Phytoplankton Vertical Distributions and Composition in Baltic Sea
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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 environ- ment (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 concen- trations (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 1250Â 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 600Â 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 750Â 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.