Cyclotella Scaldensis Spec. Nov. (Bacillariophyceae), a New Estuarine Diatom

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Nova Hedwigia 63 3—4 335—345 Stuttgart, November 1996

Cyclotella scaldensis spec. nov. (Bacillariophyceae), a new estuarine diatom

Koenraad Muylaert and Koen Sabbe Department of Morphology, Systematics and Ecology, Botanical Laboratory University of Gent K.L. Ledeganckstraat 35, B-9000 Gent, Belgium

With 17 figures and 1 table

Muylaert, K. & K. Sabbe (1996):Cyclotella scaldensis spec. nov. (Bacillariophyceae), a new estuarine diatom. - Nova Hedwigia 63: 335-345. Abstract: Cyclotella scaldensis spec. nov. is described from the freshwater tidal reaches of the Schelde estuary (Belgium/The Netherlands) where it was the dominant taxon in the spring phytoplankton com munity. A detailed light and electron microscopical description is given. The differences withC. mene ghiniana and C. gamma, two closely related taxa, are discussed. Keywords: diatom taxonomy, Bacillariophyceae, Thalassiosiraceae,Cyclotella scaldensis, Schelde estua ry, new species.

Introduction

Representatives of the genus Cyclotella are often dominant components of estuarine phytoplankton communities. Although some important taxa, such as C. meneghi niana Kiitzing and C. striata (Kützing) Grunow, have been thoroughly revised (cf. Schoeman & Archibald 1980, Hâkansson 1986, Hâkansson 1990, Lange & Syvertsen 1989), the taxonomy of other taxa still remains confusing, especially in estuarine environments (Hâkansson & Kling 1994). Recently described brackish waterCyclo tella species (e.g. C. choctawatcheeana Prasad, (Prasad et al. 1990) are often soon reported from different localities all over the world, indicating inadequacies in our knowledge of estuarine Cyclotella species. In spring 1993, a survey of the phytoplankton communities was carried out in the Schelde estuary (Belgium/The Netherlands) in the framework of the MATURE- project (Biogeochemistry of the Maximum TURbidity zone in Estuaries). Light microscopy (LM) and scanning electron microscopy (SEM) revealed the presence of several Cyclotella species, viz. C. atomus Hustedt, C. choctawatcheeana, C. mene ghiniana, C. comta (Ehrenberg) Kiitzing and C. striata. However, the most common Cyclotella species, also the most abundant taxon in the phytoplankton, could not be allocated to any known Cyclotella species. It is therefore described as a new species.

Materials and methods

The material investigated was collected in May 1993 in the Schelde estuary, a turbid coastal plain estuary situated partly in Belgium and partly in The Netherlands. The estuary is characterized by a high residence time and a longitudinal salinity gradient which is relatively stable in both time and space (De Pauw 1975). Organic and inorganic pollution are very high, and often cause near-anoxic conditions in the water column of the freshwater tidal reaches (Heip 1989). Subsurface plankton samples were collected with a plankton net (mesh size 10ßm) in 11 stations along the longitudinal salinity gradient. All samples were fixed with formaldehyde solution (final concentration 1-2%). The samples were treated with a mixture of sulphuric and nitric acid to remove the cell contents (Round et al. 1990), followed by rinsing with distilled water and alcohol. Permanent preparations were made with Naphrax. Scanning electron microscopy (SEM) was performed with a JEOL JSM-840 at 15 kV. Terminology is based on Anonymous (1975), Ross et al. (1979) and Theriot & Serieyssol (1994). For the description of the cingulum structure we used the terminology proposed by Round et al. (1990), where all elements are referred to as copulae, except for the one adjacent to the valve, which is called the valvocopula.

Observations

Cyclotella scaldensis sp. nov. Description: Valvae circulares, area centrali sine areolis et zona marginali striata; 16-34 ßm diametro, striae 7-11 in 10ßm. Area centralis transverse undulata et colliculosa, colliculae in ordine radiale ad mar- ginem zonae centralis disposita. Pars elevata areae centralis serie semicirculari fultoportularum (poris 3 satelliticis) 1-9 instructa. Area marginalis striis radiantibus elevatis et interstrias parum depressis instruc ta. Costa internae per laminam centralem obtectae. Limbus valvae circulo fultoportularum (poris 3 satel liticis) prope marginum praeditus. Processus in secunda, tertia vel quarta quaque interstria locati. Rimo- portula una, apertura interior oblique, apertura exterior globuliformis infra circulum fultoportularum marginalem positus. Arete super aperturas externas nonnullas fultoportularum spina cum anterides prae sens. Valva granulis numerosis. Valves circular, with a central area without areolae and a striated marginal area; diameter 16-34 /um, 7-11 striae in 10 /im. The central area is transversely undulate and colliculate, the colliculae being radially arranged towards the valve margin. One to nine fultoportulae (with 3 satellite pores each) are arranged in a semicircle in the raised part of the central area. In the marginal area, radiate striae are found on ridges and interstriae are situated in furrows. Internally, there are costae which are covered by a central lamina. A circle of fultoportulae (with 3 satellite pores each) is present near the valve margin. These processes are inserted at every second, third or fourth interstria. A single rimoportula with an oblique internal opening and a no- duliform external opening is positioned beneath the ring of marginal fultoportulae. Buttressed spines are present above some of the external openings of the fultoportu lae. Numerous granules cover the valve surface.

Figs 1-6. C. scaldensis, Schelde estuary. Figs 1-3: LM, Figs 4-6: SEM. Scale bar represents 10ßm. Fig. 1. Arrows indicate the position of the fultoportulae in the upper left quadrant of the valve. Fig. 2. Ar rows indicate the hyaline ring crossing the costae. Fig. 3. Arrow indicates the position of the rimoportula. Figs 4-6. External view of the valve.

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Type locality: Temse, Schelde estuary (Belgium). Plankton. Holotype: The Natural History Museum, Department of Botany, London, Eng land. Isotype: GENT1 (MATURE SI 1), The Herbarium, University of Gent, Belgium. Named after the Schelde estuary where the species was found. LM - The valves are circular with a diameter of 16.3.-33.8(20.3±4.3, n = 29) ßm. The central area is transversely undulate and distinctly colliculate, the colliculae being radially arranged near the margin of the central area. The marginal zone has 7.2-11.2(8.5 ± 1.0, n = 27) striae in 10ßm; the striae and costae are crossed by a hya line ring (Fig. 2). Fultoportulae are present near the valve margin at the end of 30 to 75% of the costae (Fig. 1-2); 1 to 9 fultoportulae can be seen in the raised part of the central valve face. Careful focussing also reveals the presence of one large marginal rimoportula (Fig. 3). SEM - As the whole valve face is externally covered by a dense sheet of granules, it is difficult to be certain about the areolation of the marginal zone. We therefore use the term ‘ridges’ and ‘furrows’ (instead of striae and interstriae) to indicate the exact position of certain structures. The marginal zone consists of ridges separated by furrows. The shape of the ridges is rectangular. Areolae perforate the cell wall in the marginal zone; their exact location, on the marginal ridges, can only be seen in broken valves (Fig. 10). Whether the striae become wider towards the valve mar gin (as in C. meneghiniana, cf. Hâkansson & Kling 1994) is impossible to assess. Granules cover the entire outer valve surface and sometimes form dendritic silica structures (Fig. 9). Spines are present at the end of some furrows on the valve face - valve mantle junction. They are conical and have a buttressed base (Figs 5, 7-8). On the valve mantle, below some furrows, the external openings of fultoportulae can be seen; they are short and tubular and surrounded by a more or less structure less aggregation of silica (Fig. 7). The external opening of the rimoportula consists of a large nodule (Fig. 9) and is also situated below a furrow in the marginal zone, though slightly closer towards the valve centre than the openings of the fultopor tulae. The internal central area is smooth. The central fultoportula(e) have 3 satellite pores each and are more or less arranged in ä semicircle. Their number appears to increase with valve diameter (cfr. Fig. 11 vs. Fig. 12). The costae in the marginal area are partially covered by a central lamina (Figs 10-13). Fultoportulae with 3 satellite pores each are present on every second to fifth costa (or on 30 to 75% of all costae).

Figs 7-12. C. scaldensis, Schelde estuary. SEM. Figs 7-10: scale bar represents 1ßm, Figs 11-12: scale bar represents 10 ßm . Fig. 7. Detail of the marginal zone showing the spines and the external openings of the fultoportulae (arrows). Fig. 8. Detail of the marginal zone showing the buttressed spines and the granules. Fig. 9. Detail of the marginal zone showing the external opening of the rimoportula. Fig. 10. Detail of a broken valve showing the position of the striae on the marginal ridges. Figs 11-12. Internal views of the valve; arrows indicate rimoportulae.

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Figs. 13-15. C. scaldensis, Schelde estuary. SEM. Fig. 13, 15: scale bar represents 10 /rm. Fig. 14: scale bar represent 1 ttm. Fig. 13. Internal view of the valve, arrow indicates rimoportula. Fig. 14. Detail of the internal opening of the rimoportula. Fig. 15. Girdle view (vc = valvocopula, cl-4 = copula 1-4); note the position of the ligulae (arrows) on c2 and c4.

They are more densely spaced when the valves are larger. The marginal rimoportula is placed on a costa opposite the central fultoportula(e). It is large and has a twisted stalk, which causes its internal slitlike opening to be obliquely orientated (Fig. 14). The structure of the cingulum can be observed in Fig. 15. The valvocopula (vc) is as broad as the valve mantle. The first copula (cl) is much smaller and open. The

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second copula (c2) is as broad as the valvocopula and has a ligula that fits in the opening of the first copula. The third copula (c3) is as broad as the second and is also open. The opening of the third copula is covered by the ligula of the fourth and last copula (c4), which again is narrow. The openings in the copulae are ar ranged in a dextral pattern (Fryxell et al. 1981). The external surface of the cingulum is completely covered with small granules, similar to but smaller than the ones on the valves.

Discussion

In recent studies on the systematics of the genus Cyclotella (e.g. Schoeman & Archi bald 1980, Hâkansson 1982, Lange & Syvertsen 1989, Hâkansson 1990, Wendker 1991, John & Economou-Amilli 1991, Hâkansson et al. 1993, Hâkansson & Kling 1994, Nikiteeva & Likhoshway 1994, Scheffler 1994) the following morphological features of the frustule are usually regarded to be of taxonomic importance: the form and arrangement of the marginal striae, all features (e.g. number, shape, satel lite pores) of the central and marginal fultoportulae and the rimoportula(e), the shape and size of the alveoli and the structure of the central area. Other features, such as the presence and the structure of spines and granules are known to be more variable and are therefore considered to be less important (Round et al. 1990). Cyclotella scaldensis clearly belongs to the C. meneghiniana species complex (com prising species with a transversely undulate central area, evenly sized marginal costae, fultoportulae with 3 satellite pores and one marginal rimoportula), and shows considerable resemblance to this species and to C. gamma Sovereign. Al though the colliculate central area is reminiscent of C. striata, the evenly sized mar ginal costae clearly discern it from the latter species, which has thick and thin costae (Hâkansson 1986, Lange & Syvertsen 1989). Within C.the meneghiniana- group, the distinction at the species level is often difficult; LM is usually not sufficient for un ambiguous identification. In Table I, the morphology and ecology of C. meneghi niana, C. gamma and C. scaldensis are compared. During our survey of the phytoplankton of the Schelde estuary, C. scaldensis was first identified as C. meneghiniana. The latter species was described in detail by Lowe (1975), Schoeman & Archibald (1980), Hâkansson (1982), Battarbee et al. (1984), and Hâkansson (1990). However, thorough LM and SEM examination of our material revealed the presence of two different taxa, C. meneghiniana (Figs 16-18) and the newly describedC. scaldensis. C. scaldensis mainly differs from C. meneghiniana in the following respects: (1) the central area is distinctly colliculate; (2) the alveolar openings are smaller (relative to the radius of the valve) due to the presence of an internal central lamina; (3) the marginal fultoportulae are only pres ent on 50 to 75% of the costae; (4) the marginal rimoportula is large and twisted and has a distinct, knob-like external opening. Other striking differences between the two species concern the external ornamentation of the valve: the valves and the cingulum in C. scaldensis are covered by a dense layer of small granules, while the marginal spines have a distinctly different shape than the ones in C. meneghiniana

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Table I. Morphological comparison of the Cyclotella species discussed in the text.

C. scaldensis C. meneghiniana C. gamm a

present study after Schoeman & Archibald (1980), after Lowe (1981), Hâkansson (1990), Hâkansson (1982), Hâkansson (1990) Hâkansson & Kline (1994)

Diameter (jam) 16-34 5-43 18-33

Striae in 10 pm 7-11 6-10 5-7

Location and shape of the on the rigdes, shape unknown on the ridges, broadening towards the in the furrows, straight marginal striae valve margin Marginal fultoportulae on 30 to 75 % of the costae on 95% of the costae on 75 % of the costae

Central lamina present absent, when present very small present

Structure of central area colliculate with radially arranged bumps and hollows forming radial radial ridges colliculae near the margin of the ridges central area Rimoportula large and twisted with a knob-like small small external opening Valve face fultoportulae 1-9 1-3 1-3

Ecology eutrophic and brackish waters eutrophic and brackish waters eutrophic waters

and are never accompanied by minor spines. Although the latter features might not be of great taxonomic value, they appeared to be very stable within the studied population. Cyclotella gamma was first described by Sovereign (1963) and was described in detail by Lowe (1981), Hâkansson (1990) and Hâkansson & Kling (1994).C. scal densis mainly differs from this species in (1) the presence of a large and twisted ri moportula with its typical external opening, (2) the location of the striae on the mar ginal ridges instead of in the furrows and (3) the structure of the central area. In addition, the marginal fultoportulae are more widely spaced and the alveolar open ings are relatively smaller, although these characters tend to be less obvious in the larger valves ofC. scaldensis. However, even in those valves, there are always more central fultoportulae and there are never fultoportulae present on four neighbouring costae. An important similarity with C. gamma concerns the hyaline ring crossing the marginal striae, which can be observed in LM. C. scaldensis can easily be distinguished from some other common estuarine Cyclo tella species: apart from being larger, it differs from C. atomus, C. choctawatcheea na and C. caspia Grunow, in having three instead of two satellite pores on the mar ginal fultoportulae (Lowe 1975, Genkai & Kiss 1993, Hâkansson et al. 1993). C. scaldensis, C. meneghiniana and C. gamma can be found in similar environ ments. Cyclotella meneghiniana is known to prefer brackish waters (Hustedt 1930) or eutrophic waters with high conductivity (Hâkansson 1990) and appears to be very common in the freshwater tidal reaches of estuaries (Filardo & Dunstan 1985, De Sève 1993). The ecological preferences of C. gamma are as yet poorly known but it also appears to occur in eutrophic waters (Hâkansson 1990). C. scaldensis was the most abundant diatom species (up to 7500 cells/ml) in the spring phytoplankton communities of the freshwater and oligohaline tidal reaches (salinity 0 to 3%o) of the

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Figs 16-18. C. meneghiniana, Schelde estuary. Fig. 16 and Fig. 18: scale bar represents IOß m. Fig. 17: scale bar represents 1 /jm. Fig. 16: LM, Fig 17-18: SEM. Fig. 17. External view of the valve; note the striae which are broadening towards the valve mantle. Fig. 18. Internal view of the valve; arrow indicates the rimoportula.

Schelde estuary. Other important planktonic taxa included several green algae (e.g. Nannochloris coccoides Naumann and Scenedesmus spp.). Turbidity in these reaches was extremely high, with particulate suspended matter concentrations up to 100 mg/1. Oxygen levels in the water column were low (20% saturation); nutrient concentrations were very high (ammonium-nitrogen up to 380 mM, phosphate- phosphorus up to 14 mM, silicate-silicon up to 220 mM). It is clear, however, that in the past, during ecological surveys of eutrophic and estuarine waters these three species have often been lumped together and have been identified as C. meneghiniana. Correct identification of these species might yield more detailed information on their specific ecological requirements.

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Acknowledgement

Many thanks are due to Dr. Hannelore Hâkansson and an anonymous referee for the valuable sug gestions on the manuscript. K. Muylaert is a research assistant of the National Fund for Scientific Re search (Belgium). This study was supported by the European Union (MATURE project, nr. EV 5V- CT92-0064). Financial support was provided by FKFO project no. 2.0094.92.

References

ANONYMOUS (1975): Proposals for a standardisation of diatom terminology and diagnosis. - Beih. Nova Hedwigia 53: 323-354. BATTARBEE, R.W., C.W. KEISTER & J.P. BRADBURRY (1984): The frustular morphology and taxonomic relationships of Cyclotella quillensis Bailey. - In: D. MANN (ed.): Proc. of the 7th Interna tional Diatom Symposium: 173-184. Otto Koeltz, Koenigstein. DE PAUW, N. (1975): Bijdrage tot de kennis van milieu en plankton in het Westerschelde estuarium. PhD Thesis, University of Gent, Belgium. DE SÈVE, M.A. (1993): Diatom bloom in the tidal freshwater zone of a turbid and shallow estuary, Rupert Bay (James Bay, Canada). - Hydrobiologia 269/270: 225-233. FILARDO, M .J. & W.M. DUNSTAN (1985): Hydrodynamic control of phytoplankton in low salinity waters of the James River Estuary, Virginia, U.S.A. - Estuarine, Coastal & Shelf Sei. 21: 653-667. FRYXELL, G.A., G.F. HUBBARD & T.A. VILLAREAL (1981): The genusThalassiosira: variations of the cingulum. - Bacillaria 4: 41-63. GENKAL, S.I. & K.T. KISS (1993): Morphological variability of the diatom Cyclotella atomus Hustedt var. atomus and C. atomus var. gracilis var. nov. - Hydrobiologia 269/270: 39-42. HÂKANSSON, H. (1982): Taxonomical discussion on four diaton taxa from an ancient lagoon in Spjälkö, South Sweden. - Dep. Quat. Geol. Rep. 22: 65-81. HAKANSSON, H. (1986): A study of theDiscoplea species (Bacillariophyceae) described by Ehrenberg. - Diatom Res. 1: 33-56. HÂKANSSON, H. (1990):Cyclotella meneghiniana Kiitz. (Bacillariophyceae), its morphology and reap praisal of similar species. - Beih. Nova Hedwigia 100: 19-37. HÂKANSSON, H ., S. HAD JU, P. SNOEIJS & L. LOGINOVA (1993): C. hakanssoniae Wendker and its relationship to C. caspia and other similar brackish water Cyclotella species. - Diatom Res. 8: 333-347. HÂKANSSON, H. & H. KLING (1994): Cyclotella agassizensis sp. nov. and its relationship to C. quil lensis Bailey and other Prairie Cyclotella species. - Diatom Res. 9: 289-301. HEIP, C. (1989): The ecology of the estuaries of Rhine, Meuse and Scheldt in the Netherlands. - Sei. Mar. 53: 457-463. HUSTEDT, F. (1930): Die Kieselalgen Deutschlands, Österreichs und der Schweiz. - In: RABEN HORST, L. (ed.): Kryptogamenflora von Deutschland, Österreich und der Schweiz: Bd. 7, Teil 1. Akad. Verlagsges., Leipzig. JOHN, J. & A. ECONOMOU-AMILLI (1991): Morphology and ultrastructure of the centric diatom Cyclotella distinguenda. - Diatom Res. 6: 307-315. LANGE, C.B. & E.E. SYVERTSEN (1989):Cyclotella litoralis sp. nov. (Bacillariophyta), and its rela tionship to C. striata and C. stylorum. - Nova Hedwigia 48: 341-356. LOWE, R.L. (1975): Comparative ultrastructure of the valves of someCyclotella species (Bacillariophy ceae). - J. Phycol. 11: 415-424. LOWE, R.L. (1981): The frustular morphology and distributionCyclotella of gamma Sov. (Bacillario phyceae). Proc. Iowa Acad. Sei. 88: 82-84.

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NIKITEEVA, T.A. & Y.V. LIKHOSHWAY (1994): Cyclotella gracilis sp. nov. from Pleistocene mate rial of Lake Baikal, Russia. - Diatom Res. 9: 349-353, PRASAD, A.K.S.K., J.A. NIENOW & R.J. LIVINGSTON (199Ó): The genusCyclotella (Bacillariophy ta) in Choctawatchee Bay, Florida, with special reference to C. striata and C. choctawhatcheeana sp. nov. Phycologia 29: 418-436. ROSS, R„ E.J. COX, N.I. KARAYEVA, D.G. MANN, T.B.B. PADDOCK, R. SIMONSEN & P.A. SIMS (1979): An amended terminology for the silicious components of the diatom cell. - Beih. Nova Hedwigia 64: 513-533. ROUND, F.E., R.M. CRAWFORD & D.G. MANN (1990): The diatoms. Cambridge Univ. Press, Cambridge. SCHEFFLER, W. (1994): Cyclotella pseudocomensis nov. sp. (Bacillariophyceae) aus Norddeutschen Seen. - Diatom Res. 9: 355-369. SCHOEMAN, F.R. & R.E.M. ARCHIBALD (1980): The diatom flora of Southern Africa. - Nat. Inst. Water Research, Council Sei. and Industrial Res., Special Report 6. SOVEREIGN, H.E. (1963): New and rare diatoms from Oregon and Washington. - Proc. California Acad. Sei., 4th ser. 33(14): 349-368. THERIOT, E. & K. SERIEYSSOL (1994): Phylogenetic systematics as a guide to understanding features and potential morphological characters of the centric diatom family Thalassiosiraceae. - Diatom Res. 9: 429-450. WENDKER, S. (1991):Cyclotella hakanssoniae sp. nov. (Bacillariophyceae) — eine kleine Cyclotella- Art aus dem Schlei-Äestuar (BRD). - Nova Hedwigia 52: 359-363.

Received 21 February 1996, accepted in revised form 2 May 1996.

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The Diatom Genus Thalassiosira (Bacillariophyta) in the Estuaries of the Schelde (Belgium/The Netherlands) and the Elbe (Germany)

K. Muylaert* and K. Sabbe

Department of Morphology, Systematics and Ecology, Botanical Laboratory, University of Gent, K. L. Ledeganckstraat 35, B-9000 Gent, Belgium * Corresponding author

A taxonomic and ecological study of the phytoplankton of the estuaries of the Schelde (Belgium/The Nether lands) and the Elbe (Germany) revealed the presence of numerous species belonging to the diatom genus Thalassiosira. Its representatives were a dominant component of the phytoplankton spring bloom in these estuaries. A total of 13 species were identified. All of these were present in the Schelde, while only 10 were found in the Elbe. Seven and five species respectively had previously not been reported from the Schelde and Elbe. A detailed light and electron microscopical description of these species is given. Most species had their distributional optimum in the polyhaline reaches of the estuaries. Community struc ture in these reaches was different from the one in the euhaline zone, indicating that the lower and middle reaches of these estuaries harbour specific assemblages which do not occur in the neighbouring coastal envi ronment. Only one species, Thalassiosira proschkinae, had its maximal abundance in the mesohaline zone and can thus be regarded as a truly brackish water species.Thalassiosira pseudonana was restricted to the tidal freshwater zone of the estuaries. Some taxa often appeared to be associated with sediment and detritus particles. The significance of this phenomenon is discussed.

Introduction was carried out. This study revealed the presence of numerous representatives of the diatom genusThal The tidal parts of the Western European rivers assiosira Cleve, most of which had a specific distri Schelde and Elbe constitute well mixed coastal plain bution within the estuaries. Some species were locally estuaries. Extensive mud- and sandflats are present dominant, concerning both abundance and biomass, in the meso- and polyhaline zones. A turbidity maxi in the phytoplankton communities. The genusThal mum occurs in the upper reaches of both estuaries. assiosira comprises more than one hundred species As nutrient concentrations are high in both estuaries (Roundet al. 1990). It is often a major component (and especially in the Schelde), phytoplankton of the phytoplankton assemblages of marine (e.g. growth is probably mainly light-limited (Brockmann Lange et al. 1992, Hallegraeff and Jeffrey 1993), estu 1992, Soetaertet al. 1994). The Schelde mainly differs arine (e.g. Belcher and Swale 1986, Sancetta 1990) from the Elbe in water residence time, which is higher and limnetic water bodies (e. g. Belcher and Swale in the Schelde because of the tenfold lower river dis 1977). Despite this widespread occurrence, the identi charge (Brockmann 1992, Soetaert and Herman 1995) and in pollution levels, both organic (e. g. sew fication of Thalassiosira species is often difficult. This age) and inorganic (e.g. heavy metals), which are is mainly due to the need for electron microscopy to very high in the Schelde estuary (Heip 1988). The observe certain distinguishing characteristics and the high organic loading causes near-anoxic conditions in scatter of taxonomic information over many papers the oligohaline and limnetic reaches of this estuary (cf. Gaulet al. 1993). throughout the year. At the time of sampling a sig The aim of this paper is to give a detailed account nificant difference in water temperature between the of the observed Thalassiosira species, with remarks two estuaries was observed, the Schelde (13-17 °C) on their taxonomy, morphology and ecology. Their being on average about 6 °C warmer than the Elbe occurrence within the studied estuaries is compared (7-10.5 °C); this was probably caused by both lati to data from other estuaries worldwide. tudinal and seasonal (the Elbe was sampled in April, the Schelde in May) factors. In the framework of the MATURE-project (Bio Materials and Methods geochemistry of the MAximum TURbidity zone in Estuaries), an ecological survey of the phytoplankton Subsurface water column samples were taken along spring bloom in the estuaries of the Schelde and Elbe a longitudinal transect in the estuaries of the Schelde