Gomez 2007 Climate Diatoms

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Unusual diatoms linked to climatic events in thenortheastern English Channel

Fernando Gómez ⁎, Sami Souissi

Station Marine de Wimereux, Université des Sciences et Technologies de Lille-Lille1,

FRE 2816 ELICO CNRS, 28 Avenue Foch, F-62930 Wimereux, France

Received 26 February 2007; accepted 2 August 2007Available online 21 August 2007

Abstract

The composition of the micro-phytoplankton was investigated at two fixed stations in the northeastern English Channel from November 1997 to December 2005. In late summer-autumn 2005, proliferations of the centric diatoms Proboscia indica and Rhizosolenia hebetata f. semispina were recorded for the first time in this area. The weather in 2005 was abnormal: a coldwinter and early arrival of summer conditions that extended to late autumn. This resulted in an unusually long, sustained calm andwarm period. We suggest that the reduction in mixing may have allowed diatoms of more-stratified waters to be competitive in thenormally highly mixed northeastern English Channel in summer-autumn 2005. In 2003 and in 2005, two warm-water diatomspecies were recorded for the first time. A colony of Eucampia cornuta was observed after an exceptional heat wave in September of 2003, and again in September 2005. Simultaneously at both fixed stations, Chaetoceros peruvianus was recorded in the warm

October of 2005. The climatic events are associated with the northward spreading of thermophilic diatoms such as E. cornuta andCh. peruvianus in the European Atlantic waters, and the proliferation of diatoms of stratified environments such as P. indica and

R. hebetata f. semispina in the usually highly turbulent waters of the northeastern English Channel.© 2007 Elsevier B.V. All rights reserved.

Keywords: Diatoms; Thermophilic and warm-water phytoplankton; Time series; Climate change; Interannual variability

1. Introduction

It is generally accepted that we are in a climate phaseof accelerated warming coupled with increased highfrequency of temperature variation (Schär et al., 2004).Meteorological or climatic factors affect the tempera-ture, salinity, and physical structure of the ocean water column. Climate change will influence the boundariesof biogeographical regions and thus the ranges of distribution of individual species. This hypothesis is

widely supported today, and has been reinforced byrecent studies that show responses of marine organisms

to climatic features of the northeastern Atlantic (e.g.,Alheit and Hagen, 1997; Beaugrand et al., 2000;Hawkins et al., 2003).

The summer of 2003 was characterised by excep-tionally hot weather in Europe, with the meantemperature exceeding that of any summer for the past 500 years (Luterbacher et al., 2004). A heat waveoccurred from 1 to 15 August 2003, which causedexcessive human mortality throughout Europe, to acatastrophic extent in France (Vandentorren et al.,2004). The year 2005 was also anomalous in Europe,

Journal of Sea Research 58 (2007) 283 –290www.elsevier.com/locate/seares

⁎ Corresponding author. E-mail address: [email protected] (F. Gómez).

1385-1101/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.seares.2007.08.002

mailto:[email protected]

http://dx.doi.org/10.1016/j.seares.2007.08.002

http://dx.doi.org/10.1016/j.seares.2007.08.002

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with a severe winter that caused snowfalls extending tounusual places such as Algeria. In late spring-summer,

rainfall and snowmelt caused severe flooding of centraland eastern European rivers, and multi-month drought conditions and forest fires also affected much of WesternEurope during July, August, and September (WorldMeteorological Organization, 2005).

Long-term monitoring is essential for advances inunderstanding the interannual variability and climaticlinkages of marine plankton. The French nationalSOMLIT (Service d'Observation en Milieu LITtoral)monitoring program was established off Boulogne-sur-Mer (NE English Channel) in November 1997. The

present study describes qualitative changes in thediatom composition since that time. Eucampia cornuta

(Cleve) Grunow was recorded for the first time in 2003,and Chaetoceros peruvianus Brightwell in 2005. In late

summer and autumn 2005, unusual rhizosolenioiddiatoms reached high abundances. We describe andillustrate these observations of unusual diatoms, whichreveal changes in the structure of the marine ecosystemin European Atlantic waters.

2. Material and methods

Overall, 158 cruises were carried out on board RV‘Sepia II’ from November 1997 to December 2005 off Boulogne-sur-Mer in the northeastern English Channel

(Fig. 1). Two fixed stations were sampled during hightides. One station was located 2 km offshore (50°40′

Fig. 1. Map of the English Channel. The arrow indicates the sampling stations (solid circles).

Fig. 2. Temporal distribution (1998–2005) of mean daily air temperature (°C) at Boulogne-sur-Mer.

284 F. Gómez, S. Souissi / Journal of Sea Research 58 (2007) 283 – 290



75N; 1°31′17E; 21 m depth) under the influence of the‘coastal flow’ (Brylinski et al., 1991). The other station,8 km offshore (50°40′75N; 1°24′60E, 50 m depth), wasoften separated by a tidal coastal front. The samplingfrequency was intended to be biweekly, but cruises weresometimes cancelled or were restricted to the shorestation because of bad weather. Conductivity-tempera-ture-depth (CTD) profiles were measured by a SeaBirdSBE 25 probe. However, in order to avoid erroneoustendencies due to the undersampled periods or mal-

functions leading to missing data during the eight years,the meteorological variability was determined from themeteorological station closest to the sampling stations.Continuous records of air temperature were provided bythe French Meteorological Agency (Météo-France)from a meteorological station located at Boulogne-sur-Mer (50°44′ N, 01°36′E, altitude 73 m).

Seawater samples were collected with a Niskin bottleat the surface and at one metre above the bottom. Lugol-fixed samples of 25 or 50 ml were settled in compositesettling chambers. The entire chamber was scanned at

200 × magnification with an Olympus IX71 invertedmicroscope, and specimens were photographed at 400×magnification. The sample analyses of the 8–y timeseries were carried out by the first author using the samemethod.

3. Results

3.1. Air temperature at Boulogne-sur-Mer (November

1997 — December 2005)

The lowest mean daily air temperatures wererecorded from mid-December to February 2003, with

a minimum of −4.56 °C on 8 January 2003. In winter 1998 and during the winter of 2005, lower temperatureswere also recorded (Fig. 2). The years with the highest number of days when mean air temperature was lower than 0 °C were 1998 (11 d), 2003 (9 d), and 2005 (9 d).The fewest days with mean daily temperature b0 °Cwere recorded in 1999 (1 d), 2000 (2 d), and 2004 (2 d).In winter 2005, mean air temperature fell below 0 °C on3 March, which was the latest temperature below 0 °C inthe 8–y time series (Fig. 2).

The highest mean daily air temperature of the 8–ytime series occurred on 10 August 2003, 28.35 °C. Thewarmest years, based on the number of days with meandaily air temperatures higher than 22 °C, were 2003(8 d) and 2005 (6 d). In contrast, 1998 and 2000 hadmean daily air temperatures higher than 22 °C on onlytwo days. As usual, the highest temperatures wererecorded in July, August, and early September. In 2005,the temperature reached daily means of up to 25.32 °Cin early June. These data indicate that 2003 and 2005were anomalous years. The winters were very cold in

both years, and extremely high temperatures wererecorded in the first half of August 2003. In 2005, thelow temperatures extended to March; high temperatureswere reached unusually early, in June, and continued tolate autumn (Fig. 2).

3.2. General diatom assemblage characteristics

The sampling stations were located near the Strait of Dover, which constitutes the maritime corridor betweenthe English Channel and the North Sea. This region issubject to strong tidal currents and winds (Salomon andBreton, 1991; Prandle et al., 1993). Within this highly

Fig. 3. Abundance (×103 cells L−1) of Proboscia indica, Rhizosolenia hebetata f. semispina, and Neocalyptrella robusta in autumn 2005. Note thedifferent scale of Fig. 3d.




turbulent environment, a few species of diatomsdominate the phytoplankton assemblage, even insummer. The spring diatom bloom is dominated byGuinardia delicatula (Cleve) Hasle, Rhizosolenia

imbricata var. shrubsolei (Cleve) Schröder, Guinardia

striata (Stolterfoth) Hasle, and Guinardia flaccida

(Castracane) H. Peragallo. Although these diatomsmay also re-appear in late summer-early autumn

Fig. 4. Photomicrographs of unusual diatoms off Boulogne-sur-Mer (NE English Channel), bright-field optics. (a –d) Rhizosolenia hebetata

f. semispina. (e–f) Proboscia alata. (g–h) Proboscia indica. (i) Neocalyptrella robusta. (j) Corethron cf. hystrix. (k–l) Chaetoceros peruvianus.(m–n) Eucampia cornuta. Dates of records: (n) 11/09/2003; (a, e) 08/08/2005; (b, c, f) 06/09/2005; (d, g, i, m) 19/09/2005; (h, j, k, l) 18/10/2005.Scale bars represent 50 μm.




proliferations, other species are more characteristic of this period, such as Pseudo-nitzschia spp. accompanied by Ditylum brightwellii (West) Grunow, Thalassionema

nitzschioides (Grunow) Grunow ex Hustedt, Eucampia

zodiacus Ehrenberg, Meuniera membranacea (Cleve) P.

C. Silva, and, towards winter, the tychoplanktonicspecies Paralia sulcata (Ehrenberg) Cleve and Rapho-

neis amphiceros (Ehrenberg) Ehrenberg. Rhizosolenia

setigera Brightwell was also a common component of autumn assemblage, with an exceptionally high abun-dance in October 2003.

3.3. Unusual rhizosolenioid diatom assemblage in late

summer-autumn 2005

From August to November 2005, unusual members of

the family Rhizosoleniaceae Petit were observed. InSeptember, October, and November 2005, Proboscia

indica (H. Peragallo) Hernández-Becerril (= Rhizosolenia

alata var. indica) was recordedforthe first time in the 8–ytime series. Rhizosolenia hebetata f. semispina (Hensen)Gran was very rare before 2005. However, these two largespecies reached high densities in autumn 2005. Also thelarge diatom Neocalyptrella robusta (Norman) Hernán-dez-Becerril et Meave (= Rhizosolenia robusta), veryrarely observed previously, reached a remarkable abun-dance in autumn 2005. This assemblage of large rhi-zosolenioid diatoms in late summer and autumn 2005 is

highly unusual.

3.3.1. Rhizosolenia hebetata f. semispina ( Fig. 3d,

Fig. 4a – d)

Associated with Proboscia indica was Rhizosolenia

hebetata f. semispina. Whereas P. indica is morpholog-ically distinctive, R. hebetata f. semispina may bemisidentified with congeneric species such as Rhizoso-

lenia styliformis Brightwell, especially the variety R. styliformis var. longispina Hustedt. R. hebetata f. semispina is characterised by two columns of segments

and claspers, and a pointed otaria along the proximal part of the process (Fig. 4a–d). The specimens of R. hebetata f. semispina reached up to 800 μm in length. No proliferation of R. hebetata f. semispina wasobserved before autumn 2005. This taxon showed thehighest abundance at the beginning of October, reaching20 000 cells L−1 at the subsurface depth of the onshorestation (Fig. 3d).

3.3.2. Proboscia indica (=Rhizosolenia alata var.

indica) ( Fig. 4 g – h)

Very few specimens of Proboscia alata (Brightwell)Sündstrom (= Rhizosolenia alata var. alata) were re-

corded in late summer 2005 (Fig. 4e, f). However, thedominant species of that genus was Proboscia indica

(= Rhizosolenia alata var. indica). This diatom differsfrom other rhizosolenioid diatoms by the proboscis, anelongated part of the valve with truncate tip and no

process. P. indica exhibits a number of characteristicsthat distinguish it from P. alata and related taxa: itsfrustule and valve dimensions are larger than in other species, and as in P. alata the chain arrangement isasymmetrical because of ‘displaced claspers’. The long proboscis is more strongly sloped than in P. alata

(Fig. 4e–h). No misidentification is possible becauseof this distinctive morphology (Jordan et al., 1991;Hernández-Becerril, 1995), although the nomenclatureof the species is complicated because it has been oftenconsidered a variety of R. alata. During this 8–y period,

P. indica was only recorded in 2005, persisting betweenAugust and late October. In September 2005, P. indica

reached an abundance of 10 000 cells L−1, becomingone of the main components of the autumnal diatomassemblage (Fig. 3).

3.3.3. Neocalyptrella robusta (=Rhizosolenia robusta)

( Fig. 4i)

This is a highly distinctive and very large species. Asingle record in April 2002 and two records in February2005 were the only observations before autumn 2005. Neocalyptrella robusta reached a maximum abundance

of 150 cells L−1 in September and October 2005(Fig. 3). Although its abundance was low comparedwith the previous taxa, it should be taken into account that the diatom is very large and thick, and contributesconsiderably to the phytoplankton biomass. This speciescompleted the unusual diatom assemblage of largerhizosolenioid diatoms observed in autumn 2005.

3.4. First records of tentative warm – water diatoms

The 2005 autumnal rhizosolenioid assemblage was

composed of diatoms that are common in surroundingwaters such as the North Sea and in the French Atlantic.The diatoms appeared for the first time in our study areaand reached high abundances, coming to dominate theassemblage. In 2003 and 2005, two diatoms, Eucampia

cornuta and Chaetoceros peruvianus, were for the first time recorded in the 8–y time series. In contrast to theunusual rhizosolenioid diatoms, E. cornuta andCh. peruvianus were restricted to a few records and ashort period in later summer or early autumn. Thesespecies are here considered to be biological indicators of warmer conditions. Specimens of the genus Corethron

Castracane, tentatively identified as C. hystrix Hensen




(Fig. 4 j) appeared for the first time. The first recordscoincided with Ch. peruvianus and re-appeared later inmid December. The consideration of Corethron cf. hy-

strix as a thermophilic species requires further research.

3.4.1. Chaetoceros peruvianus ( Fig. 4k –

l)Ch. peruvianus is easily identified, and was recorded

simultaneously at both sampling stations and bothdepths on 18 October 2005. The specimens appearedas solitary cells (Fig. 4k), with the exception of onetwin-cell chain, probably recently divided (Fig. 4l).Ch. peruvianus belongs to the Phaeoceros subgroup,with thick setae that contain mobile chloroplasts. Thecurved convex setae reached up to 400 μm in length.The maximum abundance was only 5 specimens per sample in the offshore surface station (100 cells L−1).

The offshore seawater temperature was 16.8 °C, and thesalinity was 34.9. Slightly lower values were observedonshore (16.4 °C and salinity of 34.6).

3.4.2. Eucampia cornuta ( Fig. 4m – n)

A colony of a diatom tentatively identified as Eu-

campia cornuta was recorded for the first time on 11September 2003 (seawater temperature 18.5 °C andsalinity 34.4) and reappeared on 19 September 2005(seawater temperature 18.8 °C and salinity 34.2). In bothyears, E. cornuta appeared as single 4-cell chains in theonshore station, at the surface in 2003, and at 21 m depth

in 2005. The cells were slightly curved in broad girdleview, elevations long, narrow and apertures tall andelliptical. The apical length of each cell was ∼12 μmand the pervalvar axis ∼50 μm (Fig. 4m, n). Another congeneric species, Eucampia zodiacus Ehrenberg, alsohas a concave valve face. It is quite common in autumn but may occasionally appear throughout the year. Theribbed bands of E. cornuta were not observed due to theoptical limitations. The identification is based on theslightly curved shape of the colony (Fig. 4n) and the talland elliptical apertures of E. cornuta (Fig. 4m, n) when

compared with E. zodiacus.

4. Discussion

Climate and meteorological events are well-knownregulators of phytoplankton growth, i.e., seasonal windregulation of the annual spring bloom, interannualvariations in this event and in mini-blooms, and speciessuccessions (Robinson and Hunt, 1986). According toDickson et al. (1988), the timing of stratification of thewater column and the resulting spring diatom outbreakis crucial in determining the extent of primary productivity during a particular year.

In the North Atlantic, sea-surface temperature, precipitation, and wind and weather patterns arestrongly influenced by atmospheric circulation patternsindexed as the North Atlantic Oscillation (NAO). High(positive) winter (December to February) NAO index

values imply increased westerly wind strength andmixing, increased precipitation, and milder winters.Stronger mixing (increased winds) and nutrient levels(increased river runoff) should favour diatoms rather than flagellates. Low (negative) NAO index values areassociated with southerly or south-easterly winds andwith colder winters. Highly negative monthly NAOvalues in summer are associated with increased seasonalstratification, shallower mixed depths, and risingtemperatures (Pingree, 2005).

The winter NAO index values from 1998 to 2005

were highly positive in 1999 (1.70) and 2000 (2.80), but there was a single highly negative value in 2001(−1.89). The values were 0.20 in 2003,−0.07 in 2004,and 0.12 in 2005. Consequently these NAO winter index values close to ∼0 cannot be associated with thehighly positive or negative values. Negative valuescould be expected, taking into account the cold wintersin 2003 and 2005. The relationship between values of the NAO index and the meteorology in our study area isnot obvious. Consequently, the relationship between the NAO index values and the unusual qualitative changesof the phytoplankton composition first observed in 2003

is also not immediately apparent.

4.1. Autumnal rhizosolenioid diatom assemblage

The occurrence, persistence, and high abundance of two species, Rhizosolenia hebetata f. semispina and Proboscia indica, in late summer-autumn are unusualwhen compared with the previous 8–y time series.These species are cosmopolitan and associated with thesummer stratification in northern European waters( Nehring, 1998) and along the French Atlantic coast

(Paulmier, 1997). Establishing which factors caused theappearance of these species in 2005 is not easy. For unknown reasons, these species were very rare at our sampling stations, whereas they were more common insurrounding regions. The main distinctive characteristicof our study region is its high turbulence level. Areduction of mixing in summer-autumn 2005 could havefavoured the development of species that were morecommon in stratified waters of surrounding regions. Our monitoring stations are located near the Strait of Dover,which is governed by a northerly tidal flux (amplitude)of 1.06×106 m3 s−1 from the English Channel to the

North Sea, enhanced by frequent southwest winds, with




a high tidal amplitude (8 m maximum tidal range)(Salomon and Breton, 1991; Prandle et al., 1993). Thetides are periodic astronomical-forced phenomena that have little influence on the interannual variability of the phytoplankton.

A factor that affects the reduction of water columnmixing is thermal stratification (Pingree et al., 1978).The high temperatures in summer 2003 were excep-tional (Luterbacher et al., 2004). However, this was ashort two-week heat wave that did not extend to autumn.For example, the monthly mean air temperature inOctober 2003 was 9.73 °C, the lowest values in anyOctober of the 8–y time series. In contrast, the October of 2001 and of 2005 showed the highest meantemperatures in the 8–y time series (15.23 °C and15.00 °C, respectively). The summer conditions began

unusually early in 2005, and the high temperaturescontinued to late October. Consequently the summer-autumn 2005 was an exceptionally long period of sustained high temperatures, which may have favoureddeeper thermal stratification of the water column.

We hypothesise that the long periods of sustainedhigh temperatures resulted in an unusual period of stability of the water column. In 2005, species of later stages of the phytoplankton succession, such as large Rhizosolenia diatoms, proliferated (Margalef, 1978).However, in the northeastern English Channel during thenormal years, the diatom species succession was

interrupted by the frequent mixing events. The unusuallymild conditions in 2005 may have enabled the diatomswith larger vacuoles, which accumulate nutrient reservesduring mixing periods, such as Proboscia indica and Rhizosolenia hebetata f. semispina, to outcompete anddisplace the normal local diatom species. It is uncertainwhether the change in the water column stratificationwas the only factor that could have triggered and main-tained the unusual diatom assemblage.

4.2. Northward spreading of warm-water diatoms

The current climate period is characterised by morerapid warming to greater maxima than during any other period of the twentieth century (Luterbacher et al.,2004). Climate change will influence boundaries of biogeographic regions, and in consequence the north-ward displacement of the limit between the boreal-Lusitanian (southern Europe) boundaries. Two diatoms, Eucampia cornuta and Chaetoceros peruvianus, in-creased their abundances to levels high enough to bedetected. Both species are considered in the literature to be warm-water species (Hasle and Syvertsen, 1997), andto the best of our knowledge are unknown from the

North and Norwegian Seas, some of the best-investi-gated seas in the world. Both of these species wererecorded in late summer and early autumn, coincidingwith the warmer seawater conditions.

E. cornuta was not included in the atlas of diatoms

found along the French Atlantic coasts by Paulmier (1997). This author illustrated two species in the genus Eucampia, the common E. zodiacus and the boreal-arctic species E. groenlandica Cleve. E. cornuta has thevalve face concave in broad girdle view, whereas in E. groenlandica the valve face is convex or flat (Hasleand Syvertsen, 1997). E. cornuta is largely cited in theMediterranean Sea and known from the subtropicalnortheast Atlantic (Margalef, 1973) and Portugal (Moitaand Vilarinho, 1999).

Although E. cornuta could be misidentified with

E. zodiacus during routine microscopical observations,Ch. peruvianus is a highly distinctive species and itsmisidentification is most improbable. In this study,Ch. peruvianus appeared simultaneously at the inshoreand offshore stations, indicating a wide intrusion of thiswarm-water species. Ch. peruvianus is known from thesouth of the British Isles (Hendey, 1974; Hartley, 1986).In the southern English Channel, Bygrave (1911)remarked that the inflow of Atlantic water into theEnglish Channel was associated with the speciesCh. peruvianus and R. hebetata f. semispina. Paulmier (1997) did not report Ch. peruvianus along the French

Atlantic coasts. However, Paulmier (1997, p. 188)illustrated the closely related species Chaetoceros

pendulus Karsten, which in some cases shows adistinctive short tubular central process. Hasle andSyvertsen (1997) considered that Ch. pendulus may be asynonym of Ch. aequatorialis Cleve. These two speciesare morphologically close to Ch. peruvianus. In anycase, these three species are warm-water species that tothe best of our knowledge are unknown from the Northand Norwegian Seas or higher latitudes.

The results of this study indicate a link between the

spread of warm-water species such as Ch. peruvianusand E. cornuta and climatic warm episodes in thenortheast Atlantic. So far these species have shown lowabundance, and it can be assumed that they have hadlittle influence on the structure of the pelagic food web.In contrast, the proliferation of an unusual assemblageof large rhizosolenioid species, Rhizosolenia hebetata

f. semispina and Proboscia indica, could have moreconsequences for the local pelagic food web, whichneeds to be investigated. This indicates that climate andmeteorological changes can influence niche structure, pre-empted by a more-competitive species (Smayda,2002). Alterations in the species composition, as




reported in this study, may ultimately be responses toclimatic fluctuations.

Acknowledgements

Samples were collected within the context of theSOMLIT program on board RV ‘Sepia II’ (INSU-CNRS). We thank N. Degros, E. Lecuyer, D. Devreker and G. Flamme for their help in sample collection, J.W.Reid for assistance with the English, and two anony-mous J. Sea Res. reviewers for their valuable comments.This is a contribution to the French IFB ‘Biodiversité et Changement Global’ and PNEC ‘Programme d’Envir-onnement Côtier ’ programs.

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Gomez 2007 Climate Diatoms