© 2006 International Council for the Exploration of the Sea
Changes in the mesozooplankton community associated with the hydrography off the northwestern Iberian Peninsula
a Departamento de Organismos y Sistemas (Zoología) Universidad de Oviedo, Calle Catedrático Rodrigo Uría s/n 33071 Oviedo, Spain
b Instituto Español de Oceanografía, Centro Oceanográfico de Gijón Camino del Arbeyal s/n, 33212 Gijón, Spain
*Correspondence to L. Blanco-Bercial: tel: +34 985 10 48 39; fax: +34 985 10 48 68. e-mail: leocadiobb.uo{at}uniovi.es.
Changes in the mesozooplankton community were investigated along the northwestern coast of the Iberian Peninsula during OMEX II cruises in August 1998, under upwelling conditions, and in October 1999, when there was a saline, warm poleward current flowing along the slope. Six stations, located at both the shelf break and in oceanic waters, were sampled for mesozooplankton by day and by night. Samples were grouped in accordance with their species abundance levels, and the resulting groups were compared with salinity, integrated chlorophyll a, and sea surface and average temperatures. The analysis discriminated stations from different cruises into different groups, and detected the influence of hydrographic structures. The abundance of species linked to high concentrations of Chl a and low temperature increased during the upwelling session, whereas species related to warmer water were more abundant during the 1999 cruise. Further, the analysis revealed differences in the mesozooplankton composition between coastal and oceanic sites, which were influenced to different degrees by the upwelling and the poleward current.
Keywords: Iberian Peninsula, mesozooplankton community, poleward current, upwelling
Received 18 August 2005; accepted 10 March 2006.
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Introduction |
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 Top Introduction Material and methodsResultsDiscussionReferences |
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Ocean margins, and the fluxes between shelf areas and open ocean that take place there, are important to the knowledge of the global biogeochemical cycle of carbon, nutrients, and trace elements (Wollast and Chou, 2001; Huthnance et al., 2002; Verity et al., 2002), but the processes are not as well known and understood as they should be. In recent decades many projects have been launched to correct this lack of knowledge, and one of them was the EU's OMEX II Project, focused on the northwestern Iberian Peninsula.
The hydrography NW of the Iberian Peninsula shows marked seasonal variability. Off NW Spain, windstress is directed southwards in summer, and upwelling develops (Tenore et al., 1995; Castro et al., 1997; Teira et al., 2001). The entrance into the euphotic zone of deep, nutrient-rich water associated with this upwelling enhances primary production (Joint et al., 2001b). Coastal systems under the influence of seasonal upwelling are characterized by changes in the zooplankton coupled to these events (Paffenhöfer et al., 1994; Mackas et al., 2001). Moreover, coastal upwelling can generate displacement of water masses in the form of filaments, exporting the plankton associated with them (Paffenhöfer et al., 1994; Halvorsen et al., 2001; Hitchcock et al., 2002). On the other hand, a warm, saline current flowing polewards along the slope appears to be the main hydrographic feature of the waters off NW Spain during autumn and winter (Frouin et al., 1990; Haynes and Barton, 1990; Pingree and Le Cann, 1990; Huthnance et al., 2002). At the same time, a reversal of the meridional component of the wind to a northward direction induces onshore Ekman transport (Frouin et al., 1990), and primary production decreases (Tenore et al., 1995). Previous studies have investigated the influence of this poleward current on phytoplankton and zooplankton biomass, and on metabolic rates both off NW Spain (Álvarez-Salgado et al., 2003; Huskin et al., 2003; Isla and Anadón, 2004) and in the central Cantabrian Sea (Fernández and Bode, 1991; Fernández et al., 1993). However, information on the mesozooplankton community composition remains limited. Another important source of seasonal changes comes from the continental freshwater inputs from the Rías Bajas (Álvarez-Salgado et al., 2003).
Changes in the environment affect the composition and structure of plankton communities, and therefore the ecosystem dynamics (Paffenhöfer, 1980; Landry et al., 2001). Consequently, given that mesoscale hydrographic events, and the interactions and exchanges between open ocean and continental shelf waters, affect the spatial distribution and composition of mesozooplankton, environmental changes play an essential role in biogeochemical cycles (Pinca and Dallot, 1995; Pakhomov and Perissinotto, 1997; Grunewald et al., 2002; Siokou-Frangou et al., 2004). Moreover, shelf and oceanic sites have different zooplankton compositions (Siokou-Frangou et al., 1998; Gaard, 1999; Morgan et al., 2003) that can be influenced by hydrographic change to different degrees. The differences in the responses to these changes could be another source of variation.
The objective of this work was to analyse the variations in zooplankton species abundance in response to two different hydrographic conditions: a summer upwelling event and an autumn period when the poleward-flowing current is in evidence. Further, the influence of both on coastal and oceanic areas is analysed, with the purpose of detecting differences in the influence of the hydrographic events close to the coast and farther offshore. The work contributes to knowledge of the mesozooplankton community under different hydrographic conditions in the study area, an area where most earlier studies were carried out during upwelling events.
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Material and methods |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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Study area and sampling
The data used for the study were obtained during two oceanographic cruises carried out off NW Spain: OMEX-0898 from 1 to 11 August 1998 onboard RV "Professor Shtokman" and OMEX-1099 from 14 to 20 October 1999 onboard RV "Thalassa". The same six stations were sampled on each cruise (Figure 1), by day and by night, except for Station 1 in 1998 and Station 16 in 1999, which were only sampled by day.
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Continuous profiles of temperature, salinity, and fluorescence were recorded at each station, with a Neil Brown MARK-III CTD probe in 1998 and a SeaBird 25 CTD probe with fluorometer in 1999. Fluorescence measurements were transformed into chlorophyll a concentration (mg m–3) according to the fluorescence/Chl a relationships obtained by Teira et al. (2001) from each cruise.
Zooplankton samples were taken with vertical hauls from 200 m, or from near the bottom in shallower water, to the surface. A triple WP2 net of 200-µm mesh and 40 cm (OMEX-0898) or 60 cm (OMEX-1099) diameter was used. Although different nets were used on the two cruises, we consider that this fact did not influence the results, because both abundance and biomass were higher during OMEX-0898 (40-cm diameter net), when the gear would have been less efficient than if the 60-cm diameter net had been used, as it was during OMEX-1099 (Sameoto et al., 2000). Therefore, if there was any effect, it would have contributed to reducing the differences between both years. No flowmeter was used, assuming the volume of filtered water as the entire column. Samples from one codend were preserved in 4% buffered formalin–seawater solution for taxon identification. The other codends were set aside for other experiments. In the laboratory, the zooplankton were identified to species level in most cases, and counted under a stereomicroscope. Subsamples taken were no less than 1/10 of the total sample. Abundance was expressed as number m–2.
Data analysis
Rare species (those in <10% of the samples) were excluded from the analysis. The structure of the mesozooplankton community was examined by multivariate techniques using the PRIMER software package (Clarke and Warwick, 1994). Both classification and ordination procedures were employed to group samples with similar zooplankton composition.
Prior to analysis, all data were transformed using the function X' = X1/4 to reduce the weighting of abundant species. Cluster analysis was based on the Bray–Curtis similarity measure and average linkage classification. Similarity level was adjusted to obviously meaningful groupings, rather than forcing clusters to a fixed level (Hunt et al., 2002). Moreover, the similarity matrix was ordinated using non-metric multidimensional scaling (NMDS) in a two-axis ordination. A combination of clustering and ordination analysis allows evaluation of the adequacy and mutual consistency of both representations (Clarke and Warwick, 1994). The ordination scores of the NMDS, which summarize the zooplankton community in terms of the abundance data, were regressed against sea surface temperature, average temperature, average salinity, and integrated Chl a concentration, to determine the effect of these variables on community structure.
The percentage contribution of each species to within- and between-group dissimilarity was determined using the similarity percentages (SIMPER) procedure within the PRIMER software. The abundance of the taxa responsible for most of the similarity (within groups) and dissimilarity (between groups) was tested against the environmental variables by regression analysis, to check the affinity of the taxa for different environmental conditions.
Differences in average temperature between groups were tested using one-way ANOVA and a post hoc Tukey test (
= 0.05). Assumptions of ANOVA were checked using a Kolmogorov–Smirnov test for normality and a Levene test for homogeneity of variances. Differences between particular groups in sea surface temperature, integrated Chl a concentration, and salinity were checked by applying t-tests owing to the heterogeneity of variances. When multiple tests were carried out, sequential Bonferroni correction was applied (Rice, 1989).
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Results |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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Hydrographic conditions, phytoplankton size structure, and chlorophyll a concentration and distribution during OMEX-0898 and OMEX-1099 are described in Teira et al. (2001). During OMEX-0898, upwelling conditions prevailed, the upper water column being cooler near the coast than at oceanic stations. Coastal waters were slightly fresher than oceanic waters, so there was a thermohaline stratification. Chl a concentration was higher at coastal stations, with >2-µm phytoplankton cells accounting for >80% of total Chl a, while at open ocean stations, the subsurface Chl a maximum (50–80 m deep) was composed of small particles containing Chl a. In autumn 1999 (OMEX-1099), the thermohaline structure was characterized by the warm, saline poleward current flowing over the upper slope, at depths of 60–120 m. There was a nucleus of warm water (17°C) in the upper 50 m over the core of the poleward current. Freshwater inputs were higher than in the summer 1998 cruise, affecting the entire water column at some stations. Chl a concentration was lower than during upwelling. The poleward current favoured accumulation of phytoplankton biomass in the upper 60 m at shelf and shelf break stations, and a subsurface maximum (40–50 m deep) was found at oceanic stations. The relative contribution of >2-µm Chl a was generally <70% in the upper 40 m, and the proportion of large cells increased with depth. Picoplankton dominated the upper layer of the onshelf stations of the south transect, representing >60%.
Cluster analysis (Figure 2a) grouped samples from the same years, discriminating samples from the different cruises into two groups, except for samples from Station 19, which were grouped separately from other samples. The NMDS plot (Figure 2b) reflects the same grouping of samples. Average abundances of the mesozooplanktonic taxa within each group are shown in Table 1.
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The analysis identified six groups of samples at approximately a 65% level of similarity. Within-group similarities of groups B and C were each 66.3%, and the rest of the groups had within-group similarities >70%. Group A contained samples from Station 19 in 1998. At that station, close to the coast and with a bottom depth of 39 m, the upwelled water reached the surface. Group B included the oceanic stations (4, 10) sampled during OMEX-0898. These stations were located in or outside the upwelling front, where water was more saline and warmer than in the upwelling area, and slightly stratified. Group C comprised samples influenced directly by the summer upwelling of 1998, inshore of the upwelling front. Therefore, those samples were collected in the coolest average temperatures and the highest Chl a concentrations of the study (Table 2). Groups D, E, and F corresponded to the OMEX-1099 cruise. Group D included samples from Station 19, which was close to the coast, so its main characteristics then were a low salinity (owing to the continental freshwater inputs, affecting the entire column) and a high average temperature. Group E contained continental shelf samples, under the influence of the run-off from the Rías Bajas. Group F comprised slope and oceanic samples. Samples of that group came from Stations 4, 10, and 16 (Figures 2a, b). The main features of those stations during October 1999 were the presence of a nucleus of warm water in the upper 50 m, and the high salinity core of the poleward slope current found at depth.
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Values of the environmental variables for each group are shown in Table 2. Groups differed significantly in average temperature (F5,16 = 128.8; p < 0.001), which were ranked C = A < B = F < E < D by the post hoc Tukey test. Surface temperatures were significantly higher in D vs. A (p = 0.004) and E vs. C (p = 0.003), but were not different between C and B (p = 0.105) nor between F and E (p = 0.114). Integrated salinity was significantly higher in group B than in group C (p < 0.001), and in group E it was significantly lower than in group F (p = 0.04). Integrated Chl a in group C was significantly higher than in groups E and B (p = 0.002 and p = 0.049, respectively). The regression analysis between environmental variables and the ordination scores of the NMDS showed that Chl a explained 49% of the variation in zooplankton abundance data, salinity explained 63.8%, surface temperature accounted for 71.8%, and average temperature accounted for 58.9% of the variation (Table 3).
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Only taxa with the greatest contribution to within- and between-group dissimilarity (Table 4) were studied in detail. Group C, where during OMEX-0898 the upwelled waters caused low temperatures and high integrated salinity and Chl a, was characterized by the dominance of species linked to high phytoplankton concentrations: Calanus helgolandicus, Calanoides carinatus, Oithona helgolandica, and Oncaea media. On the other hand, during October 1999 (group E), the density of species specialized to feed on small particles such as appendicularians, Salpa fusiformis, and Oithona nana, increased. This group was also characterized by higher densities of gastropod and bivalve larvae. Meroplankton taxa, such as polychaetes and cirripede larvae, and pluteus, bivalve, and gastropod larvae, appear as indicators of very shallow water (groups A and D) in both cruises. Appendicularians, Sagitta friderici, and O. nana were scarce during upwelling, attaining their greatest densities in October 1999. Conversely, coastal upwelling indicator species such as O. helgolandica and O. media dominated during OMEX-0898. At open ocean stations (B and F), there were oceanic indicator copepods, such as Calocalanus styliremis, Mesocalanus tenuicornis, Metridia lucens, Pleuromamma gracilis, and Clausocalanus pergens during both cruises, and their abundance peaked in those groups of samples (Table 1). In general, such species were positively related to salinity (Table 5). In general, temporal changes at those stations were mainly due to the input of continental shelf species (Table 4). Whereas the populations of species related to upwelling, such as O. helgolandica, increased during OMEX-0898, taxa such as O. nana and bivalve larvae, which are typically found nearshore, were at their greatest density in October. Conversely, the densities of oceanic species did not change significantly between cruises (Table 1).
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The significance of the regression analyses carried out with average and sea surface temperatures, integrated Chl a, and salinity against the species indicated by the SIMPER analysis (Table 5) reflects the link between the main species involved in the groupings (indicated by the SIMPER analysis) and the hydrographic variables. Species related positively with Chl a and/or colder waters were representative of upwelling samples (group C): C. helgolandicus, O. helgolandica, O. media, Clausocalanus jobei, whereas the species indicators of samples from the autumn cruise were related to warmer water and low values of Chl a (S. fusiformis, O. nana, some groups of meroplankton; Tables 4 and 5). A positive relationship with salinity was an indicator of taxa related to open ocean waters, i.e. groups B and F (C. styliremis, M. lucens, P. gracilis, C. pergens), whereas taxa linked to shelf break conditions (meroplankton taxa, appendicularians, S. friderici, Paracalanus parvus) were negatively related to this factor (Tables 4 and 5).
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Discussion |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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The study showed the influence of the two main hydrographic features off the NW Iberian Peninsula on the composition of the zooplankton community. The high phytoplankton biomass generated by upwelling (Teira et al., 2001) would explain the dominance of species associated with highly productive systems near the coast in OMEX-0898, whereas the influence of the poleward current on the environment played a crucial role in determining the zooplankton community during October 1999.
During upwelling, species linked to high phytoplankton biomass were in increased numbers close to the coast; O. helgolandica and O. media are linked to coastal upwelling events or to spring blooms (Paffenhöfer, 1980; Turner and Granéli, 1992; Atkinson and Shreeve, 1995). C. helgolandicus can take advantage of spring blooms in temperate latitudes (Laabir et al., 1998; Weikert et al., 2001), and C. carinatus develops in seasonal upwelling areas (Verheye and Field, 1992; Verheye et al., 1992; Hitchcock et al., 2002) and during summer along the Atlantic coast of the Iberian Peninsula (Williams and Conway, 1988; Ceballos et al., 2004). The results of the regression analyses supported these results; such species are negatively related to temperature (a consequence of upwelling) and/or positively to Chl a (Tables 1 and 5). Although other species such as P. parvus and Acartia clausi were abundant everywhere during upwelling, their importance was less close to the coast, where Chl a concentrations and phytoplankton composition (mainly >2-µm cells; Teira et al., 2001) favoured other species adept at taking advantage of phytoplankton blooms and feeding on large phytoplankton cells (Table 1). In shallower water, the upwelled water reached the surface, but the Chl a concentrations and the >2-µm phytoplankton contribution were less than over the continental shelf, so the contribution of the larger "upwelling" species (C. helgolandicus and C. carinatus) was less important, even less than during OMEX-1099. The other two species associated with upwelling (O. helgolandica and O. media) could also feed on small phytoplankton, and they were present in great numbers. Head et al. (2002) also found communities dominated by Oithona spp. and Oncaea spp. wherever flagellates and picoplankton accounted for ca. 85% of phytoplankton standing stock. Another characteristic of such samples, which distinguished them from continental shelf or shelf break samples, was the great abundance of species linked to lower salinity, such as A. clausi and meroplankton, which are quite common inshore (Gislason and Astthorsson, 1995; Gaard, 1999).
In general, small copepods (<500 µm) dominated the offshore community during August 1998 (Table 1), and Chl a concentrations were less than near the coast and the subsurface Chl a maximum was formed largely by cells <2 µm (Teira et al., 2001). These conditions restricted the numbers of larger copepods (>500 µm) linked to coastal upwelling, which specialize on feeding on larger cells. The small size of phytoplankton would also explain the relative abundance of appendicularians in oceanic waters compared with close to the coast (Tables 1 and 4) during OMEX-0898. Oceanic indicator species were characterized by their positive relationship with salinity, a feature observed for C. styliremis and M. lucens, which had higher densities at these stations (Tables 1 and 5) than close to the coast.
Picoplankton dominated the phytoplankton community during OMEX-1099 (Teira et al., 2001), agreeing with the findings of Fernández et al. (1993), Castro et al. (1997), and Álvarez-Salgado et al. (2003), who found that the phytoplankton community in the poleward current was dominated by small autotrophic flagellates. Species such as appendicularians (Acuña and Anadón, 1992), Salpa fusiformis (Calbet et al., 2001), and Oithona nana (Nielsen and Sabatini, 1996; Nielsen and Andersen, 2002) are related to warm water (a characteristic of the poleward current), and are specialized to feed on small particles. Halvorsen et al. (2001) detected relatively high densities of O. nana during the OMEX-0898 cruise in an off-shelf upwelled filament, where the phytoplankton community also consisted of picophytoplankton, which accounted for 61–65% of the total carbon fixation (Joint et al., 2001a). Huskin et al. (2003) found high concentrations of salps NW of Spain during November 1999 in the poleward intrusion, and the relatively high temperature and salinity observed in the poleward current would also favour S. fusiformis because this species seems to have an affinity for warm saline water (Lavaniegos et al., 2002). Another feature of the OMEX-1099 cruise was the densities of gastropod and bivalve larvae, findings which support the increase in meroplankton already reported in the study zone at the end of summer and during autumn by Valdés et al. (1990). However, groups of stations during OMEX-1099 had distinguishing characteristics. Shallower stations were dominated by species linked to warm and/or low-salinity water, such as meroplankton, S. friderici, and O. nana. S. friderici and O. nana prefer shallow water, low salinity, and high temperature (Gibbons, 1994; Marazzo and Nogueira, 1996; Jamet et al., 2001; Richard and Jamet, 2001), and peaked in October 1999, when freshwater inputs were at their maximum (Teira et al., 2001). Moreover, the increased densities of small phytoplankton during OMEX-1099 favoured O. nana and particle-feeders such as appendicularians, which also favour warmer temperatures. These results are also supported by the relationships found (positive for temperatures, negative for salinity; Table 5) for these species. The abundances of C. helgolandicus and C. carinatus remained high during OMEX-1099, even higher than during the upwelling period. Over the continental shelf, the abundance of those species was lower than during OMEX-0898, but they were still relatively high. Both species can maintain relatively high abundance during October, because the upwelling season extends from March to October (Álvarez-Salgado et al., 2002). As a consequence, Ceballos et al. (2004) pointed out that C. carinatus could follow a conservative strategy to extend its reproductive season beyond the end of the upwelling season.
Oceanic waters were indicated by the presence of ocean indicator species, positively related to salinity (M. lucens, P. gracilis, C. styliremis). Moreover, offshore sites were indicated during OMEX-1099 by the presence of Scolecithricella dentata (Table 1). This copepod is a tropical/subtropical species (Bradford-Grieve et al., 1999), which would likely prefer the warm, saline poleward current that was found there. Finally, during OMEX-1099, the contribution of coastal plankton such as O. nana (Böttger-Schnack, 1995) and bivalve larvae (Gislason and Astthorsson, 1995; Gaard, 1999) offshore was greater than during the August 1998 cruise, indicating a possible displacement of coastal waters offshore.
The trends described here agree with the findings of Isla et al. (2004) and Isla and Anadón (2004), who based their analyses on the same data series. Isla et al. (2004) described a herbivorous foodweb close to the coast and an omnivorous one offshore during upwelling. Similarly, our community was dominated by large (>1000 µm) herbivorous species close to the coast, while offshore the dominant species belonged to the 200–500-µm and 500–1000-µm fractions, with their ability to feed on small phytoplankton cells and some of them usually described as omnivorous, or were large carnivorous copepods. Offshore, however, Isla and Anadón (2004) hypothesized the presence of an omnivorous foodweb (protein-orientated metabolism). Likewise, we found that the abundance of relatively small omnivorous species, particle-feeders, and meroplankton larvae increased in comparison with the upwelling cruise.
Our results emphasize the essential role that hydrographic conditions play in structuring the mesozooplankton community along the NW Iberian Peninsula.
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Acknowledgements |
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We thank the captains and crews of RV "Professor Shtokman" and RV "Thalassa", and the participants of cruises OMEX-0898 and OMEX-1099, for their support. We are also grateful for the comments on an earlier draft made by S. Ceballos and J. A. Isla, which helped to improve the manuscript. LB-B is financially supported by a PhD grant of the "III Plan Regional de I+D+I del Principado de Asturias". The work was supported by the EU in the framework of the Mast 3 Programme, contract number MAS3-CT97-0076 (OMEX II project).
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References |
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