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

413 Seasonal succession o phy oplank on species on he coas o A Co uña (Galicia, no hwes Spain) B. Casas, M. Va ela and A. Bode Cen o Oc anog á ico de A Co uña. Ins i u o Español de Oceanog a ía. Apdo. 130. 15080 A Co uña, Spain Recei ed Oc obe 1997. Accep ed Feb ua y 1998. ABSTRACT Phy oplank on species composi ion was moni o ed mon hly a wo s a ions on he con inen- al shel o A Co uña (Galicia, no hwes Spain). The deepe s a ion (80 m) was sampled be- ween May 1989 and Decembe 1992. The coas al s a ion (20 m) was s udied du ing 1992. In each sampling, wa e samples we e aken om selec ed dep hs h oughou he wa e column. Mic o lagella es o med he bulk o phy oplank on, being dominan all yea long, ollowed by di- a oms, which domina ed du ing phy oplank on blooms. The samples s udied we e a anged in- o i e g oups, acco ding o en i onmen al condi ions de ined o he main oceanog aphic pe- iods in he a ea. Each g oup showed a cha ac e is ic phy oplank on species composi ion. The mos impo an phy oplank on blooms occu ed du ing sp ing and au umn. Howe e , majo phy oplank on abundances we e also de ec ed du ing upwelling e en s in he summe . Se e al species o he genus Chae oce os, especially C. socialis (Laude , 1864) we e dominan du ing blooms, and accompanying species, e.g. Laude ia bo ealis (G an, 1900), we e impo an du ing sp ing and au umn. Lep ocylind us danicus (Cle e, 1889) made a majo con ibu ion o phy o- plank on abundance du ing summe upwelling e en s. In con as wi h s udies conduc ed in neighbou ing a eas, dino lagella es ne e domina ed du ing summe s a i ica ion pe iods, p obably because o he e y equen upwelling e en s, which caused des abilisa ion o he wa- e column. Du ing win e mixing and in deepe wa e s, we ound a mixed communi y com- p ising species ha a e p esen h oughou he yea , bu in e y low densi ies, as well as esus- pended dia oms om sedimen . In 1992, a he coas al s a ion, ed- ide e en s occu ed, and some species o dino lagella es (e.g. Gymnodinium ca ena um G aham, 1943) eached densi ies o well o e 100 cells ml–1. Conside ing he en i e se ies o obse a ions, he mos no ewo hy ea- u es a e a dec ease in he magni ude o dia om blooms du ing he s udy pe iod and, on he con a y, an inc ease in he abundance o mic o lagella es. Key wo ds: Phy oplank on, succession, seasonali y, dia oms, dino lagella es, mic o lagella es, no hwes Spain. RESUMEN Sucesión es acional de i oplanc on en la cos a de A Co uña (Galicia, no oes e de España) Se es udia la composición de especies y g upos axonómicos de i oplanc on con una ecuencia ap oxi- madamen e mensual en dos es aciones de la pla a o ma con inen al en e a A Co uña (Galicia, no oes e de España). La es ación más p o unda (80 m) ue mues eada en e mayo de 1989 y diciemb e de 1992. La es- ación cos e a (20 m) ue es udiada du an e 1992. En cada mues eo se ecogie on mues as epa idas po oda la columna de agua. El g upo de i oplanc on más nume oso ue el de los mic o lagelados, siemp e do- minan es en odas las épocas del año, seguido de las dia omeas, esponsables de las p incipales p oli e a- ciones de i oplanc on egis adas. Las obse aciones se clasi ica on en cinco g upos según las ca ac e ís icas ambien ales de inidas pa a los p incipales pe iodos oceanog á icos de es a zona. Cada uno de es os g upos p esen ó una composición de especies de i oplanc on ca ac e ís ica. Las mayo es p oli e aciones de i oplanc- Bol. Ins . Esp. Oceanog . 15 (1-4). 1999: 413-429 INSTITUTO ESPAÑOL DE OCEANOGRAFÍA ISSN: 0074-0195 © Minis e io de Ag icul u a, Pesca y Alimen ación, 1999 BOLETÍN INTRODUCTION Phy oplank on species succession is linked o changes in he s a i ica ion o wa e -column su - ace laye s (Ha is, 1986). Mo phological and phys- iological adap a ions o phy oplank on cells enable some g oups o species o compe e ad an ageously du ing he a ious s eps o he con inuous ansi- ion be ween e ical mixing and s a i ica ion o he su ace laye (Ma gale , 1978; Smayda, 1980). In empe a e wa e s, one o he main causes o s a i ica ion is he s eady su ace hea ing du ing sp ing and summe due o inc easing sola i adi- ance (Walsh, 1988). In gene al, he succession p og esses in se e al s eps, om ini ial phases whe e s ong e ical mix- ing a ou s he dominance o chain- o ming di- a oms, o ma u e phases whe e wa e column s a - i ica ion a ou s he s a egy o dino lagella es and o he lagella es ha can swim o zones ich in ligh o nu ien s (Ma gale , 1978). Howe e , in a eas a - ec ed by episodic upwelling e en s, he upwelling o cold wa e s o su ace laye s o he wa e column adically changes he seasonal pa e n o s a i ica- ion, keeping he phy oplank on communi ies o hese a eas in he ini ial s ages o succession (Es ada and Blasco, 1985). On he Galician coas (no hwes Spain), which is a ec ed by upwelling e en s essen ially be ween Ma ch and Oc obe (F aga, 1981), seasonal a ia- ion o phy oplank on species composi ion has been well s udied in he ias, specially he Rias Bajas (Ma gale , Du an and Saiz,1955; Du án e al., 1956; Nunes e al., 1984; Figuei as and Niell, 1987b; Figuei as and Ríos, 1993), bu also in he Rias Al as (Campos and Ma iño, 1984; Ma iño e al., 1985). O he s udies ha e desc ibed phy oplank on species composi ion in he ias du ing ce ain sea- sons (Ma gale , 1956; Campos and Ma iño, 1982; Va ela, 1982; Blanco, 1985; Figuei as and Niell, 1987a; Figuei as and Pazos, 1991a,b). Howe e , e - e ences o phy oplank on on he Galician con i- nen al shel o he ias a e qui e sca ce. The only da a a ailable p e iously we e o ce ain mon hs (Es ada, 1984; Va ela e al., 1987a,b), bu no s ud- ies had been conduc ed on phy oplank on succes- sion in he a ea. Fo s udies o annual phy oplank- on succession on he con inen al shel o he Bay o Biscay, see Es ada (1982) and Fe nández and Bode, Casas and Va ela (1994). The scope o he p esen s udy is o desc ibe he seasonal a iabili y o phy oplank on abundance and cha ac e is ic species o he main oceano- g aphic pe iods desc ibed o La Co uña’s con i- nen al shel (Bode, Casas and Va ela, 1994; Casas e al., 1997). MATERIALS AND METHODS Samples o wa e and phy oplank on we e aken a wo s a ions o A Co uña ( igu e 1), a app oxi- ma ely mon hly in e als. S a ion 2 (dep h o 80 m) was sampled om May 1989 o Decembe 1992. S a ion 4 (dep h o 20 m) was sampled om Janua y o Decembe 1992. In each sampling, em- pe a u e, salini y, i adiance, dissol ed nu ien s, chlo ophyll, and pa icula e ni ogen and ca bon we e measu ed. Me hods used o de e mine hese pa ame e s and de ailed in o ma ion on hese da a B. Casas, M. Va ela and A. Bode Phy oplank on succession in no hwes Spain Bol. Ins . Esp. Oceanog . 15 (1-4). 1999: 413-429 414 on se p oduje on en p ima e a y o oño, pe o ambién se egis a on ele adas abundancias du an e episodios de a lo amien o en e ano. Va ias especies del géne o Chae oce os, especialmen e C. socialis (Laude , 1864), ue on las p incipales esponsables de es as p oli e aciones, acompañadas de o as especies de dia o- meas como Laude ia bo ealis (G an, 1900) en p ima e a y o oño, y Lep ocylind us danicus (Cle e, 1889) en el a lo amien o de e ano. A di e encia de lo encon ado en o as zonas p óximas, los dino lagela- dos no llegan a hace se dominan es en el pe iodo de es a i icación é mica de e ano, p obablemen e debido a la ecuencia de los episodios de a lo amien o. Du an e el pe iodo de mezcla in e nal y en las capas p o- undas de la columna de agua se encuen a una mezcla de especies p esen es odo el año, aunque con bajas abundancias, y ambién dia omeas del sedimen o esuspendidas. En 1992, especialmen e en la es ación cos- e a, se p oduje on episodios de ma ea oja du an e el e ano, supe ando algunas especies de dino lagelados (como Gymnodinium ca ena um G aham, 1943) 100 células ml–1. Conside ando oda la se ie de obse - aciones, se ap ecia una disminución en la magni ud de las p oli e aciones de dia omeas y un inc emen o en la abundancia de mic o lagelados en los úl imos años. Palab as cla e: Fi oplanc on, sucesión, es acionalidad, dia omeas, dino lagelados, mic o lagelados, no- oes e de España. can be ound in Casas (1995) and Casas e al. (1997). Phy oplank on samples we e aken wi h Niskin bo les a s anda d dep hs o 0, 5, 10, 20, 30, 40 and 70 m om s a ion 2, be ween May 1989 and Decembe 1991. F om 1992 on, sampling dep hs we e chosen acco ding o in si u i adiance p o- iles, so ha samples we e aken a 100, 50, 25, 10 and 1% o su ace i adiance le els (Casas, 1995). Phy oplank on samples we e p ese ed wi h Lugol’s solu ion and kep in da kness un il coun - ing, using U e möhl’s echnique (1958). Obse a- ion o samples was ca ied ou using a Nikon Diapho in e ed mic oscope wi h Noma sky phase- con as sys em. Magni ica ion powe s o 100, 200 and 400 we e used, acco ding o he size o o ganisms. Resul s a e exp essed in cells ml–1. When he o ganisms we e oo small (usually smalle han 30 µm) o be classi ied a he genus o species le el, hey we e included in wide g oups, e.g. lagella es, dino lagella es, and so on. Flagella es, which can belong o di e en axo- nomic ca ego ies, we e di ided in o h ee size class- es: 2-5 µm, 5-10 µm and la ge han 10 µm. The di- a om ela i e abundance index was calcula ed by di iding he o al numbe o dia oms by he o al numbe o dia oms plus dino lagella es o each sample. To de ine he g oups o cha ac e is ic species o each oceanog aphic si ua ion, a disc iminan analy- sis was ca ied ou using he species ound du ing he pe iod 1989-1992 a s a ion 2 and du ing 1992 a s a ion 4. The g oups o samples de ined a p io i we e hose desc ibed by Casas (1995) and Casas e al. (1997), using he hyd og aphic p ope ies o he wa e column, along wi h he nu ien and chlo o- phyll concen a ions obse ed in each case. The analysis was ca ied ou wi h 387 samples and 49 species o axa, selec ing hose axa whose ela i e equency was highe han 20 %, o a oid he e ec o a e species. Da a we e loga i hmically ans- o med. The Mul a 4 (Wildi and O loci, 1990) s a- is ical package was used in he analysis. RESULTS Tempo al dis ibu ion o cellula abundance Dis ibu ion o o al abundance o dia oms and dino lagella es in s a ion 2 ( igu e 2a) shows ha highe abundances occu o e sho pe iods o ime du ing au umn (> 7 000 cells ml–1 in 1989), summe (up o 6 000 cells ml–1 in 1990) and sp ing (a ound 2 000 cells ml–1 in 1989 and 1990). These pe iods coincide wi h maximum alues in chlo o- phyll and pa icula e ma e ial concen a ions a hese s a ions (Casas, 1995; Casas e al., 1997). The ela i e abundance o dia oms was usually high (in- dex alues highe han 0.5) h oughou he pe iod o s udy, wi h dia oms becoming e en mo e domi- nan du ing he a o esaid blooms. The ela i e abundance o dia oms was low du ing win e and some pe iods o summe ( igu e 2b), essen ially due o a dec ease in dia om abundance in he o - me case, and o he ela i e inc ease o dino lagel- la es in he la e . E en hough he dis ibu ion pa - e n o dia om ela i e abundance is simila among di e en yea s and also a bo h s a ions, alues o dia om abundance highe han 1 000 cells ml–1 we e obse ed mo e o en in 1989 and 1990 han in he o he yea s. Mic o lagella es always cons i u ed he bulk o phy oplank on. Mic o lagella es o 2-5 µm, coun - ed in he samples since 1991, showed abundances well o e 10 000 cells ml–1 in some au umn and summe samples ( igu e 3a). The abundance o mi- c o lagella es measu ing 5-10 µm does no show a clea seasonal pa e n o dominance, a leas un il 1991, when abundances highe han 1 000 cells ml–1 we e eco ded in su ace laye s du ing la e B. Casas, M. Va ela and A. Bode Phy oplank on succession in no hwes Spain Bol. Ins . Esp. Oceanog . 15 (1-4). 1999: 413-429 415 Figu e 1. Loca ion o s udy a ea and sampled s a ions A Co uña summe , ex ending he laye o highe abundance o s eadily inc easing dep hs h oughou 1992 ( ig- u e 3b). Va ia ions in abundance o his size ac- ion o lagella es a s a ion 4 we e ela i ely small (always a ound 1 000 cells ml–1). La ge mic o la- gella es (> 10 µm) we e especially dominan du ing summe pe iods, e en hough hey showed abun- dances highe han 100 cells ml–1 in au umn and win e du ing 1992 ( igu e 3c). Taking in o ac- coun all ac ions, bu especially he mid-size ac- ion (5-10 m), an inc ease in mic o lagella e abundance was obse ed du ing 1992. B. Casas, M. Va ela and A. Bode Phy oplank on succession in no hwes Spain Bol. Ins . Esp. Oceanog . 15 (1-4). 1999: 413-429 416 Figu e 2. Tempo al a ia ion o o al abundance o dia oms plus dino lagella es (cells ml–1, A) and ela i e abundance o di- a oms index (%, B) B. Casas, M. Va ela and A. Bode Phy oplank on succession in no hwes Spain Bol. Ins . Esp. Oceanog . 15 (1-4). 1999: 413-429 417 Figu e 3. Tempo al a ia ion o abundance o mic o lagella es, 2-5 µm (A), 5-10 µm (B) and la ge han 10 µm (C), ex- p essed in cells ml–1 Succession o species assemblages Du ing he p esen s udy, 205 species o ca e- go ies o phy oplank on we e ecognised. A com- ple e lis can be ound in Casas (1995). To sum- ma ise he in o ma ion ob ained, a disc iminan analysis was conduc ed on he mos abundan species ( able I). Samples we e classi ied in o i e g oups acco ding o cha ac e is ics o en i on- men al a iables in selec ed oceanog aphic s ages, summa ised in able II, as de ined in Casas (1995) and Casas e al. (1997). G oup 1 con ains samples ep esen a i e o phy oplank on blooms du ing sp ing and au umn, when chlo ophyll concen a- ions we e highe han 1 mg m–3 and a sligh den- si y g adien was p esen in he uppe laye o he wa e column. G oup 2 includes samples om he he mal s a i ica ion si ua ion obse ed du ing B. Casas, M. Va ela and A. Bode Phy oplank on succession in no hwes Spain Bol. Ins . Esp. Oceanog . 15 (1-4). 1999: 413-429 418 Table I. Mean alues (cells ml–1), s anda d e o o mean (s.e.m.) and p esence (n) o he 49 axa used in he disc iminan analysis. The code used in igu e 4 is indica ed Code Species o g oup n Mean s.e.m. DIN < 30 Dino lagella es < 30 µm 357 16.32 1.54 DIN > 30 Dino lagella es > 30 µm 73 0.55 0.18 DINACU Dinophysis acumina a 34 0.19 0.17 GYRSPI Gy odinium spi ale 83 0.33 0.09 GYRGLA Gy odinium glaucum 74 0.35 0.13 MASSPP Massa ia spp. 98 2.33 0.74 MICSPP Mic acan hodinium spp. 35 0.20 0.14 PROMIC P o ocen um micans 44 0.64 0.72 PROBIP P o ope idinium bipes 42 0.22 0.15 PRODIA P o ope idinium diabolus 24 0.11 0.11 SCRTRO Sc ippsiella ochoidea 87 2.93 1.98 ASTJAP As e ionella japonica 29 1.89 2.42 BACDEL Bac e ias um delica ulum 30 2.01 2.19 CERBER Ce a aulina be goni 66 0.34 0.13 CHFCHW Chae oce os a inis (+ a . willei) 85 7.07 4.70 CHACUR Chae oce os cu ise us 15 0.62 1.38 CHADID Chae oce os didymus 70 2.96 2.74 CHASOC Chae oce os socialis 119 83.74 33.41 CHASPP Chae oce os spp. 101 6.69 4.61 CENTR Cen ic dia oms 57 0.30 0.16 PENN Penna e dia oms 81 0.43 0.13 LAUBOR Laude ia bo ealis 54 4.31 2.77 LEPDAN Lep ocylind us danicus 124 26.83 23.87 LEPMIN Lep ocylind us minimus 83 3.82 1.84 NAVSAL Na icula c . salina um 73 0.18 0.06 NAVSPP Na icula spp. 37 0.28 0.21 NITEPI Ni zschia spp. 55 1.15 0.59 NITLON Ni zschia longissima 266 4.50 0.64 PARSUL Pa alia sulca a 39 0.49 0.37 PSEDEL Pseudoni zschia delica issima 129 2.34 0.72 PSEPUN Pseudoni zschia pungens 212 21.76 8.14 RHIDEL Rhizosolenia delica ula 154 10.57 3.15 RHIFRA Rhizosolenia agilissima 51 1.70 1.12 RHISHU Rhizosolenia sh ubsolei 64 0.47 0.25 RHISET Rhizosolenia se ige a 56 0.25 0.11 RHISTO Rhizosolenia s ol e o hii 71 1.46 0.67 SCHDEL Sch oede ella delica ula 54 3.38 2.77 SKECOS Skele onema cos a um 88 7.57 5.90 THANIT Thalassionema ni zschioides 78 1.09 0.67 THAFAL Thalassiosi a allax 72 2.69 1.60 EUGL Euglenaceae 39 0.16 0.11 DISSPE Dis ephanus speculum 72 0.32 0.12 SOLSET Solenicola se ige a 91 11.76 4.35 COCO Coccoli hopho ids 43 1.67 1.59 PHAPOU Phaeocys is pouche i 75 6.72 2.81 CRYP C yp ophyceae 300 41.60 6.10 MESRUB Mesodinium ub um 87 0.46 0.20 summe , when a sha p densi y g adien was e i- den in he wa e column, concen a ions o dis- sol ed nu ien s we e low, and chlo ophyll con- cen a ion we e usually lowe han 1 mg m–3. Win e samples and hose co esponding o deep laye s o wa e column du ing di e en pe iods we e included in G oup 3. In his g oup, samples a e cha ac e ised by a well-mixed wa e column wi h ela i ely high nu ien concen a ions and low alues o chlo ophyll, as well as low abun- dances o phy oplank on cells. G oup 4 comp ises samples om summe upwelling e en s, cha ac- e ised by ela i ely cold wa e (a ound 13 °C) and ich in nu ien s (i.e. > 5 µM o ni a e) nea he su ace. Samples om his g oup show chlo o- phyll concen a ions highe han 1 mg m–3, and a o al phy oplank on abundance simila o ha ob- se ed du ing sp ing blooms. Finally, G oup 5 in- cludes wo ypes o a e episodes: ed ides and downwelling. These wo phenomena we e ob- se ed du ing la e summe o 1992, and bo h a e ela ed o he p esence o wa m and low- densi y su ace wa e s. In he case o ed ide, p oli e a- ions o dino lagella es we e obse ed, wi h abun- dances well o e 100 cells ml–1. In he case o downwelling, wa m su ace wa e was ound a lowe dep hs o he wa e column, coinciding wi h a change in he su ace di ec ion ha accumula - ed su ace wa e owa ds he coas (Casas e al., 1997). When plo ing he species in ela ion o he i s and second disc iminan unc ions ( igu e 4) i can be obse ed ha he i s disc iminan unc ion (FD1) g oups oge he , in i s posi i e pa , dia oms –including Laude ia bo ealis (G an, 1900), Thalassiosi a allax (Meunie , 1910), Chae oce os so- cialis (Laude , 1864), Chae oce os cu ise us (Cle e, 1899), Sch oede ella delica ula (Pa illa d, 1913) and Rhizosolenia delica ula (Cle e, 1898)– ha a e all dominan species du ing sp ing and au umn blooms as well as upwelling e en s. The nega i e pa o FD1 sepa a es species ypical o s a i ied B. Casas, M. Va ela and A. Bode Phy oplank on succession in no hwes Spain Bol. Ins . Esp. Oceanog . 15 (1-4). 1999: 413-429 419 Table II. Mean alues, s anda d de ia ion (s.d.), numbe o samples (n), maximum alue (max) and minimum (min) o em- pe a u e ( , °C), salini y (S, usp), concen a ions o ni i e (µmol l–1), ni a e (µmol l–1), phospha e (µmol l–1) and dissol ed silica e (µmol l–1), and chlo ophyll-ain wo size ac ions: pa icles la ge han 12 µm (Chl-a> 12 µm, mg m–3) and smalle han 12 µm (Ch-a< 12 µm, mg m–3), g ouping he obse a ions acco ding o oceanog aphic pe iods de ined by Casas (1995) and Casas e al. (1997) o he s udy a ea. Samples om win e pe iod also include hose ob ained below he pho ic zone in o he pe iods Pe iod S Ni i e Ni a e Phospha e Silica e Chl-a> 12 µm Chl-a< 12 µm Phy oplank on 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 S a i ica ion 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 Win e 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 Summe 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 ide 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 wa e s –including he dino lagella es Gy odinium glaucum (Ko oid and Swezy, 1921), P o ope idinium diabolus (Balech, 1974), P o ope idinium bipes (Balech, 1974)– and dia oms ha domina e du ing phases o elaxing upwelling, e.g. Rhizosolenia s ol e - o hii (Pe agallo, 1898), Rhizolenia agilissima (Be gon, 1903), Chae oce os a inis (Hus ed , 1930), Lep ocylind us danicus (Cle e, 1889) and Lep ocylin- d us minimus (G an, 1915). The second disc imi- nan unc ion (FD2) shows, in i s posi i e pa , mos o he dino lagella e and dia om species, whe eas in he nega i e po ion appea hose species ypical o win e mixing and/o deep wa- e s, e.g. Na icula c .Salina um (G unow, 1878), Pa alia sulca a (Cle e, 1873) and Dic yocha speculum (Eh enbe g, 1839). The dis ibu ion o phy oplank on samples wi h ega d o FD1 and FD2 p esen s a sepa a ion in he space among he i e g oups, sepa a ing he posi- i e pa o FD1 he samples co esponding o sp ing, au umn and mos o upwelling blooms om summe s a i ica ion samples and hose be- longing o ed ide and downwelling e en s in he nega i e pa ( igu e 5). The second unc ion (FD2) also sepa a es in i s posi i e pa samples o sp ing, au umn and upwelling, bu also, hose o summe s a i ica ion, ed ide and downwelling. The nega i e pa o FD2 includes win e and deep- wa e samples. The disc iminan analysis co ec ly g ouped oge he mo e han 88 % o samples in he ini ial g oups, which clea ly con i ms he de- sc ip i e u ili y o he iden i ied oceanog aphic s ages. The species cha ac e ising G oup 1 a e essen- ially dia oms, among which C. socialis,L. bo ealis, Rhizosolenia se ige a (B igh well, 1858) and S. delica - ula, which can op 1 000 cells ml–1, a e he mos no ewo hy species. C. socialis eached e y high abundances (maximum o 6 000 cells ml–1 in au- umn 1989) du ing sp ing and au umn blooms a bo h s a ions ( igu e 6a). I also appea ed in high abundances du ing upwelling e en s (> 2 000 cells ml–1 in 1991, s a ion 2). Howe e , his species eached lowe densi ies du ing 1992 han in p e i- ous yea s. Dia oms o he genus Chae oce os we e gene ally dominan in all obse ed blooms. The main di e ence be ween sp ing and au umn blooms wi h ega d o upwelling blooms was in he numbe and ype o associa ed species ha ap- pea ed along wi h Chae oce os. Some o hese species occu ed exclusi ely du ing sp ing and au- umn, e.g. L. bo ealis ( igu e 6b). O he s we e mo e equen du ing summe upwelling e en s, e.g. L. danicus ( igu e 7a). Also du ing summe upwelling B. Casas, M. Va ela and A. Bode Phy oplank on succession in no hwes Spain Bol. Ins . Esp. Oceanog . 15 (1-4). 1999: 413-429 420 Figu e 4. P ojec ion o he phy oplank on species on he i s wo axes o disc iminan analysis (FD1 and FD2). Codes o species a e shown in able I (G oup 4), Lep ocylind us minimus,Pseudoni zschia delica issima (Heiden, 1928) and C. a inis (+ a . willei) showed abundances highe han 100 cells ml–1. Pseudoni zschia c . pungens (Hasle, 1993) and he P ymnesiophyceae Phaeocys is pouche i (Lage - heim, 1893) we e also abundan du ing upwelling, bu wi h lowe densi ies. On he con a y, summe s a i ica ion phases (G oup 2) we e cha ac e ised by a mix u e o dia oms and dino lagella es, wi h somewha lowe abundances compa ed wi h hose o blooms, and which can also appea in o he pe- iods. Among dia oms, L. danicus and some species o he gene a Chae oce os and Massa ia we e he mos abundan du ing his pe iod, bu none ex- ceeded 100 cells ml–1. An illus a i e example o a dominan dino lagella e du ing his phase is Gy odinium spi ale (Ko oid and Swezy, 1921), which only once showed abundances highe han 10 cells ml–1 ( igu e 7b). The species appea ing in he samples o G oup 3 we e usually p esen h oughou he yea . Skele onema cos a um (Cle e, 1873), o ins ance, showed ela i ely high abundances in Janua y and Feb ua y 1990, e en hough i also appea ed du - ing blooms in summe and au umn. Some o he me oplank onic dia oms, e.g. Pa alia sulca a, we e ypical o hese samples, and we e p esen in deep- e wa e s, no only du ing win e mixing, bu also du ing o he seasons, when esuspension o bo - om ma e ial occu s ( igu e 8b). Species causing ed ides (G oup 5) appea ed wi h highe abundances a he insho e s a ion. Du ing July 1992, Alexand ium lusi anicum (Balech, 1985) ( igu e 9a) and P o ocen um micans (Ehe en- be g, 1837) ( igu e 9b) we e dominan , he la e obse ed h oughou he sp ing and ea ly summe , al hough ne e in high abundances. In Sep embe , Gymnodinium ca ena um (G aham, 1943) ( igu e 9c) and Sc ippsiella ochoidea (Loeblich III, 1976) ( ig- u e 9d) domina ed he dino lagella e popula ion a he ime when downwelling o wa m su ace wa e s occu ed (Casas, 1995; Casas e al., 1997). DISCUSSION The alues o phy oplank on abundance ob- se ed in he p esen s udy all wi hin he ange o hose p e iously desc ibed by o he au ho s o Galician wa e s (Va ela, 1982; Es ada, 1984; Va ela e al., 1987a,b; Valdés e al., 1991; Va ela, 1992), bu hey a e highe han hose ci ed o he Can ab ian Sea (Es ada, 1982; Fe nández and Bode, 1994). The succession model is simila o ha expec ed B. Casas, M. Va ela and A. Bode Phy oplank on succession in no hwes Spain Bol. Ins . Esp. Oceanog . 15 (1-4). 1999: 413-429 421 Figu e 5. P ojec ion o samples on he i s wo axes o he disc iminan analysis (FD1 and FD2). The numbe s indica e he g oup o which each sample was ini ially assigned and which co esponds o he oceano- g aphic pe iods de ined (see ex ): (1): phy oplank on blooms du ing sp ing and au umn; (2): summe he mal s a i ica- ion; (3): win e mixing and deep samples; (4): summe upwelling; (5): ed ides and downwelling o su ace wa e s Figuei as, F. G. and Y. Pazos. 1991a. 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