988 and 1991. Journal of Plankton Research 16:771–782.
Holligan P.M, Pingree R.D, Mardell G.T. (1985) Oceanic solitons, nutrient pulses and phytoplancton growth. Nature 314:348–350.[CrossRef]
Horstman K.R and Fives J.M. (1994) Ichthyoplankton distribution and abundance in the Celtic Sea. ICES Journal of Marine Science 51:447–460.
Huntley M. (1988) Feeding biology of Calanus: a new perspective. Hydrobiologia 167/168:83–99.
Huntley M.E and Lopez M.D.G. (1992) Temperature-dependent production of marine copepods: a global synthesis. American Naturalist 140:201–242.[CrossRef][Web of Science]
Iles T.D and Sinclair M. (1982) Atlantic herring: stock, discreteness and abundance. Science 215:627–633.
Kiørboe T. (1998) Population regulation and role of mesozooplankton in shaping marine pelagic food webs. Hydrobiologia 363:13–27.[Web of Science]
Kiørboe T, Munk P, Richardson K, Christensen V, Paulsen H. (1988) Plankton dynamics and larval herring growth, drift, and survival in a frontal area. Marine Ecology Progress Series 44:205–219.[Web of Science]
Koutsicopoulos C, Fortier L, Gagne J.A. (1991) Cross-shelf dispersion of Dover sole (Solea solea) eggs and larvae in Bay of Biscay and recruitment to inshore nurseries. Journal of Plankton Research 13:923–945.
Lasker R. (1975) Field criteria for survival of anchovy larvae: the relation between inshore chlorophyll maximum layers and successful first feeding. Fishery Bulletin 73:453–462.[Web of Science]
Last J.M. (1980) The food of twenty species of fish larvae in the west-central North Sea. Fisheries Research Technical Report 60:1–44.
Le Fèvre J. and Frontier S. (1988) Influence of temporal characteristics of physical phenomena on plankton dynamics, as shown by North-West European marine ecosystems. In Rothchild B. J. (Ed.). Toward a Theory on Biological–Physical Interactions in the World Ocean pp. 245–272.
López-Jamart E, Coombs S.H, García A, Halliday N.C, Knust R, Nellen W. (1995) The distribution and survival of larvae of sardine Sardina pilchardus (Walbaum, 1792) off the north and north-western Atlantic coast of the Iberian Peninsula, in relation to environmental conditions. Boletín del Instituto Español de Oceanografía 11:27–46.
Mackezie B.R and Leggett W.C. (1991) Quantifying the contribution of small-scale turbulence to the encounter rates between larval fish and their zooplankton prey: effects of wind and tide. Marine Ecology Progress Series 73:149–160.[Web of Science]
Munk P, Larsson P.O, Danielsen D, Moksness E. (1995) Larval and small juvenile cod Gadus morhua concentrated in the highly productive areas of a shelf break front. Marine Ecology Progress Series 125:21–30.[Web of Science]
O'Brien B and Fives J.M. (1995) Ichthyoplankton distribution and abundance off the west coast of Ireland. ICES Journal of Marine Science 52:233–245.
Pingree R.D, Mardell G.T, New A.L. (1986) Propagation of internal tides from the upper slopes of the Bay of Biscay. Nature 321:154–158.[CrossRef]
Porteiro C., Cabanas J., Valdés L., Carrera P., Franco C., Lavín A. (1996) Hydrographic features and dynamics of blue whiting, mackerel and horse mackerel in the Bay of Biscay, 1994–1996. A multidisciplinary study on SEFOS ICES CM 1996/S: 13, 29 pp.
Quevedo M and Anadón R. (2000) Spring microzooplancton composition, biomass and potential grazing in the central Cantabrian coast (southern Bay of Biscay). Oceanologica Acta 23:297–309.[CrossRef][Web of Science]
Richardson K, Jónasdóttir S.H, Hay S.J, Christoffersen A. (1999) Calanus finmarchicus egg production and food availability in the Faeroe-Shetland Channel and northern North Sea: October–March. Fisheries Oceanography 8:Suppl. 1, 153–162.
Runge J.A. (1985) Relationships of egg production of Calanus pacificus to seasonal changes in phytoplankton availability in Puget Sound, Washington. Limnology and Oceanography 30:382–396.[Web of Science]
Runge J.A. (1988) Should we expect a relationship between primary production and fisheries? The role of copepod dynamics as a filter of trophic variability. Hydrobiologia 167/168:61–71.
Runge J.A and De Lafontaine Y. (1996) Characterization of the pelagic ecosystem in surface waters of the northern Gulf of St. Lawrence in early summer: the larval redfish-Calanus-microplankton interaction. Fisheries Oceanography 5:21–37.[Web of Science]
Sabatés A and Masó M. (1990) Effect of a shelf-slope front on the spatial distribution of mesopelagic fish larvae in the western Mediterranean. Deep-Sea Research 37:1085–1098.
Serret P. (1997) Balances de producción y consumo de oxígeno por el microplancton en la capa fótica. PhD thesis, Departamento de Biología de Organismos y Sistemas, Universidad de Oviedo, Oviedo: 208 pp.
Shea R.E and Broenkow W.W. (1982) The role of internal tides in the nutrient enrichment of Monterey Bay, California. Estuarine Coastal and Shelf Science 15:57–66.[CrossRef]
Sinclair M. (1988) Marine Populations: An Essay on Population Regulation and Especiation(Washington Sea Grant Program, Seattle) 252 pp.
Solá A., Motos L., Franco C., Lago de Lanzos A. (1990) Seasonal ocurrence of pelagic fish eggs and larvae in the Cantabrian Sea (VIIIc) and Galicia (IXa) from 1987 to 1989. ICES CM 1990/H: 25, 38 pp.
Solá A, Franco C, Lago de Lanzos A, Motos L. (1992) Temporal evolution of Sardina pilchardus (Walb.) spawning on the N–NW coast of the Iberian Peninsula. Boletín del Instituto Español de Oceanografía 8:97–114.
Stefanescu C, Morales-Nin B, Massuti E. (1994) Fish assemblages on the slope in the Catalan Sea (western Mediterranean): influence of a submarine canyon. Journal of the Marine Biological Association UK 74:499–512.
Walsh M., Skogen M., Reid D. G., Svensen E., McMillan J. A. (1996) The relationship between the location of western mackerel spawning, larval drift and recruit distributions: a modelling study. ICES CM 1996/S: 33, 35 pp.
Yentsch C.S and Menzel C.S. (1963) A method for the determination of phytoplankton by fluorescence. Deep-Sea Research I 10:221–231.
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