Sucesión estacional de fitoplancton en la costa de A Coruña (Galicia, noroeste de España)

413 Seasonal succession of phytoplankton species on the coast of A Coruña (Galicia, northwest Spain) B. Casas, M. Varela and A. Bode Centro Ocranográfico de A Coruña. Instituto Español de Oceanografía. Apdo. 130. 15080 A Coruña, Spain Received October 1997. Accepted February 1998. ABSTRACT Phytoplankton species composition was monitored monthly at two stations on the continental shelf off A Coruña (Galicia, northwest Spain). The deeper station (80 m) was sampled between May 1989 and December 1992. The coastal station (20 m) was studied during 1992. In each sampling, water samples were taken from selected depths throughout the water column. Microflagellates formed the bulk of phytoplankton, being dominant all year long, followed by diatoms, which dominated during phytoplankton blooms. The samples studied were arranged into five groups, according to environmental conditions defined for the main oceanographic periods in the area. Each group showed a characteristic phytoplankton species composition. The most important phytoplankton blooms occurred during spring and autumn. However, major phytoplankton abundances were also detected during upwelling events in the summer. Several species of the genus Chaetoceros, especially C. socialis (Lauder, 1864) were dominant during blooms, and accompanying species, e.g. Lauderia borealis (Gran, 1900), were important during spring and autumn. Leptocylindrus danicus (Cleve, 1889) made a major contribution to phytoplankton abundance during summer upwelling events. In contrast with studies conducted in neighbouring areas, dinoflagellates never dominated during summer stratification periods, probably because of the very frequent upwelling events, which caused destabilisation of the water column. During winter mixing and in deeper waters, we found a mixed community comprising species that are present throughout the year, but in very low densities, as well as resuspended diatoms from sediment. In 1992, at the coastal station, red-tide events occurred, and some species of dinoflagellates (e.g. Gymnodinium catenatum Graham, 1943) reached densities of well over 100 cells ml–1. Considering the entire series of observations, the most noteworthy features are a decrease in the magnitude of diatom blooms during the study period and, on the contrary, an increase in the abundance of microflagellates. Key words: Phytoplankton, succession, seasonality, diatoms, dinoflagellates, microflagellates, northwest Spain. RESUMEN Sucesión estacional de fitoplancton en la costa de A Coruña (Galicia, noroeste de España) Se estudia la composición de especies y grupos taxonómicos de fitoplancton con una frecuencia aproximadamente mensual en dos estaciones de la plataforma continental frente a A Coruña (Galicia, noroeste de España). La estación más profunda (80 m) fue muestreada entre mayo de 1989 y diciembre de 1992. La estación costera (20 m) fue estudiada durante 1992. En cada muestreo se recogieron muestras repartidas por toda la columna de agua. El grupo de fitoplancton más numeroso fue el de los microflagelados, siempre dominantes en todas las épocas del año, seguido de las diatomeas, responsables de las principales proliferaciones de fitoplancton registradas. Las observaciones se clasificaron en cinco grupos según las características ambientales definidas para los principales periodos oceanográficos de esta zona. Cada uno de estos grupos presentó una composición de especies de fitoplancton característica. Las mayores proliferaciones de fitoplancBol. Inst. Esp. Oceanogr. 15 (1-4). 1999: 413-429 INSTITUTO ESPAÑOL DE OCEANOGRAFÍA ISSN: 0074-0195 © Ministerio de Agricultura, Pesca y Alimentación, 1999 BOLETÍN INTRODUCTION Phytoplankton species succession is linked to changes in the stratification of water-column surface layers (Harris, 1986). Morphological and physiological adaptations of phytoplankton cells enable some groups of species to compete advantageously during the various steps of the continuous transition between vertical mixing and stratification of the surface layer (Margalef, 1978; Smayda, 1980). In temperate waters, one of the main causes for stratification is the steady surface heating during spring and summer due to increasing solar irradiance (Walsh, 1988). In general, the succession progresses in several steps, from initial phases where strong vertical mixing favours the dominance of chain-forming diatoms, to mature phases where water column stratification favours the strategy of dinoflagellates and other flagellates that can swim to zones rich in light or nutrients (Margalef, 1978). However, in areas affected by episodic upwelling events, the upwelling of cold waters to surface layers of the water column radically changes the seasonal pattern of stratification, keeping the phytoplankton communities of these areas in the initial stages of succession (Estrada and Blasco, 1985). On the Galician coast (northwest Spain), which is affected by upwelling events essentially between March and October (Fraga, 1981), seasonal variation of phytoplankton species composition has been well studied in the rias, specially the Rias Bajas (Margalef, Duran and Saiz,1955; Durán et al., 1956; Nunes et al., 1984; Figueiras and Niell, 1987b; Figueiras and Ríos, 1993), but also in the Rias Altas (Campos and Mariño, 1984; Mariño et al., 1985). Other studies have described phytoplankton species composition in the rias during certain seasons (Margalef, 1956; Campos and Mariño, 1982; Varela, 1982; Blanco, 1985; Figueiras and Niell, 1987a; Figueiras and Pazos, 1991a,b). However, references to phytoplankton on the Galician continental shelf off the rias are quite scarce. The only data available previously were for certain months (Estrada, 1984; Varela et al., 1987a,b), but no studies had been conducted on phytoplankton succession in the area. For studies of annual phytoplankton succession on the continental shelf of the Bay of Biscay, see Estrada (1982) and Fernández and Bode, Casas and Varela (1994). The scope of the present study is to describe the seasonal variability of phytoplankton abundance and characteristic species of the main oceanographic periods described for La Coruña’s continental shelf (Bode, Casas and Varela, 1994; Casas et al., 1997). MATERIALS AND METHODS Samples of water and phytoplankton were taken at two stations off A Coruña (figure 1), at approximately monthly intervals. Station 2 (depth of 80 m) was sampled from May 1989 to December 1992. Station 4 (depth of 20 m) was sampled from January to December 1992. In each sampling, temperature, salinity, irradiance, dissolved nutrients, chlorophyll, and particulate nitrogen and carbon were measured. Methods used to determine these parameters and detailed information on these data B. Casas, M. Varela and A. Bode Phytoplankton succession in northwest Spain Bol. Inst. Esp. Oceanogr. 15 (1-4). 1999: 413-429 414 ton se produjeron en primavera y otoño, pero también se registraron elevadas abundancias durante episodios de afloramiento en verano. Varias especies del género Chaetoceros, especialmente C. socialis (Lauder, 1864), fueron las principales responsables de estas proliferaciones, acompañadas de otras especies de diatomeas como Lauderia borealis (Gran, 1900) en primavera y otoño, y Leptocylindrus danicus (Cleve, 1889) en el afloramiento de verano. A diferencia de lo encontrado en otras zonas próximas, los dinoflagelados no llegan a hacerse dominantes en el periodo de estratificación térmica de verano, probablemente debido a la frecuencia de los episodios de afloramiento. Durante el periodo de mezcla invernal y en las capas profundas de la columna de agua se encuentra una mezcla de especies presentes todo el año, aunque con bajas abundancias, y también diatomeas del sedimento resuspendidas. En 1992, especialmente en la estación costera, se produjeron episodios de marea roja durante el verano, superando algunas especies de dinoflagelados (como Gymnodinium catenatum Graham, 1943) 100 células ml–1. Considerando toda la serie de observaciones, se aprecia una disminución en la magnitud de las proliferaciones de diatomeas y un incremento en la abundancia de microflagelados en los últimos años. Palabras clave: Fitoplancton, sucesión, estacionalidad, diatomeas, dinoflagelados, microflagelados, noroeste de España. can be found in Casas (1995) and Casas et al. (1997). Phytoplankton samples were taken with Niskin bottles at standard depths of 0, 5, 10, 20, 30, 40 and 70 m from station 2, between May 1989 and December 1991. From 1992 on, sampling depths were chosen according to in situ irradiance profiles, so that samples were taken at 100, 50, 25, 10 and 1% of surface irradiance levels (Casas, 1995). Phytoplankton samples were preserved with Lugol’s solution and kept in darkness until counting, using Utermöhl’s technique (1958). Observation of samples was carried out using a Nikon Diaphot inverted microscope with Nomarsky phasecontrast system. Magnification powers of 100, 200 and 400 were used, according to the size of organisms. Results are expressed in cells ml–1. When the organisms were too small (usually smaller than 30 µm) to be classified at the genus or species level, they were included in wider groups, e.g. flagellates, dinoflagellates, and so on. Flagellates, which can belong to different taxonomic categories, were divided into three size classes: 2-5 µm, 5-10 µm and larger than 10 µm. The diatom relative abundance index was calculated by dividing the total number of diatoms by the total number of diatoms plus dinoflagellates for each sample. To define the groups of characteristic species for each oceanographic situation, a discriminant analysis was carried out using the species found during the period 1989-1992 at station 2 and during 1992 at station 4. The groups of samples defined a priori were those described by Casas (1995) and Casas et al. (1997), using the hydrographic properties of the water column, along with the nutrient and chlorophyll concentrations observed in each case. The analysis was carried out with 387 samples and 49 species or taxa, selecting those taxa whose relative frequency was higher than 20 %, to avoid the effect of rare species. Data were logarithmically transformed. The Mulva 4 (Wildi and Orloci, 1990) statistical package was used in the analysis. RESULTS Temporal distribution of cellular abundance Distribution of total abundance of diatoms and dinoflagellates in station 2 (figure 2a) shows that higher abundances occur over short periods of time during autumn (> 7 000 cells ml–1 in 1989), summer (up to 6 000 cells ml–1 in 1990) and spring (around 2 000 cells ml–1 in 1989 and 1990). These periods coincide with maximum values in chlorophyll and particulate material concentrations at these stations (Casas, 1995; Casas et al., 1997). The relative abundance of diatoms was usually high (index values higher than 0.5) throughout the period of study, with diatoms becoming even more dominant during the aforesaid blooms. The relative abundance of diatoms was low during winter and some periods of summer (figure 2b), essentially due to a decrease in diatom abundance in the former case, and to the relative increase of dinoflagellates in the latter. Even though the distribution pattern of diatom relative abundance is similar among different years and also at both stations, values of diatom abundance higher than 1 000 cells ml–1 were observed more often in 1989 and 1990 than in the other years. Microflagellates always constituted the bulk of phytoplankton. Microflagellates of 2-5 µm, counted in the samples since 1991, showed abundances well over 10 000 cells ml–1 in some autumn and summer samples (figure 3a). The abundance of microflagellates measuring 5-10 µm does not show a clear seasonal pattern of dominance, at least until 1991, when abundances higher than 1 000 cells ml–1 were recorded in surface layers during late B. Casas, M. Varela and A. Bode Phytoplankton succession in northwest Spain Bol. Inst. Esp. Oceanogr. 15 (1-4). 1999: 413-429 415 Figure 1. Location of study area and sampled stations A Coruña summer, extending the layer of higher abundance to steadily increasing depths throughout 1992 (figure 3b). Variations in abundance of this size fraction of flagellates at station 4 were relatively small (always around 1 000 cells ml–1). Larger microflagellates (> 10 µm) were especially dominant during summer periods, even though they showed abundances higher than 100 cells ml–1 in autumn and winter during 1992 (figure 3c). Taking into account all fractions, but especially the mid-size fraction (5-10 m), an increase in microflagellate abundance was observed during 1992. B. Casas, M. Varela and A. Bode Phytoplankton succession in northwest Spain Bol. Inst. Esp. Oceanogr. 15 (1-4). 1999: 413-429 416 Figure 2. Temporal variation of total abundance of diatoms plus dinoflagellates (cells ml–1, A) and relative abundance of diatoms index (%, B) B. Casas, M. Varela and A. Bode Phytoplankton succession in northwest Spain Bol. Inst. Esp. Oceanogr. 15 (1-4). 1999: 413-429 417 Figure 3. Temporal variation of abundance of microflagellates, 2-5 µm (A), 5-10 µm (B) and larger than 10 µm (C), expressed in cells ml–1 Succession of species assemblages During the present study, 205 species or categories of phytoplankton were recognised. A complete list can be found in Casas (1995). To summarise the information obtained, a discriminant analysis was conducted on the most abundant species (table I). Samples were classified into five groups according to characteristics of environmental variables in selected oceanographic stages, summarised in table II, as defined in Casas (1995) and Casas et al. (1997). Group 1 contains samples representative of phytoplankton blooms during spring and autumn, when chlorophyll concentrations were higher than 1 mg m–3 and a slight density gradient was present in the upper layer of the water column. Group 2 includes samples from the thermal stratification situation observed during B. Casas, M. Varela and A. Bode Phytoplankton succession in northwest Spain Bol. Inst. Esp. Oceanogr. 15 (1-4). 1999: 413-429 418 Table I. Mean values (cells ml–1), standard error of mean (s.e.m.) and presence (n) of the 49 taxa used in the discriminant analysis. The code used in figure 4 is indicated Code Species or group n Mean s.e.m. DIN < 30 Dinoflagellates < 30 µm 357 16.32 1.54 DIN > 30 Dinoflagellates > 30 µm 73 0.55 0.18 DINACU Dinophysis acuminata 34 0.19 0.17 GYRSPI Gyrodinium spirale 83 0.33 0.09 GYRGLA Gyrodinium glaucum 74 0.35 0.13 MASSPP Massartia spp. 98 2.33 0.74 MICSPP Micracanthodinium spp. 35 0.20 0.14 PROMIC Prorocentrum micans 44 0.64 0.72 PROBIP Protoperidinium bipes 42 0.22 0.15 PRODIA Protoperidinium diabolus 24 0.11 0.11 SCRTRO Scrippsiella trochoidea 87 2.93 1.98 ASTJAP Asterionella japonica 29 1.89 2.42 BACDEL Bacteriastrum delicatulum 30 2.01 2.19 CERBER Cerataulina bergoni 66 0.34 0.13 CHFCHW Chaetoceros affinis (+ var. willei) 85 7.07 4.70 CHACUR Chaetoceros curvisetus 15 0.62 1.38 CHADID Chaetoceros didymus 70 2.96 2.74 CHASOC Chaetoceros socialis 119 83.74 33.41 CHASPP Chaetoceros spp. 101 6.69 4.61 CENTR Centric diatoms 57 0.30 0.16 PENN Pennate diatoms 81 0.43 0.13 LAUBOR Lauderia borealis 54 4.31 2.77 LEPDAN Leptocylindrus danicus 124 26.83 23.87 LEPMIN Leptocylindrus minimus 83 3.82 1.84 NAVSAL Navicula cf. salinarum 73 0.18 0.06 NAVSPP Navicula spp. 37 0.28 0.21 NITEPI Nitzschia spp. 55 1.15 0.59 NITLON Nitzschia longissima 266 4.50 0.64 PARSUL Paralia sulcata 39 0.49 0.37 PSEDEL Pseudonitzschia delicatissima 129 2.34 0.72 PSEPUN Pseudonitzschia pungens 212 21.76 8.14 RHIDEL Rhizosolenia delicatula 154 10.57 3.15 RHIFRA Rhizosolenia fragilissima 51 1.70 1.12 RHISHU Rhizosolenia shrubsolei 64 0.47 0.25 RHISET Rhizosolenia setigera 56 0.25 0.11 RHISTO Rhizosolenia stolterforthii 71 1.46 0.67 SCHDEL Schroederella delicatula 54 3.38 2.77 SKECOS Skeletonema costatum 88 7.57 5.90 THANIT Thalassionema nitzschioides 78 1.09 0.67 THAFAL Thalassiosira fallax 72 2.69 1.60 EUGL Euglenaceae 39 0.16 0.11 DISSPE Distephanus speculum 72 0.32 0.12 SOLSET Solenicola setigera 91 11.76 4.35 COCO Coccolithophorids 43 1.67 1.59 PHAPOU Phaeocystis poucheti 75 6.72 2.81 CRYP Cryptophyceae 300 41.60 6.10 MESRUB Mesodinium rubrum 87 0.46 0.20 summer, when a sharp density gradient was evident in the water column, concentrations of dissolved nutrients were low, and chlorophyll concentration were usually lower than 1 mg m–3. Winter samples and those corresponding to deep layers of water column during different periods were included in Group 3. In this group, samples are characterised by a well-mixed water column with relatively high nutrient concentrations and low values of chlorophyll, as well as low abundances of phytoplankton cells. Group 4 comprises samples from summer upwelling events, characterised by relatively cold water (around 13 °C) and rich in nutrients (i.e. > 5 µM of nitrate) near the surface. Samples from this group show chlorophyll concentrations higher than 1 mg m–3, and a total phytoplankton abundance similar to that observed during spring blooms. Finally, Group 5 includes two types of rare episodes: red tides and downwelling. These two phenomena were observed during late summer of 1992, and both are related to the presence of warm and lowdensity surface waters. In the case of red tide, proliferations of dinoflagellates were observed, with abundances well over 100 cells ml–1. In the case of downwelling, warm surface water was found at lower depths of the water column, coinciding with a change in the surface direction that accumulated surface water towards the coast (Casas et al., 1997). When plotting the species in relation to the first and second discriminant functions (figure 4) it can be observed that the first discriminant function (FD1) groups together, in its positive part, diatoms –including Lauderia borealis (Gran, 1900), Thalassiosira fallax (Meunier, 1910), Chaetoceros socialis (Lauder, 1864), Chaetoceros curvisetus (Cleve, 1899), Schroederella delicatula (Pavillard, 1913) and Rhizosolenia delicatula (Cleve, 1898)– that are all dominant species during spring and autumn blooms as well as upwelling events. The negative part of FD1 separates species typical of stratified B. Casas, M. Varela and A. Bode Phytoplankton succession in northwest Spain Bol. Inst. Esp. Oceanogr. 15 (1-4). 1999: 413-429 419 Table II. Mean values, standard deviation (s.d.), number of samples (n), maximum value (max) and minimum (min) of temperature (t, °C), salinity (S, usp), concentrations of nitrite (µmol l–1), nitrate (µmol l–1), phosphate (µmol l–1) and dissolved silicate (µmol l–1), and chlorophyll-ain two size fractions: particles larger than 12 µm (Chl-a> 12 µm, mg m–3) and smaller than 12 µm (Ch-a< 12 µm, mg m–3), grouping the observations according to oceanographic periods defined by Casas (1995) and Casas et al. (1997) for the study area. Samples from winter period also include those obtained below the photic zone in other periods Period t S Nitrite Nitrate Phosphate Silicate Chl-a> 12 µm Chl-a< 12 µm Phytoplankton n 38 38 64 60 64 33 64 64 blooms mean 13.21 35.69 0.40 2.30 0.42 0.62 1.98 0.72 s.d. 0.84 0.30 0.24 2.07 0.28 0.55 2.24 0.71 max 15.47 35.93 0.94 9.28 1.77 2.82 11.10 3.71 min 12.27 34.38 0.10 0.10 0.11 0.12 0.10 0.10 Stratification n 31 21 37 36 41 26 46 46 mean 15.27 35.75 0.45 0.98 0.30 0.52 0.61 0.85 s.d. 1.19 0.23 0.30 1.03 0.14 0.27 0.59 0.66 max 17.86 36.68 0.90 4.60 0.70 1.20 3.24 2.85 min 12.90 35.44 0.10 0.12 0.15 0.18 0.10 0.12 Winter n 114 105 174 170 174 104 180 180 mean 13.31 35.61 0.48 5.03 0.54 1.61 0.38 0.39 s.d. 0.65 0.26 0.28 2.02 0.23 1.30 0.36 0.27 max 14.90 36.20 1.55 14.31 1.55 5.38 3.29 1.76 min 11.51 34.65 0.11 0.77 0.15 0.19 0.10 0.00 Summer n 33 33 63 68 68 34 68 68 upwelling mean 14.68 35.67 0.39 3.13 0.42 1.19 1.52 0.83 s.d. 1.59 0.16 0.21 2.71 0.19 1.03 1.41 0.80 max 17.50 36.23 0.85 9.84 0.87 3.47 6.71 3.94 min 11.68 35.35 0.11 0.12 0.12 0.10 0.10 0.00 Red tide and n 29 29 29 29 29 26 29 29 downwelling mean 16.97 35.63 0.44 1.84 0.38 1.27 0.64 0.56 s.d. 1.53 0.13 0.27 2.10 0.16 1.08 0.39 0.36 max 18.98 35.88 0.96 7.29 0.74 4.90 1.68 1.90 min 13.49 35.38 0.11 0.15 0.17 0.10 0.11 0.14 waters –including the dinoflagellates Gyrodinium glaucum (Kofoid and Swezy, 1921), Protoperidinium diabolus (Balech, 1974), Protoperidinium bipes (Balech, 1974)– and diatoms that dominate during phases of relaxing upwelling, e.g. Rhizosolenia stolterfothii (Peragallo, 1898), Rhizolenia fragilissima (Bergon, 1903), Chaetoceros affinis (Hustedt, 1930), Leptocylindrus danicus (Cleve, 1889) and Leptocylindrus minimus (Gran, 1915). The second discriminant function (FD2) shows, in its positive part, most of the dinoflagellate and diatom species, whereas in the negative portion appear those species typical of winter mixing and/or deep waters, e.g. Navicula cf.Salinarum (Grunow, 1878), Paralia sulcata (Cleve, 1873) and Dictyocha speculum (Ehrenberg, 1839). The distribution of phytoplankton samples with regard to FD1 and FD2 presents a separation in the space among the five groups, separating the positive part of FD1 the samples corresponding to spring, autumn and most of upwelling blooms from summer stratification samples and those belonging to red tide and downwelling events in the negative part (figure 5). The second function (FD2) also separates in its positive part samples of spring, autumn and upwelling, but also, those of summer stratification, red tide and downwelling. The negative part of FD2 includes winter and deepwater samples. The discriminant analysis correctly grouped together more than 88 % of samples in the initial groups, which clearly confirms the descriptive utility of the identified oceanographic stages. The species characterising Group 1 are essentially diatoms, among which C. socialis,L. borealis, Rhizosolenia setigera (Brightwell, 1858) and S. delicatula, which can top 1 000 cells ml–1, are the most noteworthy species. C. socialis reached very high abundances (maximum of 6 000 cells ml–1 in autumn 1989) during spring and autumn blooms at both stations (figure 6a). It also appeared in high abundances during upwelling events (> 2 000 cells ml–1 in 1991, station 2). However, this species reached lower densities during 1992 than in previous years. Diatoms of the genus Chaetoceros were generally dominant in all observed blooms. The main difference between spring and autumn blooms with regard to upwelling blooms was in the number and type of associated species that appeared along with Chaetoceros. Some of these species occurred exclusively during spring and autumn, e.g. L. borealis (figure 6b). Others were more frequent during summer upwelling events, e.g. L. danicus (figure 7a). Also during summer upwelling B. Casas, M. Varela and A. Bode Phytoplankton succession in northwest Spain Bol. Inst. Esp. Oceanogr. 15 (1-4). 1999: 413-429 420 Figure 4. Projection of the phytoplankton species on the first two axes of discriminant analysis (FD1 and FD2). Codes for species are shown in table I (Group 4), Leptocylindrus minimus,Pseudonitzschia delicatissima (Heiden, 1928) and C. affinis (+ var. willei) showed abundances higher than 100 cells ml–1. Pseudonitzschia cf. pungens (Hasle, 1993) and the Prymnesiophyceae Phaeocystis poucheti (Lagerheim, 1893) were also abundant during upwelling, but with lower densities. On the contrary, summer stratification phases (Group 2) were characterised by a mixture of diatoms and dinoflagellates, with somewhat lower abundances compared with those of blooms, and which can also appear in other periods. Among diatoms, L. danicus and some species of the genera Chaetoceros and Massartia were the most abundant during this period, but none exceeded 100 cells ml–1. An illustrative example of a dominant dinoflagellate during this phase is Gyrodinium spirale (Kofoid and Swezy, 1921), which only once showed abundances higher than 10 cells ml–1 (figure 7b). The species appearing in the samples of Group 3 were usually present throughout the year. Skeletonema costatum (Cleve, 1873), for instance, showed relatively high abundances in January and February 1990, even though it also appeared during blooms in summer and autumn. Some other meroplanktonic diatoms, e.g. Paralia sulcata, were typical of these samples, and were present in deeper waters, not only during winter mixing, but also during other seasons, when resuspension of bottom material occurs (figure 8b). Species causing red tides (Group 5) appeared with higher abundances at the inshore station. During July 1992, Alexandrium lusitanicum (Balech, 1985) (figure 9a) and Prorocentrum micans (Eherenberg, 1837) (figure 9b) were dominant, the latter observed throughout the spring and early summer, although never in high abundances. In September, Gymnodinium catenatum (Graham, 1943) (figure 9c) and Scrippsiella trochoidea (Loeblich III, 1976) (figure 9d) dominated the dinoflagellate population at the time when downwelling of warm surface waters occurred (Casas, 1995; Casas et al., 1997). DISCUSSION The values of phytoplankton abundance observed in the present study fall within the range of those previously described by other authors for Galician waters (Varela, 1982; Estrada, 1984; Varela et al., 1987a,b; Valdés et al., 1991; Varela, 1992), but they are higher than those cited for the Cantabrian Sea (Estrada, 1982; Fernández and Bode, 1994). The succession model is similar to that expected B. Casas, M. Varela and A. Bode Phytoplankton succession in northwest Spain Bol. Inst. Esp. Oceanogr. 15 (1-4). 1999: 413-429 421 Figure 5. Projection of samples on the first two axes of the discriminant analysis (FD1 and FD2). The numbers indicate the group to which each sample was initially assigned and which corresponds to the oceanographic periods defined (see text): (1): phytoplankton blooms during spring and autumn; (2): summer thermal stratification; (3): winter mixing and deep samples; (4): summer upwelling; (5): red tides and downwelling of surface waters Figueiras, F. 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