Phytoplankton Vertical Distributions and Composition in Baltic Sea

Harmful Algae 6 (2007) 189–205 www.elsevier.com/locate/hal

Phytoplankton vertical distributions and composition in Baltic Sea cyanobacterial blooms Susanna Hajdu *, Helena Ho¨glander, Ulf Larsson Department of Systems Ecology, Marine and Brackish Water Ecology, Stockholm University, SE-106 91 Stockholm, Sweden Received 17 January 2006; received in revised form 27 July 2006; accepted 31 July 2006

Abstract We studied the vertical structure of the phytoplankton community in two toxic cyanobacterial blooms in the offshore Baltic Sea. In 1994, vertically separated potentially toxic, diazotrophic and mixotrophic species (belonging to Cyanophyceae, Dinophyceae and Prymnesiophyceae) dominated. In 1997, picocyanobacteria, mainly in colonies, made up 40–50% of the total phytoplankton carbon biomass in the top 20 m both day and night. Colony-forming species of picocyanobacteria seem to be occasionally important and hitherto underestimated in the Baltic Sea. We found species-specific depth distribution patterns. Nodularia spumigena and Anabaena spp. were observed mainly above 10 m depth, while Aphanizomenon sp. was mostly found deeper, especially at night. Dinophysis norvegica was only abundant near the seasonal pycnocline and showed very limited diurnal migration. Other flagellates, including small Cryptophyceae and 10 identified Chrysochromulina species, occurred down to 40 m depth. Their vertical migration may help to retrieve nutrients from below the summer pycnocline. We conclude that considerable differences in dominating functional groups may occur between years/bloom stages, and that the vertical distribution pattern of many species is recurring at similar environmental conditions, suggesting species-specific niche- separation. # 2006 Elsevier B.V. All rights reserved.

Keywords: Baltic Sea; Chrysochromulina; Picocyanobacteria; Phytoplankton; Vertical distribution

1. Introduction lead to a nutrient-depleted euphotic zone, isolated from the nutrient-rich water below. This condition strongly Species-specific nutrient requirements are key influences the species composition (Smayda, 1997) and factors in regulating the phytoplankton community the vertical distribution of phytoplankton (Cushing, (Tilman, 1982) and will lead to modifications in the 1989) affecting the coupling between primary and community structure when nutrient availability changes secondary production. It may also lead to harmful algal (Sommer, 1989). Physical and biological interactions blooms (Smayda, 1997) and favour diazotrophs, also determine the success of different species (e.g. mixotrophs or phytoplankton species with other Cushing, 1989; Hansen et al., 1995; Grane´li et al., 1995; qualities, e.g. ability to migrate vertically, or possession Suikkanen et al., 2004). Strong water stratification may of a high surface to volume ratio that gives them competitive advantages in a nutrient-depleted environment (Kilham and Kilham, 1980; Smayda, 1997). * Corresponding author. Tel.: +46 8 161730/18 425827; In the brackish Baltic Sea proper, noxious blooms of fax: +46 8 158417. diazotrophic cyanobacteria are common (Kononen, E-mail address: [email protected] (S. Hajdu). 1992; Wasmund, 1997) due to nitrogen limitation of the

1568-9883/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.hal.2006.07.006 190 S. Hajdu et al. / Harmful Algae 6 (2007) 189–205 spring bloom, which leaves unused dissolved inorganic different summer species in the open Baltic Sea. Here, phosphorus (DIP) that favours N2-fixing cyanobacteria we report on phytoplankton composition, including in summer (Larsson et al., 2001). Their intensive growth picocyanobacteria, and the vertical distribution patterns depletes DIP in the illuminated water column (Walve, during toxic, N-fixing cyanobacteria blooms in the open 2002) while leakage of nitrogen during growth and the Baltic Sea. decomposition of the bloom add new nitrogen (Larsson et al., 2001). The bloom co-occurs with a rapid build up 2. Materials and methods of heterotrophic biomass (Johansson et al., 2004) and fish biomass (Hjerne and Hansson, 2002) that result in 2.1. Study areas and sampling methods increased grazing pressure and nutrient sequestering (e.g. Hjerne and Hansson, 2002). These factors affect In 1994, we visited the station WGB (588220N, the phytoplankton community during a cyanobacteria 188280E), depth 128 m, in the western Gotland basin bloom and influence the structure of the pelagic food and in 1997, the station EGB (578210N, 198400E), depth web. Several Baltic studies have focused on cyano- 110 m, in the eastern Gotland basin (Fig. 1). In 1994, bacteria blooms and N2-fixation (e.g. Niemisto¨ et al., samples were collected daily, between 11 a.m. and 3 1989; Kononen, 1992; Larsson et al., 2001), but only p.m., from 24 to 29 July. In 1997, samples were taken at Kononen et al. (1998) have studied the phytoplankton noon and at mid-night for two 24-h periods (8–9 community change during a bloom in the northern August). Baltic proper. Wind speed data are from the Swedish Meteorolo- The Baltic Sea proper surface water is separated gical and Hydrological Institute’s (SMHI) weather from the deep water by a permanent halocline at 60– station at Landsort (1994) and from ship readings 70 m depth and in summer a seasonal pycnocline (1997). separates an upper mixed layer of 10–20 m depth from Salinity and temperature were measured by CTD the underlaying winter water. This winter water casts (Meerestechnik Elektronik GmbH). Vertical contains some nutrients, particularly phosphorus that could be a source of nutrients for vertically migrating phytoplankton when mixed layer nutrients are exhausted (e.g. Niemisto¨ et al., 1989; Carpenter et al., 1995). This nutricline may contribute to the deep chlorophyll maxima observed in several parts of the Baltic Sea (Niemi et al., 1970; Kuosa, 1990a; Kononen et al., 1998), involving also potentially toxic species (Kaas et al., 1991; Carpenter et al., 1995; Hajdu et al., 1996). Physical and biological mechanisms have been invoked to explain such subsurface cell concentrations (e.g. Kononen et al., 1998) and their role in population dynamics (Kuosa, 1990a; Maestrini and Grane´li, 1991). The vertical distributions of many species differ between day and night, indicating either migration or differences in production/mortality rates. Due to sampling difficulties and the time consuming analysis, studies of phytoplankton vertical distributions have been limited to individual species in the Baltic Sea and elsewhere (e.g. Sommer, 1982; Olsson and Grane´li, 1991; Olli et al., 1998; Olli, 1999), and performed mostly in coastal areas (Olli et al., 1998; Olli, 1999), in the laboratory (Arvola et al., 1991), or in mesocosm experiments (Olli and Seppa¨la¨, 2001). However, little is still known about the phytoplankton community composition in offshore cyanobacterial blooms, as well Fig. 1. Study areas. Station Western Gotland Basin (WGB) sampled as the depth preferences and migration patterns of the in 1994 and station Eastern Gotland Basin (EGB) sampled in 1997. S. Hajdu et al. / Harmful Algae 6 (2007) 189–205 191 profiles of irradiance were measured with a Li-193SA directly with an Olympus VANOX-T microscope with Spherical Quantum Sensor (Li-Cor Bioscience). a 100 W mercury lamp and a green filter set (excitation In 1994, nutrients were analysed from the surface to 545 nm, barrier 590 nm) at 1250 magnification. In 80 m depth (0, 1, 2, 5 m, then every fifth meter down to 1997, we used the same microscope and filter set as in 30 m, and thereafter every 10th meter) and chlorophyll 1994, but counts were performed after transferring a every fifth meter down to 20 m depth. In 1997, we epifluorescence images to an image analyser through a used CTD-data and data on nutrient concentrations (0, grey level camera (MTI-SIT 66). Cell sizes of 50 cells 20, 40, 60, 80, 100 and 125 m depth) collected at the per sample were measured with the OPTIMAS 5.0 same time by the Baltic Sea Research Institute software (cell length ranged from 0.4 to 2.37, cell width (Warnemu¨nde, Germany) at a nearby station from 0.24 to 1.71 mm). In both years, at least 300 cells (578170N, 208050E). were counted per filter (MacIssac and Stockner, 1993). Phytoplankton (including picoplankton) were ana- Mucilaginous colony-forming taxa with cells <2 mm lysed from discrete and integrated water samples (henceforth called colony-forming picocyanobacteria, (0–20 m) collected with a Ruttner water bottle and a for species see Table 1) were counted with a NIKON 20 m long plastic hose (inner diameter 19 mm), inverted microscope and phase contrast at 600 respectively. In 1994, discrete samples were taken magnification in samples preserved with acid Lugol’s from 0, 1, 2, 5, 10, 15, 30 m for picoplankton, and solution in 1994. In 1997, colony-forming taxa were from every 2 m down to 20 m depth for phytoplankton counted on the same filter as single-celled picocyano- >2 mm. On July 25, additional samples were taken bacteria in epifluorescence (Olympus VANOX-T micro- every 6 h, from 3 a.m. to 11 p.m., to study diurnal scope) in two diagonals, at 750 magnification. Number vertical migration of Dinophysis spp. In 1997, we of cells per colony, colony size and individual cell size collected phytoplankton samples on August 8, at were determined from epifluorescence images (Table 2). 13:30 h GMT, from the surface and then every second Cell density of the colonies was calculated as number of meter to 20 m depth and on 9 August, at 0:30, 12:00 cells per colony divided by the colony area measured in and 23:00 h GMT, from every 5 to 30 m and at 40 and the two dimensional epifluorescence images (Table 2). 60 m. In daytime, additional samples from every To simplify counting, colonies were enumerated in three meter between 10 and 20 m depth were taken for groups with different cell densities (Table 2) and cell enumeration of Dinophysis cells. volumes (Table 1): compact colonies (Fig. 2a), loose colonies (Fig. 2b) and colonies with cells organised in 2.2. Analytical methods rows (Aphanothece parallelliformis Cronberg) (Fig. 2c). Picoplankton cell volumes were calculated either as In 1994, phosphate and inorganic nitrogen (ammo- spheres or as ovoid cells (V = p [(W2 L)/4 W3/ nium, nitrite, and nitrate) concentrations were measured 12]) (Hagstro¨m et al., 1979). Cells with length/width on ship using standard methods (Grasshoff et al., 1983). ratio >0.8 and <1.20 were considered as spheres Detection limits for phosphate, ammonium, nitrite and and with ratio 1.20 as ovoid. In 1994 cell volume of nitrate were 0.016, 0.07, 0.02 and 0.02 mM, respec- single-celled picocyanobacteria was not measured tively. Chlorophyll a samples (2 l) were filtered on and therefore the mean cell volume of 516 cells from 47 mm Whatman GF/F filters and stored frozen 1997 was used. Average cell volumes of single-celled (20 8C) over silica gel until analysis. Filters were and colony-forming picocyanobacteria are shown in homogenised in 90% acetone in a piston grinder, Table 1. centrifuged and the clear supernatant analysed in a Nanoplankton (cell size 2–20 mm) and microplankton Hitachi U2000 spectrophotometer. Calculations fol- (cell size 20–200 mm) were counted in samples lowed Jeffrey and Humphrey (1975). preserved with acid Lugol’s solution, after sedimentation Picoplankton (cell size <2 mm) were fixed with in Utermo¨hl chambers using a NIKON inverted micro- paraformaldehyde solution (final concentration 0.2%) scope with phase contrast. Microplankton was counted in directly after sampling and stored at 4 8C. Subsequently, diagonals or on the half/whole chamber bottom at 150 4–20 ml of the samples was filtered onto black 0.2 mm magnification. Dinophysis was always enumerated on the polycarbonate membrane filters (diameter 25 mm). The whole chamber bottom. For nanoplankton 1–4 diagonals filters were placed on glass slides and a small drop of non- were counted at 600 magnification. Several taxa were fluorescent immersion oil and a cover slip added. The counted in size groups, some of them including several slides were stored frozen (20 8C) until enumeration. In species in each (Table 1). Micro- and nanoplankton 1994, single-celled picocyanobacteria were counted biomass was calculated by multiplying the cell number 192 S. Hajdu et al. / Harmful Algae 6 (2007) 189–205

Table 1 Average cell volume (mm3) of all taxon included in the calculation of the total phytoplankton biomass Taxon Volume 1994 Volume 1997 Cyanophyta (cyanobacteria) Nostocophyceae Single picocyanobacteria 0.30 0.19–0.51 Colonial picocyanobacteria (‘‘loose’’)a 0.88 0.81 Colonial picocyanobacteria (‘‘compact’’)a 0.88 0.77 Woronichinia spp. 66 Aphanothece parallelliformis Cronberg 1.03 Pseudanabaena limnetica (Lemmermann) Koma´rek 6 Anabaena spp. (mostly A. lemmermannii P. Richter) 117 117 Aphanizomenon sp. 87 87 Nodularia spumigena Mertens (diameter 11 and 9 mm, resp.) 313 171 Cryptophyta Cryptophyceae Hemiselmis virescens Droop 10 Plagioselmis prolonga Butcher (6–7 4 mm) 40 35 P. prolonga (8 4.5 mm) 53 Rhodomonas cf. baltica Karsten 612 Teleaulax amphioxeia (Conrad) Hill 160 117 T. acuta (Butcher) Hill 260 238 Dinophyta (Dinophyceae) Prorocentrum minimum (Pavillard) Schiller 1400 Dinophysis acuminata Clapare´de and Lachmann 12,180 10,350 D. norvegica Clapare´de and Lachmann 28,000 28,000 Gymnodinium simplex (Lohmann) Kofoid and Swezy 113 256 Gymnodinium cf. sanguineum Hirasaka 24,800 Gymnodinium sp. (45–55 mm 23–27 mm) 10,400 Gymnodiniales (diameter <10 mm) 580 Gymnodiniales (diameter 10–15 mm) 820 Gymnodiniales (diameter 20–25 mm) 2000 1700 Gyrodinium spp. (10–15 mm 7–10 mm) 370 Gyrodinium spp. (25–35 mm 18–23 mm) 5270 Heterocapsa rotundata (Lohmann) Hansen 100 228 Heterocapsa triquetra (Ehrenberg) Stein 960 Lingulodinium cf. polyedrum (Stein) Dodge 14,100 Peridiniales spp. (diameter 10–15 mm) 1400 Peridiniales spp.(diameter 15–20 mm) 2700 Haptophyta Prymnesiophyceae Chrysochromulina spp. (2–4 mm)b 14 14 Chrysochromulina spp. (4–6 mm)c 60 60 Chrysochromulina spp. (>6 mm)d 133 230 Chrysophyta Chrysophyceae Uroglena/Lepidochrysis 73 Dinobryon faculiferum (Willeń) Willeń5050 Pseudopedinella tricostata (Rouchijajnen) Thomsen 34 Apedinella radians (Lohmann) Campbell 268 Diatomophyceae Attheya septentrionalis (Østrup) Crawford 96 Chaetoceros danicus P.T.Cleve 900 2050 C. impressus K.G. Jensen and Moestrup 3500 C. throndsenii (Marino, Montresor and Zingone) Marino, Montresor and Zingone 44 Coscinodiscus granii Gough 74,500 71,400 Cyclotella choctawhatcheeana Prasad 108 106 Thalassiosira baltica (Grunow) Ostenfeld 62,800 S. Hajdu et al. / Harmful Algae 6 (2007) 189–205 193 Table 1 (Continued ) Taxon Volume 1994 Volume 1997 Nitzschia sp. (40 mm 7 mm) 773 Nitzschia longissima (Bre´bisson) Ralfs 455 N. paleacea (Grunow) Grunow 149 Pseudonitzschia sp. 369 Euglenophyta Euglenophyceae Eutreptiella spp. 500 430 Chlorophyta Prasinophyceae (Micromonadophyceae) Pyramimonas spp. (7 mm 5 mm and 5 mm 4 mm, respectively) 110 77 Pyramimonas spp. (9 mm 7 mm) 270 Chlorophyceae cf. Chlamydomonas sp. 23 Monoraphidium contortum (Thuret) Koma´rkova´-Legnerova´ 255 Monoraphidium cf. komarkovae Nygaard 12 Oocystis spp. 200 175 Planctonema lauterbornii Schmidle 95 Ciliophora Litostomatea Mesodinium rubrum (Lohmann) Hamburger and Buddenbrock diameter 14–16 mm 2200 2200 diameter 20–27 mm 7000 7000 diameter 27–33 mm 14,100 14,100 diameter 33–37 mm 22,400 diameter 37–45 mm 33,500 Others Unidentified flagellates 2–3 mm (sphere) 8 3–5 mm (ellipsoid) 34 36 5–7 mm (ellipsoid) 92 7–10 mm (ellipsoid) 220 10–15 mm (ellipsoid) 517 Miscellaneous 3–5 mm (sphere) 21 10–15 mm (sphere) 536 5–7 mm (ellipsoid) 81 7–10 mm (ellipsoid) 265 a Included mainly Cyanodictyon balticum Cronberg, C. imperfectum Cronberg and Weibull, C. planctonicum Meyer, but also Cyanonephron styloides Hickel, Aphanothece bachmanii Komarkova´-Legenerova´ and Cronberg, Aphanocapsa delicatissima W. and G. S. West, Snowella septentrionalis Koma´rek and Hinda´k and Lemmermaniella pallida (Lemmermann) Geitler. b In 1994 included C. minor Parke et Manton, C. brachycylindra Ha¨llfors et Thomsen. c In 1994 included C. simplex Estep, Davis, Hargraves et Sieburth em. Birkhead et Pienaar, C. ephippium Parke et Manton, C. fragaria Eikrem et Edwardsen, C. scutellum Eikrem et Moestrup and C. cymbium Leadbeater et Manton. d In 1994 included C. polylepis Manton et Parke, C. hirta Manton, C. ericina Parke et Manton. with standard mean volumes from the ongoing monitor Species identification of Chrysochromulina was programme on a nearby station or from own measure- made on a sample collected on 29 July 1994 ments of 25 cells. Cell volumes were calculated from preserved with 2% osmium tetroxide (nine drops to geometric shapes and formulas recommended by the 100-ml sample) and concentrated by centrifugation. Baltic Monitoring Programme (HELCOM, 1988). Drops of material were transferred to Formvar/ Carbon biomass was estimated with the equations of carbon-coated copper grids, dried, rinsed in distilled Menden-Deuer and Lessard (2000) and used to compare water, dried again and shadowcast with chromium phytoplankton community compositions. and examined in a JEM 100SX electron microscope 194 S. Hajdu et al. / Harmful Algae 6 (2007) 189–205

Table 2 Dimensions of the colony-forming picocyanobacteria Group N Cell length (mean 1 S.D.) Cell width (mean 1 S.D.) Cell density (mean 1 S.D.) Compact colonies 188 1.35 0.3 0.94 0.18 0.306 0.189 Loose colonies 97 1.42 0.24 0.94 0.16 0.087 0.036 A. parallelliformis 48 1.65 0.29 1.0 0.24 0.336 0.128 Cell length (mm), cell width (mm) and cell density (cells mm2). by Professor Ø. Moestrup at the University of Diel changes in vertical distribution are estimated Copenhagen. from the weighed mean depth (WMD) of individual The Baltic Sea Aphanizomenon, previously reported populations (Pearre, 1973): as A. ﬂos-aquae (Linne´) Ralfs, is here called sp. due to P taxonomic uncertainties (Janson et al., 1994), although it n d WMD ¼ P i i (1) has been suggested to be a genotype of the freshwater A. ni ﬂos-aquae (Laamanen et al., 2002). This paper follows the nomenclature and system of Ha¨llfors (2004). where ni is cell number per litre seawater at depth di.

Fig. 2. Epiﬂourescence images recorded by video camera showing the three dominating types of colonial picocyanobacteria observed in the samples in August 1997 at station EGB: (a) ‘‘compact’’ colony, (b) ‘‘loose’’ colony and (c) Aphanothece parallelliformis with cells organized in rows. Bars = 10 mm. S. Hajdu et al. / Harmful Algae 6 (2007) 189–205 195

water temperature was exceptionally high and occasionally reached 25 8C in the top surface layer in daytime. The 1% irradiance level occurred at 10 m depth from 24 to 26 July and at 15 m depth on 27 July, corresponding to 25 and 5.7 mmol quanta m2 s1. Dissolved inorganic phosphorus (DIP) and nitrogen (DIN) concentrations were low in the mixed layer (below 0.05 and 0.4 mM, respectively) except towards the end of the bloom when the ammonium concentration increased (from 0.11 to 0.27 mM). Below the seasonal pycnocline, DIP concentrations increased sharply to 0.2–0.5 mM while DIN concentrations were only moderately higher (<0.7 mM). Salinity ranged from 6.3 to 6.8 and the chlorophyll a from 1.5 to 6.5 mgl1 in the mixed layer, with a deep maximum (6.5 mgl1) at 15 m on 24 July. The hepatotoxic Nodularia spumigena just started to accumulate on the surface when we arrived at the sampling site. Filamentous cyanobacteria (mainly N. spumigena and Aphanizomenon sp.) and dinoflagellates dominated the phytoplankton community (as carbon biomass, Fig. 3a, Table 3). The biomass of N. spumigena varied between 31 and 34 mgCl1. The biomass of unidentified flagellates was very low, and colony-forming picocyanobacteria constituted less than 1% of the total phytoplankton carbon (Table 3). The phytoplankton communities differed considerably between the surface and in the seasonal pycnocline (15 m depth) (Fig. 3b and c). Filamentous nitrogen-fixing cyanobacteria decreased at the surface towards the end of the cruise (Fig. 3b), while the biomass of Prymnesiophyceae (Chrysochromulina spp.) and Dinophyceae (Dinophysis norvegica) increased in the seasonal pycnocline (Fig. 3c). Depth distributions of the occurring taxa, mostly toxic and potentially toxic species, differed considerably. Most of the single-celled picocyanobacteria were 1 Fig. 3. Phytoplankton biomass (incl. picocyanobacteria) (mgCl )in found above the seasonal pycnocline, with abundances (a) integrated (0–20 m) and discrete water samples (b) surface and (c) 8 1 15 m in 24–29 July 1994 at the station WGB. DINO: Dinophyceae varying between 1.7 and 4 10 cells l (Fig. 4a). N. (autotrophic dinoflagellates); PRYM: Prymnesiophyceae (Chryso- spumigena population accumulated mainly in the top chromulina spp.); UNID < 15 mm: unidentified nanoflagellates 5 m of the water mass, while Aphanizomenon sp. was (mostly Chrysophyceae); OTHERS: Pyramimonas spp. (Chlorophy- found in the whole trophogenic layer and had bimodal ceae), P. prolonga, T. acuta, T. amphioxeia (Cryptophyceae) and E. vertical distributions (Fig. 4b and c). Their weighted gymnastica (Euglenophyceae) as most important; CYAN (fil.): filamentous cyanobacteria; CYAN (p.col.): colony-forming picocyano- mean depth (WMD) also showed distinct differences bacteria; CYAN (p.s.): single-celled picocyanobacteria. (Table 4). Anabaena spp. (mostly Anabaena lemmermannii P. Richter) also had bimodal abundance depth 3. Results distributions with the deeper peak somewhat shallower compared to Aphanizomenon sp. (Fig. 4d, Table 4). D. 3.1. Western Gotland Basin, 1994 norvegica was abundant in the seasonal pycnocline (18– 34 103 cells l1), but virtually absent above 10 m The weather during the sampling period was calm depth (Fig. 4e, Table 4). It migrated upward in the and sunny, with wind speeds mostly below 6 m s1. The morning and downward in the afternoon, but only 196 S. Hajdu et al. / Harmful Algae 6 (2007) 189–205

Table 3 Cyanobacterial and total phytoplankton biomass (mgCl1) in 0–20 m samples at the stations WGB (July 1994) and EGB (August 1997) and % of total phytoplankton biomass in brackets Date Western Gotland Basin (WGB) Eastern Gotland Basin (EGB) 24 July (day) 26 July (day) 29 July (day) 8 August (day) 9 August (night 1) 9 August (day) 9 August (night 2) CYAN (fil.) 78 (42) 88 (52) 41 (37) 34 (22) 48 (28) 63 (32) 44 (26) CYAN (p.col) 1 (0.5) 1 (0.5) 1 (0.5) 41 (26) 57 (34) 57 (29) 51 (31) CYAN (p.s.) 20 (11) 11 (6) 10 (9) 35 (23) 25 (15) 27 (14) 35 (21) Total phyto 185 171 109 157 167 197 168 CYAN (fil.): filamentous cyanobacteria; CYAN (p.col.): colony-forming picocyanobacteria; CYAN (p.s.): single-celled picocyanobacteria; Total phyto.: total phytoplankton including picocyanobacteria. between 10 and 20 m (abundance maximum was at 17 Most of the total phytoplankton and the single-celled and 12 m depth at 3 and 9 a.m., respectively, and at 15 and colony-forming picocyanobacteria biomass were and 20 m depth at 3 and 9 p.m., respectively). found above the seasonal pycnocline (Fig. 6a–c). Chrysochromulina cells <6 mm resided above the Picocyanobacteria contributed a large part (40–50%) seasonal pycnocline, with a tendency to be less of the total phytoplankton carbon biomass above 20 m abundant in the near surface layer (Fig. 4f, Table 4). both day and night (Table 3). About one third of the total Their abundance increased during the study from 2.2 to phytoplankton carbon was in the form of colony- 3.7 106 cells l1 in integrated samples. At the end of forming picocyanobacteria (Table 3), and included the cruise, Chrysochromulina cells >6 mm (dominated several Chroococcal taxa whose cells were embedded in by the potentially toxic C. polylepis) had a pronounced mucilage (Table 1). Colonies with long-oval cells maximum at 12 m depth (1.2 106 cells l1, Fig. 4g). organised in ‘‘rows’’ (Fig. 2c) belong to a newly Among other nanoflagellates, Eutreptiella gymnastica described species A. parallelliformis Cronberg (Cron- occurred mostly above 15 m depth, with maxima berg, 2003). Compact and loose colonies were mostly around 6 m, while small cryptophycean species species of the genus Cyanodiction: C. imperfectum, C. (Plagioselmis prolonga, Teleaulax amphioxeia and T. planctonicum and the newly described C. balticum acuta) had their maxima at 20 m depth (altogether Cronberg (Cronberg, 2003). Other species (Table 1) 1.4 106 cells l1, data not shown). occurred only in low numbers. Altogether, 10 Chrysochromulina species were Filamentous cyanobacteria biomass was lower identified from the sample collected on 29 July compared to 1994 (Table 3), especially due to (Table 1). Nine of them are known from the area considerably lower biomass of N. spumigena (Hajdu et al., 1996); C. cymbium is new for the northern (0.5 mgCl1 compared to 34 mgCl1). Baltic Sea proper. Vertical distributions of the most important species are shown in Fig. 7. Colony-forming picocyanobacteria 3.2. Eastern Gotland Basin, 1997 resided mainly above 20 m depth and colonies of Aphanothece were less common, while compact and Weather conditions in early August 1997 were loose colonies were equally common (Fig. 7a). N. similar to those in 1994. The mixed layer was 17–18 m spumigena and A. lemmermannii occurred mainly deep, with rather uniform temperatures between 19 and above 5 m depth, but N. spumigena filaments were 21 8C and a strong seasonal pycnocline. The 1% occasionally observed deeper, especially at night irradiance level was between 12 and 14 m depth. DIP (Fig. 7b). In contrast, the depth distribution of and DIN concentrations were low in the mixed layer Aphanizomenon sp. was centred around 10 m depth, (below 0.04 and 0.5 mM, respectively) and increased to with few cells at the surface or below the seasonal 0.3 and 0.7 mM at 40 m depth. Salinity ranged from 6.8 pycnocline (Fig. 7c, Table 4). D. norvegica was to 7.0 above 20 m depth and was slightly higher (about confined between 10 and 20 m, both day and night, 7.1) between 20 and 40 m; there was a deep chlorophyll with high cell concentrations found in thin layers maximum at 12–15 m depth (2–3 mgl1 chlorophyll a). (Fig. 7d and e; Table 4). The autotrophic ciliate Cyanobacteria and unidentified nanoflagellates Mesodinium rubrum and the nanoflagellate E. gymnas- <15 mm (mostly chrysophycean taxa) dominated the tica differed in their diurnal vertical distributions, with phytoplankton community (as carbon biomass, Fig. 5). high cell numbers in or below the seasonal pycnocline at S. Hajdu et al. / Harmful Algae 6 (2007) 189–205 197

night (Fig. 7f and g; Table 4). Cryptophycean flagellates were found in high numbers down to 30 m depth a (Fig. 7h–j), with distinct inter-specific differences in vertical distribution (Table 4). Chrysochromulina spp. were less abundant (ten to several hundred thousand cells l1) in 1997 than in 1994, with no significant differences in vertical distribution between size groups. Size group >6 mm was, however, dominated by larger cells compared to 1994 (Table 1). The small diatom, 9.1 9.9 4.2 M. rubrum E. gymnastica C. throndsenii Chaetoceros throndsenii occurred mainly near the surface (Table 4), with maximum abundance of m m 1.2 106 cells l1. b Teleaulax 7–15 4. Discussion m m Determination of the vertical distribution and Plagioselmis 5–7 migration patterns of phytoplankton is easily biased by sampling errors. Phytoplankton may accumulate in m m relatively thin layers (Lindholm, 1992) making it Hemiselmis 3–5 difficult to resolve the real vertical distribution. Additional factors which may introduce bias are water exchange (due to turbulent mixing and/or horizontal

m advection of the water mass), disruption of vertical m

6 structures by strong wind mixing and patchiness. The Chrysochromulina < depth distributions of D. norvegica from 1994 and 1997 show the need for high depth resolution to accurately m

m resolve its vertical abundance pattern. However, the 6

> general vertical distribution patterns of most species

spp. were similar in both years despite differences in sampling intervals and are in agreement with earlier studies (see below).

4.1. Community composition

Low concentrations of inorganic nitrogen in the – 16.7 7.0 7.5surface layer, 16.7 a 23.1 strong seasonal 14.9 16.2 pycnocline 9.9 and warm, sunny and calm weather, as in 1994 and 1997, favour the formation of Baltic Sea cyanobacterial surface accumu-

a lations (Wasmund, 1997), as well as the build up of phytoplankton concentrations near the seasonal pycno-

spp., since a large part of the biomass is found below 20 m. cline (e.g. Carpenter et al., 1995). Total phytoplankton carbon biomass, species composition (0–20 m) and

Teleaulax vertical distribution of species were similar between Aphanizomenon N. spumigena Anabaena D. norvegica Chrysochromulina 1994 and 1997, but the proportion of the dominating species, as carbon biomass, differed considerably. One (m) reason may be differences in nutrient availability during different stages of the cyanobacterial bloom. During intensive cyanobacterial growth, phosphorus limitation may occur (Walve, 2002), which favours species with 26 July29 July 0–20 0–20 6.8 8.3 1.8 3.0 3.8 8.0 14.1 15.2 5.7 6.7 6.0 10.9 – – – – – – – – – – – – 9 August 12:00 h9 August 23:00 h 0–40 0–40 10.1 14.3 1.6 2.1 – – 16.0 15.3 6.2 7.8 6.0 7.3 5.7 14.9 12.0 18.0 20.5 20.9 12.7 17.0 7.7 17.7 1.8 1.7 9 August 0:30 h 5–40 10.8 abilities to use nutrient resources at depth or have

The missing 0 m sample0–20 m is important is for irrelevant depth the interval depth for distribution of the species. alternative nutritional modes (e.g. mixo- and phagotro- a b 1994 24 July1997 8 August (day) 0–20 0–20 8.9 10.7 3.5 3.1 7.0 2.3 16.8 14.9 7.4 7.7 8.6 7.0 – 10.1 9.5 – – – – – –: No data or very low abundances. Table 4 Diel changes in vertical distribution calculatedYear as weighted mean depth Time (WMD) of individual populations Depth phy) and species producing toxin at nutrient limitation 198 S. Hajdu et al. / Harmful Algae 6 (2007) 189–205 , D. norvegica spp., (e) Anabaena sp., (d) ) (different scales). 1 Aphanizomenon , (c) ), others as cells l 1 N. spumigena m (ﬁlamentous species counted as meters per litre (m l m 6 > spp. Chrysochromulina m and (g) m 6 < spp. Chrysochromulina Fig. 4. Vertical distribution of dominating species in July 1994 at the station WGB. (a) Picocyanobacteria, single-cells, (b) (f) S. Hajdu et al. / Harmful Algae 6 (2007) 189–205 199

siderably higher than in 1994 (Table 3), primarily due to the much higher biomass of colony-forming picocyanobacteria. It is not clear whether their higher biomass in 1997 was due to a late bloom stage, inter-annual variability or to other factors. The amount of picocyanobacteria may, however, vary considerably between years and summer months (Albertano et al., 1997). Data from the Landsort Deep (NW Baltic proper) in Larsson et al. (2001) indicate that we actually sampled close to the bloom peak in 1994 and at a considerably later stage in 1997. Data from this station show also that colony- forming picocyanobacteria increased following the filamentous cyanobacteria peak in both years, and the total carbon biomass of the colony-forming species was much higher in 1997 than in 1994 (Hajdu, unpublished Fig. 5. Phytoplankton biomass (incl. picocyanobacteria) (mgCl1)in the integrated (0–20 m) day sample on 8 August 1997 at the station data; Larsson et al., 1998). These data suggest that the EGB (abbreviations as in Fig. 3). differences between 1994 and 1997 may be related to successional stage, and perhaps inter-annual variability. (Grane´li et al., 1995; Legrand et al., 1996; Johansson and Stal et al. (1999, 2003) suggested that picocyanobacteria Grane´li, 1999; Hajdu, 2002). During the decomposition may be nitrogen-limited and, consequently, may be stage of a bloom nanoflagellates, small diatoms and favoured by fixed nitrogen released from diazotrophs. single-celled and colony-forming picocyanobacteria, i.e. We found that aggregates of N. spumigena were efficient competitors for nutrients, are likely to be highly colonised by bacteria, the diatom Nitzschia favoured. Phytoplankton community structures may paleacea and microzooplankton in 1997, in agreement therefore be highly influenced by the N2-fixing cyano- with Gabrielson and Hamel (1985) and Hoppe (1981) bacteria blooms. In 1994, the relative contribution of the who observed a rapid colonisation and decomposition different cyanobacterial groups to the total phytoplank- of N. spumigena filaments. The decomposing bloom ton carbon (Table 3) and the relative proportions of may have favoured the development of unidentified diazotrophic species (as carbon biomass, N. spumigena, nanoflagellates and picoplankton and the high abun- 55% and Aphanizomenon sp., 44%) were very similar to dance of the small diatom C. throndsenii (1.2 those found in intensive blooms in the northern Baltic 106 cells l1 near the surface). Many nanoflagellates proper (Niemisto¨ et al., 1989; Kononen et al., 1998). At are able to ingest bacteria and those with high cell the same time, only a few potentially toxic, motile and surface to volume ratio may have benefited from mixotrophic species (D. norvegica and Chrysochromu- nutrients released during decomposition. lina spp. >6 mm) were abundant near the seasonal We conclude that considerable differences in pycnocline. In 1997, the fraction of cyanobacteria present dominating functional groups may occur between the in the total phytoplankton carbon biomass was con- growth and the decomposition phase of a cyanobacterial

Fig. 6. Vertical distribution of single-celled (CYAN p.s.) and colony-forming (CYAN p.col.) picocyanobacteria and total phytoplankton biomass (incl. picocyanobacteria) in mgCl1 on 9 August 1997 at station EGB. (a) Night 1 = 0:30 h, (b) day = 12:00 h and (c) night 2 = 23:00 h. 200 S. Hajdu et al. / Harmful Algae 6 (2007) 189–205

Fig. 7. Vertical distribution of different species during day and night on 8 and 9 August 1997 at the station EGB. (a) Colonial picocyanobacteria (103 colonies l1) on 9 August (day at 12:00 h); (b and c) N. spumigena and Aphanizomenon sp. (meters per litre, m l1) on 9 August; (d and e) D. norvegica (103 cells l1) on 8 and 9 August (different scales); (f and g) M. rubrum and E. gymnastica and (h, i and j) the cryptophyceans Hemiselmis (3–5 mm), Plagioselmis (5–7 mm) and Teleaulax (7–15 mm) (103 cells l1) on 9 August (night 1 = 00:30 h; day = 12:00 h; night 2 = 23:00 h; different scales). bloom and that the phytoplankton community composi- result. The vertical distribution patterns of single-celled tion in 1994 likely represent the growth phase, and in picocyanobacteria agreed with Kuosa’s (1988) results. 1997 the decomposition stage of a Nodularia bloom. He found them grow fast (m = 1.09 day1) even at 1% of surface light intensity. Adaptation to low light may be 4.2. Single-celled and colony-forming the prime reason for their relatively high abundance picocyanobacteria near the seasonal pycnocline. Colony-forming species occurred sparsely in 1994, Single-celled picocyanobacteria were important in but in high numbers in 1997. We found a considerably both years (Table 3). Maximum abundances (4 108 higher abundance of colonies in 1997 (2–2.5 106 l1 and 6 108 cells l1 in 1994 and 1997, respectively) compared to 2.2–4.3 104 l1) than Albertano et al. were similar to earlier studies in the northern Baltic (1997) (Middle Bank, Cental Baltic in August 1995), (Kuosa, 1988, 1990b; Kononen et al., 1998), but lower the only Baltic Sea study that reports colony-forming by 1–2 orders of magnitude than reported from the picocyanobacteria abundance. The amount of colony- central Baltic (Albertano et al., 1996). Single-celled forming picocyanobacteria may vary considerably picocyanobacteria grow fast (Kuosa, 1988; Stal et al., between summer months and between years (Albertano 1999), are sensitive to grazing (Kononen et al., 1998) et al., 1997; Hajdu unpublished data; Larsson et al., and may respond rapidly to upwelling (Kuosa, 1988). 1998). High abundance of colony-forming picocyano- Thus, pronounced spatial and temporal variability may bacteria may result from an effective nutrient uptake S. Hajdu et al. / Harmful Algae 6 (2007) 189–205 201 due to their small size (Probyn et al., 1990), and reduced by Niemisto¨ et al. (1989) and Kononen et al. (1998). N. sinking velocity and low grazing pressure due to their spumigena preferred surface water (WMD above 3 m), large mucilaginous sheaths (Walsby and Reynolds, while Aphanizomenon sp. had a maximum around 10 m 1980; Pearl, 1988). depth (WMD between 7 and 14 m with very small Despite higher picocyanobacteria abundances, our diurnal differences), in agreement with earlier studies total carbon biomass (single and colony-forming) was (e.g. Walsby et al., 1995; Heiskanen and Olli, 1996; considerably lower than the total biomass reported by Kononen et al., 1998). The consistent differences in Albertano et al. (1997) (76–86 mgCl1 compared to vertical distribution patterns indicate niche-separation 266 mgCl1). This difference may at least partly between the two species. N. spumigena is able to grow depend on inclusion of larger cells (maximum diameter at low DIP concentration due to its affinity to low 3 mm compared 2 mm) and the use of a higher carbon phosphorus level (Ks 0.016 mM, Wallstro¨m et al., conversion factor (0.294 pg C cell1 compared to 0.22– 1992). High temperature and irradiation stimulate its 0.24 pg C cell1)byAlbertano et al. (1997). According growth (Wasmund, 1997). Thus, living near the surface to their Table IV and VII, 24% of their largest cells is advantageous for N. spumigena. In contrast, (Class c) in August had an average cell volume of Aphanizomenon sp. has a wide temperature tolerance 2.9 1.6 mm3 compared to our highest and consider- (Wasmund, 1997), stores phosphorus (Larsson et al., ably less common average cell volume of 1.03 mm3 2001) and grows at low light (De Nobel et al., 1998) (A. parallelliformis). consistent with its observed vertical distribution and presence during the entire season. 4.3. Vertical distribution of nano- and D. norvegica cells occurred in high numbers in both microplankton years, but only in a thin layer at and below the 1% irradiance level, and exhibited very limited diurnal DIP and DIN concentrations remained low in the migration, as shown also by Carpenter et al. (1995) and mixed layer during both sampling periods. Nutrients for Gisselson et al. (2002). According to Gisselson et al. phytoplankton growth were probably obtained from (2002), photosynthesis supports a low Dinophysis growth regenerative processes or internal storage (Larsson rate (m = 0.10–0.17 day1) in the Baltic Sea seasonal et al., 2001; Walve, 2002) and/or from heterotrophic pycnocline, heterotrophic nutrition is needed for higher nutrition. Substantial amounts of DIP were, however, growth rates (m = up to 0.4 day1). The alloxanthin present below the seasonal pycnocline. content (a carotenoid typical of cryptophytes) of the Co-existing species in stratified and nutrient poor Baltic D. norvegica indicates adaptation to the low light environments have different survival strategies. Many at depth (Meyer-Harms and Pollehne, 1998). Janson phytoplankton flagellates are able to rapidly swim (2004) has recently shown that in the Baltic D. norvegica vertically, to satisfy their light as well as nutrient plastids are likely newly acquired from the free-living demands (e.g. Olsson and Grane´li, 1991; Passow, 1991). Teleaulax amphioxeia, a cryptophycean species, which in Mixotrophic species adapted to low light levels, our study co-occurred with D. norvegica below the 1% however, do not necessarily need to migrate, but can irradiance level. The consistent depth distribution stay near the seasonal pycnocline during long periods patterns irrespective of the seasonal pycnocline depth and form distinct abundance peaks at depth (Lindholm, suggest light determines the vertical distribution of D. 1992 and references therein; Carpenter et al., 1995). norvegica. Ingestion of Teleaulax may be an adaptation Vertical niche-separation of co-occurring phytoplank- to low light that sustains a higher heterotrophic growth ton species has also been documented both in limnetic rate. Other cryptophycean species, which had their and marine waters (e.g. Sommer, 1982; Taylor and maximum abundance between 10 and 20 m, could also Pollingher, 1987; Olli et al., 1998; Olli and Seppa¨la¨, serve as food resources. 2001) and is likely essential in maintaining vertical Chrysochromulina spp. were also abundant and structures in phytoplankton communities. unevenly distributed with depth, especially in 1994. In both years, we observed clear species-specific Abundances of Chrysochromulina spp. up to several patterns in the vertical distribution (Table 4) of several million cells per litre are not unusual in the Baltic Sea species. Despite their seemingly similar requirements when water temperature is above 13 8Candthewater for bloom development (Wasmund, 1997), the vertical mass is stratified (Hajdu et al., 1996; Hajdu, 1997; distribution of the co-occurring nitrogen-fixing cyano- Kononen et al., 1998). High subsurface cell concentra- bacteria (N. spumigena, Aphanizomenon sp. and tions have also been noted (Kaas et al., 1991; Hajdu et al., Anabaena spp.) differed considerably, as also found 1996; Kononen et al., 1998). Hajdu (2002) found that the 202 S. Hajdu et al. / Harmful Algae 6 (2007) 189–205 increase in Chrysochromulina spp. abundance in the the species-specific distribution pattern are not clear from northern Baltic proper often coincided with the period of our study, but light has been considered the most intensive growth of diazotrophic cyanobacteria. During important external factor regulating diurnal vertical the cyanobacteria bloom, phosphorus limitation may migration and vertical niche-separation of cryptophytes occur (Walve, 2002), which can stimulate phagotrophy (Sommer, 1982; references in Arvola et al., 1991). The and toxin production of Chrysochromulina spp. (e.g. chloroplasts of H. virescens, P. prolonga and Teleaulax Legrand et al., 1996; Johansson and Grane´li, 1999). Late spp. have different colours (Hill, 1992). This indicates in the 1994 cruise, Chrysochromulina spp. >6 mm had a different pigment compositions and suggests different pronounced maximum (1.2 106 cells l1)at12m light requirements, which would influence their vertical depth. Three species, C. polylepis (dominant), C. ericina distribution patterns. However, it is difficult to envisage and C. hirta, were involved, according to the SEM how these small species can perform diurnal vertical observations (Moestrup personal communication). All migrations of considerable distance since, generally, three species can grow at low light intensities (Johnsen swimming speed is proportional to size (Throndsen, et al., 1992; Rhodes and Burke, 1996), ingest detritus, 1973; Sommer, 1988). Perhaps these observations are bacteria and nanoplankton (Nygaard and Tobiesen, 1993; biased by sinking cells and water exchange or are the Jones et al., 1994; Hajdu, 2002), and are lightly grazed result of vertical migrations undertaken on less than a (Jebram, 1980; Hansen et al., 1995). Low grazing diurnal basis. pressure led to higher Chrysochromulina abundance in We conclude that considerable differences in the study of Kononen et al. (1998). The abundance dominant functional groups may occur between years increase of large Chrysochromulina (>6 mm) at the end and/or cyanobacterial bloom stages, and that the vertical of the 1994 study may be due to regenerated nutrients and segregation patterns of phytoplankton are species- increasing bacterial production (Larsson, unpublished specific, and appear to recur at similar environmental data), but the increase was too great to be explained by conditions. The differences in day and night vertical population growth alone, and water exchange may have distributions of some species, e.g. small cryptophycean contributed. flagellates, suggest migrational nutrient retrieval from Several highly motile flagellates (M. rubrum, E. depth. Additional factors, e.g. phytoplankton hetero- gymnastica and small cryptophyceae species) occurred and mixotrophy, toxicity, pigmentation, etc., may in significant numbers below 15 m, especially at night, further contribute to a complex and dynamic vertical suggesting their migration to depth to acquire nutrients. structure in Baltic Sea pelagic food webs. Both M. rubrum and E. gymnastica seem to be well adapted to exploit stratified waters. They have wide Acknowledgements temperature, salinity and light tolerances (Lindholm and Mo¨rk, 1990; Lindholm, 1995; Olli et al., 1996), are We would like to thank Prof. Ø. Moestrup fast swimmers (Throndsen, 1973; Lindholm, 1985) and (Biological Institute, University of Copenhagen) for are able to migrate to layers rich in nutrients (Lindholm his valuable help to identify Chrysochromulina spp. and and Mo¨rk, 1990; Olli and Seppa¨la¨, 2001). When they his assistant L. Haukrogh for preparing the shadowcast co-occur, competition between them is expected preparations. Dr. G. Cronberg and Prof. J. Koma´rek because of their similar behaviour and requirements. kindly helped identify some of the colony-forming However, M. rubrum, at times, seems to exploit deeper picocyanobacteria. Dr. G. Nausch, The Baltic Sea layers than E. gymnastica. Research Institute in Warnemu¨nde (Germany) provided Little is known about vertical migration and niche- data on nutrients, salinity and temperature for 1997. We separation of marine cryptophycean species. We found are grateful also to Prof. R. Elmgren and Dr. G. Ejdung Hemiselmis virescens and P. prolonga to have maximum for valuable suggestions on the manuscript and for abundances of 0.3 and 5.6 105 cells l1, respectively, linguistic corrections. We would like to thank R. below the seasonal pycnocline at night (Fig. 7h–i), while Mattsson (National Veterinary Institute, Uppsala, Teleaulax spp. occurred there in high numbers (2.0– Sweden) for toxicity analyses, Dr. B. Witek 2.8 105 cells l1 at 20–30 m depth) also during the (PHYTO-LaB, Poland) and M. Tireń for careful day, in contrast to the observation of Olli (1999) (Fig. 7j). phytoplankton analyses, and all technical personnel The high cryptophyceaen abundance below the seasonal involved in this study. Funding was provided by the pycnocline suggests that these small nanoflagellates European Union (MAST III/BASYS program MAS3- exhibit deep nutrient retrieval behaviour, as shown in lake CT96-0058), the Swedish EPA’s Marine Monitoring populations (Salonen et al., 1984). The factors regulating Program and the Swedish Foundation for Strategic S. Hajdu et al. / Harmful Algae 6 (2007) 189–205 203

Environmental Research (MISTRA: SUCOZOMA). Heiskanen, A.-S., Olli, K., 1996. Sedimentation and buoyancy of [TS] Aphanizomenon cf. flos aquae (Nostocales, Cyanophyta) in a nutrient-replete and nutrient-depleted coastal area of the Baltic Sea. Phycologia 35, 94–101. References HELCOM, 1988. Guidelines for the Baltic Monitoring Programme for the third stage. Part D. Biological determinants.In: Baltic Sea Albertano, P., Di Somma, D., Leonardi, D., Canini, A., Grilli Caiola, Environment Proceedings 27D. Helsinki Commission, Helsinki. M., 1996. Cell structure of planktic cyanobacteria in the Baltic Hill, D.R.A., 1992. Baltic Sea phytoplankton identification sheet nos. Sea. Algol. Stud. 83, 29–54. 7, 8, 12 and 13. Ann. Bot. Fenn. 29 163–166, 173–176. Albertano, P., Di Somma, D., Capucci, E., 1997. Cyanobacterial Hjerne, O., Hansson, S., 2002. The role of fish and fisheries in the picoplankton from the Central Baltic Sea: cell size classification Baltic Sea nutrient dynamics. Limnol. Oceanogr. 47, 1023– by image-analyzed fluorescence microscopy. J. Plankton Res. 19, 1032. 1405–1416. Hoppe, H.-G., 1981. Blue-green algae agglomeration in surface water: Arvola, L., Ojala, A., Barbosa, F., Heaney, S.I., 1991. Migration a microbiotope of high bacterial activity. Kieler Meeresforsch. 5, behaviour of three cryptophytes in relation to environmental 291–303. gradients: an experimental approach. Br. Phycol. J. 26, 361–373. Janson, S., 2004. Molecular evidence that plastids in the toxin- Carpenter, E.J., Janson, S., Boje, R., Pollehne, F., Chang, J., 1995. The producing dinoflagellate genus Dinophysis originate from the dinoflagellate Dinophysis norvegica: biological and ecological free-living cryptophyte Teleaulax amphioxeia. Environ. Micro- observations in the Baltic Sea. Eur. J. Phycol. 30, 1–9. biol. 6, 1102–1106. Cronberg, G., 2003. New and interesting cyanoprokaryotes from Janson, S., Carpenter, E.J., Bergman, B., 1994. Fine structure and temperate, brackish ponds and the Baltic Sea. Algol. Stud. 109, immunolocalisation of proteins in Aphanizomenon sp. from the 197–211. Baltic Sea. Eur. J. Phycol. 29, 203–211. Cushing, D.H., 1989. A difference in structure between ecosystems in Jebram, D., 1980. Prospection for a sufficient nutrition for the strongly stratified waters and in those that are only weakly cosmopolitic marine bryozoan Electra pilosa (Linnaeus). Zool. stratified. J. Plankton Res. 11, 1–13. Jahrb., Abt. Syst. Okol. Geogr. Tiere 107, 386–390. De Nobel, W.T., Matthijs, H.C.P., Von Elert, E., Mur, L.R., 1998. Jeffrey, S.W., Humphrey, G.F., 1975. New spectrophotometric equa- Comparison of the light-limited growth of the nitrogen-fixing tions for determining chlorophylls a, b, c-1 and c-2 in higher cyanobacteria Anabaena and Aphanizomenon. New Phytol. 138, plants, algae and natural phytoplankton. Biochem. Physiol. Pflan- 579–587. zen 167, 191–194. Gabrielson, J.O., Hamel, K.S., 1985. Decomposition of the Cyano- Johansson, N., Grane´li, E., 1999. Cell density, chemical composition bacterium Nodularia spumigena. Bot. Mar. 28, 23–27. and toxicity of Chrysochromulina polylepis (Haptophyta) in rela- Gisselson, L.-A˚ ., Carlsson, P., Grane´li, E., Pallon, J., 2002. Dinophysis tion to different N:P supply ratios. Mar. Biol. 135, 209–217. blooms in the deep euphotic zone of the Baltic Sea: do they grow Johansson, M., Gorokhova, E., Larsson, U., 2004. Annual variability in the dark? Harmful Algae 1, 401–418. in ciliate community structure, potential pray and predators in the Grane´li, E., Anderson, D.M., Carlsson, P., Finenko, G., Maestrini, open Baltic Sea proper. J. Plankton Res. 26, 67–80. S.Y., Sampayo, M.A.de M., Smayda, T.J., 1995. Nutrition, growth Johnsen, G., Sakshaug, E., Vernet, M., 1992. Pigment composition, rate and sensibility to grazing for the dinoflagellates Dinophysis spectral characterization and photosynthetic parameters in Chry- acuminata, D. acuta and D. norvegica. La mer 33, 149–156. sochromulina polylepis. Mar. Ecol. Prog. Ser. 83, 241–249. Grasshoff, K., Ehrhardt, M., Kremling, K. (Eds.), 1983. Methods of Jones, H.L.J., Leadbeater, B.S.C., Green, J.C., 1994. Mixotrophy in Seawater Analysis. Verlag Chemie GmbH, Weinheim. haptophytes. In: Green, J.C., Leadbeater, B.S.C. (Eds.), The Hagstro¨m, A˚ ., Larsson, U., Ho¨rstedt, P., Normark, S., 1979. Frequency Haptophyte Algae. Systematics Association Special, vol. 51. of dividing cells, a new approach to the determination of bacterial Clarendon Press, Oxford, pp. 247–263. growth rates in aquatic environments. Appl. Environ. Microbiol. Kaas, H., Larsen, J., Mohlenberg, F., Richardson, K., 1991. The 37, 805–812. Chrysochromulina polylepis bloom in the Kattegat (Scandinavia) Hajdu, S., 1997. Chrysochromulina species (Haptophyta, Prymnesio- May–June 1988. Distribution, primary production and nutrient phyceae) in the phytoplankton of the Baltic Sea. Licentiate Thesis, dynamics in the late stage of the bloom. Mar. Ecol. Prog. Ser. 79, Stockholm University, Stockholm. 151–161. Hajdu, S., 2002. Phytoplankton of Baltic environmental gradients: Kilham, P., Kilham, S., 1980. The evolutionary ecology of phyto- observations on potentially toxic species. PhD Thesis, Stockholm plankton. In: Morris, I. (Ed.), The Physiological Ecology of University, Stockholm. Phytoplankton. Blackwell Scientific Publication, Oxford, pp. Hajdu, S., Larsson, U., Moestrup, Ø., 1996. Seasonal dynamics of 571–597. Chrysochromulina species (Prymnesiophyceae) in a coastal area Kononen, K., 1992. Dynamics of the toxic cyanobacterial blooms in and a nutrient-enriched inlet of the Northern Baltic Proper. Bot. the Baltic Sea. Finn. Mar. Res. 261, 3–36. Mar. 39, 281–295. Kononen, K., Ha¨llfors, S., Kokkonen, M., Kuosa, H., Laanemets, J., Ha¨llfors, G., 2004. Checklist of Baltic Sea phytoplankton species Pavelson, J., Autio, R., 1998. Development of a subsurface (including some heterotrophic protists). In: Baltic Sea Environ- chlorophyll maximum at the entrance to the Gulf of Finland, ment Proceedings, vol. 95, pp. 1–208, available at http://www.hel- Baltic Sea. Limnol. Oceanogr. 43, 1089–1106. com.fi/stc/files/Publications/Proceedings/bsep95.pdf). Kuosa, H., 1988. Occurrence of autotrophic picoplankton along an Hansen, P.J., Nielsen, T.G., Kaas, H., 1995. Distribution and growth of open sea–inner archipelago gradient in the Gulf of Finland, Baltic protists and mesozooplankton during a bloom of Chrysochromu- Sea. Ophelia 28, 85–93. lina spp. (Prymnesiophyceae Prymnesiales). Phycologia 34, 409– Kuosa, H., 1990a. Subsurface chlorophyll maximum in the northern 416. Baltic Sea. Arch. Hydrobiol. 118, 437–447. 204 S. Hajdu et al. / Harmful Algae 6 (2007) 189–205

Kuosa, H., 1990b. Picoplanktonic cyanobacteria in the northern Baltic Olli, K., Heiskanen, A.-S., Seppa¨la¨, J., 1996. Development and fate of Sea: role in the phytoplankton community. In: Barnes, M., Gib- Eutreptiella gymnastica bloom in nutrient enriched enclosures in son, R.N. (Eds.), Trophic Relationships in the Marine Environ- the coastal Baltic Sea. J. Plankton Res. 18, 1587–1604. ment. Proceedings of the 24th European Marine Biology Olli, K., Heiskanen, A.-S., Lohikari, K., 1998. Vertical migration of Symposium. University Press, Aberdeen, pp. 11–17. autotrophic micro-organisms during a vernal bloom at the coastal Laamanen, M.J., Forsstro¨m, L., Sivonen, K., 2002. Diversity of Baltic Sea—coexistence through niche separation. Hydrobiologia Aphanizomenon flos-aquae (Cyanobacterium) populations along 363, 179–189. a Baltic Sea salinity gradient. Appl. Environ. Microbiol. 5296– Olsson, P., Grane´li, E., 1991. Observations on diurnal vertical migra- 5303. tion and phased cell division for three coexisting marine dino- Larsson, U., Hajdu, S., Andersson, L., Edler, L., 1998. Den fria flagellates. J. Plankton Res. 13, 1313–1324. vattenmassan. In: O¨ stersjo¨’97, Report on Environmental Monitor- Passow, U., 1991. Vertical migration of Gonyaulax catenata and ing, SMF Stockholm University, Stockholm, pp. 28–33, In Swed- Mesodinium rubrum. Mar. Biol. 110, 455–463. ish with English summary and legends, http://www.smf.su.se. Pearl, H., 1988. Nuisance phytoplankton blooms in coastal, estuarine, Larsson, U., Hajdu, S., Walve, J., Elmgren, R., 2001. Baltic nitrogen and inland waters. Limnol. Oceanogr. 33, 823–847. fixation estimated from the summer increase in upper mixed layer Pearre Jr., S., 1973. Vertical migration and feeding in Sagitta elegans total nitrogen. Limnol. Oceanogr. 46, 811–820. Verrill. Ecology 54, 300–314. Legrand, C., Saemunsdottir, S., Grane´li, E., 1996. Phagotrophy in Probyn, T., Waldron, H., James, A., 1990. Size-fractionated mea- Chrysochromulina polylepis (prymnesiophyceae): ingestion of surements of nitrogen uptake in aged upwelled waters: implica- fluorescent labelled algae (FLA) under different nutrient conditions for pelagic food web structures. Limnol. Oceanogr. 35, 202– tions. In: Yasumoto, T., Oshima, Y., Fukuyo, Y. (Eds.), Harmful 210. and Toxic Algal Blooms. IOC of UNESCO, Paris, pp. 339–342. Rhodes, L., Burke, B., 1996. Morphology and growth characteristics Lindholm, T., 1985. Mesodinium rubrum a unique photosynthetic of Chrysochromulina species (Haptophyceae = Prymnesiophy- ciliate. Adv. Aquat. Microbiol. 3, 1–48. ceae) isolated from New Zealand coastal waters. New Zeal. J. Lindholm, T., 1992. Ecological role of depth maxima of phytoplank- Mar. Freshw. Res. 30, 91–103. ton. Ergebn. Limnol.: Adv. Limnol. 35, 33–45. Salonen, K., Jones, R.I., Arvola, L., 1984. Hypolimnetic phosphorus Lindholm, T., 1995. Green water caused by Eutreptiella gymnastica retrival by diel vertical migration of lake phytoplankton. Freshw. (Euglenophyceae) in a stratified Baltic Sea inlet. In: Lassus, P., Biol. 14, 431–438. Arzul, G., Erard-Le Denn, E., Gentien, P., Marcaillou-Le Baut, C. Smayda, T.J., 1997. Harmful phytoplankton blooms: their ecophy- (Eds.), Harmful Marine Algal Blooms. Lavoisier, Paris, pp. 181– siology and general relevance to phytoplankton blooms in the sea. 186. Limnol. Oceanogr. 42, 1137–1153. Lindholm, T., Mo¨rk, A.C., 1990. Depth maxima of Mesodinium Sommer, U., 1982. Vertical niche separation between two closely rubrum (Lohmann) Hamburger and Buddenbrock—examples related planktonic flagellate species (Rhodomonas lens and from a stratified Baltic Sea inlet. Sarsia 75, 53–64. Rhodomonas minuta var. planctonica). J. Plankton Res. 4, MacIssac, E.A., Stockner, J., 1993. Enumeration of phototrophic 137–142. picoplankton by autofluorescence microscopy. In: Kemp, P., Sommer, U., 1988. Some size relationships in phytoflagellate motility. Sherr, B., Sherr, E., Cole, J. (Eds.), Handbook of Methods in Hydrobiologia 161, 125–131. Aquatic Microbial Ecology. Part II. Identification, Enumeration Sommer, U., 1989. The role of competition of resources in phyto- and Diversity. Lewis Publishers, Boca Raton, Florida, USA, pp. plankton succession. In: Sommer, U. (Ed.), Plankton Ecology. 187–197. Succession in Plankton Communities. Springer–Verlag, Berlin, Maestrini, S.Y., Grane´li, E., 1991. Environmental conditions and pp. 57–106. ecophysiological mechanisms which led to the 1988 Chrysochro- Stal, L.J., Staal, M., Villbrandt, M., 1999. Nutrient control of cyano- mulina polylepis bloom: a hypothesis. Oceanol. Acta 14, 397–413. bacterial blooms in the Baltic Sea. Aquat. Microb. Ecol. 18, 165– Menden-Deuer, S., Lessard, E.J., 2000. Carbon to volume relation- 173. ships for dinoflagellates, diatoms and other protist plankton. Stal, L.J., Albertano, P., Bergman, B., Von Bro¨ckel, K., Gallon, J.R., Limnol. Oceanogr. 45, 569–579. Hayes, P.K., Sivonen, K., Walsby, A.E., 2003. BASIC: Baltic Sea Meyer-Harms, B., Pollehne, F., 1998. Alloxanthin in Dinophysis cyanobacteria. An investigation of the structure and dynamics of norvegica (Dinophysiales, Dinophyceae) from the Baltic Sea. J. water blooms of cyanobacteria in the Baltic Sea—responses to a Phycol. 34, 280–285. changing environment. Cont. Shelf Res. 23, 1695–1714. Niemi, A˚ ., Skuja, H., Willeń, T., 1970. Phytoplankton from the Suikkanen, S., Fistarol, G.O., Grane´li, E., 2004. Allelopathic effects of Pojoviken—Tva¨rminne area, S. coast of Finland. Mem. Soc. the Baltic cyanobacteria Nodularia spumigena. Aphanizomenon Fauna Flora Fenn. 44, 14–28. flos-aque and Anabaena lemmermanii on algal monocultures. J. Niemisto¨, L., Rinne, I., Melvasalo, T., Niemi, A˚ ., 1989. Blue-green Exp. Mar. Biol. Ecol. 308, 85–101. algae and their nitrogen fixation in the Baltic Sea in 1980, 1982 Taylor, F.J.R., Pollingher, U., 1987. Ecology of Dinoflagellates. In: and 1984. Meri 17, 1–59. Taylor, F.J.R. (Ed.), The Biology of Dinoflagellates. Blackwell Nygaard, K., Tobiesen, A., 1993. Bacterivory in algae: a survival Scientific, Oxford, pp. 398–502. strategy during nutrient limitation. Limnol. Oceanogr. 38, 273– Throndsen, J., 1973. Motility in some nanoplankton flagellates. Norw. 279. J. Zool. 21, 192–200. Olli, K., 1999. Diel vertical migration of phytoplankton and hetero- Tilman, D., 1982. Resource Competition and Community Structure. trophic flagellates in the Gulf of Riga. J. Mar. Syst. 23, 145–163. Princenton University Press, Princenton, New Jersey. Olli, K., Seppa¨la¨, J., 2001. Vertical niche separation of phytoplankton: Wallstro¨m, K., Johansson, S., Larsson, U., 1992. Effects of nutrient large-scale mesocosm experiments. Mar. Ecol. Prog. Ser. 217, enrichment in planktonic blue-green algae in the Baltic Sea. Acta 219–233. Phytogeogr. Suec. 78, 25–31. S. Hajdu et al. / Harmful Algae 6 (2007) 189–205 205

Walsby, A.E., Reynolds, C.S., 1980. Sinking and ﬂoating. In: Morris, Walve, J., 2002. Nutrient limitation and elemental ratios in Baltic Sea I. (Ed.), The Physiological Ecology of Phytoplankton. Blackwell, plankton. PhD Thesis, Stockholm University, Stockholm. Oxford, pp. 371–431. Wasmund, N., 1997. Occurrence of cyanobacterial blooms in the Walsby, A.E., Hayes, P.K., Boje, R., 1995. The gas vesicles, buoyancy Baltic Sea in relation to environmental conditions. Int. Rev. Ges. and vertical distribution of cyanobacteria in the Baltic Sea. Eur. J. Hydrobiol. 82, 169–184. Phycol. 30, 87–94.