Spatial distribution of phytoplankton in the eastern part of the North ...

getting deeper towards the north. It includes the Skagerrak with depths up to 725 metres. Surfacewater temperature varies between 0 and 20 °C, depending on the season and the part of the sea, withless variation in the north. Salinity displays few variations in the open North Sea (32-34.5 ‰). Inthe coastal areas of Skagerrak salinity ranges between 25 and 34 and in the Wadden Sea it is usuallyless than 30. Temperature and salinity show variability at annual, seasonal and decadal scales. Thecoastlines display a large variety of habitats. In Scotland and Norway the coastlines aremountainous and rocky, often dissected by deep fjords. The coasts of northern England andScotland have a variety of cliffs, pebble beaches and mud flats. The background information wastaken from ÆRTEBJERG et al. (2001).2.2. SamplingSampling was carried out between 2 nd and 4 th of March 2004, with a total of nine samplingstations along a transect, located in the western, shallow part of the sea (Fig. 1). The stations wereregularly spaced along 55°30´N, between 3°40´E and 7°40´E. The distances between the stationswere 30´ (circa 32 km).Figure 1. Location of the sampling stations in the North Sea.4

and fixed net samples. For the investigation of differences between the algal composition of bottomand surface water levels, only the fixed bottle samples were used.The diatom frustules were cleaned using the following procedure: 10 ml of the sample wasmixed with 2 ml of 30% sulphuric acid (H 2 SO 4 ) and 10 ml of saturated potassium permanganate(KMnO 4 ). After 24 hours, the oxidation was induced by addition of 10 ml saturated oxalic acid(H 2 C 2 O 4·2H 2 O). The solution was rinsed 3 times with distilled water and a drop was placed onto acoverslip and dried. Afterwards, the coverslip was placed onto a slide with a drop of Naphraxmounting medium. Finally, the slide was carefully heated to remove the toluene from the medium.Epifluorescence microscopy – For observation of thecal morphology and plate tabulation ofdinophyte species, several samples were stained with calcofluor white solution (FRITZ & TRIEMER1985) and examined under the Olympus BX-60 microscope equipped with epifluorescenceillumination lamp Olympus U-RFL-T-200. The UV filter arrangement was for 330-380 nmexcitation and 420 nm emission wavelength (Calcofluor absorbs UV radiation in the 340-400 nmrange and re-emits visible blue light.).Transmission electron microscopy (TEM) – Species of choanoflagellates and diatoms weredetermined using JEOL 1010 transmission electron microscope. 2 sets of grids were used: The gridsmade on board from the 10 µm sample and grids from acid-cleaned material. The later was used toexamine frustules of small diatoms, which were observed without shadowcasting.Scanning electron microscopy (SEM) – The two samples with the highest richness of dinophytespecies were examined by JEOL JSM-6400 scanning electron microscope. 1 ml of the Lugol-fixedsample was washed in distilled water and mounted on 8 µm Millipore filters. Dehydration throughan ethanol series (15 minutes in 30%, 50% and 70% ethanol, 20 minutes in 96% ethanol and 30minutes in 99% ethanol and 99% ethanol with molecular sieves) was followed by critical-pointdrying with carbon dioxide (BAL-TEC CPD 030). Filters were mounted on 0.5´´ aluminiumspecimen stubs (Agar scientific) and sputter-coated with platinum-palladium for 30 seconds using aJEOL JFC 2300 HR.2.5. Data analysisThe data for species abundance and the environmental characteristics were statistically analysedusing a multivariate analysis in the program Canoco for Windows 4.5 (TER BRAAK & ŠMILAUER1998). The program CanoDraw for Windows 4.0 (TER BRAAK & ŠMILAUER 2002) was used for7

The values varied between 32.7 and 35.0 PSU 1(approximately corresponds to ‰). Higher valueswere observed at the stations in the central part of thesea whereas low salinity was noted in the stationsnear the coast. This decrease is probably caused byinfluence of the estuaries on the west coast ofDenmark.Distinct fluctuation of fluorescence occurredalong the whole transect (Fig. 4). Values ofchlorophyll a ranged from 0.13 to 0.54 mg/m 3 . Thehighest chlorophyll values were measured at stations157 and 147, located near both ends of the transect. Inthe middle part of the transect, very low chlorophyllconcentrations were found.Figure 4. Vertical profile of water fluorescence.3.2. Distribution of the algal abundanceThere was a distinct gradient in phytoplankton concentration of cells per ml along the wholetransect. The counted concentrations of cells are given in Tab. 2.Number of cells per ml143 145 147 149 151 153 155 157 123Bacillariophyceae 39000 14844 5188 1995 4993 4058 87950 388800 83067Dictyochophyceae 4 4 4 4 21 283 250 36 20Dinophyceae 684 244 175 119 50 217 200 144 188Prasinophyceae 4 12 13 0 0 41 0 4 0Chlorophyceae 8 6 6 5 0 0 0 0 0Percentual abundance143 145 147 149 151 153 155 157 123Bacillariophyceae 98.2 98.2 96.3 94.0 98.6 88.2 99.5 100.0 99.8Dictyochophyceae 0.0 0.0 0.1 0.2 0.4 6.2 0.3 0.0 0.0Dinophyceae 1.7 1.6 3.2 5.6 1.0 4.7 0.2 0.0 0.2Prasinophyceae 0.0 0.1 0.2 0.0 0.0 0.9 0.0 0.0 0.0Chlorophyceae 0.0 0.0 0.1 0.2 0.0 0.0 0.0 0.0 0.0Table 2. The cell concentrations of different phytoplankton groups, shown as number of cells perml and as a relative percentual abundance.1 PSU means practical salinity unit, after UNESCOs Practical Salinity Scale 1978 (FOTONOFF & MILLARD JR. 1983).9

3.3. Phytoplankton composition of the transectA total of 144 different specieswere found at all stations along thewhole transect. The species list withvalues of relative abundances at allstations and references to plates are givenin Tab. 7 at the end of the document.With 85 species, diatoms were therichest group. In comparison, only 44of the species found weredinoflagellates. Other algal groupsFigure 6. Total species numbers of different groups ofwere much less abundant (Fig. 6). The marine phytoplankton, found in the transect.highest number of algal species werefound at stations 153 and 151 (79taxa), located in the middle part of thetransect. At both ends of the transect,where higher fluorescence wasmeasured, a lower number of specieswas found (Fig. 7). This differencewas caused mainly by changes in thenumber of dinoflagellate species.Whereas the number of diatom speciesfound was similar at all stations, theFigure 7. Spatial variation in the species number ofnumbers of dinoflagellate species were diatoms, dinoflagellates and other algal groups.highest at stations in the middle of thetransect.‣ Craspedophyceae – Two species of colourless flagellates – Calliacantha simplex andParvicorbicula socialis – were found in samples from stations 125 and 153. Both taxa bear anextracellular lorica, which structure serves as important taxonomic feature. The species were foundonly in TEM grids, prepared during the sampling. Due to the small size of choanoflagellate species11

However, no single Chaetocerosspecies was found at all stations. Thespecies C. contortus and C. convolutuswere common at the western end ofthe transect, whereas C. danicus andC. diadema appeared in the easternpart of transect, near the coast.Chaetoceros subtilis was observed at Figure 8. Spatial variation in the relative abundance ofthe stations at both ends of the transect, four most common species of Chaetoceros.but not in the middle part. The genus Thalassiosira was represented by 10 species. Contrary toChaetoceros, some species of this genus were found at all stations. The two most common species,T. angulata and T. eccentrica, had maximum relative abundance in the middle part of the transect.However, the highest number of species (with very abundant T. nordenskioeldii) was found atstation 143, located near the coast. Therefore, the middle and eastern part of transect were the areaswith the highest abundance of Thalassiosira species (Fig. 9).The number of pennate diatomspecies was clearly less than centricones (only 25 % of all species),although some species occurred inhigh abundance. Among the mostabundant species were Nitzschialongissima and Thalassionemanitzschioides. High levels offluorescence in the eastern part of thetransect were probably caused by theirFigure 9. Spatial variation in the relative abundanceblooms. Other pennate diatom species, of Thalassiosira species.Asterionellopsis glacialis and Pseudonitzschiapungens, were very frequent at stations in the western part of the transect. Cells ofRhaphoneis amphiceros appeared in quite high abundance at all stations. They were observed inplankton attached to sand grains or to the frustules of other diatoms.A high number of species belonged to the genera Navicula and Nitzschia. In total, 9 species ofNavicula were identified. Navicula distans was the most common species. The Nitzschia genus13

was represented by 6 species, with themost common species, being Nitzschialongissima and N. constricta. Alongthe transect, distinct changes inNitzschia abundance was observed.Whereas all species were observed inthe western part, only N. longissimaoccurred on the eastern side of thetransect (Fig. 10).Figure 10. Spatial variation in the relative abundance‣ Dinophyceae – A total of 44 of Nitzschia species.different species of dinoflagellates were found in the transect. The most abundant species wereAlexandrium tamarense in the western part, Protoperidinium achromaticum and Diplopelta bombain the eastern part, and Pentapharsodinium dalei at all stations along the transect with maximumabundance in the central area. At all stations, Minuscula bipes, Protoperidinium pellucidum andZygabikodinium lenticulatum occurred in relatively constant abundance.Most of the dinoflagellate species found belong to the thecate genera Ceratium andProtoperidinium (only 5 athecate species were found). In total, 7 species of Ceratium occurred inthe transect, with a distinct gradient of abundance. All 7 species were found in the central area,while no species of this genus were found at two stations in the western part of the transect (Fig.11). A similar decrease in number of species was noted in the eastern end of transect, where only 2species occurred.With 17 species found, the genusProtoperidinium was the richest genusof algae, and was found throughout thetransect. P. achromaticum, the mostnumerous species, was very abundantat stations 143 and 145, located nearthe coast. After Thalassionemanitzschioides, P. achromaticum was Figure 11. Spatial variation in the relative abundanceof Ceratium species.the second most dominant alga inthese samples. However, there were no cells found at the western end of the transect. Interestingly,only P. pellucidum occurred at all stations along the transect. All other species appeared either in14

part of the transect (e.g. P. pyriformeor P. subinerme) or only at one station(e.g. P. marielebouriae, quite commonat station 145, but not found at nearbystations). Generally, the highestoccurrence of Protoperidinium specieswas noted at stations 153 and 151, inthe central part of the transect.Towards both ends of the transect,decreasing total relative abundance of Figure 12. Spatial variation in the relative abundancespecies was noted. Only at the coastal of Protoperidinium species.station 145, high abundance of P. achromaticum together with the presence of 3 unique speciescaused the increase of species abundance (Fig. 12).‣ Prasinophyceae – In total, 5 species belonging to this group were found in the transect.Apart from Halosphaera viridis, found in the western part of the transect, all other species belongedto the genus Pterosperma. Species ofthis genus are mainly distinguished bythe structure of the phycoma stage,bearing a distinct equatorial keel or asystem of keels, dividing its surfaceinto polygons. Pterosperma cristatum,appeared in the western and centralparts of the transect, and the threeother species were found only in thecentral part of the study area (Fig. 13). Figure 13. Spatial variation in the relative abundanceof Pterosperma species.This is in accordance with cellabundance, which was highest in the same, central area of the transect (Fig. 5c).‣ Chlorophyceae – Only three species of this group were found – Pediastrum boryanum,Pediastrum cf. kawraiskyi and Scenedesmus sp. Pediastrum boryanum was found at the six stationsin the eastern part of the transect, while Scenedesmus sp. and Pediastrum cf. kawraiskyi were onlyfound at stations 145 and 143, respectively, nearest the coast.15

3.4. Differences in algal composition of bottom and surface water levelsAt four stations (every second station in the transect), the differences between bottom andsurface species composition were investigated. In this survey, a total of 70 species was foundaltogether at the four stations (46 diatoms, 17 dinoflagellates, 7 species belonging to other groups ofalgae). Most species exhibited the same abundance in both bottom and surface water samples,however several species, in the two largest groups of algae, showed a distinct vertical gradient. Therelative abundances of species, occurred in the bottom and surface water samples, are given in Tab. 8.Interestingly, several diatoms were found to be more abundant in bottom water samples(Asterionellopsis glacialis, Pleurosigma lanceolatum, Neostreptotheca subindica and Nitzschialongissima), while only Podosira stelliger was found more often in surface water samples. Theopposite pattern was found in two dinoflagellate species (Ceratium lineatum and Protoperidiniumachromaticum), which were more abundant in surface water samples. No dinoflagellate speciesoccurred with higher abundance in bottom water samples.In Fig. 14, showing the vertical gradient of diatom species abundance, the greatest differencebetween bottom and surface water species composition is apparent at station 153. There, the totalnumber of species occurring in surface or bottom water samples was 24 and 30, respectively. At thesame station, the inverse vertical gradient of dinoflagellate species was noted (Fig. 15). Due to thevery small number of species found from other algal classes, vertical gradients were interpretedonly for the two previously mentioned algal groups.Figure 14. Horizontal and vertical variationin the number of diatom species found. Redcolour indicates the areas with the highestspecies richness.Figure 15. Horizontal and vertical variationin the number of dinoflagellate speciesfound. Red colour indicates the areas withthe highest species richness.16

3.5. Results of statistical analyses3.5.1. Inner structure of dataAn indirect gradient analysis PCA was used to detect the inner structure of the data obtained.The results are given in Tab. 3:**** Summary ****Axes 1 2 3 4 Total varianceEigenvalues : 0.386 0.214 0.123 0.096 1.000Cumulative percentage variance of species data : 38.6 60.0 72.3 81.8Sum of all eigenvalues 1.000Table 3. Part of the PCA analysis output.The two first ordination axes togetherexplained 60 % of the total variability.The positions of the samples in the spaceof these two axes are illustrated in Fig. 16.On the ordination diagram, the samplinglocalities are arranged more or less incorrespondence to their physical orderalong transect, forming the shape of anoverturned letter U. The sampling stationsform three distinct groups in the diagram: Figure 16. Positions of the samples in the space ofthe first two ordination axes. PCA analysis.The group of coastal stations 143, 145,147 and 149; the group of oceanic stations 123, 155 and 157; and the group of two stations 151 and153, situated in the middle part of the transect. Distinct separation of these three groups along thefirst ordination axis reflects to the important role of the distance from the coast for the speciescomposition at each station. However, the uneven arrangement of the stations along the first axisindicates the considerable changes of phytoplankton composition in the areas between stations 149-151 and 153-155.The ratio of species composition of the samples and the orientation of environmental variablesin ordination space are shown in Fig. 17. The association of coastal distance with the first axis iswell shown. Similarly, the second ordination axis dividing the stations 151 and 153 from all other17

ones is linked positively with the depth of seaand negatively with fluorescence. At stations,sited in negative part of second ordinationaxis (stations 123, 157, 143), high values offluorescence were measured. At the samestations, there were also higher numbers ofdiatom species.Figure 17. Positions of the samples and theenvironmental variables in the space of thefirst two ordination axes. PCA analysis. Theratio of the species composition of allsamples is shown (yellow – diatoms, green –dinoflagellates, grey – others).-1.0 1.0coast145149147143151depthfluorescence-1.0 1.0153123temperaturesalinity155abundance1573.5.2. Relationships between phytoplankton and environmental characteristicsThe influence of several environmental characteristics on species composition was statisticallytested using RDA analysis. Separate analyses were made to detect the influence of temperature,salinity, fluorescence, distance from the coast, depth and the total abundance of algae. Interestingly,only the influence of distance from coast proved to be significant, with a p-value of 0.001. All othercharacteristics here found not to be statistically significant for the transect. The results of analysiswith distance from the coast as the tested environmental variable are given in Tab. 4.**** Summary ****Axes 1 2 3 4 Total varianceEigenvalues : 0.343 0.201 0.132 0.090 1.000Species-environment correlations : 0.972 0.000 0.000 0.000Cumulative percentage varianceof species data : 34.3 54.4 67.6 76.6of species-environment relation : 100.0 0.0 0.0 0.0Sum of all eigenvalues 1.000Sum of all canonical eigenvalues 0.343**** Summary of Monte Carlo test ****Test of significance of all canonical axes: Trace = 0.343F-ratio = 3.658P-value = 0.0010Table 4. Part of the RDA analysis output.18

The first ordination axis, characterizing the influence of coast distance, explained almost 35 %of the total variability. In Fig. 18., the relation of individual species to the coastal or oceanicenvironment is shown. The species names abbreviations, used in following figure, are given in Tab. 5.Figure 18. Positions of the species and their relation to the coastal or oceanic environment in the spaceof the first two ordination axes. RDA analysis. The 78 most fitted species are shown in the diagram.The species with affinity to coastal environment are situated in the right part of the diagram.Chaetoceros diadema, Chaetoceros danicus, Emiliania huxleyi, Protoperidinium achromaticum,Actinoptychus senarius and Pediastrum boryanum belong among the typical coastal species.Similarly, 5 of 6 species of Thalassiosira, shown in the diagram, are positively correlated withcoastal environment. The species Chaetoceros convolutus, Chaetoceros contortus, Pleurosigmalanceolatum, Gyrosigma fascicola, Skeletonema costatum and other species in the left part ofdiagram show an affinity for oceanic environment. Species displayed in the upper part of the19

4. DiscussionAlong the transect, vertical and horizontal gradients of algal concentration and species numberwere found. Three groups of stations were found based on different phytoplankton composition:Most of the species were most abundant either at the coastal or at the oceanic end of the transect,while some species occurred mainly at stations located in the deeper area in the middle of thetransect. High concentrations of diatom cells per ml were noted at both ends of the transect.However, the algal composition of stations situated at the eastern and the western end of thetransect, respectively, were different. Generally, the highest species diversity of all algae groupswas found at station with the lowest number of algae per ml. This phenomenon is known not onlyfrom marine environment but also from all types of natural conditions, as has been published e. g.by LEVIN et al. (2001). A vertical gradient in species number of diatoms and dinoflagellates wasfound in the middle part of the transect. The gradient was probably caused by slight stratification,appeared as small vertical gradients of temperature and salinity at station 151 (Figs 2., 3.).The level of temperature and salinity was more or less constant along the whole transect, apartfrom two stations at the west coast of Denmark. There the water was colder and less saline probablydue to the influence of fresh water from estuaries along the coast of Denmark. A very differentpattern was found in the spatial distribution of fluorescence values. Two distinct peaks offluorescence occurred at stations 157 and 147, located at the opposite ends of the transect. In themiddle part of the transect, very low chlorophyll concentrations were found. This distribution ofchlorophyll was obviously associated with the water depth. High values of chlorophyll weremeasured in the shallow parts of the transect, while in the deepest part (station 151) the lowestconcentration was found. While the eastern end of the transect was influenced by coastal proximity,the small depth at the western end of the transect was caused by the vicinity of the shallow areaDogger Bank (JOHNS & REID 2001). These two zones were separated by an area with depths of 35 –40 meters. The high algal abundance at both ends of the transect was probably caused by thenutrients richness in shallow, mixed parts of the sea, that evoked the phytoplankton bloom.However, we did not measure any nutrients concentrations to support this hypothesis.At both ends of the transect, the high values of fluorescence were probably caused by the largenumber of diatoms, which are typical organisms of spring blooms in marine environments (JOHNS& REID 2001). However, we could not absolutely rule out that these values were caused by otherorganisms, for example by those, which are nanoplanktonic. They could contribute to the measured21

values of fluorescence but they were not found. However, diatoms seemed to be really dominantorganisms, because we found them in huge numbers, for example circa 400 millions cells per litreof sampled water at station 157. Moreover, the diatom blooms at the different ends of the transectwere produced by different species. At the eastern end of the transect, the diatoms Porosiraglacialis, Thalassionema nitzschioides and Thalassiosira nordenskioeldii were dominant. Incontrast, the bloom at the western end of the transect was mainly caused by Skeletonema costatum,Nitzschia longissima and Porosira glacialis. The spatial distribution of Porosira glacialis showedan atypical pattern, with high abundances at both ends of the transect. Such a distribution was notfound in any other algal species. Some other species, e.g. Coscinodiscus asteromphalus andPleurosigma normanii, occurred with almost same abundance at all stations.The bloom of Skeletonema costatum in the sea far from the coast is quite surprising. Occurrenceof Skeletonema costatum is usually associated with the coastal areas; blooms have been reportedvery frequently during spring from the coastal areas in Norway (BRAARUD et al. 1973; ERGA &HEIMDAL 1984), Canada (CONOVER & MAYZAUD 1984), Alaska (WAITE et al. 1992) and BritishColumbia (HAIGH et al. 1992). Furthermore, SMAYDA (1957) suggested that Skeletonema costatumgrows best in semi-enclosed waters and it is not often found in high concentrations in oceanicenvironments. In our study, high abundances were noted at stations near the coast. However, thehighest abundance of this species was observed at the western end of the transect, in the middle partof the North Sea. Additionally, the diatom cell concentration in this part of transect was 10x higherthan the bloom near the coast. The high production of Skeletonema costatum in the open sea mightbe caused by the relatively shallow water in this part of the sea, which simulates coastal conditions,and the slightly warmer water temperature. The higher oceanic abundance could be also associatedwith eddies reflecting the coastal origin of the water. However, our data did not support thehypothesis about ecological preference of S. costatum for semi-enclosed, coastal waters, as has beenproposed by SMAYDA (1957).The spatial distribution of dinoflagellates was similar to that of diatoms. However, the highestnumber of dinoflagellate cells was found at station 143, whereas the highest bloom of diatoms wasobserved at station 157. The lowest abundance of dinoflagellates was found in the middle of thetransect (station 151). It increased again toward the oceanic end of the transect. The speciescompositions of the coastal and oceanic ends of transect were also different. Among the speciesfound more in the coastal area were mainly Protoperidinium achromaticum, Phalacromarotundatum, Prorocentrum micans and most species of the genus Ceratium. Protoperidinium22

achromaticum has previously been found by e. g. DODGE (1982) from British Isles and byTRIGUEROS et al. (2000) in the estuary of Urdaibai in northern Spain. It seems that this species isable to tolerate wide range of salinity in marine and brackish waters. The species found mainly atthe oceanic end of the transect were Protoceratium reticulatum, Protoperidinium cerasus andProtoperidinium pyriforme. Minuscula bipes, Pentapharsodinium dalei, Protoperidiniumpellucidum and Zygabikodinium lenticulatum were found at all stations of the transect, but theircontribution to the total quantity of algae was rather low.The species of the genus Dictyocha (mainly the species D. speculum) showed a different spatialdistribution along the transect. Contrary to diatoms and dinoflagellates, they were mainly found inthe middle part of the transect. Only the skeleton bearing stadium was found in all samples,although this genus is able to form other stages, which don’t require silicate and therefore don’tcompete with diatoms for it.Three distinct groups of stations were found based on differences in species composition – threestations at each end of the transect, and two stations in the middle. Different algal speciesdominated in each group. Spatial differences of phytoplankton composition along the transect wassignificant in RDA analysis, showing differences between coastal and oceanic species. REID et al.(1978), who investigated the spatial distribution of phytoplankton ofCalifornia, published similar results. They found that the planktoncomposition at stations 100 km apart in the along shore direction wasmore similar that at stations hundreds of meters apart in the offshoredirection.The dependence of depth, salinity, temperature and fluorescenceon the species composition was not proved. For illustration, the p-values from the results of RDA analyses are given in Tab. 6.environmentalvariablesp-valuescoast 0.001depth 0.18temperature 0.29salinity 0.41fluorescence 0.69Table 6. The results ofMonte-Carlo permutationtests of all environmentalvariables.The individual species showed four types of distribution along the transect, as shown in Fig. 18(coastal, oceanic, with a maximum of abundance in the middle part of the transect, or moreabundant at both ends of the transect.). Most of the species occurred in either the eastern or thewestern part of the transect, in coastal or open oceanic environment, respectively. With theexception of Skeletonema costatum, the ecological preferences of most other species correspondwith published data. A preference for coastal environment has previously been published for e. g.Coscinodiscus wailesii (EDWARDS & JOHNS 2002), Emiliania huxleyi (YANG et al. 2001), andThalassiosira angulata and T. nordenskioeldii (REIGSTAD et al. 2000). The genus Chaetoceros is,23

5. ConclusionsThe data for this survey were obtained during the cruise of the vessel Dana through the NorthSea in March 2004. Samples were collected at 9 stations in the transect from the shore to the middlepart of the sea. They were obtained both from the surface and from the depth near the bottom tocompare species composition between them. The main aim of this study was the determination ofthe algae species, their composition and possible variation along the transect and relation to theenvironmental variables. Altogether 144 species of algae were identified by means of light,epifluorescence and electron microscopy. Some their ecological preferences were found on thebasis of measured environmental parameters and compared with literature. Furthermore, theprogram Canoco was used for the statistical evaluation of the data. However, only distance from thecoast was significant factor of different distributions of algae along the transect. Three main areas ofthe transect were found: the coastal end, the middle area and the oceanic end. Diatoms, mainly thecentric ones, were the most abundant group of algae. The other less abundant groups wereDinophyceae, Dictyochophyceae, Prasinophyceae and Chlorophyceae. The pattern of distributionsof diatoms and dinophytes along the transect was more or less similar, the higher numbers of cellswere found closely to the both ends of the transect, although the species composition was different.Some species were found to prefer coastal waters, another species were characterised as oceanic,and several species were found at all stations. The species of the genus Dictyocha were foundmainly in the middle of the transect. Species of the class Prasinophyceae were not abundant andoccurred rather irregularly and we didn’t observe any preferences for coastal or oceanic conditions.Within the class Chlorophyceae, several species otherwise common in fresh waters were found attwo stations. They preferred mainly coastal waters, which is in accordance with our expectation.25

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Taxon / Number of station Fig. 123 157 155 153 151 149 147 145 143CraspedophyceaeCalliacantha simplex Manton & Oates I.1. 1Parvicorbicula socialis (Meunier) Deflandre I.2. 1ChrysophyceaeMeringosphaera mediterranea Lohmann I.3. 1PrymnesiophyceaeEmiliania huxleyi (Lohmann) Hay & Mohler I.4.-5. 2 4 1 1DictyochophyceaeDictyocha crux Ehrenberg - 1Dictyocha fibula Ehrenberg I.6. 1 1 2 2 1 1 1Dictyocha speculum Ehrenberg I.7. 2 1 3 3 3 1 1 1BacillariophyceaeActinocyclus octonarius Ehrenberg II.1. 1 1 2 3 1 1 1 1 1Actinoptychus senarius (Ehrenberg) Ehrenberg II.2.-3. 1 1 1 2 2 2 2 3Amphora ovalis (Kützing) Kützing II.4. 1 1 1Asterionellopsis glacialis (F. Castrac.) F.E. Round II.5. 2 4 2 1 1 1Asterionellopsis kariana (Grunow) F.E. Round - 1 1 1Aulacodiscus argus (Ehrenberg) A. Schmidt - 1Bacillaria paxillifer (O.F. Müller) Hendey II.6.-7. 2 1 1 1 1Bellerochea malleus (Brightwell) Van Heurck II.8.-9. 2Biddulphia cf. rhombus (Ehrenberg) W. Smith II.10. 1 1Brachysira cf. aponina Kützing - 1Brockmanniella brockmannii (Hustedt) Hasle et al. II.11. 3Chaetoceros borealis J.W. Bailey II.12. 1 1 2 1Chaetoceros contortus Schütt II.13. 1 2 2 2 1Chaetoceros convolutus Castracane II.14. 2 2 3 2 2Chaetoceros danicus Cleve II.15. 1 2 1 1 2 2Chaetoceros debilis Cleve III.1. 1 2Chaetoceros decipiens Cleve III.2. 1 2 1Chaetoceros diadema (Ehrenberg) Gran III.3.-4. 3 3 3 4Chaetoceros didymus Ehrenberg III.5. 1Chaetoceros pseudocrinitus Ostenfeld III.6. 2 2Chaetoceros cf. similis Cleve III.7. 1 2 2Chaetoceros socialis Lauder - 1Chaetoceros subtilis Cleve III.8. 2 1 1 2 2 1 1Cocconeis sp. III.9. 1Corethron criophilum Castracane III.10. 2 2 1 2 1 1Coscinodiscus asteromphalus Ehrenberg III.11. 1 1 1 1 1 1 1 1 1Coscinodiscus wailesii Gran & Angs III.12.-13. 1 1 1 1 1 1Diploneis smithii (Brébisson) Cleve III.14. 1 1 1 1 1Diploneis cf. coffaeiformis (Schmidt) Cleve - 1Ditylum brightwellii (T. West) Grunow III.15. 2 4 2 1 2 3Entomoneis sp. IV.1.-2. 1 1 1 1Eucampia zodiacus Ehrenberg IV.3. 2 2Fallacia forcipata (Greville) Stickle & Mann IV.4. 1 1 1Fragilaria cf. islandica Grunow ex Van Heurck - 1Fragilariopsis cf. cylindrus (Grunow) Krieger IV.5. 1 2 2 1 1 1 1Table 7. Species list with the relative abundances and references to the figures in the plates.29

Taxon / Number of station Fig. 123 157 155 153 151 149 147 145 143Thalassiosira tenera Proschkina-Lavrenko VII.16. 1 1 1 1 1 1 1Triceratium alternans J.W. Bailey VII.17. 1 1 1 1 2 2 1 1Triceratium favus Ehrenberg - 1DinophyceaeAlexandrium tamarense (Lebour) Balech VIII.1.-2. 2 2 2 3 2 2 2Amphidoma caudata Halldal VIII.3.-4. 1 1 2 2 1 1Amylax triacantha (Jørgensen) Sournia VIII.5. 1 1 1 1 1 1Ceratium furca (Ehrenberg) Claparède et Lachmann VIII.6. 1 1 1 1 2 1Ceratium fusus (Ehrenberg) Dujardin VIII.7. 1 1 2 1Ceratium horridum (Cleve) Gran - 2Ceratium lineatum (Ehrenberg) Cleve VIII.8. 1 1 1Ceratium longipes (Bailey) Gran VIII.9. 2 1 1 2 2 1Ceratium macroceros (Ehrenberg) Vanhöffen VIII.10. 1 1Ceratium tripos (O. F. Müller) Nitzsch VIII.11. 1 2 1 1Dinophysis acuminata Claparède et Lachmann - 1 1 1Diplopelta bomba Stein ex Jorgensen VIII.12. 1 2 2 3 3 2 1Dissodinium pseudolunula Swift ex Elbrächter et Drebes VIII.13. 1Gonyaulax spinifera (Claparède et Lachmann) Diesing - 1 1 1 1 1Gymnodinium sp. - 1 1 1 1Gyrodinium cf. undulans Hulbert - 1 2 2 1 2Gyrodinium sp. - 1Minuscula bipes Lebour VIII.14. 1 1 2 2 1 2 2 2 1Noctiluca scintillans (Macartney) Kofoid et Swezy VIII.15. 1 1Pentapharsodinium dalei Indelicato et Loeblich IX.1.-3. 1 1 2 3 3 2 2 1 1Phalacroma cf. mitra Schütt - 1Phalacroma rotundatum (Clap. et Lach.) Kof. et Michen. IX.4. 1 1 1 2 2 1Prorocentrum micans Ehrenberg IX.5. 2 1 3 1 1 1Proterythropsis vigilans Marshall - 1Protoceratium reticulatum (Clap. et Lach.) Butschli IX.6.-7. 1 1 2 3Protoperidinium achromaticum (Levander) Balech IX.8.-11. 1 2 2 2 3 4 4Protoperidinium brevipes (Paulsen) Balech IX.12.-14. 1 1 1 1 1 1Protoperidinium cerasus (Paulsen) Balech X.1.-3. 1 1 1Protoperidinium conicum (Gran) Balech X.4.-5. 1 1 1Protoperidinium denticulatum (Gran et Braarud) Balech X.6. 1 1Protoperidinium depressum (Bailey) Balech X.7. 1 1 2 1Protoperidinium excentricum (Paulsen) Balech - 1Protoperidinium cf. granii (Ostenfield) Balech X.8.-9. 1 1Protoperidinium leonis (Pavillard) Balech X.10.-11. 1 2 1Protoperidinium marielebouriae (Paulsen) Balech X.12.-13. 2Protoperidinium oblongum (Aurivillius) Parke et Dodge X.14. 1 1 1Protoperidinium pellucidum Bergh XI.1.-2. 2 2 1 2 2 2 1 1 1Protoperidinium pentagonum (Gran) Balech XI.3.-5. 1 1 1Protoperidinium pyriforme (Paulsen) Balech XI.6.-9. 1 1 2 2 2Protoperidinium subinerme (Paulsen) Loeblich XI.10.-11. 1 1 1 1Protoperidinium sp. XI.12. 1Protoperidinium sp. B Hansen & Larsen XI.13. 1Pyrophacus horologium Stein XII.1.-2. 1 1 1 1 1Table 7 (cont.). Species list with the relative abundances and references to the figures in the plates.31

Taxon / Number of station Fig. 123 157 155 153 151 149 147 145 143Zygabikodinium lenticulatum Loeblich Jr. & Loeblich XII.3.-6. 1 1 1 1 1 1 1 2 1PrasinophyceaeHalosphaera viridis Schmitz - 1 1 1 2Pterosperma cristatum Schiller - 1 2 1 1 2 2 1Pterosperma moebii (Jørgensen) Ostenfeld XII.7. 1 1Pterosperma polygonum Ostenfeld XII.8. 1 1Pterosperma vanhoeffenii (Jørgensen) Ostenfeld XII.9.-10. 1 2 1ChlorophyceaePediastrum boryanum (Turpin) Meneghini XII.11. 1 1 1 1 1 1Pediastrum cf. kawraiskyi Schmidle XII.12. 1Scenedesmus sp. - 1Table 7 (cont.). Species list with the relative abundances and references to the figures in the plates.32

Taxon/number of station 157 153 149 145Surface/bottom water sample S B S B S B S BDictyochophyceaeDictyocha fibula Ehrenberg 1 1 1Dictyocha speculum Ehrenberg 1 1 3 4BacillariophyceaeActinoptychus senarius (Ehrenberg) Ehrenberg 1 1 1 1 1 1Amphora ovalis (Kützing) Kützing 1Asterionellopsis glacialis (F. Castracane) F.E. Round 3 1 1Asterionellopsis kariana (Grunow) F.E. Round 2Bacillaria paxillifer (O.F. Müller) Hendey 1 1 1Chaetoceros contortus Schütt 1Chaetoceros convolutes Castracane 2 2 1 1Chaetoceros danicus Cleve 1 2 1Chaetoceros diadema (Ehrenberg) Gran 1 3 5 3Chaetoceros pseudocrinitus Ostenfeld 2 3Chaetoceros cf. teres Cleve 1 1 1Corethron criophilum Castracane 1 1 1Coscinodiscus asteromphallus Ehrenberg 1 1 1 1Ditylum brightwellii (T. West) Grunow 2 2 2 3 1Entomoneis sp. 1 1Eucampia zodiacus Ehrenberg 1 1Fragilariopsis cf. cylindrus (Grunow) Krieger 1 1Guinardia flaccida (Castracane) H. Peragallo 1Gyrosigma fasciola (Ehrenberg) J.W. Griffith & Henfrey 1 1 1Leptocylindrus danicus Cleve 1 1Navicula distans (W. Smith) Ralfs 1 1 2 1Navicula meniscus Schumann 1 1Navicula sp. 1 1 1Neostreptotheca subindica von Stosch 1 3 2Nitzschia constricta (Kützing) Ralfs 1 1 1Nitzschia cf. fasciculata Ehrenberg 1 1 1Nitzschia longissima (Brébisson in Kützing) Ralfs 1 2 1 2 3 3 1 4Odontella aurita (Lyngbye) C. A. Agardh 1 1 1Odontella mobiliensis (Brébisson in Kützing) Ralfs 1 2 2 2 2 2Odontella sinensis (Greville) Grunow 1 1Paralia sulcata (Ehrenberg) Cleve 1 1 3 3 5 5 3 3Pleurosigma cf. lanceolatum Donkin 1 1Pleurosigma normanii Ralfs 1 1 2 2 2 1 1Podosira stelliger (J.W. Bailey) Mann 1 2Porosira glacialis (Grunow) E. Jorgensen 4 4 2 1 2 1 4 4Proboscia alata (Brightwell) Sündstrom 1 1 1Pseudo-nitzschia pungens (Grunow ex Cleve) Hasle 2 2 2 5Rhaphoneis amphiceros (Ehrenberg) Ehrenberg 1 3 2 1 1Rhizosolenia pungens Cleve-Euler 1 1 1 2 1Rhizosolenia styliformis Brightwell 3Skeletonema costatum (Greville) Cleve 5 5 5 5 1 1 3 1Thalassiosira cf. eccentrica (Ehrenberg) Cleve 1Table 8. Species list with the relative abundances in surface (S) and bottom (B) water samples.33

Taxon/number of station 157 153 149 145Surface/bottom water sample S B S B S B S BThalassionema nitzschioides (Grunow) Mereschkowsky 2 2 3 2 3 4 5 5Thalassiosira sp.1 2 1 1 2 1Thalassiosira sp.2 1 1 2 3 1 1Triceratium alternans J.W. Bailey 2 1 1DinophyceaeAlexandrium tamarense (Lebour) Balech 3 1Amphidoma caudata Halldal 2 1 1Ceratium furca (Ehrenberg) Claparède et Lachmann 1 1Ceratium lineatum (Ehrenberg) Cleve 1 1 1Ceratium longipes (Bailey) Gran 1 1 3 2Ceratium tripos (O. F. Müller) Nitzsch 1Diplopelta bomba Stein ex Jorgensen 1 2 2 1 1Gonyaulax spinifera (Claparède & Lachmann) Diesing 2Gonyaulax cf. diegensis Kofoid 1Gyrodinium cf. undulans Hulburt 2 2Minuscula bipes Lebour 1 1 1 1Pentapharsodinium dalei Indelicato et Loeblich 1 1 1 1Phalacroma rotundatum (Clap. et Lachm.) Kofoid et Michener 1 1 1 1Prorocentrum micans Ehrenberg 2 2 1Protoperidinium achromaticum (Levander) Balech 1 1 2 1 3 2 4 2Protoperidinium pellucidum Bergh 1 1 1 1 1 2 1Protoperidinium pentagonum (Gran) Balech 1 1 1PrasinophyceaeHalosphaera viridis Schmitz 2Phaeocystis pouchetii (Hariot) Lagerheim 1Pterosperma cristatum Schiller 1 1 1Pterosperma vanhoeffenii (Jørgensen) Ostenfeld 1 1Table 8 (cont.). Species list with the relative abundances in surface (S) and bottom (B) water samples.34

Plate I. Craspedophyceae, Chrysophyceae, Prymnesiophyceae, Dictyochophyceae.1. Calliacantha simplex – 2. Parvicorbicula socialis – 3. Meringosphaera mediterranea – 4.,5. Emiliana huxleyi – 6. Dictyocha fibula – 7,. 8. Dictyocha speculum. Scale bars: Figs 1-3: 2µm; Figs 4, 5: 0,5 µm; Figs 6-8: 10 µm.

Plate II. Bacillariophyceae 1.1. Actinocyclus octonarius – 2., 3. Actynoptychus senarius – 4. Amphora ovalis – 5.Asterionellopsis glacialis – 6., 7. Bacillaria paxillifer – 8., 9. Belleroches maleus – 10.Biddulphia cf. rhombus – 11. Brockmanniella brockmannii – 12. Chaetoceros borealis – 13.Chaetoceros contortus – 14. Chaetoceros convolutus 15. Chaetoceros danicus. Scale bars:Figs 1-3, 5, 7-10: 10 µm; Figs 4, 6, 11: 2 µm; Figs 12-15: 20 µm.

Plate III. Bacillariophyceae 2.1. Chaetoceros debilis – 2. Chaetoceros decipiens – 3., 4. Chaetoceros diadema – 5.Chaetoceros didymus – 6. Chaetoceros pseudocrinitus – 7. Chaetoceros cf. similis – 8.Chaetoceros subtilis – 9. Cocconeis sp. – 10. Corethron criophilum – 11. Coscinodiscusasteromphalus – 12., 13. Coscinodiscus willei – 14. Diploneis smithii – 15. Ditylumbrightwellii. Scale bars: Figs 1-8, 10, 14-15: 20 µm; Fig. 9: 2 µm; Figs 11-13: 100 µm.

Plate IV. Bacillariophyceae 3.1., 2. Entomoneis sp. – 3. Eucampia zodiacus – 4. Fallacia forcipata – 5. Fragilariopsis sp. –6. Guinardia flaccida – 7. Minidiscus trioculatus – 8.-10. Navicula distans – 11. Navicula cf.duerrenbergiana – 12. Navicula cf. pavillardii – 13., 14. Navicula perminuta – 15. Naviculasp. (transitans) – 16. Neostreptotheca subindica. Scale bars: Figs 1-6, 8-12, 15-16: 10 µm;Figs 7, 13, 14: 1 µm.

Plate V. Bacillariophyceae 4.1., 2. Nitzschia constricta – 3. Nitzschia cf. dissipata – 4. Nitzschia cf. fasciculata – 5.Nitzschia longissima – 6. Nitzschia pusilla – 7. Odontella aurita – 8. Odontella mobiliensis –9. Odontella sinensis – 10., 11. Paralia sulcata – 12., 13. Pleurosigma cf. lanceolatum – 14.,15. Pleurosigma normanii. Scale bars: Figs 1, 4-5, 7, 10-11: 10 µm; Figs 2-3, 6: 2 µm; Figs8, 9: 50 µm.

Plate VI. Bacillariophyceae 5.1., 2. Podosira stelliger – 3., 4. Porosira glacialis – 5. Proboscia alata – 6., 7. Nitzschiapungens – 8.-11. Rhaphoneis amphiceros – 12. Rhizosolenia pungens – 13., 14. Rhizosoleniastyliformis – 15. Skeletonema costatum – 16. Thallassionema nitzschioides. Scale bars: Figs1-5, 7-16: 10 µm; Fig. 6: 1 µm.

Plate VII. Bacillariophyceae 6.1. Thalassiosira angulata – 2. Thalassiosira anguste-lineata – 3., 4. Thalassiosira constricta– 5., 6. Thalassiosira curviseriata – 7., 8. Thalassiosira eccentrica – 9., 10. Thalassiosiranordenskioeldii – 11., 12. Thalassiosira pacifica – 13. Thalassiosira pseudonana – 14., 15.Thalassiosira punctigera – 16. Thalassiosira tenera – 17. Triceratium alternans. Scale bars:Figs 1-2, 7-8, 14-17: 10 µm; Figs 3-6, 9-12: 2 µm; Fig. 13: 1 µm.

Plate VIII. Dinophyceae 1.1., 2. Alexandrium tamarense – 3., 4. Amphidoma caudata – 5. Amylax triacantha – 6.Ceratium furca – 7. Ceratium fusus – 8. Ceratium lineatum – 9. Ceratium longipes – 10.Ceratium macroceros – 11. Ceratium tripos – 12. Diplopelta bomba – 13. Dissodiniumpseudolunula – 14. Minuscula bipes – 15. Noctiluca scintillans. Scale bars: Figs 1-6, 12-13:10 µm; Figs 7-11, 15: 50 µm.

Plate IX. Dinophyceae 2.1.-3. Pentapharsodinium dalei – 4. Phalacroma rotundatum – 5. Prorocentrum micans – 6.,7. Protoceratium reticulatum – 8.-11. Protoperidinium achromaticum – 12.-14.Protoperidinium brevipes. Scale bars: Fig. 1: 5 µm; Figs 2-14: 10 µm.

Plate X. Dinophyceae 3.1.-3. Protoperidinium cerasus – 4., 5. Protoperidinium conicum – 6. Protoperidiniumdenticulatum – 7. Protoperidinium depresum – 8., 9. Protoperidinium cf. granii – 10., 11.Protoperidinium leonis – 12., 13. Protoperidinium marielebouriae – 14. Protoperidiniumoblongum. Scale bars: Figs 1-14: 10 µm.

Plate XI. Dinophyceae 4.1., 2. Protoperidinium pellucidum – 3.-5. Protoperidinium pentagonum – 6.-9.Protoperidinium pyriforme – 10., 11. Protoperidinium subinerme – 12. Protoperidinium sp. –13. Protoperidinium sp. “B”. Scale bars: Figs 1-14: 10 µm.

Plate XII. Dinophyceae, Prasinophyceae, Chlorophyceae.1., 2. Pyrophacus horologium – 3.-6. Zygabikodinium lenticulatum – 7. Pterosperma moebii –8. Pterosperma polygonum – 9., 10. Pterosperma vanhoeffenii – 11. Pediastrum boryanum –12. Pediastrum cf. kawraiskyi. Scale bars: Figs 1-12: 10 µm.