Taxonomical Composition and Functional Structure of Phytoplankton in Two Water Supply Reservoirs in Serbia - page 04

The most dominant group of phytoplankton in species number was Bacillariophyta. The most abundant orders were Naviculales and Cymbellales (21 taxa for both orders recorded in both reservoirs). The highest species number in this division was observed in October 2006 (69 in Garaši and 33 in Bukulja) and the lowest number of taxa was present in September 2005 in both Garaši and Bukuljareservoirs (18 and 10 taxa, respectively). From the total number of taxa within Bacillariophyta, 44 were present only in Garaši, 10 only in Bukulja and 39 taxa were present in both reservoirs. In Garaši reservoir the highest number of taxa recorded belonged to genera Nitzschia (9), Navicula (8) and Cymbella (6), while in Bukulja reservoir the highest number of taxa recorded were members of Navicula (8) and Nitszchia (6) genera. Cyclotellameneghiniana, Cyclotellaocellata (only in July 2006), Asterionellaformosa, Stephanodiscus spp. and Aulacoseiragranulata (only in October 2006) were the most abundant species recorded during the study period. According to Padiśak et al. (2009) species that inhabit eutrophic water bodies which are sensitive to the onset of stratification are Cyclotellameneghiniana (Codon C), Cyclotellaocellata (Codon C), Stephanodiscus (Codon C), Asterionellaformosa (Codon C) and Aulacoseiragranulata (Codon P) and species that inhabit mesotrophic water bodies which is sensitive to the onset of stratification is Cyclotellaocellata (Codon B).

Chlorophyta was the second dominant group of phytoplankton. According to the number of taxa, order Chlorococcales (24 taxa) stood out. The highest number of taxa within this division was recorded in November 2005 (23 taxa) in Garaši reservoir while the highest number of taxa in Bukulja was in July 2006 (27 taxa). The lowest number of taxa in both reservoirs was recorded in October 2006 (6 in Garaši and 8 in Bukulja). From the total number of taxa within Chlorophyta 9 were present only in Garaši, 15 only in Bukulja while 30 taxa were common. Scenedesmus (11 taxa in both reservoirs) and Monoraphidium (5 taxa in Garaši and 6 in Bukulja) were among the genera with the highest species number in both reservoirs. There were significant number of cells of species and varietes of Scenedesmus, Monoraphidium and Tetraedron during the studied period. Padiśak et al. (2009) indicated that these taxa are abundant in shallow, highly enriched systems - Scenedesmus (Codon J) and Tetraedron (Codon J) and in shallow eu-hypertrophic environments - Monoraphidium (Codon X1).

Another important group of phytoplankton was Cyanobacteria. Chroococcales (13 taxa) and Oscillatoriales (10 taxa) were the orders with the highest number of taxa recorded in both reservoirs. The highest number of taxa within Cyanobacteria was observed in July 2006 in Garaši reservoir (12 taxa), and in November 2005 16 taxa were observed within the same division in Bukulja reservoir. The lowest number of taxa observed within this division was in October 2006 (4 taxa) in Garaši and in September 2005 (4 taxa) in Bukulja reservoir. From the total number of taxa, 6 were present only in Bukulja, 5 only in Garaši while 15 were common. In both reservoirs genus Microcystis had the highest species number (4 taxa). In both reservoirs dominant blue-green algae were Aphanizomenonflos-aquae, Microcystisaeruginosa, Planktothrixagardhii and Anabaena affinis which are distinctive for highly eutrophic waters (Reynolds et al., 2002) and in high densities are the cause of "blooming". All of these species can cause water quality problems in lakes and reservoirs because of their possibility to generate toxins harmful for the water lifeforms (Codd et al., 2005). The most numerous species in both reservoirs was Planktothrixagardhii which is very common in shallow eutrophic lakes and rivers (Chorus, 2005) and it can produce hepatotoxic microcystins (Tonk et al., 2005). Using this kind of water for drinking, having in mind the presence of the aforementioned toxic species, requires special purification methods (Karadžić et al., 2010).

Division Euglenophyta had 10 taxa in both reservoirs together. The number of recorded taxa was quite uniform in the both reservoirs during the studied period (about 4 taxa). The smallest number of taxa was recorded in October 2006 (1 taxa) in Garaši and in July 2006 in Bukulja reservoir (1 taxa). From the total number of taxa, 7 were present in both reservoirs, while 3 taxa were present only in Garaši. All the taxa from this division belong to the order Euglenales. Two genera (Trachelomonas and Euglena) were observed within this division. Trachelomonas had higher species number than Euglena (8 taxa in Garaši and 7 in Bukulja). The presence of nutrients, especially nitrogen and phosphorus, stimulates the increase of abundance and development of Euglenophyta, particularly species of Trachelomonas (Subakov-Simić et al., 2004). According to Padiśak et al. (2009) species of Euglena inhabit ponds and water bodies rich in organic matter from husbandry or sewages (Codon W1) while species of Trahelomonas inhabit shallow, meso-eutrophic ponds and water bodies (Codon W2).

Chrysophyta was represented with orders Chromulinales and Synurales (2 and 6, respectively). Species diversity of this division was quite uniform in September 2005, November 2005 and July 2006 (3 taxa) while the lowest number of species was observed in October 2006 (1 taxa) in Bukulja. In Garaši reservoir the highest species diversity was in July 2006 while in October 2006 there were no taxa from this division. From the total number of taxa, 2 species were present only in Garaši, 2 species were present only in Bukulja while 4 species were common for both reservoirs.

Dinophyta was represented with only 3 species (Ceratiumhirundinella, Peridiniopsis sp. and Peridinium sp.) but C. hirundinella and Peridinium sp. were recorded in both reservoirs. In September 2005, November 2005 and July 2006 both species were recorded in Garaši while in October 2006, only Ceratiumhirundinella was observed. In Bukulja reservoir both species were recorded in September 2005 and July 2006, in November 2005 only Peridinium sp. was observed, while in October 2006 only Ceratiumhirundinella was present. Significant abundance of Ceratiumhirundinella was observed in October and July 2006. Ceratiumhirundinella inhabit oligo- to eutrophic water bodies (Codon Lo) and eutrophic to hypereutrophic water bodies when it's co-occurring with Microcystis spp. (Codon LM) according to Padiśak et al. (2009).

All taxa whose relative biomasses represented ≥ 5% of the total biomass in the samples from different depths were considered and their functional groups were determined. A total of 11 functional groups were identified in these two reservoirs during the study period. The list of all functional groups recorded in Garaši and Bukulja reservoirs together with their description and the species belonging to the functional group (FG) is presented in Table 2. Phytoplankton functional group structure for Garaši and Bukulja reservoirs during the study period is shown in Figure 3. There were no significant differences in phytoplankton functional group structure for these two reservoirs. Coda H1, C and J were the most dominant in September 2005 in Garaši reservoir and coda H1 and J in Bukulja reservoir. Coda C and H1 were the most dominant in November 2005 in Garaši, and coda H1 was the most dominant in Bukulja. In July 2006 the most dominant coda were C and S1 in Garaši and coda C, S1 and LM in Bukulja. In October 2006 the most dominant coda in Garaši were LM, C and S1 and in Bukulja S1 and LM.

The changes in composition and abundance experienced by phytoplankton communities in aquatic ecosystems are often unpredictable and chaotic (Benincá et al., 2008). However, they can be, at least partly, explained as the result of variations in the physical (light climate) and the chemical (nutrient availability) constraints for algal growth (Reynolds, 1984; Margalef, 1997). These changes can be recognized at the floristic level (species succession), in the phytoplankton functional structure and in the size or biomass distribution of the photo-autotrophic assemblages (e.g., Lindenschmidt and Chorus, 1998; Reynolds et al., 2002; Padisák et al., 2003).

 

03
Figure 3: Phytoplankton functional group structure for Garaši and Bukulja reservoirs during the study period.

 

Table 2: Phytoplankton functional groups (FG) recorded in Garaši and Bukulja reservoirs
Tab02