Coccolithophore species as indicators of surface oceanographic
conditions in the vicinity of Azores islands
Q2
A. Silva
a,*, V. Brotas
a,b, A. Valente
c, C. Sá
a, T. Diniz
a, R.F. Patarra
d, N.V. Álvaro
d, A.I. Neto
d aCentro de Oceanografia, Faculdade de Ciências da Universidade de Lisboa, Campo Grande, 1749-016 Lisboa, PortugalbPlymouth Marine Laboratory, Prospect Place, Plymouth PL1 3DH, United Kingdom
cCentro do Clima, Meteorologia e Mudanças Globais (CCMMG), Universidade dos Açores, Campus de Angra do Heroísmo, Terra-Chã, 9701-851 Angra do Heroísmo, Açores, Portugal dAquatic Insular Research, Centro Interdisciplinar de Investigação Marinha e Ambiental (AIRe CIIMAR/CIMAR), CIRN & Departamento de Biologia, Universidade dos Açores Apartado 1422, 9501-801 Ponta Delgada, Açores, Portugal
a r t i c l e i n f o
Article history: Received 14 March 2012 Accepted 28 December 2012 Available online xxx Keywords: coccolithophores phytoplankton HPLC pigments Azores archipelago Atlantic Oceana b s t r a c t
During summer 2008 and spring 2009, surface oceanographic surveys were carried out around three islands of the Azores archipelago (Terceira, São Miguel and Santa Maria) to assess the phytoplankton distribution and associated physico-chemical processes. The Azores archipelago is a major feature in the biogeochemical North Atlantic Subtropical Gyre (NAST) province although its influence on the produc-tivity of the surrounding ocean is poorly known. Surface phytoplankton was studied by microscopy and HPLC (High Precision Liquid Chromatography). The mean values for biomass proxy Chlorophyll a (Chla) ranged from 0.04 to 0.55mg L1(Chla maximum¼ 0.86mg L1) and coccolithophores were the most abundant group, followed by smallflagellates, Cyanobacteria, diatoms and dinoflagellates being the least abundant group. The distribution of phytoplankton and coccolithophore species in particular presented seasonal differences and was consistent with the nearshore influence of warm subtropical waters from the south Azores current and colder subpolar waters from the north. The satellite-derived circulation patterns showed southward cold water intrusions off Terceira and northward warm water intrusions off Santa Maria. The warmer waters signal was confirmed by the subtropical coccolithophore assemblage, being Discosphaera tubifera a constant presence under these conditions. The regions of enhanced bio-mass, either resulting from northern cooler waters or from island induced processes, were characterized by the presence of Emiliania huxleyi. Diatoms and dinoflagellates indicated coastal and regional processes of nutrient enrichment and areas of physical stability, respectively.
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1. Introduction
The Azores archipelago (36e390N, 25e310W) consists of nine volcanic islands forming three groups (western, central and east-ern) located within the North Atlantic Subtropical Gyre
bio-geochemical province (NAST; Longhurst et al., 1995). The
archipelago lies in a transition zone between the North Atlantic Current (NAC) to the northwest, the Azores Current (AC), a jet-like current, ca.34N, to the south, and a region of weak circulation to the northeast (Juliano and Alves, 2007). Related to the main jet of the Azores Current there is an important thermohaline front, sep-arating fresher and colder waters to the north and warmer and saltier water masses to the south (Gould, 1985). The islands are not in the direct eastward path of the main jet, but are affected by the
recirculation patterns and eddies that originate from its
meandering. Due to convergent southward and northwardflows from the NAC and AC, respectively, strong thermal gradients are typical of the region (Lafon et al., 2004). Topographically-induced turbulence significantly modifies the physical and biological con-ditions adjacent to islands, which often result in higher marine productivity (Bakker et al., 2007; Hasegawa et al., 2008). In the NAST province, wintertime mixing provides the seasonal replen-ishment of nutrients to the euphotic zone while in spring, thermal stratification favours phytoplankton growth, which progressively leads to surface nutrient depletion by late summer. In the North Atlantic, blooms and seasonal mass flux of coccolithophores are known to occur (Holligan et al., 1993;Broerse et al., 2000;Sprengel et al., 2000) and most of the annual production takes place during spring (Schiebel et al., 2011).
Coccolithophores are a calcareous nannoplanktonic group which widespread distribution in the ocean, range from oligo-trophic subtropical gyres to temperate and high latitude euoligo-trophic regimes.
* Corresponding author.
E-mail addresses:asilva@fc.ul.pt,amsilva@ipma.pt,adsilva@fc.ul.pt(A. Silva).
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Estuarine, Coastal and Shelf Science
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / e c s s
0272-7714/$e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.ecss.2012.12.010 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110
The importance and motivation for studying coccolithophore dynamics, is that according to particular environmental conditions characteristic assemblages are found, which can be distinguished by their coccolith types and coccosphere morphology. As the group is known to be driven by oceanographic changes, reflecting on a fine scale, ecological patterns, and may be sensitive to climate change and ocean acidification (Broerse et al., 2000;Tortell et al., 2002;Rost et al., 2003;Smyth et al., 2004;Andruleit, 2007;Silva et al., 2008;
Tyrrell, 2008) it is always relevant to gather ecological information on individual species to determine which are capable of providing key significant responses. In this sense, Emiliania huxleyi is probably one of the best-studied phytoplankton species that is of relevance in the ocean. It is the most predominant coccolithophore and blooms have been reported from different settings of the North Atlantic and Pacific (Beaufort and Heussner, 2001;Beaufort et al., 2008), under conditions of high turbulence, during an early stage of the phyto-plankton succession in spring, as well as during calm and stratified conditions following the spring bloom. i.e., during MayeJuly in the North Atlantic (Silva et al., 2008, 2009;Schiebel et al., 2011). On the other hand, in warm waters depleted in nitrate and under very high light intensities, as off Bermuda (N Atlantic) the coccolithophore assemblage is different and species such as Discosphaera tubifera are observed (Haidar and Thierstein, 2001).
The present work is an output of project CAMAG, character-ization of coastal water masses in the vicinity of the islands of Terceira (Central group), São Miguel and Santa Maria (Oriental group). Our aim is to assess the abundance and diversity of the phytoplankton assemblage and describe the major physical pat-terns and regional processes by using coccolithophores as in-dicators of surface oceanographic changes and seasonal variations. 2. Methods
2.1. Surveyed area and sampling strategy
During summer 2008 (JulyeAugust) and spring 2009 (Maye June) three surveys were carried out, covering three islands of the Azores archipelago, Terceira (TER, Central group), São Miguel (SM,
Oriental group) and Santa Maria (SMA, Oriental group) (Fig. 1). As sampling was defined in the context of CAMAG project (related to the European Water Framework Directive), 44 samples were col-lected on board a small vessel, using a Niskin bottle to collect the surface water layer for phytoplankton microscopy observation and cell counting, pigment analysis and nutrient concentrations. Details on the water samples processing for the different analysis are described below. During summer, five stations were sampled around Terceira (stations 1,4 and 5 in the south; station 3 in the north and station 2 in the east), eleven around São Miguel (stations 1e5 and station G in the south and stations 6e10 in the north) and two in Santa Maria (station 1 and 2, in the south and east, respec-tively). During spring, four additional sites were sampled around Terceira (stations I1, P1 and P2 in the south and station I2 in the east), and two both in São Miguel (stations IN in the north and IS in the south) and Santa Maria (stations I1 and I2 in the south and east, respectively) (Fig. 3). Most of the stations were near the coast with 40 m depth, some stations were at ca.100 m depths (I1 at Terceira and IN and IS at São Miguel, I1 and I2 at Santa Maria) and a few at depths greater than 200 m (G at São Miguel, P1 and P2 at Terceira). 2.2. Physico-chemical parameters
Surface temperature was determined in situ with a Multi-parameter Water Quality Portable Meter Hanna HI-9828. Water for nutrient determination wasfiltered through a 0.45
m
m Millipore filter and stored at 4C for subsequent colorimetric analyses with a Tecator FIAstarÔ 5000 Analyser. Nitrite (NO2) plus nitrate (NO3) were determined according toGrassoff (1976), phosphate (PO34 ) was determined according toMurphy and Riley (1962)and silicate (Si(OH)4) according to Fanning and Pilson (1973). The detection limit for seawater analysis was 0.5m
M for silicate, 0.11m
M for nitriteþ nitrate and 0.1m
M for phosphate.2.3. Phytoplankton analysis
The phytoplankton assemblage was identified and counted through microscopy (Section2.3.1) and photosynthetic pigments
Fig. 1. Location of the Azores archipelago in the NE Atlantic Ocean context. The islands and stations sampled are highlighted: Terceira (TER, Central Group), São Miguel (SM, Oriental Group) and Santa Maria (SMA, Oriental Group).
111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240
were biochemically quantified by HPLC (Section 2.3.2). These complementary methodologies were fundamental to quantita-tively evaluate and characterize the phytoplankton community, in particular the smaller size fraction, known to be present in oceanic waters and normally underestimated by microscopy. A Principal Component Analysis (Section2.3.3) was used to statistically high-light potential groups of species regarding their temporal and horizontal distribution in all stations around the three islands. 2.3.1. Microscopy analysis
Phytoplankton samples were preserved with hexamethylene-tetramine buffered formalin to a final concentration of 2% (Throndsen, 1978). Phytoplankton species were identified and enumerated in subsamples of 50 ml by the Utermöhl technique (Hasle, 1978), using a Zeiss IM35 inverted microscope with phase contrast and brightfield illumination. A magnification of 160 and 400 was used to analyse the phytoplankton assemblage with
a detection limit of 60 cells L1and 3000 cells L1, respectively at a 95% confidence level (Bollmann et al., 2002). When possible, the cells were identified to species level according to Hasle and Syvertsen (1996),Dodge (1982)andYoung et al. (2003). A scan-ning electron microscope (JEOL-5200) was used to complete the identifications, in particular for the nannoplanktonic phores (e.g. holococcolithophores). Cells recognized as coccolitho-phores but that could not be further identified were included in the category“Undetermined species”. In addition, it was not possible to identify several small phytoplankton cells, which are designated hereafter as smallflagellates/others.
2.3.2. HPLC pigment analysis
The biomass and composition of phytoplankton were bio-chemically determined by HPLC, i.e, through the identification and quantification of various pigments and carotenoids from the dif-ferent classes of microalgae. Water samples (1.5 L) werefiltered Fig. 2. Satellite-derived SST and surface geostrophic currents averaged: a) during the 2008 observation period (1 Julye31 August 2008); and b) during the 2009 observation period (13 Maye21 June 2009). Stations are represented by crosses. White arrow point to coastal feature. In the lower left corner of each figure is represented a horizontal reference vector for a current speed of 10 cm/s.
Fig. 3. Distribution of Chla (mg L1) in São Miguel, Santa Maria and Terceira (from left to right), during summer 2008 (a,b and c) and spring 2009 (d,e and f). Q 3 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370
onto Whatman GF/Ffilters (nominal pore size of 0.7
m
m and 25 mm diameter), under vacuum pressure lower than 500 mbar. Thefilters were immediately frozen and stored at 80 C. Phytoplankton pigments were extracted with 3 mL of 95% cold-buffered methanol (2% ammonium acetate) for 30 min at20C, in the dark. Samples were sonicated (Bransonic, model 1210, w: 80, Hz: 47) for 1 min at the beginning of the extraction period. The samples were then centrifuged at 1100 g for 15 min, at 4 C. Extracts werefiltered (Fluoropore PTFE filter membranes, 0.2m
m in pore size) and immediately injected into the HPLC. Pigment extracts were ana-lysed using a Shimadzu HPLC comprised of a solvent delivery module (LC-10ADVP) with system controller (SCL-10AVP), a pho-todiode array (SPD-M10ADVP), and a fluorescence detector (RF-10AXL). Chromatographic separation was carried out using a C18 column for reverse phase chromatography (Supelcosil; 25 cm long; 4.6 mm in diameter; 5 mm particles) and a 35 min elution pro-gramme. The solvent gradient followedKraay et al. (1992)adapted byBrotas and Plante-Cuny (1996)with aflow rate of 0.6 mL min1 and an injection volume of 100m
L. The limit of detection (LOD) and limit of quantification (LOQ) of this method were calculated and discussed inMendes et al. (2007). Pigments were identified from absorbance spectra plus retention times and concentrations cal-culated from the signals in the photodiode array detector (Ex. 430 nm; Em. 670 nm). Calibration of the HPLC peaks was performed using commercial standards, namely, chlorophyll a (Chla) and chlorophyll b from Sigma, chlorophyll c2, chlorophyll c3, peridinin, fucoxanthin, diadinoxanthin, diatoxanthin, 190 -hexanoyloxyfucox-anthin, neox-hexanoyloxyfucox-anthin, prasinox-hexanoyloxyfucox-anthin, violax-hexanoyloxyfucox-anthin, allox-hexanoyloxyfucox-anthin, 190 -butanoyloxyfucoxanthin and zeaxanthin from DHI (Institute for Water and Environment, Denmark).Some pigments are exclusive of specific phytoplankton groups and can be used as taxonomic indicators (Jeffrey et al., 1997). For example, 190-Hexanoyloxyfucoxanthin (Hex-fuco) is exclusive of Prymnesiophytes and was used in this study as an indicator of coccolitophores. Coccolithophores cell counts were positively cor-related with Hex-fuco (r2¼ 0.369, p < 0.05), but not with fucox-anthin (also present in coccolithophores). Whereas fucoxfucox-anthin presented a significant correlation with diatoms cell counts (r2¼ 0.535, p < 0.001). Fucoxanthin was therefore used as a proxy for diatoms. Peridin is exclusive to dinoflagellates and was used as their marker, however the inverse is not necessarily the case (Jeffrey et al., 1997). The presence of this group was not always coincident with peridin concentration meaning that some di-noflagellates species found lacked this pigment or had the pigment in concentrations lower than the HPLC detection limit. Alloxanthin (biomarker for cryptophytes), prasinoxantin (exclusive for prasi-nophytes), chlorophyll b (present in clorophytes, prasinophytes and euglenophytes) and 190-Butanoyloxyfucoxanthin (in crysophytes and prymnesiophytes) were used as a proxy forflagellates while zeaxanthin (in clorophytes and cyanobacteria) was used as pro-karyotes indicator. This decision was based on the observation that
the source of zeaxanthin presented a distinct temporal and spatial distribution from the other three pigments, being most probably of cyanobacteria. Unicellular marine cyanobacteria belong mainly to two genera, Synechococcus and Prochlorococcus, which differ in the form of chlorophyll a, monovinyl and divinyl, respectively. The HPLC method used does not allow the separation of these pigments and therefore were treated as a group, prokaryotes.
2.3.3. Principal component analysis
Principal component analysis (PCA;Hair et al., 1998) was con-ducted in order to identify potential groups of species regarding their temporal and spatial distribution in all stations around the three islands of Azores, during summer and spring (after data standardization and logþ 1 transformation) using Primer 6 soft-ware (Clarke and Gorley, 2006). The input variables for the PCA were cell counts, pigment concentrations and stations (separated by seasons). The analysis was carried out with the species that occurred at least in 20% of the samples. PCA axis labels (PC1 asso-ciated with seasonality and PC2 with nutrient availability) derived from the interpretation of overall results.
2.4. Satellite derived data
Satellite-derived maps of sea surface temperature (SST) and surface currents were used to investigate oceanographic conditions in the region. The SST maps were obtained from the“North Atlantic Regional Sea Surface Temperature” (NAR SST) product, provided by the EUMETSAT’s Ocean and Sea Ice Satellite Application Facility (OSI-SAF;CMS, 2009). The NAR SST product consists of four daily SST maps (approximately 02 h, 10 h, 12 h and 20 h UTC) calculated from the infra-red (IR) channels of the National Oceanic and At-mospheric Administration/Advanced Very High Resolution Radio-meter (NOAA/AVHRR) sensors and re-mapped onto a stereopolar grid at 2 km resolution. Geostrophic velocityfields were derived from the delayed time‘‘Up-to-date’’ global gridded product of sea level anomalies and produced by Ssalto/Duacs at Collecte Local-ization Satellites (CLS, 2009). This product is generated every 7 days at 1/3resolution and was obtained from the AVISO website (http:// www.aviso.oceanobs.com/en/data.html). The mean maps of SST and surface currents were computed by averaging all data between 1 Julye 31 August 2008 (Fig. 2a) and 13 Maye 21 June 2009 (Fig. 2b), which correspond to thefirst and last day of the summer and spring observation periods, respectively.
3. Results
3.1. Physico-chemical data
In situ surface temperatures from summer 2008 ranged from 17 to 18.5C, and were on averagew3C higher than in spring 2009 (>20C) (Table 1). The averaged surface circulation denoted the
Table 1
Average values for temperature (C) and nutrients (Nitriteþ Nitrate, Silicate and Phosphate,mM), during summer 2008 and spring 2009 surveys, in the three islands (N-north, S-South and E-east), () means not measured.
São Miguel (SM, Oriental group) Santa Maria (SMA, Oriental group) Terceira (T, Central group)
Summer 08 Spring 09 Summer 08 Spring 09 Summer 08 Spring 09
N S N S S E S E N S E N S E Temperature (oC) 21.9 21.4 18.5 18.3 e e 18.5 18.1 22.6 20.4 22.0 17.6 17.0 17.4 Nitritesþ Nitrates (mM) e 0.50 1.06 0.88 e e 0.63 0.71 0.00 0.00 0.00 0.54 0.94 1.39 Silicates (mM) e 12.90 6.83 7.32 e e 6.41 9.90 5.77 6.11 14.48 6.54 11.40 8.92 Phosphates (mM) 0.24 0.27 0.66 0.58 e e 0.74 0.66 0.21 0.20 0.22 0.94 0.86 0.85 Number of stations 5 6 6 7 1 1 2 2 1 3 1 1 6 2 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500
influence of the Azores Current on the southernmost islands, and the analysis of averaged SST maps during the sampling periods revealed patches of colder waters in the vicinity of the islands, more visible at the S-SW Terceira (highlighted by a white arrow in
Fig. 2a). The sum of surface nitrate plus nitrite, mostly determined during spring recorded the higher concentrations in Terceira and São Miguel (Table 1). Phosphate concentrations, when determined during both seasons, were higher during spring and around Ter-ceira. Silicate was always largely available particularly at the S-SE sides of the islands, being the higher concentrations determined during summer south of São Miguel and at the east side of Terceira (Table 1). The spring concentrations of silicate were at least three times higher than those recorded during summer.
3.2. Phytoplankton assemblage
Phytoplankton biomass, given by its proxy, Chla concentration, ranged in average between 0.04
m
g L1at the east side of Santa Maria, during summer and 0.55m
g L1at the eastern stations of Terceira, during spring, where the maximum was recorded (0.86m
g L1in # 2 at east Terceira,Fig. 3f).Coccolithophores, diatoms and dinoflagellates were usually the dominant groups, accounting for 90% of total counted phyto-plankton (TF) (Fig. 4,Table 2). The highest abundances (cells L1) were observed during spring while during summer the number of species identified was higher. Coccolithophores overall presented the greatest abundances, reaching a maximum of 93% of total phytoplankton counted in Santa Maria, during spring and
a minimum of 1% in Terceira, during summer, when the assemblage was dominated by dinoflagellates and small flagellates (Table 2). Dinoflagellates were most abundant during summer while diatoms increased during spring.
The analysis of chromatograms identified a total of 16 pigments (Table 2). Overall, the pigment signatures obtained by HPLC con-firmed the distinct cell distribution patterns identified by micro-scopy but revealed also the presence of a prokaryotes-picoplankton community, unable to be identified by microscopy (Table 2). Although almost no zeaxantin was observed in spring, this pigment had, in average, relatively high concentrations in summer (0.07
m
g L1), particularly in Terceira and at the southernmost is-land of Santa Maria (Oriental group), indicating that the pro-karyotes community could be significant and probably dominant in this area, during this period, concomitantly with the lowest total-cell and Chla values (0.04m
g L1).The most diversified pigment’s set was found in the northern stations of São Miguel, during summer, confirming the microscopy observations of euglenophyceae cells and small flagellates, and allowing a further resolution within the smallflagellates’ assem-blage. This community seems to be constituted by cryptophytes (biomarker alloxanthin), prasinophytes, or prasinophytes plus chlorophytes (as prasinoxanthin, Chlb, violaxanthin and neo-xanthin were detected), and probably chrysophytes (its presence, however, cannot be confirmed, as Chlc3, diadinoxanthin, fucox-anthin and 19’ButFuco, can be present also in other groups iden-tified by microscopy as coccolitophores and diatoms). The most abundant carotenoid was fucoxanthin, especially in Terceira
Fig. 4. Microscopy observations of phytoplankton groups ( 103cells L1) in a) São Miguel, b) Santa Maria and c) Terceira. [N-north, S-south, E-east, S08-summer 2008 and Sp09-spring 2009]. Note scales are different.
501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630
(0.018e0.022
m
g L1), where diatoms were more abundant (Table 2,Fig. 4). The second most abundant was Hex-fuco, always present (Table 2), which is in accordance with the ubiquitous presence of coccolithophores, the maximum (0.15
m
g L1) corresponded to thehighest concentration of E. huxleyi, 35 103cell L1at #1, north of Terceira, in spring (Table 2,Fig. 5). Moreover, the highest concen-trations of But-fuco and Hex-fuco (characterizing Haptophytes type 6, 7 and 8, Zapata et al., 2004) were coincident, reinforcing the Table 2
Average values for phytoplankton cell counts by microscopy ( 103cells L1), coccolithophore species % in relation to total coccolithophores (TC) and pigments determined (mg L1) during summer 2008 and spring 2009 surveys, by geographical location (N-north, S-South and E-east) per island. () means not measured.
( 103 cells L1) São Miguel (SM, Oriental group) Santa Maria (SMA, Oriental group) Terceira (T, Central group)
Summer 08 Spring 09 Summer 08 Spring 09 Summer 08 Spring 09
N S N S S E S E N S E N S E Total phytoplankton (TF) 10.5 7.2 18.9 14.0 5.0 3.9 8.5 4.5 27.4 31.0 31.8 17.4 26.0 28.9 Coccolithophores (TC) 0.6 4.7 11.5 8.9 4.3 3.3 6.9 4.2 0.2 11.0 12.2 5.2 17.7 17.3 Coccolithophores (% TF) 6 65 61 64 86 85 81 93 1 35 38 30 68 60 Diatoms 2.4 1.5 0.6 3.3 0.1 0.1 1.1 0.3 0.1 8.4 2.6 11.9 7.8 10.3 Dinoflagellates 1.7 0.2 0.1 0.2 0.6 0.4 0 0 23.2 9.0 14.5 0.3 0.3 0.3 Chryptophytaþ Prasinophyta þ smallflagellates þ Cyanobacteria 5.7 0.8 6.8 1.7 0.0 0.1 0.5 0.0 4.0 4.0 2.5 0.0 0.2 1.0
Number of species identified 27 29 19 20 24 16 19 12 27 30 34 32 28 28
Number of stations 5 6 6 7 1 1 2 2 1 3 1 1 6 2 (%TC) C.leptoporus e 0.5 0.5 0.1 1.4 e 0.4 e 13.3 0.1 0.3 e 0.2 0.3 C.quadriperforatus e 0.2 e e e e e 0.7 e 0.1 e 0.4 0.5 0.7 D.tubifera e 1.0 0.2 0.1 9.8 12.1 1.1 1.2 46.7 0.5 0.2 e e e E.huxleyi 97.1 66.7 98.0 89.6 e e 87.5 83.9 e 97.1 73.8 96.7 60.7 98.1 H.carteri e 0.2 e e e e e e e e e e e 0.1 Ophiaster sp. e 0.2 0.1 e 2.1 1.8 0.1 0.1 e e e e e e U.sibogae e 0.6 0.1 0.2 e e 0.2 0.5 e e e 0.6 0.1 0.1 Syracosphaera spp. e 4.0 0.2 0.2 3.3 8.2 0.4 0.6 e e e 1.0 0.1 0.3 Holococcolithophore spp. 2.3 4.9 1.0 2.0 83.0 17.5 10.1 13.0 26.7 2.1 1.1 1.4 38.3 0.4 Undetermined species 0.6 21.7 e 7.9 0.5 60.4 0.2 e 13.3 0.1 24.6 e e e Pigments (mg L1) (maximum) Chlorophyll a (0.86) 0.33 0.27 0.23 0.27 0.07 0.04 0.16 0.12 0.14 0.37 0.15 0.25 0.40 0.55 Chlorophyll b (0.11) 0.09 0.03 0.05 0.03 e e 0.01 e e 0.02 e e 0.02 0.03 Fucoxanthin (0.34) 0.04 0.09 0.02 0.08 e e 0.06 0.05 e 0.15 0.03 0.07 0.18 0.22 Peridinin (0.14) 0.04 e e e e e e e 0.03 0.01 0.02 0.14 0.01 e 190-Hexanoyloxyfucoxanthin (0.15) 0.03 0.03 0.04 0.05 0.01 0.01 0.01 0.03 0.02 0.05 0.02 0.03 0.07 0.10 190-Butanoyloxyfucoxanthin (0.06) 0.01 0.01 e 0.02 e e 0.01 0.01 e 0.01 e 0.02 0.03 0.04 Alloxanthin (0.02) 0.01 e 0.01 e e e e e e e e e e e Zeaxanthin (0.08) 0.06 0.04 e 0.01 0.06 0.07 e e 0.06 0.04 0.07 e e e Prasinoxanthin (0.03) 0.01 e 0.01 0.01 e e e e e e e e e e
Other pigments detected and respective maximum concentration, (mg L1), were: Chlorophyll c3 (0.23), Chlorophyll c1 plus c2 (0.19), Diadinoxanthin (0.06), Diatoxanthin (0.01), Violaxanthin (0.01), Neoxanthin (0.01) andb,b-Caroteno (0.02).
() means not observed for % TC and BDL for pigments.
Fig. 5. Distribution of coccolithophore species by station ina) São Miguel, b) Santa Maria and c) Terceira (cells L1). Holococcolithophores and other‘undetermined species’ are not represented here, however its contribution to the total coccolithophore assemblage is shown inTable 2. [N-north, S-south, E-east, S08-summer 2008 and Sp09-spring 2009]. 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682 683 684 685 686 687 688 689 690 691 692 693 694 695 696 697 698 699 700 701 702 703 704 705 706 707 708 709 710 711 712 713 714 715 716 717 718 719 720 721 722 723 724 725 726 727 728 729 730 731 732 733 734 735 736 737 738 739 740 741 742 743 744 745 746 747 748 749 750 751 752 753 754 755 756 757 758 759 760
observation of the dominance of coccolithophores in the studied area.
The analysis of each of the three islands (Table 2,Fig. 4) showed that around São Miguel (Oriental group), the northern side recor-ded always higher cell concentrations and the differences in the phytoplankton assemblage from summer 2008 to spring 2009 were: i) coccolithophore cell counts increased on both sides of the island, accounting more than 60% of TF; ii) diatom maxima changed from N (#6) to S (#5), iii) dinoflagellates declined at the north side, and iv) the smallflagellates assemblage increased and were still more abundant on the northern side of the island, dominating the phytoplankton assemblage together with coccolithophores (36% and 61% of TF, respectively). The southernmost island of the Ori-ental group, Santa Maria, showed Chla values in average always extremely low (0.04
m
g L1in summer and 0.16m
g L1in spring), microscope observations indicated a dominance of coccolitho-phores (>80% of TF) and the HPLC analysis suggested also the presence of a strong prokaryote community. From summer to spring the diatom assemblage increased while dinoflagellates dis-appeared. On the other hand, the island from the Central group, Terceira, exhibited the highest Chla concentrations (0.55m
g L1at the east side, average value # 2 and I2, during spring) and cell counts (31.8 103cells L1). The composition and distribution of phytoplankton groups changed spatially around the island, be-tween summer 2008 and spring 2009 as: i) coccolithophores increased, especially due to higher concentrations of Emiliania huxleyi, and were distributed preferably at the south and east sides of the island, reaching 68% of TF in the south; ii) diatoms increased at the north (68% of TF) dominating the phytoplankton assemblage, iii) dinoflagellates clearly decreased (from 46% in the north to< 2% of TF) and iv) small flagellates decreased to a minor pres-ence (<4%TF).Regarding the distributing of each coccolithophore species (Fig. 5, Table 2), distinct spatial and temporal maxima were observed around each island. E. huxleyi, present in most of the samples, always accounted >60% of the total coccolithophore assemblage (TC). The species increased in abundance from summer to spring, except in the south of Terceira. This island and São Miguel recorded the highest concentrations while in Santa Maria, during summer, this species was absent from surface samples. Other identified coccolithophores (Fig. 6), less abundant (<47% of TC) than E. huxleyi, were mostly of a subtropical to temperate origin, as several species from the genus Syracosphaera (grouped as SUM Syracosphaera spp.), Discosphaera tubifera, Calcidiscus leptoporus, Calcidiscus quadriperforatus, Umbilicosphaera sibogae, Helicosphaera carteri and Ophiaster spp. São Miguel and Santa Maria presented a greater number of species and an important contribution of the holoccolithophore fraction (they are produced during the haploid phase of the life-cycle of a wide range of coccolithophores that bear heterococcoliths in their diploid life-cycle phase) to the total coc-colithophore assemblage (e.g.83% of TC in Santa Maria during summer). The Syracosphaera spp. assemblage could not be analysed in the perspective offinding markers for hydrological conditions, since it is composed of several species with a small and random occurrence.
D. tubifera distribution (Table 2,Fig. 5) was higher during sum-mer, at the south of Santa Maria and north of Terceira, but absent during spring conditions around Terceira. The coccolithophores Calcidiscus leptoporus and Calcidiscus quadriperforatus, occurred during both seasons in all islands, with the former being most abundant at the north of Terceira during summer (13.3% TC) and the latter occurring preferentially during spring around this island in particular. U. sibogae peaked only at the south of São Miguel during summer and was distributed preferably during spring
-6 -4 -2 0 2 4 PC1 (38.4%) -8 -6 -4 -2 0 2 PC 2 (2 1. 3%) S-SM S-SM S-SM S-SM S-SM N-SM N-SM N-SM N-SM N-SM S-SM S-SM S-SM S-SM S-SM N-SM N-SM N-SM N-SM S-T E-T N-T S-T S-T S-T S-T E-T E-T E-TN-T S-T S-T S-SMA E-SMA S-SMA E-SMA S-SMA E-SMA D.tubifera E.huxleyi Small flagellates 16 4 7 Chaetoceros spp. C.leptoporus C.quadriperforatus 11 10 1 8 56 12 13 2 143 15 CC32+C1 Perid Hexa-fuco Fuco Buto-fuco Prasin Zea Chlb Chla 9 S-T
Fig. 6. Distribution of phytoplankton species and pigments in the space defined by the first (PC1) and second (PC2) components. 761 762 763 764 765 766 767 768 769 770 771 772 773 774 775 776 777 778 779 780 781 782 783 784 785 786 787 788 789 790 791 792 793 794 795 796 797 798 799 800 801 802 803 804 805 806 807 808 809 810 811 812 813 814 815 816 817 818 819 820 821 822 823 824 825 826 827 828 829 830 831 832 833 834 835 836 837 838 839 840 841 842 843 844 845 846 847 848 849 850 851 852 853 854 855 856 857 858 859 860 861 862 863 864 865 866 867 868 869 870 871 872 873 874 875 876 877 878 879 880 881 882 883 884 885 886 887 888 889 890
around Santa Maria and Terceira. Ophiaster species were more abundant during summer in São Miguel and Santa Maria and ab-sent around Terceira.
Concerning the other phytoplankton groups (Fig. 4), diatoms as the chain-forming species from the genera Chaetoceros and Pseu-donitzschia were the most abundant, while Guinardia, Dactyliosolen, Leptocylindrus and Thalassiotrix exhibited lower concentrations. These species were recurrent components of diatoms peaks during spring in the south of São Miguel and Santa Maria and during summer in the south of Terceira. The dinoflagellates peaks were mostly composed by the genus Ceratium, Prorocentrum and Proto-peridinium as in Terceira during summer (#3). Unidentified
di-noflagellates comprised small thecated as well as naked
dinoflagellates but never as a dominant fraction of this phyto-plankton component (seeTable 3for all species identified).
3.3. Principal component analysis
Thefirst three components explained 79.4% of the total variation in the data and thefirst two are represented in Fig. 6. Thefirst component (PC1) explained 38.4% of total variability and separated the spring and summer conditions. E. huxleyi was strongly asso-ciated with spring while small flagellates appeared related to summer, related to the São Miguel northern stations. The second component (PC2) explained 21.3% of total variability and separated all stations of São Miguel þ small flagellates and Terceiraþ E. huxleyi from the southernmost island of Santa Maria, during summer, in turn associated with Discosphaera tubifera.
Considering the distinct ecological preferences that these species are known to have from the literature (see Discussion) as well as the observations in this study, this axis was interpreted as a nutrient availability gradient. The third component (PC3, 19.7%), plot not shown, highlighted a fourth group of species, diatoms Pseudonitzschia spp. and Chaetoceros spp., related to the south station of Terceira during summer (#1). All the other species and pigments plotted, presented an indistinguishable distribution along axes, with the exception of the two species from the genus Calcidiscus that were slightly detached towards D. tubifera.
4. Discussion
The complementarity of microscopy observations and HPLC photosynthetic pigment analysis showed that phytoplankton and coccolithophores in particular, the most abundant group, presented temporal differences between summer 2008 and spring 2009, as well as spatial differences in the surface distribution around the three islands (Terceira, São Miguel and Santa Maria). Surface spatial differences between islands appear related to large scale circula-tion, such as the transport of warm subtropical waters (typically nutrient poor) from the south and colder subpolar waters from the north (seeFig. 2), whereas nearshore small scale differences were associated with the existence of colder water patches representing most probably the signature of upwelling or mixing processes (S-SW of Terceira, # 1e2 and E side of São Miguel, # 1,Fig. 2). Using satellite andfield data,Lafon et al. (2004) observed episodes of lower temperatures and higher chlorophyll concentrations, on the Table 3
List of the species identified by microscopy.
Bacillariophyta (Diatoms) Dinophyta (Dinoflagellates) Prorocentrum spp. Ciliatea
Acnanthes spp. Amphidoma caudatum Prorocentrum triestinum Mesodinium rubrum
Actinophycus senarius Ceratium azoricum Protoperidinium bipes
Bacteriastrum hyalinum Ceratium candelabrum Protoperidinium claudicans
Biddulphia spp. Ceratium falcatum Protoperidinium crassipes
Chaetoceros spp. Ceratium furca Protoperidinium depressum
Cocconeis spp. Ceratium fusus Protoperidinium diabolum
Cylindrotheca closterium Ceratium massiliense Protoperidinium leonis
Dactyliosolen fragilissimus Ceratium minimum Protoperidinium ovum
Detonula pumila Ceratium pentagonum Protoperidinium pellucidum
Diploneis spp. Ceratium teres Protoperidinium pentagonum
Ditylum brightwellii Ceratium tripos Protoperidinium quinquecorne
Grammatophora spp. Cladopyxis brachiolata Protoperidinium spp.
Guinardia cf. delicatula Dinophysis cf. acuminata Protoperidinium steinii
Guinardia cf. striata Dinophysis spp. Protoperidinium tuba
Guinardiaflaccida Diplopsalis spp. Pseliodinium vaubani
Gyrosigma spp. Gonyaulax polygramma Scripsiella cf. trochoidea
Hemiaulus hauckii Gonyaulax spinifera Prymnesiophyceae (Coccolithophores)
Hemiaulus sinensis Gonyaulax spp. Acanthoica quattrospina
Lauderia annulata Gymnodinium spp. Algirosphaera robusta
Leptocylindrus danicus Gyrodinium fusiforme Braarudosphaera bigelowii
Leptocylindrus minimus Gyrodinium spp. Calcidiscus leptoporus
Licmophora spp. Lingulodinium polyedrum Calcidiscus quadriperforatus
Navicula spp. Noctiluca sintilans Coronosphaera mediterranea
Nitzschia longissima Ornithocercus magnificus Discosphaera tubifera
Paralia sulcata Ostreopsis cf. ovata Emiliania huxleyi
Planktionella sol Ostreopsis cf. siamensis Gephyrocapsa spp.
Pleurosigma spp. Ostreopsis heptagona Helicosphaera carteri
Proboscia alata Oxytoxum laticeps Ophiaster spp.
Pseudo-nitzschia spp. Oxytoxum scolopax Rhabdosphaera clavigera
Rhizosolenia setigera Oxytoxum spp. Syracosphaera prolongata
Rhizosolenia spp. Phalacroma rotundata Syracosphaera pulchra
Skeletonema costatum Podolampas spp. Syracosphaera spp.
Striatella unipunctata Pronoctiluca spinifera Umbilicosphaera sibogae
Surirella spp. Prorocentrum compressum Dictyochophyceae (Chrysophyta)
Thalassionema frauenfeldii Prorocentrum gracile Dictyochafibula
Thalassionema nitzschioides Prorocentrum lima Prasinophyta
Thalassiosira spp. Prorocentrum micans Pterosperma sp.
Thalassiotrix sp. Prorocentrum scuttelum Pyramimonas spp.
891 892 893 894 895 896 897 898 899 900 901 902 903 904 905 906 907 908 909 910 911 912 913 914 915 916 917 918 919 920 921 922 923 924 925 926 927 928 929 930 931 932 933 934 935 936 937 938 939 940 941 942 943 944 945 946 947 948 949 950 951 952 953 954 955 956 957 958 959 960 961 962 963 964 965 966 967 968 969 970 971 972 973 974 975 976 977 978 979 980 981 982 983 984 985 986 987 988 989 990 991 992 993 994 995 996 997 998 999 1000 1001 1002 1003 1004 1005 1006 1007 1008 1009 1010 1011 1012 1013 1014 1015 1016 1017 1018 1019 1020
south coasts of São Miguel and Santa Maria islands, than in offshore waters, proposing the existence of wind-driven upwelling south of these islands.
The satellite imagery (Fig. 2), physical-chemical in situ data, pigment results and species distribution, show clearly that Santa Maria is more strongly influenced by warmer oligotrophic waters from the AC northward incursions, than Terceira and São Miguel. Northward intrusions of warm subtropical water directly in the path of the Santa Maria and São Miguel, and southward intrusions of cold water near Terceira, combine to form a dipole-like structure which enhances gradients in the region. These gradients influenced the distribution of coccolithophore species (Fig. 5,Table 2). Overall results underlined the effect of island-induced biomass enhance-ment in oligotrophic oceanic regions and the important con-tribution of the nanoplankton fraction to the pool of Chla instead a picoplankton dominated assemblage. This shift is particularly evident at all islands during spring, when zeaxanthin is absent (Table 2). The average and standard deviation Chla values obtained in the present paper were: Terceira: 0.37 0.22, São Miguel 0.27 0.08 and Santa Maria 0.11 0.04
m
g l1. Except for Santa Maria island, these values are much higher than those reported byAiken et al. (2009), in a decadal study for the same latitudes in the NAST-E region (<0.25
m
g l1), where phytoplankton is dominated by prokaryotes and picoflagellates. In accordance with the spatial and temporal pattern for phytoplankton, surface nutrient results showed enrichment during spring more noticeable around Terceira (most influenced by colder subpolar waters). The values obtained in spring are similar to the ones found bySchiebel et al. (2011)for a NeS transect along 20W (3300.0390Ne 4659.6070N) in the North Atlantic. The relative abundance of silicates in relation to the other nutrients registered in this study was also observed by these authors. The fact that values reported here are, in some cases, double might be explained by the vicinity to the coast and the volcanic nature of these islands.During summer, the coccolithophore assemblage (Fig. 6,Table 2) was mainly composed by umbelliform species (k-selected), as Dis-cosphaera tubifera, in contrast with the dominance of placolith-bearing species during spring, as Emiliania huxleyi (r-selected).
Young (1994)defined three ecological communities of coccolitho-phores associated with three distinct environments: i) placolith-bearing cells such as E. huxleyi, Gephyrocapsa, Calcidiscus, found in coastal or mid-ocean upwelling regions; ii) umbelliform cells such as D. tubifera, Rhabdosphaera clavigera and Umbilicosphaera sibogae, found in more oligotrophic and calm waters and iii)floriform cells, such as Florisphaera profunda, associated with deep photic-zone assemblages.
In the present work, the subtropical coccolithophore assem-blage comprising the umbelliform Discosphaera tubifera, detached by the PCA analysis, indicates the influence of surface warmer waters of the Azores current (AC) around the islands, as well as summer conditions of reduced mixing and low nutrient concen-tration. This is particularly noticeable during summer, around Santa Maria (21.9% TC) and at the northern side of Terceira (46.7% TC). The coastal/upwelling related species Emiliania huxleyi was absent from these samples in both islands (Table 2,Fig. 5). D. tubifera has been associated with warm waters (in this study, 22.6C, the highest temperature) depleted in nitrate and having a preference for very high light intensities byHaidar and Thierstein (2001)around Ber-muda. We should emphasize that the highest abundance of D. tubifera coincided with the absence of nitrates in surface waters. D. tubifera maximum was also coincident with Calcidiscus lep-toporus maximum concentration in the north of Terceira during summer (13.3%TC, 22.6C, and absence of nitrates). This coccoli-thophore is usually observed in oceanic warm stratified and nutrient depleted conditions in the N Atlantic (e.g. Haidar and
Thierstein, 2001; in Bermuda, Renaud et al., 2002; in NABE-48,
Silva et al., 2009; in Lisbon bay). In the genus Calcidiscus, Calci-discus quadriperforatus presence coincided with higher nitrate and Chla averages, most evident at the east side of Terceira during
spring (0.7%TC, 17.4 C), where the highest biomass
(Chla¼ 0.55
m
g L1) and nitrate (1.39m
M) concentrations were observed (mean values for stations 2 and I2). This species is con-sidered to be more opportunistic than C. leptoporus (e.g.Haidar and Thierstein, 2001; in Bermuda,Renaud et al., 2002; in NABE-48,Silva et al., 2009; in Lisbon bay). In the genus Calcidiscus, C. quad-riperforatus is considered to be more suitable to emphasise the onset of the spring bloom or more productive environments, opposite to D. tubifera and C. leptoporus.The larger concentrations of holococcolithophores in Santa Maria (83% TC in the south) and Terceira (26.7% TC in the north) during summer, which were mainly due to Discosphaera tubifera and C. leptoporus, also confirm these species as indicators of warm oligothophic conditions. Several authors (e.gKleijne, 1991;Brand, 1994; Renaud and Klaas, 2001; Haidar and Thierstein, 2001) found more frequently the fragile haploid phase of coccolitho-phores in oligotrophic waters, higher temperatures and light in-tensities or associated with the beginning of water stratification and subsequent nutrients depletion.
Conversely, and as demonstrated by the PCA results, the greater development of diatoms and placolith-bearing Emiliania huxleyi during spring suggested a change in the hydrological regime to-wards more eutrophic conditions. E. huxleyi is considered to have an opportunistic behaviour usually reported during the early stage of phytoplankton spring production (Schiebel et al., 2011for the North Atlantic,Silva et al., 2008for Lisbon Bay). During summer the colder water patches observed (# 1e2 in Terceira, # 1 in São MIguel) were dominated by E. huxleyi and diatoms (e.g. chain forming Chaetoceros spp.) illustrating the enhancement effect of these colder patches on phytoplankton biomass. The boundaries of these patches were characterized by subtropical coccolithophores, like Discosphaera tubifera, and dinoflagellates (Fig. 5,Table 2). Di-noflagellates were in most stations the less abundant phyto-plankton group with a surface distribution similar to the subtropical assemblage of coccolithophores, indicating stratified and intermediate to oligotrophic conditions (Fig. 4,Table 2). The greater abundances, in the north of Terceira during summer coin-cided with the higher concentrations of the subtropical D. tubifera. The present findings seem to indicate that coccolithophore species could have a role in the study of the surface circulation patterns and hydrological variability around the Azores archipelago and that they are an important contribution to the pool of Chla, especially during spring. This work has contributed to the knowl-edge on the phytoplankton component of this biogeochemical ocean province which is still rather limited.
Uncited reference
Jeffrey and Hallegraeff, 1987. Q1
Acknowledgements
Field work was supported by the Projects “Caracterização das massas de água costeiras (CAMAG) das ilhas do grupo oriental (ORI) e Terceira (TER)” funded by Direcção Regional do Ordenamento do Território e Recursos Hídricos. The surveys performed in the pre-sent study comply with the current laws of Portugal. We thank Steve Groom, from Plymouth Marine Laboratory, for reading and commenting the manuscript. This work was partly funded by PEst-OE/MAR/UI0199/2011 (FCT), Projecto Estratégico e Centro de Oceanografia (CO/FC/UL) e 2011e2012, and by the ESA CoastColour 1021 1022 1023 1024 1025 1026 1027 1028 1029 1030 1031 1032 1033 1034 1035 1036 1037 1038 1039 1040 1041 1042 1043 1044 1045 1046 1047 1048 1049 1050 1051 1052 1053 1054 1055 1056 1057 1058 1059 1060 1061 1062 1063 1064 1065 1066 1067 1068 1069 1070 1071 1072 1073 1074 1075 1076 1077 1078 1079 1080 1081 1082 1083 1084 1085 1086 1087 1088 1089 1090 1091 1092 1093 1094 1095 1096 1097 1098 1099 1100 1101 1102 1103 1104 1105 1106 1107 1108 1109 1110 1111 1112 1113 1114 1115 1116 1117 1118 1119 1120 1121 1122 1123 1124 1125 1126 1127 1128 1129 1130 1131 1132 1133 1134 1135 1136 1137 1138 1139 1140 1141 1142 1143 1144 1145 1146 1147 1148 1149 1150
Project. Vanda Brotas had a sabbatical grant from FCT (SFRH/BSAB/ 1044/2010). We also thank the anonymous referees who com-mented the manuscript.
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